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Nickel nanoparticles supported by commercial carbon paper as a catalyst for urea electro-oxidation

Nickel nanoparticles supported by commercial carbon paper as a catalyst for urea electro-oxidation Nickel nanoparticles supported by commercial carbon paper (CP) are prepared by pulsed laser deposition with deposition time of 3, 6, and 12 min as a catalyst for urea electro-oxidation. The surface conditions and the morphologies of the pre- pared electrodes have been characterized by Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Urea electro-oxidation reaction in KOH solution on the Ni/CP electrodes is investigated by cyclic voltammetry and chronoamperometry. The results show that the electrode with less Ni nanoparticle agglomeration shows higher peak current density, which was achieved in the 3 min deposition samples when normalized by electroactive surface areas. How- ever, the highest current normalized by the area of the carbon paper was achieved in the 6 min deposition sample due to the larger quantity of Ni nanoparticles. All the samples show good stability. Our results suggest that the low density, low cost, and environmental friendly CP can be used as support for Ni nanoparticle as a catalyst for urea electro-oxidation. It thus has great potential for many applications involving urea oxidation, such as wastewater treatments. Keywords Nickel nanoparticles · Carbon paper · Urea electro-oxidation · Catalyst Introduction The generation of clean energy and the wastewater treatment are two major challenges faced and pursued due to the heavy dependence on fossil fuels, non-renewable energy sources, Electronic supplementary material The online version of this which cause environmental impacts through the greenhouse article (https ://doi.org/10.1007/s4024 3-020-00180 -8) contains gas emissions, and the constant contamination of rivers due supplementary material, which is available to authorized users. to the absence of an adequate wastewater treatment [1]. Urea is a substance found in domestic wastewater, as it * Júlio César M. Silva juliocms@id.uff.br is the main component of human and animal urine, con- taining about 2–2.5 wt% of urea [2, 3], and in industrial * Yutao Xing xy@id.uff.br effluents, such as a large amount of urea-rich wastewater generated by the process of urea synthesis as agricultural Departamento de Engenharia Química e de Petróleo, fertilizer and animal feed additive [4, 5]. When these efflu- Universidade Federal Fluminense, Niterói, RJ 24210-346, ents are discharged into rivers without treatment, they can Brazil generate serious environmental contamination and human Laboratório de materiais da UFF (LaMUFF), Instituto health problems because urea, despite being non-toxic, de Química, Universidade Federal Fluminense, Campus Valonguinho, Niterói, RJ 24020-141, Brazil can naturally decompose into toxic ammonia and others nitrogenous pollutants [1, 6–8]. The treatment of waste- COMAN, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, RJ 22290-180, Brazil water through traditional methods, such as aeration tanks, presents itself as a great consumer of energy [9]. Urea Laboratório de Microscopia Eletrônica de Alta Resolução, Centro de Caracterização Avançada para Indústria is conventionally treated by nitrification and denitrifica- de Petróleo (LaMAR/CAIPE), Universidade Federal tion, which are sophisticated and high energy consuming Fluminense, Niterói, RJ 24210-346, Brazil Vol.:(0123456789) 1 3 20 Page 2 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 biological processes, generating an excessive cost [7]. As presence of a background gas. The species that suffer abla- an alternative, urea electro-oxidation has proven to be an tion are deposited on the support surface [34]. effective way for its degradation. Moreover, it is consid- In this work, commercial CP, which is cheap and easy to ered as a low-cost technology, dispenses sophisticated obtain, was tested as support for Ni nanoparticles synthe- equipment and can be applied for long periods of time sized by using the PLD technique. Optimizing the balance [10, 11]. The conversion occurs from the following elec- between the Ni loading quantities and catalytic behavior for trochemical oxidation reactions [7, 12, 13]: the purpose of real applications is another main goal of this study. The support and the synthesized catalysts were char- − − Anode:CO NH + 6OH → N + 5H O + CO + 6e 2 2 2 2 2 acterized by the techniques of Raman spectroscopy, scan- (1) ning electron microscopy (SEM), and transmission electron − − microscopy (TEM). The electrocatalytic activity of the mate- Cathode:6H O + 6e → 3H + 6OH (2) 2 2 rials towards the urea electro-oxidation reaction (UER) in alkaline medium was evaluated using cyclic voltammetry Overall:CO NH + H O → N + 3H + CO (3) 2 2 2 2 2 (CV) and chronoamperometry (CA) measurements. Several catalysts such as metals (Ni, Fe, Cu, Mn, Zn, Pt, etc.), metal oxides and metal free have been