Thermochemical hydrogen production from water using reducible oxide materials: a critical review

Lawrence D’Souza

doi:10.1007/s40243-013-0007-0

Thermochemical hydrogen production from water using reducible oxide materials: a critical review

D’Souza, Lawrence 2013-02-01 00:00:00 Mater Renew Sustain Energy (2013) 2:7 DOI 10.1007/s40243-013-0007-0 REVIEW PAPER Thermochemical hydrogen production from water using reducible oxide materials: a critical review Lawrence D’Souza Received: 17 September 2012 / Accepted: 3 January 2013 / Published online: 1 February 2013 The Author(s) 2013. This article is published with open access at Springerlink.com Abstract This review mainly focuses on summarizing power system using fuel cells. The non-polluting by- the different metal oxide systems utilized for water-split- product ‘water’ upon hydrogen combustion has attracted ting reaction using concentrated solar energy. Only two or world attention to save ever polluting earth environment three cyclic redox processes are considered. Particle size for sustainable future. Currently, the hydrogen is derived effect on redox reactions and economic aspect of hydrogen from fossil fuels. The smallest molecule of universe sees production via concentrated solar energy are also brieﬂy highest demand due to its non-polluting end product as discussed. Among various metal oxides system CeO sys- well as its remarkable chemical and physical properties. tem is emerging as a promising candidate and researchers There are number of chemical transformation in which have demonstrated workability of this system in the solar hydrogen is used as hydrogenating or reducing agent. cavity-receiver reactor for over 500 cycles. The highest Moreover, present trend to harvest CO into useful chem- solar thermal process efﬁciency obtained so far is about icals demands hydrogen. Many scientists around the world 0.4 %, which needs to be increased for real commercial are pessimistic about CO hydrogenation since they see applications. Among traditionally studied oxides, thin-ﬁlm raising demand for hydrogen and currently there are no real ferrites looks more promising and could meet US Depart- alternatives to fulﬁll other than fossil fuels. Researchers ment of energy target of $2.42/kg H by 2025. The cost is have been looking at different possibilities to generate mainly driven by high heliostat cost which needs to hydrogen by biological and chemical means. Electrolysis reduced signiﬁcantly for economic feasibility. Overall, of water is one of the easy and greener route to generate more work needs to be done in terms of redox material hydrogen only if electricity comes from wind, tidal, engineering, reactor technology, heliostat cost reduction photovoltaics, geothermal or hydropower. The other and gas separation technologies before commercialization greener routes are photoelectrochemical water splitting [1], of this technology. by direct splitting of water [2, 3] and solar thermochemical cycles. It is hoped that combination of several technologies Keywords Hydrogen Water splitting Solar thermal can fulﬁll future hydrogen demand. Water splitting by low valent metal oxides at high temperature is one of the clean way of hydrogen production Introduction since the temperature needed to perform chemical reaction comes from concentrated solar thermal heat. Though the Hydrogen is considered as next generation fuel to propel technology is known since more than three decades com- airplanes, automotive vehicles and virtually any stationary mercial realization is yet to happen due to numerous challenges in this technology. The off-sun hours, cloudy and rainy seasons are main drawbacks for commercial L. D’Souza (&) realization. Moreover, technology cannot be implemented SABIC Corporate Research and Innovation Center (CRI) in geographically poor sun receiving regions. at KAUST, Saudi Basic Industries Corporation, P.O. Box This review summarizes the work done in high-tem- 4545-4700, Thuwal 23955-6900, Saudi Arabia perature hydrogen production via two-step redox processes e-mail: dsouzal@sabic.com 123 Page 2 of 12 Mater Renew Sustain Energy (2013) 2:7 using various metal oxides (Table 1). Only the high-tem- M O þ yCgðÞ r ¼ xM þ yCO ð2Þ x y perature experiments demonstrated either in solar furnace M O þ yCH ¼ xM þ yð2H þ COÞ: ð3Þ x y 4 2 or in laboratory ﬁxed-bed reactor have been considered. This review does not cover the hybrid technologies or other Two-step cyclic redox processes are simplest way of forms of hydrogen production technologies. producing hydrogen by utilizing metal oxide. The solar Bilgen et al. [4] have demonstrated the possibility of reduction step is endothermic and can be written as shown splitting water directly at high temperature. The theoretical in (4), calculation for the said reaction is depicted in Fig. 1. It was 1 1 M O ! M O þ 0:5O ð4Þ x yd yd 2 ox x red found that the amount of hydrogen produced decreases Dd Dd with increase in H O partial pressure. Figure 1 gives the 2 1 1 M O þ H O ! M O þ H ð5Þ yd 2 x yd 2 red ox compounded results for partial pressure of water equal to Dd Dd 0.1 bar between 1,500 and 5,000 K. Bilgen [4] experi- 1 1 M O þ CO ! M O þ CO ð6Þ x yd 2 x yd red ox mentally demonstrated that between 2,273 and 2,773 K Dd Dd formation of about 2–3 % H when mixture of steam and (where d is non-stoichiometric coefﬁcient and Dd is change argon was passed in the crucible at the focus of the solar in non-stoichiometric coefﬁcient). furnace. The reaction (4) takes place at temperature above The dissociation of metal oxide to their respective metal 1,000 K and many metal oxide systems have been studied is written as follows [14]: over the past four decades. Several two- and three-step H O-splitting thermochemical cycles based on metal oxi- M O ¼ xM þ O : ð1Þ x y 2 des redox reactions have been reported in the literature. Nakamura [5] ﬁrst proposed the two-step redox cycle in The temperature required for few metal oxides 1977 for Fe O /FeO redox cycle; interest then diminished 3 4 conversion to their metallic form is given in Table 2. for the next two decades and thereafter a spurt of interest Except for ZnO, achieving temperature needed to reduce resulted in investigation of several other oxide systems for metal oxide to their metallic form is practically impossible thermochemical redox cycle for hydrogen generation. The due to the high temperature required. Concentration ratios high temperature required for reduction reaction can be of up to 10,000 suns have been achieved by researchers supplied by either concentrated solar energy or fossil fuels. which translate to 3,800 K. But high-temperature The solar reduction is usually carried out in the presence of operation, reactor material thermal stability and radiation an inert gas, if a reducing gas is used the reduction tem- heat losses makes the process almost impossible. The perature can be brought down substantially. The reduced temperature required to attain DG of the reaction (1) metal oxide can be oxidized back to the original state by equals zero can be substantially brought down by the use of oxidants like H OorCO .If H O is used H can be pro- 2 2 2 2 hydrocarbon as reducing agents, for example graphite or duced and if CO is used CO can be produced as shown in methane, which can be written as follows: Eqs. (5) and (6), respectively. If H O and CO are used to 2 2 oxidize the redox material alternatively or together one can produce the synthesis gas (CO ? H ) from totally renew- able sources (CO and H O) [30]. 2 2 Sibieude et al. [10] demonstrated reduction of Fe O 3 4 to FeO in a solar furnace by heating the material 300 C above its melting point. They could obtain up to 40 % conversion in air and 100 % conversion in argon atmo- sphere. Figure 2 gives the conversion rate of Fe O 3 4 to FeO as a function of temperature under argon ﬂow (20 l/h). As observed by many researchers, they also experienced that quenching the reduced oxides is very important. Table 3 summarized the FeO yield with various quenching rates. It can be seen that in presence of air up to 40 % conversion can be obtained with 373 K/s cooling rate. As per literature the reduction of Mn O to MnO occurs 3 4 Fig. 1 Theoretical composition of the different products in the above 1,773 K [31]. Sibieude et al. [10] have studied dissociation of water at high temperature; total pressure is 1 bar and partial pressure of H O is 0.1 bar reduction of Mn O to MnO in a solar furnace. They could 2 3 4 123 Mater Renew Sustain Energy (2013) 2:7 Page 3 of 12 Table 1 Summary of potential two-step water-splitting reaction systems reported in the literature Main theme T (K) for T (K) for H yield T (K) for References DG = 0 reduction (%) oxidation Fe O = 3FeO ? 1/2 O 2,500 \1,000 [5] 3 4 2 2Mn O = 6MnO ? O 2,000 1,810 0.002 900 [6] 3 4 2 -7 2Co O = 6CoO ? O 1,000 1,175 4 9 10 900 [6] 3 4 2 2Nb O = 4NbO ? O 4,000 3,600 99.7 900 [6] 2 5 2 2 ZnO = Zn ? 1/2 O 2,350 2,300 Na Na [7–9] Mn O to MnO Na 1,773 Na Na [10] 3 4 (Fe M ) O = (Fe Mn )O Na Very low 773–1,173 [11, 12] 1-x x 3 4 1-x x (Fe M ) O = (FeM)O, M = Ni, Mn, Zn Na [1,073 Na \1,073 [13] 1-x x 3 4 (Fe Mx) O = (FeM)O, M = Mn, Co, Mg Na Na Na [14] 1-x 3 4 2CdO = 2Cd ? O Na [1,473 Na Na [10] SnO = Sn ? O Na [1,873 90 % 773–873 [4, 15] 2 2 ZnFe O = Zn Fe O , Zn(g), O Na 1,173 [16] 2 4 x 3-x 4 2 x/3Fe O ? Y Zr O = Fe Y Zr O ? x/6 O Na 1,673 Na \1,273 [17, 18] 3 4 y 1-y 2-y/2 x y 1-y 2-y/2?x 2 Fe Y Zr O ? x/3H O = x/3Fe O ? Y Zr O ? x/3H x y 1-y 2-y/2?x 2 3 4 y 1-y 2-y/2 2 2CeO (s) = Ce O (s) ? 1/2O (g); Ce O (s) ? H O(g) = 2CeO (s) ? H (g) Na 2,273 Na 673–873 [19–21] 2 2 3 2 2 3 2 2 2 Ce Zr O (0 B x B 0.3) Na 1,773 Na Na [22–24] 1-x x 2 TiO = TiO x = 1.83–1.96 Na 2,573–3,073 Na Na [25] 2 x, 2 SiO = 2 SiO ? O Na 3,250 Na 2,729 [4, 26] 2 2 SiO (g) ? H O ? SiO ? H 2 2 2 WO (s) = W ? 3/2 O Na 4,183 Na 1,157 [4, 26] 3 2 W ? 3H O = WO (s) ? 3H 2 3 2 MoO = Mo ? O Na 3,986 Na 1,816 [4, 26] 2 2 Mo ? 2H O = MoO (s) ? 2H 2 2 2 3In O = In O ? 4In Na [2,780 Na 1,000 [4] 2 3 2 3 Solid acids viz. SiO ,Al O , TiO , ZnO, CaCO Na Na Na Na [27] 2 2 3 2 3 In O = In O ? O In O ? 2H O = In O ? H Na 2,473 Na 1,073 [28] 2 3 2 2 2 2 2 3 2 MnFe O ? 3CaO ? (1 - y)H O = Ca (Fe,Mn) O ? (1 - y)H Na 1,273 Na 873 [29] 2 4 2 3 3 8-y 2 Ca (Fe,Mn) O = MnFe O ? 