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Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as highly active and stable catalysts for urea electrooxidation

Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as... Keywords: NiCr-oxide; coupling effect; urea electrooxidation; electrocatalysis; fuel cells Ni-based catalysts. The first one is a direct mechanism in Introduction which the intermediate nickel oxyhydroxide (NiOOH) ini- Growing energy demand and serious pollution issues have tiates urea oxidation in a series of electrochemical steps prompted the development of alternative environmen- [10]. The other, which is an indirect mechanism proposed tally friendly and sustainable energy sources [1], such as through density functional theory (DFT), suggests that the hydrogen fuel, which is an ideal replacement for conven- indirect route of urea oxidation is that the urea reacts with tional energy sources due to its high energy density and NiOOH to form the final product in a chemical step [11]. To pollution-free products [2 3, ]. Urea [CO(NH ) ] has proven to 2 2 address the high UOR overpotential challenge, bimetallic be an effective H carrier and CO-storage medium for con- 2 2 catalysts composed of Ni and other transition metals have tinuous energy supply due to its inherent characteristics, been developed such as NiCo O nanowire array/Ni foam −1 2 4 such as high energy density (16.9 MJ L , 10.1 wt% of H ), [12], NiMoO nanosheets [13], NiMn/carbon nanofibres [14], non-flammability, non-toxicity, ease of transportation and Ni-Mo/grapheme [15], Ni&Mn/carbon nanofibres [16], etc. low storage cost [45 , ]. As an essential intermediate in ni- The studies suggest that the bimetallic catalyst-based UOR trogen and carbon cycling in nature, urea is formed by com- follows both the direct- and the indirect-mechanism paths bining NH and CO ; thereby, it can efficiently stabilize NH 3 2 3 [17, 18]. and fix CO while storing H with high density. The stored 2 2 Special attention needs to be paid to the NiCr bimet- energy in urea can be retrieved either by releasing H ther - allic system in which Cr modifies the d-band electronic mally and catalytically or via a direct urea fuel cell (DUFC). structure by weakening the Ni–O interaction, thereby The DUFC also can be used to oxidize urea-based organics improving the UOR rate [19]. Furthermore, Cr shows strong from the wastewater stream to generate energy as well as resistance towards the quaternary ammonium functional pretreat the wastewater. Developing a high-performance group-initiated reaction inhibition [20]. Moreover, NiCr bi- anode catalyst is a crucial step towards achieving an effi- metallic catalysts also show enhanced methanol oxidation cient DUFC system. and hydrogen evolution under alkaline medium [2021 , ]. A series of studies have shown that noble metal cata- However, Cr-based materials have been rarely reported as lysts such as Pt- and Pd-based composites as anode mater - catalysts for urea oxidation. A recent study revealed that ials have the high catalytic activity of the urea-oxidation 40% Cr of NiCr on carbon support exhibits a high current reaction (UOR) [6–8]. Nevertheless, its industrial applica- −1 density of 2933 mAmg for urea oxidation at a potential Ni tion is limited by cost and scarcity. Therefore, continued of 0.55 V, which is 3.6-fold higher than that of Ni/C [22]. efforts are being made to find affordable, earth-abundant However, the interaction between Ni and Cr is still un- and non-precious-metal catalysts for UOR. clear and the poor catalytic stability needs to be further In the past few decades, researchers have discovered improved. that Ni-based transition metal catalysts have compar - Herein, we take a simple hydrothermal approach to able catalytic performance and stability to these of noble- synthesize NiCr-oxide-carbon nanotubes (CNTs). The metal-based catalyst for UOR. However, most Ni-based catalysts were characterized extensively using X-ray catalysts are often limited by high UOR overpotentials [9]. powder diffraction (XRD), scanning electron microscopy Moreover, two types of UOR mechanisms were reported on (SEM), transmission electron microscopy (TEM), Raman, Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 Gan et al. | 3 –1 X-ray photoelectron spectroscopy (XPS) and electro- temperature rate of 2.5°C min from 20 to 800°C under chemical methods to achieve the best-performing cata- continuous O flow. The true ratio of the different metals in lyst in terms of electrocatalytic active surface area and the catalyst was examined by inductively coupled plasma electrocatalytic current density by varying the Ni/Cr ratio.atomic emission spectr ometry (ICP-AES, IRIS Intrepid II The results demonstrate that NiCr -oxide-CNTs show su- XSP). The chemical states of the different elements were perior performance that gives the highest electrochem- probed by XPS (JPS-9 010 Mg Kα). The binding energy was 2 −1 ically active surface area (ESA) (50.7 m g ) and the highest calibrated based on a 284.8-eV (C–C bond) of the C 1s peak −2 electrocatalytic current density (115.6 mA cm ). Moreover, and a standard deviation of approximately ±0.05 eV. The the optimized catalyst reveals long-term stability for UOR true ratios of Ni and Cr for different materials were de- in 1.0 M KOH + 0.33 M urea solution. The exceptional cata- termined by inductively coupled plasma (ICP, PekinElmer lytic performance is ascribed to the fast charge-transfer FLexar-NexION300X). kinetics, large active surface area and better dispersion of Ni nuclei [22]. 