widely Experimental studied over the years in electrochemical processes [14]. Among them, nickel (Ni) is extensively used because of Plasma treatment of carbon paper surface its important characteristics such as its large abundance in nature, low cost (non-precious metal) and low toxic- The original Teflon treated CP was obtained from Carbon ity [15]. Additionally, Ni electrocatalysts demonstrate Paper AvCarb. The surface of CP is highly hydrophobic and excellent catalytic activity for urea electro-oxidation in hence is not suitable for the applications of the current study. alkaline solutions. As presenting good chemical stabil- To improve the contact between the CP surface and the reac- ity [12], high ductility, good thermal conductivity, and tion solution, the CP was treated with ammonia plasma in considerable electrical conductivity [15], Ni electrocata- −2 a vacuum chamber with a base pressure of ~ 2 × 10 Torr. lysts are usually used as anodic electrodes in the process Ammonia plasma etching has several advantages over air [7]. However, several factors can influence the catalytic plasma etching or chemical etching: it removes fluorine more activity of a metal, such as the metal support interaction efficiently [35], etches the surface more homogeneously, and [16, 17]. In order to increase the stability of the material no chemical residues on the highly porous CP. For plasma and increase the area of exposed active sites of Ni cata- etching, a piece of CP with a dimension of 10 cm × 2 cm was lysts, several authors have studied carbon-based supports, mounted perpendicular to the sample holder of the plasma such as carbon nanotubes [18, 19], graphene [19, 20], and chamber with half of it (10 cm × 1 cm) covered by Al plates. carbon fiber paper (CP) [21, 22] allowing an increase in Plasma etching was performed with the CP mounted on the activity and catalytic performance [23]. CP can be used for cathode of a capacitively coupled RF chamber, with a NH the electrochemical application, such as in fuel cells and plasma discharge. The discharge was established at 0.10 Torr electrocatalysts, acting as microporous layer support for NH and − 300 V self-bias (37 W RF power) for 20 min. catalysts. Its characteristics are the efficient transportation The illustration of the process and a plasma-etching image of gases and liquids, good thermal, and electrical conduc- can be found in Fig. 1. After plasma-etching, a small piece tion in high corrosive environments and high-temperature of the CP was tested by dropping water onto the surface. conditions [24–26]. For these reasons, CP is a suitable The treated CP was then cut into pieces with a dimension option as support for Ni nanoparticles for electrocatalysis of 2 cm × 0.5 cm as support of Ni nanoparticles. The hydro- applications. philic area of each sample was 1 cm × 0.5 cm. There are several techniques for the production of Ni nanoparticles [15], which may include synthesis by ther- Nanoparticles production mal decomposition [27], chemical reduction [28], microe- mulsion [29], sol–gel method [30], sonochemical synthesis The hydrophobic part of the final CP support was covered [31], Pulsed Laser Deposition (PLD) [32], etc. Among the by a piece of kapton tape and was mounted onto the sub- mentioned techniques, PLD is known for its good control strate holder of PLD system for Ni deposition. The back- of the distribution and particle size during its production −6 ground pressure of the PLD system was ~ 10 Torr. The Ni [33]. This technique is based on the ablation of a Ni tar- nanoparticles were deposited from pure Ni (99.99%) target get induced by a high power pulsed laser inside the PLD onto the hydrophilic part of the CP in the presence of an Ar chamber, that can occur in vacuum conditions or in the background atmosphere (1.0 Torr) at room temperature. The 1 3 Materials for Renewable and Sustainable Energy (2020) 9:20 Page 3 of 11 20 Fig. 1 Left: Illustration of the electrode preparation. Upper right: carbon papers under plasma etching. Lower right: Wetting test for the etched and non-etched CP surfaces by drooping water on −1 target ablation was done using the first harmonic (1064 nm recorded from 0.0 V and 0.75 V vs Hg/HgO in 1 mol L 2 −1 wavelength) of a pulsed Nd:YAG laser with 0.12 J/mm KOH + 0.33  mol  L urea solution. The concentration of energy density, 7 ns pulse duration, and 10 Hz repetition 0.33 M urea was used during the experiment to simulate the rate. The laser beam was focused onto the Ni target with an concentration of urea in urine waste and it is the urea con- incidence angle of 45° and the ablated Ni atoms expanded centration usually studied [2–4, 10, 36–39]. In both cases, in the direction of the substrate, with a fixed 3.5 cm distance the last cycle is shown. Chronoamperometric experiments from the target. Samples with deposition time of 3, 6 and were carried out at 0.55 V vs Hg/HgO for 60 min. The cur- 12 min were prepared to obtain the different amount of Ni rent from the urea