3CaO ? (1 - y)/2 O 3 3 8-y 2 4 2 Na data not available Process is practically not feasible reaction, i.e., formation of Mn O again. They have not 3 4 Table 2 Approximate temperature required for which DG of the reaction (1) equals zero performed more extensive work on this system. 0 Ehrensberger et al. [11] have studied non-stoichiometric Metal oxide T (K) for DG = 0 FeMn oxides for two-step water-splitting reaction. They Fe O 3,700 2 3 calculated DG values for two-step Nakamura cycles FeO– Al O [4,000 2 3 Fe O and MnO–Mn O and the plotted results are shown 3 4 3 4 MgO 3,700 in the Figs. 3, 4. Figure 3 indicates that DG equals to zero ZnO 2,335 for the reduction of Mn O to MnO is at least 500 K less 3 4 TiO [4,000 2 than that of Fe O to FeO system. However, Fig. 4 indi- 3 4 SiO 4,500 2 cates that FeO can produce hydrogen between 873 and CaO 4,400 1,073 K but MnO system is unable to produce hydrogen in a signiﬁcant levels. This led the authors to think of the Fe O , TiO and SiO decompose to lower valence oxides before 2 3 2 2 possibility of combining Fe and Mn oxides to reduce spinel complete dissociation to the ﬁnal at lower temperature as well as produce hydrogen in sig- obtain about 80 % conversion at 2,173 K under atmo- niﬁcant amount in oxidation step. Authors demonstrated the oxidation of Fe O and (Fe Mn ) O(x B 0.3) in spheric pressure of air with a cooling rate of 373 K/s. The 1-y 1-x x 1-y quenching of the MnO is very important to stop backward a laboratory tubular furnace and monitored gaseous 123 Page 4 of 12 Mater Renew Sustain Energy (2013) 2:7 Fig. 2 Conversion rate of magnetite versus temperature Fig. 4 Gibbs free enthalpy DG for the water splitting reaction of 0 0 0 FeO (A ), Fe O(B ) and MnO (C ) as a function of temperature 0.947 Table 3 Inﬂuence of cooling rate on FeO yield in air and argon atmosphere -1 0 0 0.1, 0.3) to (Fe Mn ) O with x \ x forming molecular Atmosphere Quenching speed (K s ) %mol FeO 1-x x 3 4 hydrogen. The substitution of iron with 10–30 % Mn in the Air 278 0 wuestite phase did not lower the total amount of hydrogen 293 25 formed, but it changed the kinetics of the process signiﬁ- 373 40 cantly. It was also found that the process is thermody- 1,273 50 namically controlled at high temperature. The rate of water Argon 278 40 splitting decreased with increase in manganese 293 45 concentration. 373 55 They also found that during water-splitting reaction 1,273 60 (Fe Mn ) O forms manganese-rich rock salt phase and 1-x x 1-y an iron-rich spine phase due to phase segregation processes Residence time = 1 min, temperature = 2,173 K, ﬂow rate = 20 l/h [12]. Tamura et al. [13] extended the work to ‘NiMnFe’ system, as shown in the reaction schemes 7 and 8. activation Ni Mn Fe O ! Ni Mn Fe O þ O 0:5 0:5 2 4 0:5 0:5 2 4d 2 at [ 1;073 K ð7Þ watersplitting Ni Mn Fe O þ dH O ! Ni Mn Fe O þ dH 0:5 0:5 2 4d 2 0:5 0:5 2 4 2 at\1;073 K ð8Þ They performed the above two-step reaction in a solar reactor at 1,073 K. In the ﬁrst endothermic step, Ni Mn Fe O was thermally activated to get oxygen- 0.5 0.5 2 4 deﬁcient compound, in the second step the oxygen-deﬁcient compound was oxidized using H O to produce H . Since, O 2 2 2 and H were produced in two different steps, high- temperature separation of those gases can be eliminated in Fig. 3 Gibbs free enthalpy DG for the decomposition of Fe O (A, R 3 4 the proposed method. They have demonstrated the B) and Mn O (C) to FeO (A), Fe O(B) and MnO (C) as a function 3 4 0.947 workability of two-step water-splitting reaction with of temperature NiFe O ,Ni Mn Fe O and Ni Zn Zn O systems 2 4 0.5 0.5 2 4 0.5 0.5 2 4 products using mass spectrometer. At atmospheric pres- using thermogravimetric experiments. They found that sure, water with a partial pressure of about 4,200 Pa in NiFe O system needs lower reactivation rate (conducted 2 4 nitrogen was able to oxidize (Fe Mn ) O(x = 0.0, after the water-splitting reaction) compared to 1-x x 1-y 123 Mater Renew Sustain Energy (2013) 2:7 Page 5 of 12 Ni Mn Fe O system. The oxygen released during to ‘peak instantaneous efﬁciency’ but after averaging the 0.5 0.5 2 4 reduction step in NiFe O ,Ni Mn Fe O and efﬁciency over 80 % of the fuel production, the actual 2 4 0.5 0.5 2 4 Ni Zn Zn O systems were 0.2, 0.3 and 0.4 %, efﬁciency is just 0.4 %. He recalculated the solar thermal 0.5 0.5 2 4 respectively. They also demonstrated the workability of the process efﬁciency and found that the value is still lower two-step hydrogen production in solar reactors. They than that of reported by Chueh et al. [20], mainly because performed two redox cycles to prove the oxygen and the later authors did not consider the energy need for large hydrogen evolution in activation (reduction) and amount of purge gas used in redox processes. Purge gas reactivation (oxidation) processes. The activation was takes up lot of solar heat and hence results in lower solar conducted at 1,373 K in presence of Ar and reactivation thermal efﬁciency. was conducted in presence of (steam ? Ar) ﬂow at 573 K. In Kang et al. [23] have extended the work on CeO sys- the case of ZnFe O , reduction follows two pathways [16]as tem. They synthesized Ce Zr O (x = 0.6, 0.7, 0.8, 1.0) 2 4 x 1-x 2 shown in Eqs. 9 and 10. solid solutions and tested for redox reactions. They found that the reduced Ce Zr O (x = 0.5, 0.6, 0.7, 0.8, 1.0) x 1-x 2-d 3ZnFe O ¼ 3Zn þ 2Fe O þ 2O ð9Þ samples exhibited higher hydrogen production ability for 2 4 3 4 2 water splitting due to improved oxygen diffusion through 6ZnFe O ¼ 6ZnO þ 4Fe O þ O ð10Þ 2 4 3 4 2 the bulk. Kaneko et al. [22] have extended the work on 4? The reduction and oxidation steps have been Ce Zr O solid solution system. They introduced Zr x 1-x 2-d demonstrated using Xe beam experiment and solar on various ratios in CeO lattices and found that the oxygen furnace experiments. It took less than 60 s for the Zn- releasing capacity or extent of CeO reduction increases 4? ferrite to release the expected amount of O from the lattice with the increase of Zr ions similar to Kang et al.’s [23] at 1,750 K. Authors have seen deposition of Zn on the observations. The highest oxygen release was found at reactor walls during reduction step and have measured O x = 2 (Ce Zr O ) at 1,773 K in air and the amount of 2 0.8 0.2 2 released using mass spectrometer. reduced cerium was found to be about 11 % which is seven Abanades et al. [19] examined CeO /Ce O redox pairs times higher than just with bare CeO . The enhancement of 2 2 3 and demonstrated the feasibility in a solar reactor featuring an the O -releasing reaction with CeO –ZrO oxide is found 2 2 2 4? inert gas atmosphere at T = 2,273 K, P = 100–200 mbar. It to be caused by an introduction of Zr , which has smaller 3? 4? consists of two chemical steps: (1) reduction, 2CeO ? ionic radius than Ce or Ce in the ﬂuorite structure. Ce O ? 0.5O ; (2) hydrolysis, Ce O ? H O ? 2CeO ? Le Gal and Abanades [24] doped trivalent lanthanides, 2 3 2 2 3 2 2 H . The reduction step is endothermic and takes place at viz. La, Sm and Gd in CeO to form binary oxides and used 2 2 T = 2,273 K, P = 100–200 mbar; however, oxidation step in hydrogen production by solar thermal redox cycles. takes place at 673–873 K resulting in pure hydrogen which They found that trivalent lanthanide-doped material can be directly used in fuel cells application. The main improves the thermal stability of the material during con- advantages of the process are low cost material which is secutive redox cycles, but hydrogen production remains the abundantly available in nature and the process uses non- same as ceria. They also doped trivalent lanthanides in corrosive chemicals. The reduced phase is very stable at CeO –ZrO to form ternary oxides. They found that with 2 2 ambient temperature and nonreactive to moisture and oxygen 1 % gadolinium to ceria–zirconia solid solutions nearly which makes this material ideal for on-site hydrogen gener- 338.2 lmol of hydrogen per gram during one cycle with ation which in turn overcomes problem associated with the O -releasing step at 1,400 C and the H -generation 2 2 transportation. However, this technology has few drawbacks, step at 1,050 C. This quantity of hydrogen is more than a maximum heat input temperature slightly higher than with CeO –ZrO system. They also found that the addition 2 2 2,273 K, the cycle working temperature of the endothermic of lanthanum enhances the thermal stability of ceria–zir- step must be optimized to be compatible with dish or tower conia solid solution similar to as observed in cases of technologies, and to reduce sample vaporization. High lanthanum-doped CeO binary oxides. molecular weight of cerium oxides poses problem in the ﬂow Lipinski et al. [21] applied ﬁrst and second laws of of solids in the process. thermodynamics to analyze the potential of applying heat Chueh et al. [20] have extended the work on CeO recovery for realizing high efﬁciency in solar-driven CeO - system. They demonstrated the O evolution during based non-stoichiometric redox cycles to split H Oor CO . 2 2 2 reduction step, CO and H generation during oxidation step They found that at 2,000 K, with 80 % solid phase heat using the solar cavity-receiver reactor over 500 cycles. recovery, advanced materials can only increase efﬁciency They could achieve solar-to-fuel efﬁciencies of 0.7–0.8 % from 16 to 20 %, while, at 1,850 K, advanced materials and concluded that efﬁciency is limited by the system scale can improve efﬁciency from 14 to 23 %, a higher maxi- and design rather than by chemistry. However, Rager [32] mum value because of decreased re-radiation and gas pointed out that the efﬁciency 0.7–0.8 % efﬁciency refers heating at the lower value of T . red 123 Page 6 of 12 Mater Renew Sustain Energy (2013) 2:7 Inoue et al. [33] demonstrated effectiveness of a ZnO/ up to a temperature where its oxygen partial pressure is MnFe O system in a lab furnace at 1,273 K. When H O higher than in atmosphere (0.21 atm). It was found that 2 4 2 was contacted with ZnO/MnFe O at 1,273 K H forma- though FeO–Fe O and NbO –Nb O give higher yield 2 4 2 3 4 2 2 5 tion happens with the expense of oxidation of ZnO/ they need to be heated above their melting point to reduce MnFe O . The later forms spinel kind of material con- them. On the other hand, MnO–Mn O and CoO–Co O 2 4 3 4 3 4 II II III III taining Zn ,Mn ,Mn and Fe ions. The reaction hap- systems can be reduced below their melting point but II pens by incorporation of Zn ions into MnFe O crystal hydrogen yield in these systems are very low (Table 4). 