1.4 Electrochemical measurements All electrochemical measurements were performed in a 1 Experimental standard three-electrode cell with a multi-channel Bio- 1.1 Materials logic VMP3 as an electrochemical workstation, in which a glassy carbon electrode (GCE), graphite plate and sat- Chromic chloride hexahydrate (CrCl⋅6H O, ≥99%, Aladdin), 3 2 urated calomel electrode (SCE) were used as the working, nickel chloride hexahydrate (NiCl⋅6H O, ≥98%, Aladdin), 2 2 counter and reference electrodes, respectively. Before the ammonium fluoride (NH F, ≥96%, Xilong), urea (CO(NH) , 4 2 2 experiment, several GCEs were polished with Al O fine 2 3 ≥99%, Aladdin), polyvinylpyrrolidone (PVP, M = 58 000, powder, then washed with HSO , ethanol and HO three 2 4 2 Aladdin), Nafion solution (~5%, Alfa Aesar), ethylene glycol times. The working electrode was prepared as follows: 4.0 [(CH OH) , XILONG], anhydrous ethanol (CH OH, ≥99.6%, 2 2 2 5 mg of the catalytic material was ultrasonically dispersed Xilong). All reagents were of analytical grade and could be in 1.0 mL mixed solvent (32 μL of 5% Nafion + 200 μ L of used without further purification. CNTs (>95%) were pur - ethanol + 768 μL of H O) for 30 min to form a homoge- chased from Aladdin. neous solution. Then, 10 μL of the above catalyst inks was pipetted onto the surface of the GCE ( = 3 mm) and φ naturally dried for use. The loading of the catalyst was 1.2 Synthesis of NiCrx-oxide-CNTs composites –2 about 0.566 mg cm . Cyclic voltammetry (CV) was ana- The NiCr -oxide-CNTs hybrid composites were obtained lysed in the potential range of 0.0 to 0.8 V (vs. SCE) with through the hydrothermal method as follows: 73.3 mg –1 a scan rate of 50 mV s in 1.0 M KOH saturated with N NiCl⋅6H O, 163.5 mg CrCl⋅6H O, 0.277 8 g NH F, 0.225 2 g 2 2 3 2 4 with and without 0.33 M urea electrolyte. Since the con- CO(NH ) , 100 mg PVP and 100 mg CNTs were added to 2 2 centration of urea in human urine is approximately 0.33 a continues sonication solution of 20 mL H O/ethylene M and most previous reports on the electrocatalysis of glycol (v/v = 1/1). After 30 min, the resulting suspension urea were carried out in 1.0 M KOH solution, the urea con- was transferred to Teflon-lined stainless steel and auto- centration of 0.33 M was used for comparison purposes in claved for 9 h at 120°C. The resulting products were cen- this work [23–25]. The stability of the catalyst was tested trifuged at 6000 rpm for 10 min, rinsed with excess H O/ by chronoamperometry at a fixed potential of 0.45 V (vs. ethanol and freeze-dried over 12 h. The obtained prod- SCE) for 2.0 h in a 1.0 M KOH + 0.33 M urea solution. All ucts were nominated as NiCr-oxide-CNTs. As a com- electrochemical studies were conducted at room tem- parison, a series of NiCr-oxide-CNTs composites with perature (25 ± 1°C). different Ni/Cr molar ratios (1/1, 1/3, 2/1 and 3/1) were further prepared using a similar approach, as mentioned above. Meanwhile, the best-performing catalyst was also 2 Results and discussion made using direct hydrothermal methods for comparison 2.1 Crystal structure and thermogravimetric purposes. analysis The sheet-like NiCr-oxide-CNTs was synthesized by a 1.3 Characterizations facile one-step method, in which the precursors of NiCl , The morphology and microstructure of the catalyst were CrCl, CNTs, NH F, PVP and urea were ultrasonically dis- 3 4 investigated in detail by SEM (FEI Quanta 200 FEG) and persed in a mixed solution of HO-ethylene glycol and TEM (JEM-2100F) with X-ray energy dispersive spectros- then heated to 120°C for 9 h to achieve the final composite copy (EDS). The crystal structure of the material was c-har (Fig. 1a). Studies have shown that urea provides an alkaline acterized by XRD (Rigaku D/Max 2 500 V/PC) at a scan environment, PVP and ethylene glycol manipulate micro- −1 speed of 2.0 degree min . Thermal gravimetric measure- structures as ligands, while NH F regulates the directional ment was made on a TGA/STA409 PC module with a rising growth. Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 4 | Clean Energy, 2020, Vol. XX, No. XX NH F + PVP + Urea Hydrothermal 120 °C, 9 h NiCl CrCl CNTs NiCr -Oxide-CNTs 2 3 2 BC NiCr -Oxide-CNTs NiCr -Oxide-CNTs 2 2 0.10 Ni-hydro-CNTs TG curve Cr-hydro-CNTs DTG curve 0.05 63.8% C (002) Ni O (OH) , JCPDS: 06-0114 3 2 4 0.00 NiOOH, JCPDS: 27-0956 –0.05 Cr(OH) 3H O, JCPDS: 16-0817 3 2 –0.10 NiCr O , JCPDS: 65-3105 2 4 –0.15 457.0 °C 20 30 40 50 60 70 80 200 400 600 800 2-Theta (°) Temperature (°C) Fig. 1: (a) Schematic illustration of the preparation of NiCr -oxide-CNTs composite. (b) XRD patterns of NiCr-oxide-CNTs, Ni-hydro-CNTs and 2 2 −1 Cr-hydro-CNTs. (c) Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of NiCr -oxide-CNTs with a heating rate of 2.5°C min under O atmosphere. The XRD patterns of the optimized catalyst NiCr -oxide- 457.0°C caused by oxidative pyrolysis of CNTs [29]. As CNTs with the two catalysts prepared using hydrothermal shown in Fig. S2 in the online Supplementary Data (see methods in the absence of either Ni (Cr-hydro-CNTs) or the online Supplementary Data), the sharp Raman peaks Cr (Ni-hydro-CNTs) are shown in Fig. 1b. The rest of the of NiCr -oxide-CNTs, Ni-hydro-CNTs and Cr-hydro-CNTs −1 NiCr -oxide-CNTs catalysts show similar XRD patterns are observed at ~1341 and 1582 cm for the D and G bands [Fig. S1 in the online Supplementary Data (see the online where the ratio of the D and G bands refers to the graphitic Supplementary Data)].F ig. 1b indicates that the crystal degree [30]. One can conclude that the NiCr -oxide-CNTs structures of NiCr-oxide-CNTs are consistent with the have the highest defect structure due to the highest /II 2 D G standard model of NiCrO (JCPDS: 65–3 105) [26], including value (1.18) as compared to these of Ni-hydro-CNTs (1.13) 2 4 the typical peak of the crystal plane (002) of CNTs at and Cr-hydro-CNTs (1.06). 25.8°. Notably, the Cr-hydro-CNTs exhibits four character - istic diffraction peaks at 18.2, 19.4, 26.6 and 43.7° corres- 2.2 Morphology analysis ponding to the (001), (100), (101) and (201) crystal planes The morphology and microstructure of NiCr -oxide-CNTs of Cr(OH) ·3H O (JCPDS: 16–0 817) [27]. Simultaneously, 2 3 2 were further investigated by SEM image, TEM images and the XRD pattern of Ni-hydro-CNTs matches well with EDS mappings. Fig. 2a is an SEM image which shows that the standard models of NiOOH (JCPDS: 27–0 956) and some spherical structural species are intertwined with Ni O (OH) (JCPDS: 06–0 114) as well, suggesting the co- 3 2 4 CNTs. The TEM image of Fig. 2b further discloses the spher - existence of these two species in Ni-hydro-CNTs com- ical species composed of sheet-like species containing posite. The ICP test results of NiCr -oxide-CNTs samples two types of clear lattice stripes with the lattice spacings with different molar ratios of Ni/Cr indicate a high level of about 0.25 and 0.32 nm (Fig. 2c), which are consistent of consistency between the experimental values and the with the (311) facet of NiCrO and (002) facet of CNTs, theoretical values [Table S1 in the online Supplementary 2 4 respectively [31]. Moreover, the EDS mappings manifest Data (see the online Supplementary Data)]. that the elements of Ni, Cr and O are evenly distributed Thermogravimetric (TG) and differential throughout the entire skeleton of the NiCr -oxide-CNTs thermogravimetric (DTG) curves were applied to explore 2 composite (Fig. 2d). the oxide content of the optimal NiCr -oxide-CNTs cata- lyst under O atmosphere (Fig. 1c). In the initial stage from room temperature to around 120°C, the weight loss is 2.3 XPS analysis due to the evaporation of water. Then, a significant drop between 400 and 600°C is attributed to the oxidation of XPS analysis was adopted to probe the chemical states of carbon. In addition, the 36.2 wt% remnant of NiCr -oxide- Ni and Cr elements in NiCr-oxide-CNTs (Fig. 3). As shown 2 2 CNTs after 800°C is nickel-chromium oxide [28]. Notably, in Fig. S3a in the online Supplementary Data (see the on- there is a sharp exothermic peak on the DTG curve at line Supplementary Data), the survey XPS spectra showed Intensity (counts) Weight (%) –1 DTG (mg min ) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 Gan et al. | 5 that NiCr-oxide-CNTs with different Ni/Cr ratios con- two peaks corresponding to Cr 2p (576.4 eV) and Cr 2p x 3/2 1/2 3+ tained Ni, Cr, C and O elements. The high-resolution C 1s (586.2 eV) of Cr [33], respectively (Fig. 3b). The binding en- [Fig. S3b in the online Supplementary Data (see the online ergy of Cr 2p for NiCr-oxide-CNTs exhibits a positive 3/2 2 Supplementary Data)] of each catalyst consists of four shift of 0.18 eV as compared to that of Cr-hydro-CNTs. peaks at C=C (284.0 eV), C–C (284.8 eV), C–O (286.0 eV) and The binding-energy changes indicate the presence of C=O (288.8 eV) [32], which are used as calibration stand- electronic-coupling and electron-transfer effects between ards. As shown in Fig. 3a, the high-resolution Ni 2p spec- the components of NiCr-oxide-CNTs [35, 36]. The elec- trum of NiCr-oxide-CNTs has been deconvoluted into four tron migration between Ni and Cr is believed to promote peaks, in which the binding energies at 854.9 and 872.7 eV the electrooxidation of urea synergistically [37]. The high- are ascribed to Ni 2p and Ni 2p , respectively, and the resolution Ni 2p and Cr 2p of the rest of the NiCr -oxide- 3/2 1/2 x other two peaks are attributed to satellite peaks [34 33], . CNTs composites were also analysed for comparison [Fig. All of these characteristics indicate that the Ni species in S3c and d in the online Supplementary Data (see the on- 2+ the composite is mainly present as Ni. It is worth noting line Supplementary Data)], which show a similar electronic that the binding energy of Ni 2p in the NiCr -oxide-CNTs structure. In addition, the high-resolution O 1s of all com- 3/2 2 composite has a negative shift of 0.25 eV compared to that posites are fitted to four peaks representing metal-oxide, of Ni-hydro-CNTs. Meanwhile, the high-resolution Cr 2p vacancies, C–O and adsorbed H O (Fig. 3c), respectively [38]. region of the NiCr-oxide-CNTs composite was fitted to The results reveal that the Cr species has a positive effect on the oxygen-defect sites, while the Ni species increase the hydrophilicity of the composite. As a result, the excel- A B lent urea electrocatalytic activity of the optimized NiCr - oxide-CNTs is the fruit of an elegant balance between the oxygen-defect sites and hydrophilicity. 2.4 Electrochemical performance analysis 1 μm The CV curves of NiCr -oxide-CNTs, Ni-hydro-CNTs and 100 nm x Cr-hydro-CNTs performed in 1.0 M KOH solution were CD used to evaluate their ESA (Fig. 4a). The figures indi- Ni cate that all catalysts except Cr-hydro-CNTs present a 0.32 nm pair of redox peaks in the potential range of 0.0 to 0.8 V. C (002) The anode peak in the forward scan is consistent with the Ni(OH) species oxidized to NiOOH and the cathode Cr O peak in the reverse scan is ascribed to the reduction of 0.25 nm NiOOH to Ni(OH) [11, 39]. Generally, the ESA was cal- NiCr O (311) 2 2 4 5 nm culated by the required reduction charge in the reverse scan, which is directly proportional to the number of ac- tive sites for urea electrooxidation. The ESA values of all Fig. 2: (a) SEM, (b) TEM and (c) high-resolution TEM images of NiCr - catalysts can be estimated from the equation ESA = /mq Q oxide-CNTs. (d) EDS mappings of NiCr -oxide-CNTs with Ni, Cr and O elements [38, 40], where Q is the total charge used for the reduction AB C O 1s Ni 2p Ni 2p Cr 2p Cr 2p 3/2 3/2 C-O Vacancies NiCr -Oxide-CNTs NiCr -Oxide-CNTs 2 ads 2 2 24.4% 34.9% Sat. Ni-O 46.0% 17.9% 0.18 eV 0.25 eV Cr-hydro-CNTs Ni-hydro-CNTs 32.3% 19.2% Cr-O 888 880 872 864 856 848 590 585 580 575 570 536 534 532530 528 Binding energy (eV) Binding energy (eV) Binding energy (eV) Fig. 3: High-resolution XPS regions of (a) Ni 2p, (b) Cr 2p and (c) O 1s from NiCr -oxide-CNTs, Ni-hydro-CNTs and Cr-hydro-CNTs Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 6 | Clean Energy, 2020, Vol. XX, No. XX 1/1 50.7 1/2 NiCr -Oxide-CNTs with x 50 1/3 different Ni/Cr ratios 2/1 41.3 3/1 36.8 35.3 Ni-hydro-CNTs Cr-hydro-CNTs 30 13.7 –20 1.1 –40 0.0 0.2 0.4 0.6 0.8 1/1 1/2 1/3 2/1 3/1 Ni E/V (vs. SCE) Different Ni/Cr ratios C D 140 1/1 NiCr -Oxide-CNTs 115.6 1/2 x 120 with different 1/3 96.3 94.7 2/1 100 Ni/Cr ratios 85.0 3/1 Ni-hydro-CNTs 80 Cr-hydro-CNTs 42.6 16.1 –20 –40 0.0 0.2 0.4 0.6 0.8 1/1 1/2 1/3 2/1 3/1 Ni E/V (vs. SCE) Different Ni/Cr ratios Fig. 4: (a) CV curves of NiCr-oxide-CNTs with different Ni/Cr ratios, Ni-hydro-CNTs and Cr-hydro-CNTs in 1.0 M KOH. (b) The summarized ESA values from (a). (c) CV curves of different catalysts in 1.0 M KOH + 0.33 M urea. (d) The summarized forward current densities from (c). of NiOOH species to Ni(OH) , m is the mass of Ni in the higher catalytic performance as compared to both Ni and −2 supported catalyst and q is 257 μC cm as only one elec- Cr single-component catalysts for urea oxidation, once tron for NiOOH converted to Ni(OH) . As shown in Fig. 4b, again indicating the synergistic effect between Ni and Cr the NiCr-oxide-CNTs shows the highest ESA of 50.7 m species. In addition, we also studied the electrochemical −1 g , which is 1.44-, 3.70-, 1.23-, 1.37- and 46-fold higher performance of NiCr-oxide, CoCr -oxide-CNTs and FeCr - 2 2 2 2 −1 than NiCr-oxide-CNTs (35.3 m g ), NiCr -oxide-CNTs oxide-CNTs in 1.0 M KOH and 1.0 M KOH + 0.33 M urea 2 −1 2 −1 (13.7 m g ), Ni Cr-oxide-CNTs (41.3 m g ), Ni Cr-oxide- [Fig. S4 in the online Supplementary Data (see the online 2 3 2 −1 2 −1 CNTs (36.8 m g ) and Ni-hydro-CNTs (1.1 m g ). Studies Supplementary Data)]. The results show that the catalytic have found that the introduction of a certain amount of performances of these control catalysts are much lower Cr species can expose more active sites through syner - than that of NiCr-oxide-CNTs, which indicates not only gistic interactions of Ni and Cr species [41]. Moreover, the importance of the CNTs, but also