electro-oxidation process was normalized nanoparticles on the CP support and were named as 3-Ni/ by the electroactive surface areas (ESA), estimated accord- CP, 6-Ni/CP, and 12-Ni/CP, respectively. ing Eq. 4. ESA = Q∕q (4) Characterization 3+ 2+ where Q is the charge required to reduce Ni to Ni The morphology and thickness of the samples were investi- [NiOOH to Ni(OH) ], which can be calculated from the gated by using field-emission scanning electron microscope cyclic voltammograms in electrolyte support. In Eq.  4, q (SEM: JEOL JSM 7100F) and field-emission high-resolution is the charge related to the formation of a monolayer of transmission electron microscopy (HRTEM. JEOL JEM Ni(OH) from NiOOH that involves one electron transfer −2 2100F). The chemical composition and electronic structure and the value can be taken as 257 μC cm [10, 40, 41]. of the products were characterized by Raman spectroscopy The procedure to determine the ESA was similar to that one (Witec alpha 300R Raman microscope, with 532 nm wave- reported elsewhere [10, 36]. However, the difference is that length excitation) and energy-dispersive X-ray spectroscopy the obtained ESA was not normalized by nickel loading in (EDS). the electrode like in the literature [10, 36]. The results were also normalized by electrode geometric area (GSA) of the Electrochemical measurements work electrode. Electrochemical measurements were carried out at room temperature using a µStat400 bipotentiostat/galvanostat (Metrohm DropSens), by using a three-electrode electro- Results and discussion chemical cell, with a platinum foil as the counter electrode, Hg/HgO as reference electrode and the CP with nickel As can be seen from Fig. 1, the CP surface has turned from particles as the work electrode. CV experiments were hydrophobic to hydrophilic by the NH plasma-etching and −1 −1 performed in 1 mol L KOH at a scan rate of 50 mV s then became suitable as catalyst support for electrocatalysis from 0.0 to 0.65 V vs Hg/HgO. Prior each experiment N applications. To investigate the surface modification, the was bubbled for 15 min, and ten consecutive cycles were original and plasma-treated CP was characterized by using −1 recorded for each material. Five cycles (10 mV s ) were Raman spectroscopy, with the results being shown in Fig. 2. 1 3 20 Page 4 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 The spectrum of the original CP showed clearly the most −1 prominent features of graphite: the G band at 1582 cm , D' G' −1 −1 the D band at ~ 1350 cm , the D′ band at about 1620 cm −1 and the G′ band at ~ 2700 cm [42]. The G band is associ- ated with a doubly degenerate phonon mode for sp2 carbon Plasma-treated networks. The D and D′ bands are related to defects, and thus being usually absent in the spectrum of highly crystalline graphite. The G′ band is a feature of D band overtone in dif- ferent kinds of graphitic materials. The integrated intensity ratio I /I for the D band and G band is usually applied for D G analyzing the number of defects in graphitic materials [43]. Non-treated A relatively weak D band is clearly observed for the original CP in Fig. 2, which indicates that the CP is constructed by graphite fibers with a small number of defects. After plasma- 1000 1500 200025003000 3500 etching, the intensity of D band significantly increased, sug- -1 Raman shift (cm ) gesting a large quantity of defect generation; other bands were not obviously modified. Fig. 2 Raman spectra of the original and plasma-etched carbon paper, Detailed microstructure studies were performed for the showing the main Raman features of graphite: the D, G, D′ and G′ original and plasma-etched CP. As can be seen from Fig. 3, bands the CP is constructed by carbon fibers with diameters in the range of 5–10 μm and possesses plenty of pores and voids. From the back-scattered electron image of the original CP Fig. 3 a Low magnification secondary electron image, b low mag- the plasma-treated CP. g A low-mag back scattered SEM image of the nification back-scattered electron image and c high resolution SEM original CP and the elemental mapping with EDS for h carbon and i images of the original CP. d, e and f are the corresponding images of fluorine 1 3 Intensity (a.u.) Materials for Renewable and Sustainable Energy (2020) 9:20 Page 5 of 11 20 (Fig. 3b), a layer with a higher brightness than the carbon plasma-etching is the roughness increase on the surface of fibers appeared. Since the back-scattered electron image each carbon fiber. From Fig.  3c we can see that the carbon gives atomic mass contrast and the surface of the CP is fiber has a very flat surface. After plasma-etching, however, hydrophobic, the brighter layer might be fluorine rich and abundant nano-sized points appeared and were homogene- is responsible for the hydrophobic property. The EDS anal- ously distributed on the surface of the carbon fibers. Higher ysis is shown in Fig. 3g–i confirmed that the original CP resolution images can be found in the supplementary infor- fibers