2 4 II structure, accompanied by the partial oxidation of Mn in Therefore, none of the systems studied are suitable to fulﬁll III MnFe O to Mn . The second step, oxygen releasing can both desired conditions for the redox reactions. 2 4 be carried out using solar thermal route but this is not It was also tried to combine metal oxide which yields demonstrated experimentally by authors. Similarly, they higher H with metal oxide which can be reduced below its have also demonstrated H production using CaO (or melting point to ﬁnd out whether this fulﬁlls the need of Na CO ) and MnFe O by passing steam at 1,273 K [29]. redox cycle. Considering the spinel phase composition of 2 3 2 4 The mechanism of H formation is similar to that explained (Fe Co ) O the H yield obtained was 45 %, but 2 0.85 0.15 3 4 2 II III earlier, i.e., oxidation of Mn to Mn to form spinel kind during the oxidation of the (Fe Co )O system the 0.85 0.15 2? 3? 2? 3? of material (Ca Fe Mn Mn O ). equilibrium oxygen pressure of the redox system will 3 2.02 0.96 0.02 7.02 Roeb et al. [34] used monolith coatings for redox sys- successfully increases and the yield of the H will gradu- tem. They noticed that the potential of the monolith coat- ally decreases down to about 3 %. The opposite effect was ings to absorb oxygen from steam and to release hydrogen found during the reduction step, the spinel phase with decreased with the number of completed cycles which is composition (Fe Co ) O will start to be reduced at 0.85 0.15 3 4 due to sintering of the material which increases with the 2,020 K, but while reduction of the spinel the wuestite redox cycles. phase will become rich with iron and the oxygen partial Lundberg [6] performed computer model calculation for pressure will decrease leading to gradual increase in the various systems for two-step solar hydrogen productions, the reduction temperature of 2,135 K by the time the initial systems considered were CoO/Co O , MnO/Mn O , FeO/ composition is reached. 3 4 3 4 Fe O , NbO /Nb O and the halide systems FeX /Fe O An yttrium-stabilized cubic zirconia material coated 3 4 3 2 5 2 3 4 where X = F, Cl, Br and I. In his calculation he found that the with iron oxide was proposed to split water in the tem- ratio of H /H O is controlled by the temperature and oxygen perature range 1,273–1,673 K [35, 36]. Kodam et al. [17] 2 2 partial pressure generated by the redox system. The yield of studied supported Fe O –FeO system. Various amount of 3 4 the hydrogen is deﬁned as follows: iron oxide was supported on yttrium-supported ZrO for cyclic redox study. It was found that the Fe O reacts with 3 4 H ðformedÞ 2 2? YSZ to produce Fe -containing ZrO phase by releasing Y ð%Þ¼ 100 ð11Þ H ðmaxÞ oxygen molecules in the ﬁrst step. It was also found that 2? the Fe ions enters into the cubic YSZ lattice. In the where H max is the maximum amount of hydrogen that 2? second step, the Fe -containing YSZ generated hydrogen can be formed as per the formula: via steam splitting to reproduce Fe O on the cubic YSZ 3 4 MOðredÞþ H OðgÞ¼ MOðoxÞþ H ðgÞ: ð12Þ 2 2 support. The system showed good reproducibility. It was Calculations showed that FeO–Fe O and NbO –Nb O found that when the Fe O content was increased up to 3 4 3 4 2 2 5 30 wt% on the Fe O /YSZ sample [17], the sample became systems give more H yield at lower temperature and that 2 3 4 of MnO–Mn O and CoO–Co O systems give [1% H denser and harder mass after the thermal reduction step, 3 4 3 4 2 yield at any temperature. In reduction step, in order to similar to the unsupported Fe O . This is due to the fact 3 4 2? that the limitation of Fe solubility in the YSZ exists close reduce thermally oxidized metal oxide needs to be heated Table 4 The yield of H at System Yield H at DH /H at Reduction Melting point (K) 2 r 2 900 K for the different metal 900 K (%) 900 K (kJ) temperature oxide systems together with the Reduced phase Oxidized phase in air enthalpy of the reaction, the reduction temperature in air and NbO /Nb O 99.7 -62.7 3,600 2,175 1,785 2 2 5 the melting points of the system FeO/Fe O 63 -49.5 2,685 1,650 1,870 3 4 MnO/Mn O 0.002 17 1,810 2,115 1,835 3 4 -7 CoO/Co O 4 9 10 251.2 1,175 2,080 Decomposes at 1,175 3 4 123 Mater Renew Sustain Energy (2013) 2:7 Page 7 of 12 to the 25 wt% Fe O content in the Fe O /YSZ. When out in the presence of argon inert gas. Table 5 summarized 3 4 3 4 2? raising the Fe O content above 25 wt%, excess Fe ions the Zn yield found in the different experiments and dif- 3 4 would form FeO crystals on the ZrO surface, which in turn ferent conditions. melts at 1,713 K. Therefore, the Fe O contents should be Recently, the solar thermal ZnO dissociation was 3 4 ´ ´ limited to \25 % to avoid sintering of redox material and demonstrated by Lede et al. [39] in a quartz vessel its cyclic reproducibility. containing sintered ZnO, by Haueter et al. [40]ina The disadvantage of mixed iron oxide cycles where rotating cavity reactor type, and by Perkins et al. [41]in oxides are partially reduced and oxidized is their low molar an aerosol reactor type. Perkins et al. reported the O ratio of released oxygen to the total oxygen present in the measurement, which is the only clear indicator of the system. The major drawback of all systems using reactive ongoing thermal ZnO dissociation. The maximum net Zn coatings is their low ratio of hydrogen mass generated to yield was 17 % [41]. However, to-date there is no report support structure mass. Considering the properties of the in the literature which claims continuous dissociation of above problems, the cycle based on the ZnO/Zn redox pair ZnO monitored by product gas analysis for more than [7–9] is of special interest since no cyclic heating and few minutes. cooling is required and a pure metal state is achieved. It Palumbo et al. [25] have studied TiO system for two- consists of the solar endothermal dissociation of ZnO(s) step solar production of Zn from ZnO, the primary reaction into its elements; and the non-solar exothermal steam- schemes can be written as shown in reactions (15) and (16). hydrolysis of Zn into H and ZnO(s), and represented by TiO ðlÞ¼ TiO(l) þð1 x=2ÞO T 2; 300 K ð15Þ 2 2 Eqs. 13 and 15. TiO ðs,lÞþ ð2 xÞZnO(s) ¼ð2 xÞZn(g) þ TiO ðsÞ x 2 1st step ðsolar ZnO-decompositionÞ: ZnO ! Zn þ 0:5O T [ 1; 200 K ð13Þ ð16Þ 2nd step ðnon-solar Zn-hydrolysisÞ: Zn þ H O But the authors have not tried water splitting using ! ZnO þ H ð14Þ partially reduced TiO . The minimum values of x that the H and O are derived in different steps, thereby authors obtained experimentally were 1.91, 1.86 and 1.83 2 2 eliminating the need for high-temperature gas separation. for temperatures of 2,300, 2,500, and 2,700 K, This cycle has been proposed to be a promising route for respectively, in an Ar atmosphere at 1 bar. They used the solar H production from H O because of its potential of latter material to reduce ZnO to produce Zn as indicated in 2 2 reaching high-energy conversion efﬁciencies and thereby reaction (16). It is to be noted that the higher the degree of its economic competitiveness [37, 38]. decomposition, the greater the vaporization of TiO , this The ﬁrst step of the two-step ZnO/Zn water-splitting limits the efﬁciency of the water-splitting cycle using TiO cycle was ﬁrst demonstrated in a solar furnace in 1977 by system. Bilgen et al. [4]. They have demonstrated the decomposi- Sibieude et al. [10] have used CdO for two-step water- tion of ZnO in a solar furnace. They also found that Zn splitting reaction. They demonstrated reduction of CdO to yield increases if ZnO is diluted with other refractory Cd in a solar furnace at high temperature. The reaction materials like ZrO and Y O and if the reaction is carried scheme is shown in Eqs. 17–19. 2 2 3 Table 5 Mol% zinc content of condensed vapors from ZnO and mixed oxides ZnO–Y O , ZnO–ZrO samples heated at the focus of 2 kW solar 2 3 2 concentrator Air p (bar) Argon atmosphere p (bar) \0.001 1 \0.001 0.034 0.092 0.263 0.789 ZnO Between 20 and No Zn formation Difﬁculties exist in obtaining due to strong volatilization of ZnO Static atmosphere 30 mol% of Zn sample; the results are poorly reproducible 68 mol% was obtained for p \ 0.001 bar Ar 45 mol% was obtained for p = 0.263 bar Ar (in a ﬂow of gas) 10 mol% ZnO 70 % 62 % 60 % 25 % Static atmosphere 90 mol% Y O 71 % 76 % 66 % 68 % 65 % Gas circulation 2 3 10 mol% ZnO 67 % 60 % 60 % 30 % Static atmosphere 90 mol% ZrO 75 % 74 % 65 % 70 % 67 % Gas circulation 123 Page 8 of 12 Mater Renew Sustain Energy (2013) 2:7 When CdO alone was heated strong vaporization produces large amount of dissociated vapors which is insufﬁciently quenched by the argon ﬂow on a water cooled wall of the condenser. The problem was overcome by mixing the CdO with ZrO ; in this case vaporization rate of Cd metal was lowered by its dispersion in the refractory metal oxide matrix which permits the effective quenching of vaporized metal. It is to be noted that partial pressure of oxygen plays a main role in the Cd yield. Figure 6 gives the %Cd metal recovered in various O partial pressures. Abanades et al. [15] have studied SnO = Sn ? 