the unique effect of the various Ni/Cr ratios result in different morphology synergy between Ni and Cr towards the catalytic perform- in NiCr -oxide-CNTs composites, which also have a sig- ance of urea oxidation. nificant effect on ESA values [21]. Therefore, the optimal In the initial stage of electrocatalysis, studies found that value of the Ni/Cr ratio is required to obtain the highest the Ni(OH) species first lose one electron to form NiOOH − − ESA. Thereafter, the electrocatalytic tests of all catalysts on the catalyst surface [Ni(OH) +OH ↔ NiOOH+H O+e ] 2 2 were performed in 1.0 M KOH + 0.33 M urea (Fig. 4c). [42, 43]. Subsequently, the produced NiOOH intermediates Although all catalysts show a similar onset potential atcould adsorb h ydroxyl ions and urea molecules from the ~0.32 V except for Cr-hydro-CNTs (0.56 V), the NiCr -oxide- electrolyte. After undergoing a complex multi-electron- CNTs catalyst has the largest current density (115.6 mA transfer process, the urea molecules are finally oxidized −2 − cm ), which is about 1.36-, 2.71-, 1.20-, 1.22- and 7.2-fold to N , CO and H O on the active sites [CO(NH) +6OH → 2 2 2 2 2 higher than those of NiCr-oxide-CNTs, NiCr-oxide-CNTs, CO +N +5H O+6e ] [38, 44] while the NiOOH species is re- 3 2 2 2 Ni Cr-oxide-CNTs, Ni Cr-oxide-CNTs and Ni-hydro-CNTs, duced back to Ni(OH) species [45]. The summarized for- 2 3 2 respectively (Fig. 4d). Here, the higher onset potential of ward current densities of NiCr -oxide-CNTs are shown in the Cr-hydro-CNTs catalyst than others may be due to the Fig. 4d; the NiCr-oxide-CNTs shows the highest current fact that the catalyst lacks effective active sites to adsorbdensity . Furthermore, it is also among the top-performing urea molecules, thereby requiring a higher polarization urea-oxidation catalysts reported in the literature because potential to drive the reaction. It is also worth noting that of the lo w onset potentials and high peak current densities Cr-hydro-CNTs has almost no catalytic activity for urea, [Table S2 in the online Supplementary Data (see the online but the composite catalysts including Ni and Cr possess Supplementary Data)]. –2 –2 j (mA cm ) j (mA cm ) –2 j (mA cm ) 2 –1 ESA (m g ) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 III II Gan et al. | 7 The stabilities of NiCr-oxide-CNTs catalysts with dif- ferent Ni/Cr ratios were tested by chronoamperometry at CO 2 – I –e a constant potential of 0.45 V in 1.0 M KOH + 0.33 M urea CO H O VI 2 H N electrolyte. As shown in Fig. 5, the NiCr -oxide-CNTs cata- H C N lyst shows the highest initial current density (85.0 mA OH −2 −2 cm ) and limiting current density (38.6 mA cm ). The ini- – –e OH tial spike can be attributed to the higher urea concentra- CO OH tion on the surface of the catalysts at the beginning and H N – H the current stabilizes after some time where the system OH – –e C N CO N reaches the equilibrium [46]. – –e O H OH OH O 2 2 2 – – – –e –e –e H O III III III 2.5 Catalytic-mechanism analysis NiCr O NiOOH CrO 2 4 x As discussed above, the NiCr-oxide-CNTs catalyst ex- hibits excellent electrocatalytic performance for urea Fig. 6: The proposed possible catalytic mechanism of the electrocatalytic 2+ urea oxidation by NiCr-oxide-CNTs catalyst in alkaline media oxidation. Electrochemical studies show that the Ni species on the surface of the NiCr-oxide-CNTs cata- lyst is first oxidized to NiOOH species (lose 1e ) as the 3 Conclusion active sites (M) (Fig.  6, Step I). Then, the urea molecules In summary, the NiCr-oxide-CNTs with different Ni/Cr in the solution are adsorbed into the active sites of the ratio catalysts were prepared by a facile hydrothermal NiOOH surface and the partially positively charged H method. Various techniques were applied to investi- atoms of the urea molecules are adsorbed onto the sur - gate the crystal structures, morphologies and chemical face of the negatively charged Cr species (due to a higher states. The results indicate that the NiCr -oxide-CNTs electron density than Ni species) through electrostatic composite is composed of the sheet-like structure of interaction (Step II). After the attack of OH ions and the NiCr -oxide and CNTs. There is a visible synergistic − 2 combination of electrooxidation (loss of 1), e one H atom effect between Ni and Cr. The electrochemical studies is removed to form a free HO molecule (Step III). After show that the NiCr-oxide-CNTs catalyst exhibits three consecutive dehydrations, an intermediate state of 2 −1 the immense ESA value (50.7 m g ), highest current M-CO⋅N is formed. Similarly, after an OH attack and the 2 −2 density (115.6 mA cm ) and permanent stability for urea electrooxidation (loss of 1e) followed by losing of a N, electrooxidation in alkaline medium. The prominent another intermediate state of M-CO OH is formed (Ste ⋅ p performance of the NiCr-oxide-CNTs catalyst is mainly − 2 IV). Along with the further attack of OH ions and one ascribed to its improved chargetr ‐ ansfer kinetics, the more electrooxidation process (loss of 1), e the last inter - larger ESA, along with an elegant balance between the mediate state of M-CO is formed while releasing one oxygen-defect sites and hydrophilicity. Moreover, the molecule of HO (Step V). Finally, the active site M is re- results also demonstrate a promising application of the covered by releasing CO (Step VI) [11, 43]. non-noble-metal catalysts in water splitting, hydrogen production, fuel cells, etc. 1/1 1/2 NiCr -Oxide-CNTs with 1/3 Supplementary data different Ni/Cr ratios 2/1 Supplementary data is available at Clean Energy online. 3/1 Acknowledgements This work has been supported by the National Natural Science Foundation of China (21965005), Natural Science Foundation of Guangxi Province (2018GXNSFAA294077, 2017GXNSFGA198004), Project of High-Level Talents of Guangxi (F-KA18015, 2018ZD004) and Innovation Project of Guangxi Graduate Education (XYCSZ2019056, YCBZ2019031). 0 1200 2400 3600 4800 6000 7200 Time (s) Fig. 5: Chronoamperometric curves of NiCr -oxide-CNTs catalysts with Conflict of Interest different Ni/Cr ratios at a constant potential of 0.45 V (vs. SCE) in 1.0 M None declared. 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Electrochim Acta 2017; urea electrooxidation catalysts.J Appl Electrochem 2017; 227:210–6. 47:905–15. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Clean Energy Oxford University Press

Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as highly active and stable catalysts for urea electrooxidation