are composted by purely carbon and the bright layer mation to show it more clearly. The result is in well agree- is rich of fluorine. No other elements with a concentration ment with the conclusion of Raman spectra, which suggested higher than the level of contamination were observed. The a large number of defect generation due to plasma-etching. first effect of plasma-etching process is fluorine removal, Both transformations from a hydrophobic to a hydrophilic which is clearly shown in Fig. 3d, e. Most of the fluorine surface and surface roughness increase greatly benefit the has been removed by NH plasma and only few of them is catalytic behavior, while the former increases the contact of left in the sharp corners, where was more difficult for plasma the CP surface with a solution and the latter enhances the generation. Quantitative analyses of the average EDS spec- specific surface area. tra obtained from the elemental mapping results for origi- A detailed microstructure study for the Ni deposited CP nal and plasma-treated CP clearly demonstrated a decrease has been done with SEM and the images are shown in Fig. 4. of fluorine from ~ 30 wt% to ~ 9 wt% after plasma etching The 3-Ni/CP sample showed that many nano-sized particles (as shown in the supplementary information). The absence appeared on the surface of each nanofiber. The high-resolu- of nitrogen signal on the EDS spectrum for the treated CP tion image (Fig. 4b) reviled that the bright nanoparticles in suggested the nonexistence of residual ammonia on the CP Fig. 4a are non-regular and are constructed by smaller sized surface. The significant decrease of fluorine concentration particles (< 100  nm). The back-scattered electron image on the surface resulted in the transformation of the CP sur- (Fig.  4c) clearly shows atomic mass contrast, indicating face from hydrophobic to hydrophilic. Another effect of the that the nanoparticles are composted by heavier element Fig. 4 a Low magnification secondary electron image, b high magnification secondary electron image and c high magnification back-scattered electron image of the 3-Ni/CP. d–f are the corresponding images of the 6-Ni/CP and g–i, of 12-Ni/CP 1 3 20 Page 6 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 than C and in this work, it is apparently Ni. With 3 min perfectly correspondents to the structure and morphology of Ni deposition, the Ni nanoparticles were well dispersed of the CP as shown in Fig. 5c. The result indicates that Ni on CP surface without large agglomerations. After 6 min nanoparticles are deposited homogeneously on the surface. of Ni deposition, more agglomeration of Ni nanoparticles The black areas in Fig. 5d do not mean the absence of Ni, occurred. Moreover, the nanoparticles started to connect but less detection of Ni X-ray signal. At those places, the Ni with each other and formed bigger patterns. With a back- nanoparticles were deposited to the deeper part of the CP scattered electron image, one can still see the low-density and due to the shape effect, X-ray photons from Ni exited by morphology and each of the bigger patterns was in fact an the electron beam hardly reach the detector. The high resolu- assembly of many smaller nanoparticles. The mean size of tion mapping of Ni element in Fig. 5f shows clearly that the the agglomerated nanoparticles is higher than of the materi- agglomerations of the particles are Ni. als prepared with 3 min of Ni deposition, however, is still The SEM images suggest that the Ni layer deposited on less than 100 nm, as can be seen from Fig. 4e. With even CPs is constructed by nanoparticles with size much smaller more Ni nanoparticles deposited (12 min), the size of the than that of the patterns. With only SEM, one cannot get agglomerations significantly increased, with a mean value detailed size and microstructure of the Ni nanoparticles. In of about 200 nm. The small-sized nanoparticles in Fig. 4c, order to obtain this information, one sample has been pre- f are hardly seen and instead, much bigger and condensed pared by depositing Ni nanoparticles onto a Cu grid with C particles appeared in Fig. 4i. support for less than 1 min and studied by using HRTEM Figure 5a, b show a side-view of the deposited Ni layer with results shown in Fig. 6. From the low magnification on top of one fiber for sample 12-Ni/CP. One can see that the image, we can see that the Ni nanoparticles have spherical- agglomerated nanoparticles have columnar structure with shape and were homogeneously deposited on the surface. the height of several hundred nanometers. Almost all the The mean diameter of the nanoparticles is about 3.5 nm with surface of the CP is covered by Ni. Smaller sized nanopar- a very narrow size distribution (width at half maximum: ticles still can be seen at the border of the Ni layer and C ~ 1.5 nm). The high-resolution image in Fig.  6b demon- fiber. Although from the back-scattered electron images we strates a crystalline microstructure for the particles. During can conclude