1/2O 2 2 cycle which consists of a solar endothermic reduction of SnO into SnO(g) and O followed by a non-solar exo- 2 2 thermic hydrolysis of SnO(s) to form H and SnO (s).The 2 2 thermal reduction occurs under atmospheric pressure at about 1,873 K and over. The solar step encompasses the formation of SnO nanoparticles that can be hydrolyzed Fig. 5 Cd (metal) content of condensates versus temperature after efﬁciently in the temperature range of 500–600 C with a thermal decomposition of CdO, ﬂow rate of argon was A* 3.4 cm /s and B*10cm /s H yield over 90 %. A preliminary process design is also proposed for cycle integration in solar chemical plants. CdO ! Cd þ = O T [ 1; 200 C ð17Þ They also compared their system with literature reported 2 2 ‘Sn-Souriau’ [42] three-step cycles and inferred that the Cd þ H O ! Cd(OH) þ H ð18Þ 2 2 reaction (22) producing hydrogen from the Sn/SnO mix- Cd(OH) ! CdO þ H O T [ 375 C ð19Þ 2 2 ture produced from reaction (21) is slow and partial at 600 C which results in low H yield of \45 %. The three- They observed that subjecting CdO alone to solar step cycling process proposed by them is as follows: radiation did not reduce the oxide, but when CdO was SnO ! SnO þ 1=2O ð20Þ mixed with refractory material, in their case 20 %mol 2 2 ZrO , resulted in the formation of Cd metal in the stream of 2SnO ! Sn þ SnO ð21Þ Ar. The amount of Cd metal in the deposited condensate at Sn þ 2H O ! SnO þ H ð22Þ 2 2 2 different temperatures is shown in Fig. 5. Quenching of evaporated metal was very important in Fan et al. [43] have studied steam to hydrogen this reaction. When CdO was dissociated into Cd(g) and conversion using six different metals. It is interesting to O(g) the recombination will also takes place very fast. note that only Fe and Sn are found to generate reasonable hydrogen at 873 K as shown in Table 6. Other metals did not show a good amount of hydrogen production at 873 K. Considering melting point of different metallic and their oxides states (as shown in Table 7) of Fe and Sn it can be inferred that Fe is very suitable for given application unless there is a provision to handle liquid metal in the solar reactor similar to Zn–ZnO case. The steam to H value 2 2 (c ) of 40.82 % is lower compared to one reported by Abanades et al. [15] which is equivalent to 90 % at similar conditions. If solar reactor is designed to handle liquid metals, then both Zn and Sn seems to be better candidates for two-step redox reactions with good hydrogen yield and at low-temperature operation. Recently, Cho and Kim [27] reported production of H using solid acids such as silica gel, activated Al O , 2 3 CaCO , TiO and ZnO. This is very interesting study as it 3 2 reports on liberation of hydrogen gas at very low temper- ature. They have demonstrated the possibility of H pro- Fig. 6 Dependence of Cd (metal) content of condensates on the duction using a laboratory plug ﬂow reactor. Figure 7 gives oxygen concentration of the argon ﬂow 123 Mater Renew Sustain Energy (2013) 2:7 Page 9 of 12 Table 6 Maximum per-pass conversion of H OtoH in the regen- of H , i.e., 1,590 ppm in 1 h reaction time the activated 2 2 2 eration reactor and the stable phase obtained at 873 K for counter- Al O produces highest amount of total H at 1,073 K. 2 3 2 current gas–solid operation Though the authors demonstrated the workability of the Metal phase c (%) Oxidized phase producing hydrogen from solid acids on a laboratory-scale ﬁxed-bed reactor, replication of the results in a solar Ni 0.4 NiO reactor needs to be performed to know the feasibility of the Cd 1.83 CdO process. Cu 0 Cu O One of the main problem to tackle is overcoming sin- Co 2.27 CoO tering of redox material. Agglomeration due to sintering Sn 40.82 SnO brings down the recyclability over multiple redox cycles. MnO 0 Mn O 3 4 The key properties of the redox material should include a Fe 74.79 Fe O 3 4 high oxygen carrying capacity, good mechanical properties Fe 74.79 Fe O 2 3 and cheap and easy synthetic procedures. If redox material c conversion of H OtoH do not fulﬁll any one of these key properties it would not be 2 2 a suitable material for commercial-scale operation. Table 7 Melting points [44] of various phases of Fe and Sn Particle size or grain size effect on rate of oxidation Material Melting point (K) It is generally accepted that smaller the particle size easier Fe (cast) 1,548 is to oxidize or reduce. In case of two-phase alloys the rate Fe (pure) 1,808 of oxidation may signiﬁcantly improve with grain size FeO 1,693 reduction because both mutual solubility and diffusivity Fe O 1,538 3 4 among the system will enhance [45, 46]. But this is not Fe O 1,811 always the case, during the oxidation if the top layer acts as 2 3 Sn 504 a protective layer then the further oxidation of the metals SnO 1,353 will be hampered. SnO 1,400 Figure 8 shows the oxidation kinetics of three different alloy systems with two different grain sizes at 1,073 K. The grain size reduced Cu–Cr alloy showed very slow oxidation kinetics compared to As-cast alloy. But in case of Cu–Fe and Cu–Co oxidation kinetics found to be much faster when nano-crystals (20–30 nm) were used compared to the As-cast alloy. This is because Cr O scale formed on 2 3 alloy prevents further oxidation. This is similar to in case aluminum where external layer forms Al O and prevents 2 3 further oxidation or corrosion of the aluminum metal. The reduction kinetics of metal oxides depends on many factors such as whether they are supported or unsupported, particle size, gas atmosphere, kind of metal oxides and whether single or mixed metal oxides. There are not many reports available on high-temperature reduction of metal oxides in an inert atmosphere as in the case of solar thermal reduction, but there are plenty of studies available in the Fig. 7 H concentration in the product gas stream at a reaction time literature on reduction of metal oxides using H or CO as of 1 h and total amount of H produced versus reaction temperature reducing agents. For example, Syed-Hassan and Chun-Zhu resulted from using a wetted Al O . For the experiments, 60 g of 2 3 [47] have studied the particle size effect on reduction of Al O (5.5 wt% H O) in a stainless steel reactor and 2 ml/min of CO 2 3 2 2 carrier gas were used at atmospheric pressure NiO in H atmosphere. The reduction proﬁles for NiO particles of size 20 and 24 nm are very different from that the concentration and total amount of H liberated at var- of particle size of 3.3 nm (please refer Fig. 5a in Ref. [47]). ious temperatures using activated Al O . The proﬁles for 20 and 24 nm are almost similar and very 2 3 The concentration of hydrogen produced in product gas much resemble to that of 55 nm NiO supported on SiO stream using ﬁve different oxides at &610 K is shown in substrate. Authors concluded that reduction kinetics is the Table 8. Though CaCO shows highest concentration independent of supported or unsupported NiO, but merely 123 Page 10 of 12 Mater Renew Sustain Energy (2013) 2:7 Table 8 H concentration in the product gas stream at a reaction time of 1 h Solid acid (amount) Al O (50 g) SiO (30 g) TiO (50 g) ZnO (50 g) CaCO (50 g) 2 3 2 2 3 H O (wt%) 0 5.2 0 5.3 0 5.1 0 5.1 0 5.0 H (ppm) 0 870 0 200 0 260 0 630 0 1,590 Table 9 Hydrogen cost estimation (per kg H ) in different processes Years Hy-S CuCl Ferrite S–A ZnO CdO MnO S–I 2015 $5.68 $6.83 $4.06 $7.78 $6.07 NA $ $ 2025 $3.85 $5.39 $2.42 $4.71 $4.18 NA $4.63 $4.68 the particle) takes place. In general, solid-state diffusion requires higher activation energy [48]. The number of steps of diffusion in the solid state would appear to increase with NiO conversion, resulting in continuous increases in the activation energy. This is the reason in general the reduc- tion of bigger particles crystallites needs higher activation energy than smaller one. Fig. 8 Oxidation kinetics of As-cast and grain size reduced Cu–M Economic evaluation alloys at 1,073 K. 1 Grain size reduced Cu–Cr alloy, 2 As-cast Cu–Co alloy, 3 As-cast Cu–Cr alloy, 4 grain size reduced Cu–Fe alloy, 4 As- The US DOE has established a target of $2 to $3 per kg cast Cu–Fe alloy, 6 grain size reduced Cu–Co alloy. [45] hydrogen by 2025 to make it economically affordable. The short term, i.e., 2015, DOE target is $6/kg hydrogen. Any competitive technology to produce hydrogen considers this depends on the particle/crystal size. The E versus %NiO converted trends are very different for ﬁrst reduction ﬁgure as a reference for their process efﬁciency and eco- (particle size 3.3 nm) to that of second (particle nomic evaluation. size = 20 nm) and third reduction (particle size = 24 nm) DOE in collaboration with TIAX (TIAX is a laboratory- (please refer Fig. 5b in Ref. [47]). The E for ﬁrst reduction based technology development company with a focus on remains almost unchanged (indicating single rate-limiting clean energy) led the effort of cost calculation for solar step) throughout the whole reduction process, but for sec- thermochemical hydrogen (STCH) in many US national laboratories. They considered eight promising technologies ond and third reduction steps the E proﬁles continuously increased till the complete reduction of NiO. The main for cost calculations, viz., hybrid-sulfur (HyS), copper chloride (CuCl), thin-ﬁlm nickel ferrite (‘‘ferrite’’), sulfur- reason for the difference could be due to the difference in surface to bulk atoms in different particle size crystals. The ammonia (S–A), zinc oxide (ZnO), manganese oxide size of the metallic island which forms during initial stages (MnO), sulfur-iodine (S–I), and cadmium oxide (CdO). of reduction is bigger than particle size, i.e., 3.3 nm, or the Five out of eight technologies mentioned in Table 9 whole particle surface can be instantaneously covered by appear to meet DOE’s short-term target (by 2015) of $6/kg the metallic layer without requiring a signiﬁcant growth of hydrogen but meeting long-term target seems quite difﬁ- islands; therefore, the reduction happens immediately in cult. Only thin ﬁlm ferrite is very close to DOE’s long-term those particles size crystals. But for a big particle with requirement. Even in this case achieving the long-term signiﬁcant atoms in the interior (e.g., 20 nm) it might take targets require signiﬁcant technological advances in mul- tiple dimensions. The primary cost driver for all the pro- quite some time for the growth of island to cover the whole particle’s surface. Once the surface has been covered cesses that were analyzed was heliostats costs. Reducing the heliostats cost or increase in plant efﬁciency will bring completely by the metal product, the Ni–NiO boundary would then progressively advance inward. As reduction down the CAPEX and OPEX. Most probably mass pro- duction of heliostats and improving its efﬁciency will help continues, the hydrogen radicals moves towards Ni–NiO boundary on the surface and the diffusion of atoms (i.e., to reduce the CAPEX and help to meet DOE hydrogen cost movement and rearrangements of atoms in the interior of targets. 123 Mater Renew Sustain Energy (2013) 2:7 Page 11 of 12 7. Palumbo, R., Lede, J., Boutin, O., Elorza Ricart, E., Steinfeld, A., Conclusions Moller, S., Weidenkaff, A., Fletcher, E.A., Bielicki, J.: The production of Zn from ZnO in a high-temperature solar decom- Several metal oxides have been proposed to apprehend position quench process—I. The scientiﬁc framework for the redox cycles. To-date, the solar-to-fuel efﬁciencies of process. Chem Eng Sci 53(14), 2503–2517 (1998) 8. Palumbo, R.: Solar thermal chemical processing: Challenges and prototype reactors are low, on the order of \1 %. The main changes. J Phys IV Fr 9, Pr3-35–Pr33-40 (1999) problem in large-scale solar application would be an efﬁ- 9. Weidenkaff, A., Brack, M., Mo ¨ ller, S., Palumbo, R., Steinfeld, cient fast quenching system to hinder the reoxidation of the A.: Solar thermal production of zinc: Program strategy and status reduced metal in liquid or in vapor phase. When molten of research. J Phys IV Fr 9, Pr3-313–Pr3-318 (1999) 10. 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Publisher: Springer Journals