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© The Author(s) 2020. Published by Oxford University Press on behalf of National Institute of Clean-and-Low-Carbon Energy
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2515-4230
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10.1093/ce/zkz029
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Abstract

Keywords: NiCr-oxide; coupling effect; urea electrooxidation; electrocatalysis; fuel cells Ni-based catalysts. The first one is a direct mechanism in Introduction which the intermediate nickel oxyhydroxide (NiOOH) ini- Growing energy demand and serious pollution issues have tiates urea oxidation in a series of electrochemical steps prompted the development of alternative environmen- [10]. The other, which is an indirect mechanism proposed tally friendly and sustainable energy sources [1], such as through density functional theory (DFT), suggests that the hydrogen fuel, which is an ideal replacement for conven- indirect route of urea oxidation is that the urea reacts with tional energy sources due to its high energy density and NiOOH to form the final product in a chemical step [11]. To pollution-free products [2 3, ]. Urea [CO(NH ) ] has proven to 2 2 address the high UOR overpotential challenge, bimetallic be an effective H carrier and CO-storage medium for con- 2 2 catalysts composed of Ni and other transition metals have tinuous energy supply due to its inherent characteristics, been developed such as NiCo O nanowire array/Ni foam −1 2 4 such as high energy density (16.9 MJ L , 10.1 wt% of H ), [12], NiMoO nanosheets [13], NiMn/carbon nanofibres [14], non-flammability, non-toxicity, ease of transportation and Ni-Mo/grapheme [15], Ni&Mn/carbon nanofibres [16], etc. low storage cost [45 , ]. As an essential intermediate in ni- The studies suggest that the bimetallic catalyst-based UOR trogen and carbon cycling in nature, urea is formed by com- follows both the direct- and the indirect-mechanism paths bining NH and CO ; thereby, it can efficiently stabilize NH 3 2 3 [17, 18]. and fix CO while storing H with high density. The stored 2 2 Special attention needs to be paid to the NiCr bimet- energy in urea can be retrieved either by releasing H ther - allic system in which Cr modifies the d-band electronic mally and catalytically or via a direct urea fuel cell (DUFC). structure by weakening the Ni–O interaction, thereby The DUFC also can be used to oxidize urea-based organics improving the UOR rate [19]. Furthermore, Cr shows strong from the wastewater stream to generate energy as well as resistance towards the quaternary ammonium functional pretreat the wastewater. Developing a high-performance group-initiated reaction inhibition [20]. Moreover, NiCr bi- anode catalyst is a crucial step towards achieving an effi- metallic catalysts also show enhanced methanol oxidation cient DUFC system. and hydrogen evolution under alkaline medium [2021 , ]. A series of studies have shown that noble metal cata- However, Cr-based materials have been rarely reported as lysts such as Pt- and Pd-based composites as anode mater - catalysts for urea oxidation. A recent study revealed that ials have the high catalytic activity of the urea-oxidation 40% Cr of NiCr on carbon support exhibits a high current reaction (UOR) [6–8]. Nevertheless, its industrial applica- −1 density of 2933 mAmg for urea oxidation at a potential Ni tion is limited by cost and scarcity. Therefore, continued of 0.55 V, which is 3.6-fold higher than that of Ni/C [22]. efforts are being made to find affordable, earth-abundant However, the interaction between Ni and Cr is still un- and non-precious-metal catalysts for UOR. clear and the poor catalytic stability needs to be further In the past few decades, researchers have discovered improved. that Ni-based transition metal catalysts have compar - Herein, we take a simple hydrothermal approach to able catalytic performance and stability to these of noble- synthesize NiCr-oxide-carbon nanotubes (CNTs). The metal-based catalyst for UOR. However, most Ni-based catalysts were characterized extensively using X-ray catalysts are often limited by high UOR overpotentials [9]. powder diffraction (XRD), scanning electron microscopy Moreover, two types of UOR mechanisms were reported on (SEM), transmission electron microscopy (TEM), Raman, Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 Gan et al. | 3 –1 X-ray photoelectron spectroscopy (XPS) and electro- temperature rate of 2.5°C min from 20 to 800°C under chemical methods to achieve the best-performing cata- continuous O flow. The true ratio of the different metals in lyst in terms of electrocatalytic active surface area and the catalyst was examined by inductively coupled plasma electrocatalytic current density by varying the Ni/Cr ratio.atomic emission spectr ometry (ICP-AES, IRIS Intrepid II The results demonstrate that NiCr -oxide-CNTs show su- XSP). The chemical states of the different elements were perior performance that gives the highest electrochem- probed by XPS (JPS-9 010 Mg Kα). The binding energy was 2 −1 ically active surface area (ESA) (50.7 m g ) and the highest calibrated based on a 284.8-eV (C–C bond) of the C 1s peak −2 electrocatalytic current density (115.6 mA cm ). Moreover, and a standard deviation of approximately ±0.05 eV. The the optimized catalyst reveals long-term stability for UOR true ratios of Ni and Cr for different materials were de- in 1.0 M KOH + 0.33 M urea solution. The exceptional cata- termined by inductively coupled plasma (ICP, PekinElmer lytic performance is ascribed to the fast charge-transfer FLexar-NexION300X). kinetics, large active surface area and better dispersion of Ni nuclei [22]. 