that the deposited nanoparticles might be Ni, it PLD process, the nanoparticles were formed before they needs to be proved by EDS. Figure 5c–f show the elemental were deposited onto the substrate, and their size depends mapping results of Ni by EDS for the sample 12-Ni/CP. The only on the gas pressure in the chamber. Since the same Ni distribution in the low-magnification mapping (Fig.  5d) pressure and distance from the target to the substrate were Fig. 5 a Side-view secondary electron image and b back-scattered EDS for d Ni. e High magnification back-scattered electron image electron image of the deposited Ni nanoparticles for 12-Ni/CP. c Low and elemental mapping with EDS for f Ni magnification secondary electron image and elemental mapping with 1 3 Materials for Renewable and Sustainable Energy (2020) 9:20 Page 7 of 11 20 Fig. 6 a Low magnification image of Ni nanoparticles. Inset shows the size distribution of the particles, b HRTEM image of Ni nanoparticles used for all the three samples in this work, the Ni nanoparti- cles in the three samples are then similar. The Ar molecules in the chamber play a role not only in the formation of Ni nanoparticles but also in decreasing their velocity. The Ni 1.0 3-Ni/CP nanoparticles with less kinetic energy deposited on the CP surface have much lower impact and hence, resulted in the 0.5 formation of less dense material, namely, columnar struc- ture. In contrast, it forms a thin film in a vacuum (much 0.0 lower pressure) and extremely low-density nanofoam in much higher pressure [44]. X-ray photoelectron spectros- -0.5 copy (XPS) experiments are of high interest to exam the possible oxidation of Ni nanoparticles after the sample being 2.4 6-Ni/CP removed from the vacuum chamber. The CV curves of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP elec- 1.2 −1 trocatalysts in 1  mol  L KOH in the potential range of 0.0 V to 0.65 V are shown in Fig. 7. The shape of the CVs 0.0 is in agreement with nickel catalysts behavior reported in the same experimental condition of the current study [2, -1.2 45]. The characteristic redox feature of nickel can be seen 3.6 in CVs. The peak in the forward scan corresponds to the 12-Ni/CP 2+ 3+ Ni oxidation to Ni [Ni(OH) to NiOOH] and the peak 2.4 3+ 2+ in the backward scan is related to the Ni reduction to Ni 1.2 [2, 36]. It is important to point out that NiOOH is the phase responsible for urea electro-oxidation, and not Ni(OH) [2, 0.0 37]. As can be seen, the current related to the nickel oxi- -1.2 dation and reduction process increases from the material 3-Ni/CF to the 12-Ni/CF, which might be related to a higher -2.4 0.00.2 0.40.6 amount of nickel deposited on the CP due to longer time deposition. The calculated ESA was 3.81 cm for the 3-Ni/ E / V vs Hg/HgO 2 2 CF, 6.92 cm for 6-Ni/CF and 7.86 cm for 12-Ni/CF. As can be seen, the ESA increases almost twice (82%) from 3 to Fig. 7 Cyclic voltammograms of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP −1 −1 6 min of nickel deposition, which is reasonable. On the other electrocatalysts in 1 mol L KOH. Scan rate of 50 mV s 1 3 I / mA 20 Page 8 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 hand, from 6 to 12 min of deposition the ESA increases only onset potential, the best results were obtained using 3-Ni/CP 13.6%. As shown in Fig. 4, the mean agglomerated particle catalyst. However, the maximum current density on 3-Ni/CP size of the materials increased as the time of nickel deposi- was only about 8% higher than on 6-Ni/CP. As shown in the tion increases, suggesting a smaller specific surface area for SEM images (Fig. 4), the agglomerated nickel particles of the material prepared with longer time deposition. However, 3-Ni/CP are smaller (< 100 nm) than of the other materials longer time deposition also results in a higher amount of and did not cover the CP surface completely. On the other nickel and larger CP surface coverage. Comparing the two hand, for 12-Ni/CP catalyst, the nickel particle mean size is electrodes 3-Ni/CP and 6-Ni/CP, it is expected that 6-Ni/CP about 200 nm which forms dense agglomerations, covering to have a twice amount of nickel loading than that in 3-Ni/ almost completely the surface of CP. The nickel particles CP since the deposition time doubled, the same comparison of 6-Ni/CP are also more agglomerated than on 3-Ni/CP, can be made for 6-Ni/CP and 12-Ni/CP. Thus, although the but the agglomerated nickel particles are also smaller than nickel deposition time of 12-Ni/CP is twice longer than that 100 nm. It is reported that the materials with at least one of 6-Ni/CP, it results in only 13.6% specific surface area dimension smaller than 100 nm show different properties increase, which is related to the larger agglomerated particle from the bulk ones, such as, quantum confinement, electri- sizes observed for 12-Ni/CP catalyst. cal, magnetic [36]. Nanostructured nickel shows improved The CV curves of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP elec- catalytic activity