Copyright: Copyright © 2013 by The Author(s)
Subject: Material Science; Materials Science, general; Renewable and Green Energy; Renewable and Green Energy
ISSN: 2194-1459
eISSN: 2194-1467
DOI: 10.1007/s40243-013-0007-0
Publisher site: See Article on Publisher Site

Abstract

Mater Renew Sustain Energy (2013) 2:7 DOI 10.1007/s40243-013-0007-0 REVIEW PAPER Thermochemical hydrogen production from water using reducible oxide materials: a critical review Lawrence D’Souza Received: 17 September 2012 / Accepted: 3 January 2013 / Published online: 1 February 2013 The Author(s) 2013. This article is published with open access at Springerlink.com Abstract This review mainly focuses on summarizing power system using fuel cells. The non-polluting by- the different metal oxide systems utilized for water-split- product ‘water’ upon hydrogen combustion has attracted ting reaction using concentrated solar energy. Only two or world attention to save ever polluting earth environment three cyclic redox processes are considered. Particle size for sustainable future. Currently, the hydrogen is derived effect on redox reactions and economic aspect of hydrogen from fossil fuels. The smallest molecule of universe sees production via concentrated solar energy are also brieﬂy highest demand due to its non-polluting end product as discussed. Among various metal oxides system CeO sys- well as its remarkable chemical and physical properties. tem is emerging as a promising candidate and researchers There are number of chemical transformation in which have demonstrated workability of this system in the solar hydrogen is used as hydrogenating or reducing agent. cavity-receiver reactor for over 500 cycles. The highest Moreover, present trend to harvest CO into useful chem- solar thermal process efﬁciency obtained so far is about icals demands hydrogen. Many scientists around the world 0.4 %, which needs to be increased for real commercial are pessimistic about CO hydrogenation since they see applications. Among traditionally studied oxides, thin-ﬁlm raising demand for hydrogen and currently there are no real ferrites looks more promising and could meet US Depart- alternatives to fulﬁll other than fossil fuels. Researchers ment of energy target of $2.42/kg H by 2025. The cost is have been looking at different possibilities to generate mainly driven by high heliostat cost which needs to hydrogen by biological and chemical means. Electrolysis reduced signiﬁcantly for economic feasibility. Overall, of water is one of the easy and greener route to generate more work needs to be done in terms of redox material hydrogen only if electricity comes from wind, tidal, engineering, reactor technology, heliostat cost reduction photovoltaics, geothermal or hydropower. The other and gas separation technologies before commercialization greener routes are photoelectrochemical water splitting [1], of this technology. by direct splitting of water [2, 3] and solar thermochemical cycles. It is hoped that combination of several technologies Keywords Hydrogen Water splitting Solar thermal can fulﬁll future hydrogen demand. Water splitting by low valent metal oxides at high temperature is one of the clean way of hydrogen production Introduction since the temperature needed to perform chemical reaction comes from concentrated solar thermal heat. Though the Hydrogen is considered as next generation fuel to propel technology is known since more than three decades com- airplanes, automotive vehicles and virtually any stationary mercial realization is yet to happen due to numerous challenges in this technology. The off-sun hours, cloudy and rainy seasons are main drawbacks for commercial L. D’Souza (&) realization. Moreover, technology cannot be implemented SABIC Corporate Research and Innovation Center (CRI) in geographically poor sun receiving regions. at KAUST, Saudi Basic Industries Corporation, P.O. Box This review summarizes the work done in high-tem- 4545-4700, Thuwal 23955-6900, Saudi Arabia perature hydrogen production via two-step redox processes e-mail: dsouzal@sabic.com 123 Page 2 of 12 Mater Renew Sustain Energy (2013) 2:7 using various metal oxides (Table 1). Only the high-tem- M O þ yCgðÞ r ¼ xM þ yCO ð2Þ x y perature experiments demonstrated either in solar furnace M O þ yCH ¼ xM þ yð2H þ COÞ: ð3Þ x y 4 2 or in laboratory ﬁxed-bed reactor have been considered. This review does not cover the hybrid technologies or other Two-step cyclic redox processes are simplest way of forms of hydrogen production technologies. producing hydrogen by utilizing metal oxide. The solar Bilgen et al. [4] have demonstrated the possibility of reduction step is endothermic and can be written as shown splitting water directly at high temperature. The theoretical in (4), calculation for the said reaction is depicted in Fig. 1. It was 1 1 M O ! M O þ 0:5O ð4Þ x yd yd 2 ox x red found that the amount of hydrogen produced decreases Dd Dd with increase in H O partial pressure. Figure 1 gives the 2 1 1 M O þ H O ! M O þ H ð5Þ yd 2 x yd 2 red ox compounded results for partial pressure of water equal to Dd Dd 0.1 bar between 1,500 and 5,000 K. Bilgen [4] experi- 1 1 M O þ CO ! M O þ CO ð6Þ x yd 2 x yd red ox mentally demonstrated that between 2,273 and 2,773 K Dd Dd formation of about 2–3 % H when mixture of steam and (where d is non-stoichiometric coefﬁcient and Dd is change argon was passed in the crucible at the focus of the solar in non-stoichiometric coefﬁcient). furnace. The reaction (4) takes place at temperature above The dissociation of metal oxide to their respective metal 1,000 K and many metal oxide systems have been studied is written as follows [14]: over the past four decades. Several two- and three-step H O-splitting thermochemical cycles based on metal oxi- M O ¼ xM þ O : ð1Þ x y 2 des redox reactions have been reported in the literature. Nakamura [5] ﬁrst proposed the two-step redox cycle in The temperature required for few metal oxides 1977 for Fe O /FeO redox cycle; interest then diminished 3 4 conversion to their metallic form is given in Table 2. for the next two decades and thereafter a spurt of interest Except for ZnO, achieving temperature needed to reduce resulted in investigation of several other oxide systems for metal oxide to their metallic form is practically impossible thermochemical redox cycle for hydrogen generation. The due to the high temperature required. Concentration ratios high temperature required for reduction reaction can be of up to 10,000 suns have been achieved by researchers supplied by either concentrated solar energy or fossil fuels. which translate to 3,800 K. But high-temperature The solar reduction is usually carried out in the presence of operation, reactor material thermal stability and radiation an inert gas, if a reducing gas is used the reduction tem- heat losses makes the process almost impossible. The perature can be brought down substantially. The reduced temperature required to attain DG of the reaction (1) metal oxide can be oxidized back to the original state by equals zero can be substantially brought down by the use of oxidants like H OorCO .If H O is used H can be pro- 2 2 2 2 hydrocarbon as reducing agents, for example graphite or duced and if CO is used CO can be produced as shown in methane, which can be written as follows: Eqs. (5) and (6), respectively. If H O and CO are used to 2 2 oxidize the redox material alternatively or together one can produce the synthesis gas (CO ? H ) from totally renew- able sources (CO and H O) [30]. 2 2 Sibieude et al. [10] demonstrated reduction of Fe O 3 4 to FeO in a solar furnace by heating the material 300 C above its melting point. They could obtain up to 40 % conversion in air and 100 % conversion in argon atmo- sphere. Figure 2 gives the conversion rate of Fe O 3 4 to FeO as a function of temperature under argon ﬂow (20 l/h). As observed by many researchers, they also experienced that quenching the reduced oxides is very important. Table 3 summarized the FeO yield with various quenching rates. It can be seen that in presence of air up to 40 % conversion can be obtained with 373 K/s cooling rate. As per literature the reduction of Mn O to MnO occurs 3 4 Fig. 1 Theoretical composition of the different products in the above 1,773 K [31]. Sibieude et al. [10] have studied dissociation of water at high temperature; total pressure is 1 bar and partial pressure of H O is 0.1 bar reduction of Mn O to MnO in a solar furnace. They could 2 3 4 123 Mater Renew Sustain Energy (2013) 2:7 Page 3 of 12 Table 1 Summary of potential two-step water-splitting reaction systems reported in the literature Main theme T (K) for T (K) for H yield T (K) for References DG = 0 reduction (%) oxidation Fe O = 3FeO ? 1/2 O 2,500 \1,000 [5] 3 4 2 2Mn O = 6MnO ? O 2,000 1,810 0.002 900 [6] 3 4 2 -7 2Co O = 6CoO ? O 1,000 1,175 4 9 10 900 [6] 3 4 2 2Nb O = 4NbO ? O 4,000 3,600 99.7 900 [6] 2 5 2 2 ZnO = Zn ? 1/2 O 2,350 2,300 Na Na [7–9] Mn O to MnO Na 1,773 Na Na [10] 3 4 (Fe M ) O = (Fe Mn )O Na Very low 773–1,173 [11, 12] 1-x x 3 4 1-x x (Fe M ) O = (FeM)O, M = Ni, Mn, Zn Na [1,073 Na \1,073 [13] 1-x x 3 4 (Fe Mx) O = (FeM)O, M = Mn, Co, Mg Na Na Na [14] 1-x 3 4 2CdO = 2Cd ? O Na [1,473 Na Na [10] SnO = Sn ? O Na [1,873 90 % 773–873 [4, 15] 2 2 ZnFe O = Zn Fe O , Zn(g), O Na 1,173 [16] 2 4 x 3-x 4 2 x/3Fe O ? Y Zr O = Fe Y Zr O ? x/6 O Na 1,673 Na \1,273 [17, 18] 3 4 y 1-y 2-y/2 x y 1-y 2-y/2?x 2 Fe Y Zr O ? x/3H O = x/3Fe O ? Y Zr O ? x/3H x y 1-y 2-y/2?x 2 3 4 y 1-y 2-y/2 2 2CeO (s) = Ce O (s) ? 1/2O (g); Ce O (s) ? H O(g) = 2CeO (s) ? H (g) Na 2,273 Na 673–873 [19–21] 2 2 3 2 2 3 2 2 2 Ce Zr O (0 B x B 0.3) Na 1,773 Na Na [22–24] 1-x x 2 TiO = TiO x = 1.83–1.96 Na 2,573–3,073 Na Na [25] 2 x, 2 SiO = 2 SiO ? O Na 3,250 Na 2,729 [4, 26] 2 2 SiO (g) ? H O ? SiO ? H 2 2 2 WO (s) = W ? 3/2 O Na 4,183 Na 1,157 [4, 26] 3 2 W ? 3H O = WO (s) ? 3H 2 3 2 MoO = Mo ? O Na 3,986 Na 1,816 [4, 26] 2 2 Mo ? 2H O = MoO (s) ? 2H 2 2 2 3In O = In O ? 