1.4 Electrochemical measurements All electrochemical measurements were performed in a 1 Experimental standard three-electrode cell with a multi-channel Bio- 1.1 Materials logic VMP3 as an electrochemical workstation, in which a glassy carbon electrode (GCE), graphite plate and sat- Chromic chloride hexahydrate (CrCl⋅6H O, ≥99%, Aladdin), 3 2 urated calomel electrode (SCE) were used as the working, nickel chloride hexahydrate (NiCl⋅6H O, ≥98%, Aladdin), 2 2 counter and reference electrodes, respectively. Before the ammonium fluoride (NH F, ≥96%, Xilong), urea (CO(NH) , 4 2 2 experiment, several GCEs were polished with Al O fine 2 3 ≥99%, Aladdin), polyvinylpyrrolidone (PVP, M = 58 000, powder, then washed with HSO , ethanol and HO three 2 4 2 Aladdin), Nafion solution (~5%, Alfa Aesar), ethylene glycol times. The working electrode was prepared as follows: 4.0 [(CH OH) , XILONG], anhydrous ethanol (CH OH, ≥99.6%, 2 2 2 5 mg of the catalytic material was ultrasonically dispersed Xilong). All reagents were of analytical grade and could be in 1.0 mL mixed solvent (32 μL of 5% Nafion + 200 μ L of used without further purification. CNTs (>95%) were pur - ethanol + 768 μL of H O) for 30 min to form a homoge- chased from Aladdin. neous solution. Then, 10 μL of the above catalyst inks was pipetted onto the surface of the GCE ( = 3 mm) and φ naturally dried for use. The loading of the catalyst was 1.2 Synthesis of NiCrx-oxide-CNTs composites –2 about 0.566 mg cm . Cyclic voltammetry (CV) was ana- The NiCr -oxide-CNTs hybrid composites were obtained lysed in the potential range of 0.0 to 0.8 V (vs. SCE) with through the hydrothermal method as follows: 73.3 mg –1 a scan rate of 50 mV s in 1.0 M KOH saturated with N NiCl⋅6H O, 163.5 mg CrCl⋅6H O, 0.277 8 g NH F, 0.225 2 g 2 2 3 2 4 with and without 0.33 M urea electrolyte. Since the con- CO(NH ) , 100 mg PVP and 100 mg CNTs were added to 2 2 centration of urea in human urine is approximately 0.33 a continues sonication solution of 20 mL H O/ethylene M and most previous reports on the electrocatalysis of glycol (v/v = 1/1). After 30 min, the resulting suspension urea were carried out in 1.0 M KOH solution, the urea con- was transferred to Teflon-lined stainless steel and auto- centration of 0.33 M was used for comparison purposes in claved for 9 h at 120°C. The resulting products were cen- this work [23–25]. The stability of the catalyst was tested trifuged at 6000 rpm for 10 min, rinsed with excess H O/ by chronoamperometry at a fixed potential of 0.45 V (vs. ethanol and freeze-dried over 12 h. The obtained prod- SCE) for 2.0 h in a 1.0 M KOH + 0.33 M urea solution. All ucts were nominated as NiCr-oxide-CNTs. As a com- electrochemical studies were conducted at room tem- parison, a series of NiCr-oxide-CNTs composites with perature (25 ± 1°C). different Ni/Cr molar ratios (1/1, 1/3, 2/1 and 3/1) were further prepared using a similar approach, as mentioned above. Meanwhile, the best-performing catalyst was also 2 Results and discussion made using direct hydrothermal methods for comparison 2.1 Crystal structure and thermogravimetric purposes. analysis The sheet-like NiCr-oxide-CNTs was synthesized by a 1.3 Characterizations facile one-step method, in which the precursors of NiCl , The morphology and microstructure of the catalyst were CrCl, CNTs, NH F, PVP and urea were ultrasonically dis- 3 4 investigated in detail by SEM (FEI Quanta 200 FEG) and persed in a mixed solution of HO-ethylene glycol and TEM (JEM-2100F) with X-ray energy dispersive spectros- then heated to 120°C for 9 h to achieve the final composite copy (EDS). The crystal structure of the material was c-har (Fig. 1a). Studies have shown that urea provides an alkaline acterized by XRD (Rigaku D/Max 2 500 V/PC) at a scan environment, PVP and ethylene glycol manipulate micro- −1 speed of 2.0 degree min . Thermal gravimetric measure- structures as ligands, while NH F regulates the directional ment was made on a TGA/STA409 PC module with a rising growth. Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 4 | Clean Energy, 2020, Vol. XX, No. XX NH F + PVP + Urea Hydrothermal 120 °C, 9 h NiCl CrCl CNTs NiCr -Oxide-CNTs 2 3 2 BC NiCr -Oxide-CNTs NiCr -Oxide-CNTs 2 2 0.10 Ni-hydro-CNTs TG curve Cr-hydro-CNTs DTG curve 0.05 63.8% C (002) Ni O (OH) , JCPDS: 06-0114 3 2 4 0.00 NiOOH, JCPDS: 27-0956 –0.05 Cr(OH) 3H O, JCPDS: 16-0817 3 2 –0.10 NiCr O , JCPDS: 65-3105 2 4 –0.15 457.0 °C 20 30 40 50 60 70 80 200 400 600 800 2-Theta (°) Temperature (°C) Fig. 1: (a) Schematic illustration of the preparation of NiCr -oxide-CNTs composite. (b) XRD patterns of NiCr-oxide-CNTs, Ni-hydro-CNTs and 2 2 −1 Cr-hydro-CNTs. (c) Thermogravimetric (TG) and differential thermogravimetric (DTG) curves of NiCr -oxide-CNTs with a heating rate of 2.5°C min under O atmosphere. The XRD patterns of the optimized catalyst NiCr -oxide- 457.0°C caused by oxidative pyrolysis of CNTs [29]. As CNTs with the two catalysts prepared using hydrothermal shown in Fig. S2 in the online Supplementary Data (see methods in the absence of either Ni (Cr-hydro-CNTs) or the online Supplementary Data), the sharp Raman peaks Cr (Ni-hydro-CNTs) are shown in Fig. 1b. The rest of the of NiCr -oxide-CNTs, Ni-hydro-CNTs and Cr-hydro-CNTs −1 NiCr -oxide-CNTs catalysts show similar XRD patterns are observed at ~1341 and 1582 cm for the D and G bands [Fig. S1 in the online Supplementary Data (see the online where the ratio of the D and G bands refers to the graphitic Supplementary Data)].F ig. 1b indicates that the crystal degree [30]. One can conclude that the NiCr -oxide-CNTs structures of NiCr-oxide-CNTs are consistent with the have the highest defect structure due to the highest /II 2 D G standard model of NiCrO (JCPDS: 65–3 105) [26], including value (1.18) as compared to these of Ni-hydro-CNTs (1.13) 2 4 the typical peak of the crystal plane (002) of CNTs at and Cr-hydro-CNTs (1.06). 25.8°. Notably, the Cr-hydro-CNTs exhibits four character - istic diffraction peaks at 18.2, 19.4, 26.6 and 43.7° corres- 2.2 Morphology analysis ponding to the (001), (100), (101) and (201) crystal planes The morphology and microstructure of NiCr -oxide-CNTs of Cr(OH) ·3H O (JCPDS: 16–0 817) [27]. Simultaneously, 2 3 2 were further investigated by SEM image, TEM images and the XRD pattern of Ni-hydro-CNTs matches well with EDS mappings. Fig. 2a is an SEM image which shows that the standard models of NiOOH (JCPDS: 27–0 956) and some spherical structural species are intertwined with Ni O (OH) (JCPDS: 06–0 114) as well, suggesting the co- 3 2 4 CNTs. The TEM image of Fig. 2b further discloses the spher - existence of these two species in Ni-hydro-CNTs com- ical species composed of sheet-like species containing posite. The ICP test results of NiCr -oxide-CNTs samples two types of clear lattice stripes with the lattice spacings with different molar ratios of Ni/Cr indicate a high level of about 0.25 and 0.32 nm (Fig. 2c), which are consistent of consistency between the experimental values and the with the (311) facet of NiCrO and (002) facet of CNTs, theoretical values [Table S1 in the online Supplementary 2 4 respectively [31]. Moreover, the EDS mappings manifest Data (see the online Supplementary Data)]. that the elements of Ni, Cr and O are evenly distributed Thermogravimetric (TG) and differential throughout the entire skeleton of the NiCr -oxide-CNTs thermogravimetric (DTG) curves were applied to explore 2 composite (Fig. 2d). the oxide content of the optimal NiCr -oxide-CNTs cata- lyst under O atmosphere (Fig. 1c). In the initial stage from room temperature to around 120°C, the weight loss is 2.3 XPS analysis due to the evaporation of water. Then, a significant drop between 400 and 600°C is attributed to the oxidation of XPS analysis was adopted to probe the chemical states of carbon. In addition, the 36.2 wt% remnant of NiCr -oxide- Ni and Cr elements in NiCr-oxide-CNTs (Fig. 3). As shown 2 2 CNTs after 800°C is nickel-chromium oxide [28]. Notably, in Fig. S3a in the online Supplementary Data (see the on- there is a sharp exothermic peak on the DTG curve at line Supplementary Data), the survey XPS spectra showed Intensity (counts) Weight (%) –1 DTG (mg min ) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 Gan et al. | 5 that NiCr-oxide-CNTs with different Ni/Cr ratios con- two peaks corresponding to Cr 2p (576.4 eV) and Cr 2p x 3/2 1/2 3+ tained Ni, Cr, C and O elements. The high-resolution C 1s (586.2 eV) of Cr [33], respectively (Fig. 3b). The binding en- [Fig. S3b in the online Supplementary Data (see the online ergy of Cr 2p for NiCr-oxide-CNTs exhibits a positive 3/2 2 Supplementary Data)] of each catalyst consists of four shift of 0.18 eV as compared to that of Cr-hydro-CNTs. peaks at C=C (284.0 eV), C–C (284.8 eV), C–O (286.0 eV) and The binding-energy changes indicate the presence of C=O (288.8 eV) [32], which are used as calibration stand- electronic-coupling and electron-transfer effects between ards. As shown in Fig. 3a, the high-resolution Ni 2p spec- the components of NiCr-oxide-CNTs [35, 36]. The elec- trum of NiCr-oxide-CNTs has been deconvoluted into four tron migration between Ni and Cr is believed to promote peaks, in which the binding energies at 854.9 and 872.7 eV the electrooxidation of urea synergistically [37]. The high- are ascribed to Ni 2p and Ni 2p , respectively, and the resolution Ni 2p and Cr 2p of the rest of the NiCr -oxide- 3/2 1/2 x other two peaks are attributed to satellite peaks [34 33], . CNTs composites were also analysed for comparison [Fig. All of these characteristics indicate that the Ni species in S3c and d in the online Supplementary Data (see the on- 2+ the composite is mainly present as Ni. It is worth noting line Supplementary Data)], which show a similar electronic that the binding energy of Ni 2p in the NiCr -oxide-CNTs structure. In addition, the high-resolution O 1s of all com- 3/2 2 composite has a negative shift of 0.25 eV compared to that posites are fitted to four peaks representing metal-oxide, of Ni-hydro-CNTs. Meanwhile, the high-resolution Cr 2p vacancies, C–O and adsorbed H O (Fig. 3c), respectively [38]. region of the NiCr-oxide-CNTs composite was fitted to The results reveal that the Cr species has a positive effect on the oxygen-defect sites, while the Ni species increase the hydrophilicity of the composite. As a result, the excel- A B lent urea electrocatalytic activity of the optimized NiCr - oxide-CNTs is the fruit of an elegant balance between the oxygen-defect sites and hydrophilicity. 2.4 Electrochemical performance analysis 1 μm The CV curves of NiCr -oxide-CNTs, Ni-hydro-CNTs and 100 nm x Cr-hydro-CNTs performed in 1.0 M KOH solution were CD used to evaluate their ESA (Fig. 4a). The figures indi- Ni cate that all catalysts except Cr-hydro-CNTs present a 0.32 nm pair of redox peaks in the potential range of 0.0 to 0.8 V. C (002) The anode peak in the forward scan is consistent with the Ni(OH) species oxidized to NiOOH and the cathode Cr O peak in the reverse scan is ascribed to the reduction of 0.25 nm NiOOH to Ni(OH) [11, 39]. Generally, the ESA was cal- NiCr O (311) 2 2 4 5 nm culated by the required reduction charge in the reverse scan, which is directly proportional to the number of ac- tive sites for urea electrooxidation. The ESA values of all Fig. 2: (a) SEM, (b) TEM and (c) high-resolution TEM images of NiCr - catalysts can be estimated from the equation ESA = /mq Q oxide-CNTs. (d) EDS mappings of NiCr -oxide-CNTs with Ni, Cr and O elements [38, 40], where Q is the total charge used for the reduction AB C O 1s Ni 2p Ni 2p Cr 2p Cr 2p 3/2 3/2 C-O Vacancies NiCr -Oxide-CNTs NiCr -Oxide-CNTs 2 ads 2 2 24.4% 34.9% Sat. Ni-O 46.0% 17.9% 0.18 eV 0.25 eV Cr-hydro-CNTs Ni-hydro-CNTs 32.3% 19.2% Cr-O 888 880 872 864 856 848 590 585 580 575 570 536 534 532530 528 Binding energy (eV) Binding energy (eV) Binding energy (eV) Fig. 3: High-resolution XPS regions of (a) Ni 2p, (b) Cr 2p and (c) O 1s from NiCr -oxide-CNTs, Ni-hydro-CNTs and Cr-hydro-CNTs Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 6 | Clean Energy, 2020, Vol. XX, No. XX 1/1 50.7 1/2 NiCr -Oxide-CNTs with x 50 1/3 different Ni/Cr ratios 2/1 41.3 3/1 36.8 35.3 Ni-hydro-CNTs Cr-hydro-CNTs 30 13.7 –20 1.1 –40 0.0 0.2 0.4 0.6 0.8 1/1 1/2 1/3 2/1 3/1 Ni E/V (vs. SCE) Different Ni/Cr ratios C D 140 1/1 NiCr -Oxide-CNTs 115.6 1/2 x 120 with different 1/3 96.3 94.7 2/1 100 Ni/Cr ratios 85.0 3/1 Ni-hydro-CNTs 80 Cr-hydro-CNTs 42.6 16.1 –20 –40 0.0 0.2 0.4 0.6 0.8 1/1 1/2 1/3 2/1 3/1 Ni E/V (vs. SCE) Different Ni/Cr ratios Fig. 4: (a) CV curves of NiCr-oxide-CNTs with different Ni/Cr ratios, Ni-hydro-CNTs and Cr-hydro-CNTs in 1.0 M KOH. (b) The summarized ESA values from (a). (c) CV curves of different catalysts in 1.0 M KOH + 0.33 M urea. (d) The summarized forward current densities from (c). of NiOOH species to Ni(OH) , m is the mass of Ni in the higher catalytic performance as compared to both Ni and −2 supported catalyst and q is 257 μC cm as only one elec- Cr single-component catalysts for urea oxidation, once tron for NiOOH converted to Ni(OH) . As shown in Fig. 4b, again indicating the synergistic effect between Ni and Cr the NiCr-oxide-CNTs shows the highest ESA of 50.7 m species. In addition, we also studied the electrochemical −1 g , which is 1.44-, 3.70-, 1.23-, 1.37- and 46-fold higher performance of NiCr-oxide, CoCr -oxide-CNTs and FeCr - 2 2 2 2 −1 than NiCr-oxide-CNTs (35.3 m g ), NiCr -oxide-CNTs oxide-CNTs in 1.0 M KOH and 1.0 M KOH + 0.33 M urea 2 −1 2 −1 (13.7 m g ), Ni Cr-oxide-CNTs (41.3 m g ), Ni Cr-oxide- [Fig. S4 in the online Supplementary Data (see the online 2 3 2 −1 2 −1 CNTs (36.8 m g ) and Ni-hydro-CNTs (1.1 m g ). Studies Supplementary Data)]. The results show that the catalytic have found that the introduction of a certain amount of performances of these control catalysts are much lower Cr species can expose more active sites through syner - than