towards UER (urea electro-oxidation reac- −1 −1 trocatalysts in 1 mol L KOH + 0.33  mol  L urea in the tion) if compared with bulk nickel [2, 47]. In this sense, potential range of 0.0 V to 0.75 V are shown in Fig. 8. As the higher catalytic activity of 3-Ni/CP followed by 6-Ni/ can be seen, the onset potential, an important parameter to CP could be related to the smaller particles agglomeration be extracted from CV experiments was about 30 mV less of nickel and better dispersion on the CP support than of positive on 3-Ni/CF (0.47 V) than on 6-Ni/CP and 12-Ni/CP. 12-Ni/CP. The decrease of the onset potential is related to a change in Figure  9 shows the chronoamperometric curves in −1 −1 the required activation energy for the process, and a suitable 1 mol L KOH + 0.33 mol L urea on 3-Ni/CP, 6-Ni/CP catalyst enhances the reaction rate by decreasing the activa- and 12-Ni/CP electrocatalysts at 0.55 V during 60 min. As tion energy [46]. It is important to point out that the onset in CV experiments, better result was obtained on 3-Ni/CP potential starts at the similar potential of the conversion from catalyst. The current density from urea electro-oxidation 2+ 3+ Ni to Ni , confirmed that NiOOH is the active phase for on 3-Ni/CP at the end of CA experiment was 44% higher the urea electro-oxidation reaction [37, 38]. than on 6-Ni/CP and 133% on 12-Ni/CP. The slow decay Another import data from the CV experiments is the peak of the current density over time is induced either by the current density. The maximum current density on 3-Ni/CP gas bubbles (i.e., N, CO ) produced from urea electro- 2 2 was about 8% higher than on 6-Ni/CP and 99% on 12-Ni/CP. oxidation or by the blockage of the active site due to the Thus, considering both of the peak current density and the intermediate products [37, 48], e.g. CO can cause the deactivation of NiOOH which is actually the active phase 3-Ni/CP 1.2 6-Ni/CP 12-Ni/CP 0.9 3-Ni/CP 0.6 6-Ni/CP 0.3 12-Ni/CP 0.0 0.00.1 0.20.3 0.40.5 0.60.7 E/V vs Hg/HgO Time (min) Fig. 8 Cyclic voltammograms of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP Fig. 9 Chronoamperometric measurements at 0.55  V vs Hg/HgO −1 −1 −1 electrocatalysts in 1 mol L KOH + 0.33  mol  L urea. Scan rate of on 3-Ni/CP, 6-Ni/CP and 12-Ni/CP electrocatalysts in 1  mol  L −1 −1 10 mV s KOH + 0.33 mol L urea 1 3 -2 J / mA cm -2 J / mA cm Materials for Renewable and Sustainable Energy (2020) 9:20 Page 9 of 11 20 6-Ni/CP (a) (b) 6-Ni/CP 12-Ni/CP 40 3-Ni/CP 4 3-Ni/CP 12-Ni/CP 0 0 Fig. 10 a Peak current density from urea electrooxidation (from CV experiments), and b current density from CA at 60 min of experiments to promote urea electro-oxidation [49]. The urea consump- Conclusions tion in the electro-oxidation process also influences the drop of current over time [4, 10]. As can be seen, the cur- In summary, we employed commercial CP as support of rent density from urea electro-oxidation on 3-Ni/CP at the Ni nanoparticles as catalyst for urea electro-oxidation. The beginning of the process is much higher than on the other Ni nanoparticles have been prepared by PLD with 3, 6 and two catalysts materials. The higher current density causes 12 min of deposition time in order to get different Ni load- faster consumption of urea near to the electrode surface, ing quantity. The TEM study shows that the Ni nanopar- resulting in a more abrupt decrease of current density in ticles have an average size of 3.5 nm with a narrow size the first minutes of the experiment. Similar behaviours distribution. The SEM results reveal that agglomeration of have been reported in the materials showing higher current Ni nanoparticles occurred in all the samples and they were density from the urea electro-oxidation process [4, 10]. more agglomerated with longer deposition time. The cyclic As discussed in the results from VC (Fig. 8) the smaller voltammograms show that the 3-Ni/CP sample shows better size and better dispersion of nickel nanoparticles of 3-Ni/ catalytic activity due to less agglomeration and thus more CP might be the reason for its superior catalytic activity active sites on the surface of Ni nanoparticles. Lower onset for urea electro-oxidation compared to the 6-Ni/CP and potential has been observed in this sample as well. For real 12-Ni/CP. applications, the current normalized by the electrode area is The results shown in the FigS. 8 and 9 are normalized then become important and the experimental results show by ESA. However, considering real applications it would that the 6-Ni/CP has the best performance, with current den- be interesting to normalize the results for the electrode sity ~ 60% higher than the other two samples. The study here geometric area. The Fig. 10 shows the results of peak cur- shows that CP can be used as low cost and environmentally rent density from urea electro-oxidation on CV experi- friendly catalyst support and electrodes with intermediate Ni ments (Fig. 10a) and the current density at the end of CA loading quantity might be of