4In Na [2,780 Na 1,000 [4] 2 3 2 3 Solid acids viz. SiO ,Al O , TiO , ZnO, CaCO Na Na Na Na [27] 2 2 3 2 3 In O = In O ? O In O ? 2H O = In O ? H Na 2,473 Na 1,073 [28] 2 3 2 2 2 2 2 3 2 MnFe O ? 3CaO ? (1 - y)H O = Ca (Fe,Mn) O ? (1 - y)H Na 1,273 Na 873 [29] 2 4 2 3 3 8-y 2 Ca (Fe,Mn) O = MnFe O ? 3CaO ? (1 - y)/2 O 3 3 8-y 2 4 2 Na data not available Process is practically not feasible reaction, i.e., formation of Mn O again. They have not 3 4 Table 2 Approximate temperature required for which DG of the reaction (1) equals zero performed more extensive work on this system. 0 Ehrensberger et al. [11] have studied non-stoichiometric Metal oxide T (K) for DG = 0 FeMn oxides for two-step water-splitting reaction. They Fe O 3,700 2 3 calculated DG values for two-step Nakamura cycles FeO– Al O [4,000 2 3 Fe O and MnO–Mn O and the plotted results are shown 3 4 3 4 MgO 3,700 in the Figs. 3, 4. Figure 3 indicates that DG equals to zero ZnO 2,335 for the reduction of Mn O to MnO is at least 500 K less 3 4 TiO [4,000 2 than that of Fe O to FeO system. However, Fig. 4 indi- 3 4 SiO 4,500 2 cates that FeO can produce hydrogen between 873 and CaO 4,400 1,073 K but MnO system is unable to produce hydrogen in a signiﬁcant levels. This led the authors to think of the Fe O , TiO and SiO decompose to lower valence oxides before 2 3 2 2 possibility of combining Fe and Mn oxides to reduce spinel complete dissociation to the ﬁnal at lower temperature as well as produce hydrogen in sig- obtain about 80 % conversion at 2,173 K under atmo- niﬁcant amount in oxidation step. Authors demonstrated the oxidation of Fe O and (Fe Mn ) O(x B 0.3) in spheric pressure of air with a cooling rate of 373 K/s. The 1-y 1-x x 1-y quenching of the MnO is very important to stop backward a laboratory tubular furnace and monitored gaseous 123 Page 4 of 12 Mater Renew Sustain Energy (2013) 2:7 Fig. 2 Conversion rate of magnetite versus temperature Fig. 4 Gibbs free enthalpy DG for the water splitting reaction of 0 0 0 FeO (A ), Fe O(B ) and MnO (C ) as a function of temperature 0.947 Table 3 Inﬂuence of cooling rate on FeO yield in air and argon atmosphere -1 0 0 0.1, 0.3) to (Fe Mn ) O with x \ x forming molecular Atmosphere Quenching speed (K s ) %mol FeO 1-x x 3 4 hydrogen. The substitution of iron with 10–30 % Mn in the Air 278 0 wuestite phase did not lower the total amount of hydrogen 293 25 formed, but it changed the kinetics of the process signiﬁ- 373 40 cantly. It was also found that the process is thermody- 1,273 50 namically controlled at high temperature. The rate of water Argon 278 40 splitting decreased with increase in manganese 293 45 concentration. 373 55 They also found that during water-splitting reaction 1,273 60 (Fe Mn ) O forms manganese-rich rock salt phase and 1-x x 1-y an iron-rich spine phase due to phase segregation processes Residence time = 1 min, temperature = 2,173 K, ﬂow rate = 20 l/h [12]. Tamura et al. [13] extended the work to ‘NiMnFe’ system, as shown in the reaction schemes 7 and 8. activation Ni Mn Fe O ! Ni Mn Fe O þ O 0:5 0:5 2 4 0:5 0:5 2 4d 2 at [ 1;073 K ð7Þ watersplitting Ni Mn Fe O þ dH O ! Ni Mn Fe O þ dH 0:5 0:5 2 4d 2 0:5 0:5 2 4 2 at\1;073 K ð8Þ They performed the above two-step reaction in a solar reactor at 1,073 K. In the ﬁrst endothermic step, Ni Mn Fe O was thermally activated to get oxygen- 0.5 0.5 2 4 deﬁcient compound, in the second step the oxygen-deﬁcient compound was oxidized using H O to produce H . Since, O 2 2 2 and H were produced in two different steps, high- temperature separation of those gases can be eliminated in Fig. 3 Gibbs free enthalpy DG for the decomposition of Fe O (A, R 3 4 the proposed method. They have demonstrated the B) and Mn O (C) to FeO (A), Fe O(B) and MnO (C) as a function 3 4 0.947 workability of two-step water-splitting reaction with of temperature NiFe O ,Ni Mn Fe O and Ni Zn Zn O systems 2 4 0.5 0.5 2 4 0.5 0.5 2 4 products using mass spectrometer. At atmospheric pres- using thermogravimetric experiments. They found that sure, water with a partial pressure of about 4,200 Pa in NiFe O system needs lower reactivation rate (conducted 2 4 nitrogen was able to oxidize (Fe Mn ) O(x = 0.0, after the water-splitting reaction) compared to 1-x x 1-y 123 Mater Renew Sustain Energy (2013) 2:7 Page 5 of 12 Ni Mn Fe O system. The oxygen released during to ‘peak instantaneous efﬁciency’ but after averaging the 0.5 0.5 2 4 reduction step in NiFe O ,Ni Mn Fe O and efﬁciency over 80 % of the fuel production, the actual 2 4 0.5 0.5 2 4 Ni Zn Zn O systems were 0.2, 0.3 and 0.4 %, efﬁciency is just 0.4 %. He recalculated the solar thermal 0.5 0.5 2 4 respectively. They also demonstrated the workability of the process efﬁciency and found that the value is still lower two-step hydrogen production in solar reactors. They than that of reported by Chueh et al. [20], mainly because performed two redox cycles to prove the oxygen and the later authors did not consider the energy need for large hydrogen evolution in activation (reduction) and amount of purge gas used in redox processes. Purge gas reactivation (oxidation) processes. The activation was takes up lot of solar heat and hence results in lower solar conducted at 1,373 K in presence of Ar and reactivation thermal efﬁciency. was conducted in presence of (steam ? Ar) ﬂow at 573 K. In Kang et al. [23] have extended the work on CeO sys- the case of ZnFe O , reduction follows two pathways [16]as tem. They synthesized Ce Zr O (x = 0.6, 0.7, 0.8, 1.0) 2 4 x 1-x 2 shown in Eqs. 9 and 10. solid solutions and tested for redox reactions. They found that the reduced Ce Zr O (x = 0.5, 0.6, 0.7, 0.8, 1.0) x 1-x 2-d 3ZnFe O ¼ 3Zn þ 2Fe O þ 2O ð9Þ samples exhibited higher hydrogen production ability for 2 4 3 4 2 water splitting due to improved oxygen diffusion through 6ZnFe O ¼ 6ZnO þ 4Fe O þ O ð10Þ 2 4 3 4 2 the bulk. Kaneko et al. [22] have extended the work on 4? The reduction and oxidation steps have been Ce Zr O solid solution system. They introduced Zr x 1-x 2-d demonstrated using Xe beam experiment and solar on various ratios in CeO lattices and found that the oxygen furnace experiments. It took less than 60 s for the Zn- releasing capacity or extent of CeO reduction increases 4? ferrite to release the expected amount of O from the lattice with the increase of Zr ions similar to Kang et al.’s [23] at 1,750 K. Authors have seen deposition of Zn on the observations. The highest oxygen release was found at reactor walls during reduction step and have measured O x = 2 (Ce Zr O ) at 1,773 K in air and the amount of 2 0.8 0.2 2 released using mass spectrometer. reduced cerium was found to be about 11 % which is seven Abanades et al. [19] examined CeO /Ce O redox pairs times higher than just with bare CeO . The enhancement of 2 2 3 and demonstrated the feasibility in a solar reactor featuring an the O -releasing reaction with CeO –ZrO oxide is found 2 2 2 4? inert gas atmosphere at T = 2,273 K, P = 100–200 mbar. It to be caused by an introduction of Zr , which has smaller 3? 4? consists of two chemical steps: (1) reduction, 2CeO ? ionic radius than Ce or Ce in the ﬂuorite structure. Ce O ? 0.5O ; (2) hydrolysis, Ce O ? H O ? 2CeO ? Le Gal and Abanades [24] doped trivalent lanthanides, 2 3 2 2 3 2 2 H . The reduction step is endothermic and takes place at viz. La, Sm and Gd in CeO to form binary oxides and used 2 2 T = 2,273 K, P = 100–200 mbar; however, oxidation step in hydrogen production by solar thermal redox cycles. takes place at 673–873 K resulting in pure hydrogen which They found that trivalent lanthanide-doped material can be directly used in fuel cells application. The main improves the thermal stability of the material during con- advantages of the process are low cost material which is secutive redox cycles, but hydrogen production remains the abundantly available in nature and the process uses non- same as ceria. They also doped trivalent lanthanides in corrosive chemicals. The reduced phase is very stable at CeO –ZrO to form ternary oxides. They found that with 2 2 ambient temperature and nonreactive to moisture and oxygen 1 % gadolinium to ceria–zirconia solid solutions nearly which makes this material ideal for on-site hydrogen gener- 338.2 lmol of hydrogen per gram during one cycle with ation which in turn overcomes problem associated with the O -releasing step at 1,400 C and the H -generation 2 2 transportation. However, this technology has few drawbacks, step at 1,050 C. This quantity of hydrogen is more than a maximum heat input temperature slightly higher than with CeO –ZrO system. They also found that the addition 2 2 2,273 K, the cycle working temperature of the endothermic of lanthanum enhances the thermal stability of ceria–zir- step must be optimized to be compatible with dish or tower conia solid solution similar to as observed in cases of technologies, and to reduce sample vaporization. High lanthanum-doped CeO binary oxides. molecular weight of cerium oxides poses problem in the ﬂow Lipinski et al. [21] applied ﬁrst and second laws of of solids in the process. thermodynamics to analyze the potential of applying heat Chueh et al. [20] have extended the work on CeO recovery for realizing high efﬁciency in solar-driven CeO - system. They demonstrated the O evolution during based non-stoichiometric redox cycles to split H Oor CO . 2 2 2 reduction step, CO and H generation during oxidation step They found that at 2,000 K, with 80 % solid phase heat using the solar cavity-receiver reactor over 500 cycles. recovery, advanced materials can only increase efﬁciency They could achieve solar-to-fuel efﬁciencies of 0.7–0.8 % from 16 to 20 %, while, at 1,850 K, advanced materials and concluded that efﬁciency is limited by the system scale can improve efﬁciency from 14 to 23 %, a higher maxi- and design rather than by chemistry. However, Rager [32] mum value because of decreased re-radiation and gas pointed out that the efﬁciency 0.7–0.8 % efﬁciency refers heating at the lower value of T . red 123 Page 6 of 12 Mater Renew Sustain Energy (2013) 2:7 Inoue et al. [33] demonstrated effectiveness of a ZnO/ up to a temperature where its oxygen partial pressure is MnFe O system in a lab furnace at 1,273 K. When H O higher than in atmosphere (0.21 atm). It was found that 2 4 2 was contacted with ZnO/MnFe O at 1,273 K H forma- though FeO–Fe O and NbO –Nb O give higher yield 2 4 2 3 4 2 2 5 tion happens with the expense of oxidation of ZnO/ they need to be heated above their melting point to reduce MnFe O . The later forms spinel kind of material con- them. On the other hand, MnO–Mn O and CoO–Co O 2 4 3 4 3 4 II II III III taining Zn ,Mn ,Mn and Fe ions. The reaction hap- systems can be reduced below their melting point but II pens by incorporation of Zn ions into MnFe O crystal hydrogen yield in these systems are very low (Table 4). 