that of NiCr-oxide-CNTs, which indicates not only gistic interactions of Ni and Cr species [41]. Moreover, the importance of the CNTs, but also the unique effect of the various Ni/Cr ratios result in different morphology synergy between Ni and Cr towards the catalytic perform- in NiCr -oxide-CNTs composites, which also have a sig- ance of urea oxidation. nificant effect on ESA values [21]. Therefore, the optimal In the initial stage of electrocatalysis, studies found that value of the Ni/Cr ratio is required to obtain the highest the Ni(OH) species first lose one electron to form NiOOH − − ESA. Thereafter, the electrocatalytic tests of all catalysts on the catalyst surface [Ni(OH) +OH ↔ NiOOH+H O+e ] 2 2 were performed in 1.0 M KOH + 0.33 M urea (Fig. 4c). [42, 43]. Subsequently, the produced NiOOH intermediates Although all catalysts show a similar onset potential atcould adsorb h ydroxyl ions and urea molecules from the ~0.32 V except for Cr-hydro-CNTs (0.56 V), the NiCr -oxide- electrolyte. After undergoing a complex multi-electron- CNTs catalyst has the largest current density (115.6 mA transfer process, the urea molecules are finally oxidized −2 − cm ), which is about 1.36-, 2.71-, 1.20-, 1.22- and 7.2-fold to N , CO and H O on the active sites [CO(NH) +6OH → 2 2 2 2 2 higher than those of NiCr-oxide-CNTs, NiCr-oxide-CNTs, CO +N +5H O+6e ] [38, 44] while the NiOOH species is re- 3 2 2 2 Ni Cr-oxide-CNTs, Ni Cr-oxide-CNTs and Ni-hydro-CNTs, duced back to Ni(OH) species [45]. The summarized for- 2 3 2 respectively (Fig. 4d). Here, the higher onset potential of ward current densities of NiCr -oxide-CNTs are shown in the Cr-hydro-CNTs catalyst than others may be due to the Fig. 4d; the NiCr-oxide-CNTs shows the highest current fact that the catalyst lacks effective active sites to adsorbdensity . Furthermore, it is also among the top-performing urea molecules, thereby requiring a higher polarization urea-oxidation catalysts reported in the literature because potential to drive the reaction. It is also worth noting that of the lo w onset potentials and high peak current densities Cr-hydro-CNTs has almost no catalytic activity for urea, [Table S2 in the online Supplementary Data (see the online but the composite catalysts including Ni and Cr possess Supplementary Data)]. –2 –2 j (mA cm ) j (mA cm ) –2 j (mA cm ) 2 –1 ESA (m g ) Downloaded from https://academic.oup.com/ce/advance-article-abstract/doi/10.1093/ce/zkz029/5698410 by guest on 18 February 2020 III II Gan et al. | 7 The stabilities of NiCr-oxide-CNTs catalysts with dif- ferent Ni/Cr ratios were tested by chronoamperometry at CO 2 – I –e a constant potential of 0.45 V in 1.0 M KOH + 0.33 M urea CO H O VI 2 H N electrolyte. As shown in Fig. 5, the NiCr -oxide-CNTs cata- H C N lyst shows the highest initial current density (85.0 mA OH −2 −2 cm ) and limiting current density (38.6 mA cm ). The ini- – –e OH tial spike can be attributed to the higher urea concentra- CO OH tion on the surface of the catalysts at the beginning and H N – H the current stabilizes after some time where the system OH – –e C N CO N reaches the equilibrium [46]. – –e O H OH OH O 2 2 2 – – – –e –e –e H O III III III 2.5 Catalytic-mechanism analysis NiCr O NiOOH CrO 2 4 x As discussed above, the NiCr-oxide-CNTs catalyst ex- hibits excellent electrocatalytic performance for urea Fig. 6: The proposed possible catalytic mechanism of the electrocatalytic 2+ urea oxidation by NiCr-oxide-CNTs catalyst in alkaline media oxidation. Electrochemical studies show that the Ni species on the surface of the NiCr-oxide-CNTs cata- lyst is first oxidized to NiOOH species (lose 1e ) as the 3 Conclusion active sites (M) (Fig.  6, Step I). Then, the urea molecules In summary, the NiCr-oxide-CNTs with different Ni/Cr in the solution are adsorbed into the active sites of the ratio catalysts were prepared by a facile hydrothermal NiOOH surface and the partially positively charged H method. Various techniques were applied to investi- atoms of the urea molecules are adsorbed onto the sur - gate the crystal structures, morphologies and chemical face of the negatively charged Cr species (due to a higher states. The results indicate that the NiCr -oxide-CNTs electron density than Ni species) through electrostatic composite is composed of the sheet-like structure of interaction (Step II). After the attack of OH ions and the NiCr -oxide and CNTs. There is a visible synergistic − 2 combination of electrooxidation (loss of 1), e one H atom effect between Ni and Cr. The electrochemical studies is removed to form a free HO molecule (Step III). After show that the NiCr-oxide-CNTs catalyst exhibits three consecutive dehydrations, an intermediate state of 2 −1 the immense ESA value (50.7 m g ), highest current M-CO⋅N is formed. Similarly, after an OH attack and the 2 −2 density (115.6 mA cm ) and permanent stability for urea electrooxidation (loss of 1e) followed by losing of a N, electrooxidation in alkaline medium. The prominent another intermediate state of M-CO OH is formed (Ste ⋅ p performance of the NiCr-oxide-CNTs catalyst is mainly − 2 IV). Along with the further attack of OH ions and one ascribed to its improved chargetr ‐ ansfer kinetics, the more electrooxidation process (loss of 1), e the last inter - larger ESA, along with an elegant balance between the mediate state of M-CO is formed while releasing one oxygen-defect sites and hydrophilicity. Moreover, the molecule of HO (Step V). Finally, the active site M is re- results also demonstrate a promising application of the covered by releasing CO (Step VI) [11, 43]. non-noble-metal catalysts in water splitting, hydrogen production, fuel cells, etc. 1/1 1/2 NiCr -Oxide-CNTs with 1/3 Supplementary data different Ni/Cr ratios 2/1 Supplementary data is available at Clean Energy online. 3/1 Acknowledgements This work has been supported by the National Natural Science Foundation of China (21965005), Natural Science Foundation of Guangxi Province (2018GXNSFAA294077, 2017GXNSFGA198004), Project of High-Level Talents of Guangxi (F-KA18015, 2018ZD004) and Innovation Project of Guangxi Graduate Education (XYCSZ2019056, YCBZ2019031). 0 1200 2400 3600 4800 6000 7200 Time (s) Fig. 5: Chronoamperometric curves of NiCr -oxide-CNTs catalysts with Conflict of Interest different Ni/Cr ratios at a constant potential of 0.45 V (vs. SCE) in 1.0 M None declared. 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Clean EnergyOxford University Press

Published: Apr 4, 2020

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