interest for real applications of experiments (Fig. 10b). As can be seen, better results were urea electro-oxidation. obtained using 6-Ni/CP, considering this normalization. Acknowledgements The authors gratefully acknowledge FAPERJ From the results shown in Fig. 10 it is seen that the current for the support with grant numbers of E-26/010.000978/2019, density from urea electro-oxidation on 6-Ni/CP is about E-26/010.001550/2019, E-26/211371/2019 and E-26/200153/2020, and 70% higher than on 12-Ni/CP and 80% on 3-Ni/CP. Con- CNPQ for the support with grant number 422614/2018-1. This study sidering the twice more nickel on 6-Ni/CP than in 3-Ni/ was also supported, in part, by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. CP due to doubled deposition time, it would be expected the current density twice higher. However, this relation Compliance with ethical standards would be true only if both materials have the same nickel agglomerated nanoparticle sizes but they do not. The SEM Conflict of Interest The authors declare that they have no conflict of analysis and the ESA results suggest that the nickel nano- interest. particle sizes of 6-Ni/CP are slightly larger. The current density from urea electro-oxidation on 12-Ni/CP is almost Open Access This article is licensed under a Creative Commons Attri- the same with that on 3-Ni/CP (Fig. 10), which is mainly bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long due to the larger (> 200 nm) aggregated nickel nanoparti- as you give appropriate credit to the original author(s) and the source, cles in 12-Ni/CP. 1 3 -2 J / mA cm -2 J / mA cm 20 Page 10 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 provide a link to the Creative Commons licence, and indicate if changes 13. Boggs, B.K., King, R.L., Botte, G.G.: Urea electrolysis: Direct were made. The images or other third party material in this article are hydrogen production from urine. Chem. Commun. (2009). https included in the article’s Creative Commons licence, unless indicated ://doi.org/10.1039/b9059 74a otherwise in a credit line to the material. If material is not included in 14. Mohammed-Ibrahim, J., Xiaoming, S.: Recent progress on earth- the article’s Creative Commons licence and your intended use is not abundant electrocatalysts for hydrogen evolution reaction (HER) permitted by statutory regulation or exceeds the permitted use, you will in alkaline medium to achieve efficient water splitting—a review. need to obtain permission directly from the copyright holder. To view a J. Energy Chem. 34, 111–160 (2019). https ://doi.org/10.1016/j. copy of this licence, visit http://creativ ecommons .or g/licenses/b y/4.0/.jeche m.2018.09.016 15. Banik, S., Mahajan, A., Kumar, Bhattacharya S.: Size control syn- thesis of pure Ni nanoparticles and anodic-oxidation of Butan-1-ol in alkali. Mater. Chem. Phys. 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Nickel nanoparticles supported by commercial carbon paper as a catalyst for urea electro-oxidation

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Abstract

Nickel nanoparticles supported by commercial carbon paper (CP) are prepared by pulsed laser deposition with deposition time of 3, 6, and 12 min as a catalyst for urea electro-oxidation. The surface conditions and the morphologies of the pre- pared electrodes have been characterized by Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. Urea electro-oxidation reaction in KOH solution on the Ni/CP electrodes is investigated by cyclic voltammetry and chronoamperometry. The results show that the electrode with less Ni nanoparticle agglomeration shows higher peak current density, which was achieved in the 3 min deposition samples when normalized by electroactive surface areas. How- ever, the highest current normalized by the area of the carbon paper was achieved in the 6 min deposition sample due to the larger quantity of Ni nanoparticles. All the samples show good stability. Our results suggest that the low density, low cost, and environmental friendly CP can be used as support for Ni nanoparticle as a catalyst for urea electro-oxidation. It thus has great potential for many applications involving urea oxidation, such as wastewater treatments. Keywords Nickel nanoparticles · Carbon paper · Urea electro-oxidation · Catalyst Introduction The generation of clean energy and the wastewater treatment are two major challenges faced and pursued due to the heavy dependence on fossil fuels, non-renewable energy sources, Electronic supplementary material The online version of this which cause environmental impacts through the greenhouse article (https ://doi.org/10.1007/s4024 3-020-00180 -8) contains gas emissions, and the constant contamination of rivers due supplementary material, which is available to authorized users. to the absence of an adequate wastewater treatment [1]. Urea is a substance found in domestic wastewater, as it * Júlio César M. Silva juliocms@id.uff.br is the main component of human and animal urine, con- taining