2 4 II structure, accompanied by the partial oxidation of Mn in Therefore, none of the systems studied are suitable to fulﬁll III MnFe O to Mn . The second step, oxygen releasing can both desired conditions for the redox reactions. 2 4 be carried out using solar thermal route but this is not It was also tried to combine metal oxide which yields demonstrated experimentally by authors. Similarly, they higher H with metal oxide which can be reduced below its have also demonstrated H production using CaO (or melting point to ﬁnd out whether this fulﬁlls the need of Na CO ) and MnFe O by passing steam at 1,273 K [29]. redox cycle. Considering the spinel phase composition of 2 3 2 4 The mechanism of H formation is similar to that explained (Fe Co ) O the H yield obtained was 45 %, but 2 0.85 0.15 3 4 2 II III earlier, i.e., oxidation of Mn to Mn to form spinel kind during the oxidation of the (Fe Co )O system the 0.85 0.15 2? 3? 2? 3? of material (Ca Fe Mn Mn O ). equilibrium oxygen pressure of the redox system will 3 2.02 0.96 0.02 7.02 Roeb et al. [34] used monolith coatings for redox sys- successfully increases and the yield of the H will gradu- tem. They noticed that the potential of the monolith coat- ally decreases down to about 3 %. The opposite effect was ings to absorb oxygen from steam and to release hydrogen found during the reduction step, the spinel phase with decreased with the number of completed cycles which is composition (Fe Co ) O will start to be reduced at 0.85 0.15 3 4 due to sintering of the material which increases with the 2,020 K, but while reduction of the spinel the wuestite redox cycles. phase will become rich with iron and the oxygen partial Lundberg [6] performed computer model calculation for pressure will decrease leading to gradual increase in the various systems for two-step solar hydrogen productions, the reduction temperature of 2,135 K by the time the initial systems considered were CoO/Co O , MnO/Mn O , FeO/ composition is reached. 3 4 3 4 Fe O , NbO /Nb O and the halide systems FeX /Fe O An yttrium-stabilized cubic zirconia material coated 3 4 3 2 5 2 3 4 where X = F, Cl, Br and I. In his calculation he found that the with iron oxide was proposed to split water in the tem- ratio of H /H O is controlled by the temperature and oxygen perature range 1,273–1,673 K [35, 36]. Kodam et al. [17] 2 2 partial pressure generated by the redox system. The yield of studied supported Fe O –FeO system. Various amount of 3 4 the hydrogen is deﬁned as follows: iron oxide was supported on yttrium-supported ZrO for cyclic redox study. It was found that the Fe O reacts with 3 4 H ðformedÞ 2 2? YSZ to produce Fe -containing ZrO phase by releasing Y ð%Þ¼ 100 ð11Þ H ðmaxÞ oxygen molecules in the ﬁrst step. It was also found that 2? the Fe ions enters into the cubic YSZ lattice. In the where H max is the maximum amount of hydrogen that 2? second step, the Fe -containing YSZ generated hydrogen can be formed as per the formula: via steam splitting to reproduce Fe O on the cubic YSZ 3 4 MOðredÞþ H OðgÞ¼ MOðoxÞþ H ðgÞ: ð12Þ 2 2 support. The system showed good reproducibility. It was Calculations showed that FeO–Fe O and NbO –Nb O found that when the Fe O content was increased up to 3 4 3 4 2 2 5 30 wt% on the Fe O /YSZ sample [17], the sample became systems give more H yield at lower temperature and that 2 3 4 of MnO–Mn O and CoO–Co O systems give [1% H denser and harder mass after the thermal reduction step, 3 4 3 4 2 yield at any temperature. In reduction step, in order to similar to the unsupported Fe O . This is due to the fact 3 4 2? that the limitation of Fe solubility in the YSZ exists close reduce thermally oxidized metal oxide needs to be heated Table 4 The yield of H at System Yield H at DH /H at Reduction Melting point (K) 2 r 2 900 K for the different metal 900 K (%) 900 K (kJ) temperature oxide systems together with the Reduced phase Oxidized phase in air enthalpy of the reaction, the reduction temperature in air and NbO /Nb O 99.7 -62.7 3,600 2,175 1,785 2 2 5 the melting points of the system FeO/Fe O 63 -49.5 2,685 1,650 1,870 3 4 MnO/Mn O 0.002 17 1,810 2,115 1,835 3 4 -7 CoO/Co O 4 9 10 251.2 1,175 2,080 Decomposes at 1,175 3 4 123 Mater Renew Sustain Energy (2013) 2:7 Page 7 of 12 to the 25 wt% Fe O content in the Fe O /YSZ. When out in the presence of argon inert gas. Table 5 summarized 3 4 3 4 2? raising the Fe O content above 25 wt%, excess Fe ions the Zn yield found in the different experiments and dif- 3 4 would form FeO crystals on the ZrO surface, which in turn ferent conditions. melts at 1,713 K. Therefore, the Fe O contents should be Recently, the solar thermal ZnO dissociation was 3 4 ´ ´ limited to \25 % to avoid sintering of redox material and demonstrated by Lede et al. [39] in a quartz vessel its cyclic reproducibility. containing sintered ZnO, by Haueter et al. [40]ina The disadvantage of mixed iron oxide cycles where rotating cavity reactor type, and by Perkins et al. [41]in oxides are partially reduced and oxidized is their low molar an aerosol reactor type. Perkins et al. reported the O ratio of released oxygen to the total oxygen present in the measurement, which is the only clear indicator of the system. The major drawback of all systems using reactive ongoing thermal ZnO dissociation. The maximum net Zn coatings is their low ratio of hydrogen mass generated to yield was 17 % [41]. However, to-date there is no report support structure mass. Considering the properties of the in the literature which claims continuous dissociation of above problems, the cycle based on the ZnO/Zn redox pair ZnO monitored by product gas analysis for more than [7–9] is of special interest since no cyclic heating and few minutes. cooling is required and a pure metal state is achieved. It Palumbo et al. [25] have studied TiO system for two- consists of the solar endothermal dissociation of ZnO(s) step solar production of Zn from ZnO, the primary reaction into its elements; and the non-solar exothermal steam- schemes can be written as shown in reactions (15) and (16). hydrolysis of Zn into H and ZnO(s), and represented by TiO ðlÞ¼ TiO(l) þð1 x=2ÞO T 2; 300 K ð15Þ 2 2 Eqs. 13 and 15. TiO ðs,lÞþ ð2 xÞZnO(s) ¼ð2 xÞZn(g) þ TiO ðsÞ x 2 1st step ðsolar ZnO-decompositionÞ: ZnO ! Zn þ 0:5O T [ 1; 200 K ð13Þ ð16Þ 2nd step ðnon-solar Zn-hydrolysisÞ: Zn þ H O But the authors have not tried water splitting using ! ZnO þ H ð14Þ partially reduced TiO . The minimum values of x that the H and O are derived in different steps, thereby authors obtained experimentally were 1.91, 1.86 and 1.83 2 2 eliminating the need for high-temperature gas separation. for temperatures of 2,300, 2,500, and 2,700 K, This cycle has been proposed to be a promising route for respectively, in an Ar atmosphere at 1 bar. They used the solar H production from H O because of its potential of latter material to reduce ZnO to produce Zn as indicated in 2 2 reaching high-energy conversion efﬁciencies and thereby reaction (16). It is to be noted that the higher the degree of its economic competitiveness [37, 38]. decomposition, the greater the vaporization of TiO , this The ﬁrst step of the two-step ZnO/Zn water-splitting limits the efﬁciency of the water-splitting cycle using TiO cycle was ﬁrst demonstrated in a solar furnace in 1977 by system. Bilgen et al. [4]. They have demonstrated the decomposi- Sibieude et al. [10] have used CdO for two-step water- tion of ZnO in a solar furnace. They also found that Zn splitting reaction. They demonstrated reduction of CdO to yield increases if ZnO is diluted with other refractory Cd in a solar furnace at high temperature. The reaction materials like ZrO and Y O and if the reaction is carried scheme is shown in Eqs. 17–19. 2 2 3 Table 5 Mol% zinc content of condensed vapors from ZnO and mixed oxides ZnO–Y O , ZnO–ZrO samples heated at the focus of 2 kW solar 2 3 2 concentrator Air p (bar) Argon atmosphere p (bar) \0.001 1 \0.001 0.034 0.092 0.263 0.789 ZnO Between 20 and No Zn formation Difﬁculties exist in obtaining due to strong volatilization of ZnO Static atmosphere 30 mol% of Zn sample; the results are poorly reproducible 68 mol% was obtained for p \ 0.001 bar Ar 45 mol% was obtained for p = 0.263 bar Ar (in a ﬂow of gas) 10 mol% ZnO 70 % 62 % 60 % 25 % Static atmosphere 90 mol% Y O 71 % 76 % 66 % 68 % 65 % Gas circulation 2 3 10 mol% ZnO 67 % 60 % 60 % 30 % Static atmosphere 90 mol% ZrO 75 % 74 % 65 % 70 % 67 % Gas circulation 123 Page 8 of 12 Mater Renew Sustain Energy (2013) 2:7 When CdO alone was heated strong vaporization produces large amount of dissociated vapors which is insufﬁciently quenched by the argon ﬂow on a water cooled wall of the condenser. The problem was overcome by mixing the CdO with ZrO ; in this case vaporization rate of Cd metal was lowered by its dispersion in the refractory metal oxide matrix which permits the effective quenching of vaporized metal. It is to be noted that partial pressure of oxygen plays a main role in the Cd yield. Figure 6 gives the %Cd metal recovered in various O partial pressures. Abanades et al. [15] have studied SnO = Sn ? 1/2O 2 2 cycle which consists of a solar endothermic reduction of SnO into SnO(g) and O followed by a non-solar exo- 2 2 thermic hydrolysis of SnO(s) to form H and SnO (s).The 2 2 thermal reduction occurs under atmospheric pressure at about 1,873 K and over. The solar step encompasses the formation of SnO nanoparticles that can be hydrolyzed Fig. 5 Cd (metal) content of condensates versus temperature after efﬁciently in the temperature range of 500–600 C with a thermal decomposition of CdO, ﬂow rate of argon was A* 3.4 cm /s and B*10cm /s H yield over 90 %. A preliminary process design is also proposed for cycle integration in solar chemical plants. CdO ! Cd þ = O T [ 1; 200 C ð17Þ They also compared their system with literature reported 2 2 ‘Sn-Souriau’ [42] three-step cycles and inferred that the Cd þ H O ! Cd(OH) þ H ð18Þ 2 2 reaction (22) producing hydrogen from the Sn/SnO mix- Cd(OH) ! CdO þ H O T [ 375 C ð19Þ 2 2 ture produced from reaction (21) is slow and partial at 600 C which results in low H yield of \45 %. The three- They observed that subjecting CdO alone to solar step cycling process proposed by them is as follows: radiation did not reduce the oxide, but when CdO was SnO ! SnO þ 1=2O ð20Þ mixed with refractory material, in their case 20 %mol 2 2 ZrO , resulted in the formation of Cd metal in the stream of 2SnO ! Sn þ SnO ð21Þ Ar. The amount of Cd metal in the deposited condensate at Sn þ 2H O ! SnO þ H ð22Þ 2 2 2 different temperatures is shown in Fig. 5. Quenching of evaporated metal was very important in Fan et al. [43] have studied steam to hydrogen this reaction. When CdO was dissociated into Cd(g) and conversion using six different metals. It is interesting to O(g) the recombination will also takes place very fast. note that only Fe and Sn are found to generate reasonable hydrogen at 873 K as shown in Table 6. Other metals did not show a good amount of hydrogen production at 873 K. Considering melting point of different metallic and their oxides states (as shown in Table 7) of Fe and Sn it can be inferred that Fe is very suitable for given application unless there is a provision to handle liquid metal in the solar reactor similar to Zn–ZnO case. The steam to H value 2 2 (c ) of 40.82 % is lower compared to one reported by Abanades et al. [15] which is equivalent to 90 % at similar conditions. If solar reactor is designed to handle liquid metals, then both Zn and Sn seems to be better candidates for two-step redox reactions with good hydrogen yield and at low-temperature operation. Recently, Cho and Kim [27] reported production of H using solid acids such as silica gel, activated Al O , 2 3 CaCO , TiO and ZnO. This is very interesting study as it 3 2 reports on liberation of hydrogen gas at very low temper- ature. They have demonstrated the possibility of H pro- Fig. 6 Dependence of Cd (metal) content of condensates on the duction using a laboratory plug ﬂow reactor. Figure 7 gives oxygen concentration of the argon ﬂow 123 Mater Renew Sustain Energy (2013) 2:7 Page 9 of 12 Table 6 Maximum per-pass conversion of H OtoH in the regen- of H , i.e., 1,590 ppm in 1 h reaction time the activated 2 2 2 eration reactor and the stable phase obtained at 873 K for counter- Al O produces highest amount of total H at 1,073 K. 2 3 2 current gas–solid operation Though the authors demonstrated the workability of the Metal phase c (%) Oxidized phase producing hydrogen from solid acids on a laboratory-scale ﬁxed-bed reactor, replication of the results in a solar Ni 0.4 NiO reactor needs to be performed to know the feasibility of the Cd 1.83 CdO process. Cu 0 Cu O One of the main problem to tackle is overcoming sin- Co 2.27 CoO tering of redox material. Agglomeration due to sintering Sn 40.82 SnO brings down the recyclability over multiple redox cycles. MnO 0 Mn O 3 4 The key properties of the redox material should include a Fe 74.79 Fe O 3 4 high oxygen carrying capacity, good mechanical properties Fe 74.79 Fe O 2 3 and cheap and easy synthetic procedures. If redox material c conversion of H OtoH do not fulﬁll any one of these key properties it would not be 2 2 a suitable material for commercial-scale operation. Table 7 Melting points [44] of various phases of Fe and Sn Particle size or grain size effect on rate of oxidation Material Melting point (K) It is generally accepted that smaller the particle size easier Fe (cast) 1,548 is to oxidize or reduce. In case of two-phase alloys the rate Fe (pure) 1,808 of oxidation may signiﬁcantly improve with grain size FeO 1,693 reduction because both mutual solubility and diffusivity Fe O 1,538 3 4 among the system will enhance [45, 46]. But this is not Fe O 1,811 always the case, during the oxidation if the top layer acts as 2 3 Sn 504 a protective layer then the further oxidation of the metals SnO 1,353 will be hampered. SnO 1,400 Figure 8 shows the oxidation kinetics of three different alloy systems with two different grain sizes at 1,073 K. The grain size reduced Cu–Cr alloy showed very slow oxidation kinetics compared to As-cast alloy. But in case of Cu–Fe and Cu–Co oxidation kinetics found to be much faster when nano-crystals (20–30 nm) were used compared to the As-cast alloy. This is because Cr O scale formed on 2 3 alloy prevents further oxidation. This is similar to in case aluminum where external layer forms Al O and prevents 2 3 further oxidation or corrosion of the aluminum metal. The reduction kinetics of metal oxides depends on many factors such as whether they are supported or unsupported, particle size, gas atmosphere, kind of metal oxides and whether single or mixed metal oxides. There are not many reports available on high-temperature reduction of metal oxides in an inert atmosphere as in the case of solar thermal reduction, but there are plenty of studies available in the Fig. 7 H concentration in the product gas stream at a reaction time literature on reduction of metal oxides using H or CO as of 1 h and total amount of H produced versus reaction temperature reducing agents. For example, Syed-Hassan and Chun-Zhu resulted from using a wetted Al O . For the experiments, 60 g of 2 3 [47] have studied the particle size effect on reduction of Al O (5.5 wt% H O) in a stainless steel reactor and 2 ml/min of CO 2 3 2 2 carrier gas were used at atmospheric pressure NiO in H atmosphere. The reduction proﬁles for NiO particles of size 20 and 24 nm are very different from that the concentration and total amount of H liberated at var- of particle size of 3.3 nm (please refer Fig. 5a in Ref. [47]). ious temperatures using activated Al O . The proﬁles for 20 and 24 nm are almost similar and very 2 3 The concentration of hydrogen produced in product gas much resemble to that of 55 nm NiO supported on SiO stream using ﬁve different oxides at &610 K is shown in substrate. Authors concluded that reduction kinetics is the Table 8. Though CaCO shows highest concentration independent of supported or unsupported NiO, but merely 123 Page 10 of 12 Mater Renew Sustain Energy (2013) 2:7 Table 8 H concentration in the product gas stream at a reaction time of 1 h Solid acid (amount) Al O (50 g) SiO (30 g) TiO (50 g) ZnO (50 g) CaCO (50 g) 2 3 2 2 3 H O (wt%) 0 5.2 0 5.3 0 5.1 0 5.1 0 5.0 H (ppm) 0 870 0 200 0 260 0 630 0 1,590 Table 9 Hydrogen cost estimation (per kg H ) in different processes Years Hy-S CuCl Ferrite S–A ZnO CdO MnO S–I 2015 $5.68 $6.83 $4.06 $7.78 $6.07 NA $ $ 2025 $3.85 $5.39 $2.42 $4.71 $4.18 NA $4.63 $4.68 the particle) takes place. In general, solid-state diffusion requires higher activation energy [48]. The number of steps of diffusion in the solid state would appear to increase with NiO conversion, resulting in continuous increases in the activation energy. This is the reason in general the reduc- tion of bigger particles crystallites needs higher activation energy than smaller one. Fig. 8 Oxidation kinetics of As-cast and grain size reduced Cu–M Economic evaluation alloys at 1,073 K. 1 Grain size reduced Cu–Cr alloy, 2 As-cast Cu–Co alloy, 3 As-cast Cu–Cr alloy, 4 grain size reduced Cu–Fe alloy, 4 As- The US DOE has established a target of $2 to $3 per kg cast Cu–Fe alloy, 6 grain size reduced Cu–Co alloy. [45] hydrogen by 2025 to make it economically affordable. The short term, i.e., 2015, DOE target is $6/kg hydrogen. Any competitive technology to produce hydrogen considers this depends on the particle/crystal size. The E versus %NiO converted trends are very different for ﬁrst reduction ﬁgure as a reference for their process efﬁciency and eco- (particle size 3.3 nm) to that of second (particle nomic evaluation. size = 20 nm) and third reduction (particle size = 24 nm) DOE in collaboration with TIAX (TIAX is a laboratory- (please refer Fig. 5b in Ref. [47]). The E for ﬁrst reduction based technology development company with a focus on remains almost unchanged (indicating single rate-limiting clean energy) led the effort of cost calculation for solar step) throughout the whole reduction process, but for sec- thermochemical hydrogen (STCH) in many US national laboratories. They considered eight promising technologies ond and third reduction steps the E proﬁles continuously increased till the complete reduction of NiO. The main for cost calculations, viz., hybrid-sulfur (HyS), copper chloride (CuCl), thin-ﬁlm nickel ferrite (‘‘ferrite’’), sulfur- reason for the difference could be due to the difference in surface to bulk atoms in different particle size crystals. The ammonia (S–A), zinc oxide (ZnO), manganese oxide size of the metallic island which forms during initial stages (MnO), sulfur-iodine (S–I), and cadmium oxide (CdO). of reduction is bigger than particle size, i.e., 3.3 nm, or the Five out of eight technologies mentioned in Table 9 whole particle surface can be instantaneously covered by appear to meet DOE’s short-term target (by 2015) of $6/kg the metallic layer without requiring a signiﬁcant growth of hydrogen but meeting long-term target seems quite difﬁ- islands; therefore, the reduction happens immediately in cult. Only thin ﬁlm ferrite is very close to DOE’s long-term those particles size crystals. But for a big particle with requirement. Even in this case achieving the long-term signiﬁcant atoms in the interior (e.g., 20 nm) it might take targets require signiﬁcant technological advances in mul- tiple dimensions. The primary cost driver for all the pro- quite some time for the growth of island to cover the whole particle’s surface. Once the surface has been covered cesses that were analyzed was heliostats costs. Reducing the heliostats cost or increase in plant efﬁciency will bring completely by the metal product, the Ni–NiO boundary would then progressively advance inward. As reduction down the CAPEX and OPEX. Most probably mass pro- duction of heliostats and improving its efﬁciency will help continues, the hydrogen radicals moves towards Ni–NiO boundary on the surface and the diffusion of atoms (i.e., to reduce the CAPEX and help to meet DOE hydrogen cost movement and rearrangements of atoms in the interior of targets. 123 Mater Renew Sustain Energy (2013) 2:7 Page 11 of 12 7. 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Thermochemical hydrogen production from water using reducible oxide materials: a critical review

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