about 2–2.5 wt% of urea [2, 3], and in industrial * Yutao Xing xy@id.uff.br effluents, such as a large amount of urea-rich wastewater generated by the process of urea synthesis as agricultural Departamento de Engenharia Química e de Petróleo, fertilizer and animal feed additive [4, 5]. When these efflu- Universidade Federal Fluminense, Niterói, RJ 24210-346, ents are discharged into rivers without treatment, they can Brazil generate serious environmental contamination and human Laboratório de materiais da UFF (LaMUFF), Instituto health problems because urea, despite being non-toxic, de Química, Universidade Federal Fluminense, Campus Valonguinho, Niterói, RJ 24020-141, Brazil can naturally decompose into toxic ammonia and others nitrogenous pollutants [1, 6–8]. The treatment of waste- COMAN, Centro Brasileiro de Pesquisas Físicas, Rio de Janeiro, RJ 22290-180, Brazil water through traditional methods, such as aeration tanks, presents itself as a great consumer of energy [9]. Urea Laboratório de Microscopia Eletrônica de Alta Resolução, Centro de Caracterização Avançada para Indústria is conventionally treated by nitrification and denitrifica- de Petróleo (LaMAR/CAIPE), Universidade Federal tion, which are sophisticated and high energy consuming Fluminense, Niterói, RJ 24210-346, Brazil Vol.:(0123456789) 1 3 20 Page 2 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 biological processes, generating an excessive cost [7]. As presence of a background gas. The species that suffer abla- an alternative, urea electro-oxidation has proven to be an tion are deposited on the support surface [34]. effective way for its degradation. Moreover, it is consid- In this work, commercial CP, which is cheap and easy to ered as a low-cost technology, dispenses sophisticated obtain, was tested as support for Ni nanoparticles synthe- equipment and can be applied for long periods of time sized by using the PLD technique. Optimizing the balance [10, 11]. The conversion occurs from the following elec- between the Ni loading quantities and catalytic behavior for trochemical oxidation reactions [7, 12, 13]: the purpose of real applications is another main goal of this study. The support and the synthesized catalysts were char- − − Anode:CO NH + 6OH → N + 5H O + CO + 6e 2 2 2 2 2 acterized by the techniques of Raman spectroscopy, scan- (1) ning electron microscopy (SEM), and transmission electron − − microscopy (TEM). The electrocatalytic activity of the mate- Cathode:6H O + 6e → 3H + 6OH (2) 2 2 rials towards the urea electro-oxidation reaction (UER) in alkaline medium was evaluated using cyclic voltammetry Overall:CO NH + H O → N + 3H + CO (3) 2 2 2 2 2 (CV) and chronoamperometry (CA) measurements. Several catalysts such as metals (Ni, Fe, Cu, Mn, Zn, Pt, etc.), metal oxides and metal free have been widely Experimental studied over the years in electrochemical processes [14]. Among them, nickel (Ni) is extensively used because of Plasma treatment of carbon paper surface its important characteristics such as its large abundance in nature, low cost (non-precious metal) and low toxic- The original Teflon treated CP was obtained from Carbon ity [15]. Additionally, Ni electrocatalysts demonstrate Paper AvCarb. The surface of CP is highly hydrophobic and excellent catalytic activity for urea electro-oxidation in hence is not suitable for the applications of the current study. alkaline solutions. As presenting good chemical stabil- To improve the contact between the CP surface and the reac- ity [12], high ductility, good thermal conductivity, and tion solution, the CP was treated with ammonia plasma in considerable electrical conductivity [15], Ni electrocata- −2 a vacuum chamber with a base pressure of ~ 2 × 10 Torr. lysts are usually used as anodic electrodes in the process Ammonia plasma etching has several advantages over air [7]. However, several factors can influence the catalytic plasma etching or chemical etching: it removes fluorine more activity of a metal, such as the metal support interaction efficiently [35], etches the surface more homogeneously, and [16, 17]. In order to increase the stability of the material no chemical residues on the highly porous CP. For plasma and increase the area of exposed active sites of Ni cata- etching, a piece of CP with a dimension of 10 cm × 2 cm was lysts, several authors have studied carbon-based supports, mounted perpendicular to the sample holder of the plasma such as carbon nanotubes [18, 19], graphene [19, 20], and chamber with half of it (10 cm × 1 cm) covered by Al plates. carbon fiber paper (CP) [21, 22] allowing an increase in Plasma etching was performed with the CP mounted on the activity and catalytic performance [23]. CP can be used for cathode of a capacitively coupled RF chamber, with a NH the electrochemical application, such as in fuel cells and plasma discharge. The discharge was established at 0.10 Torr electrocatalysts, acting as microporous layer support for NH and − 300 V self-bias (37 W RF power) for 20 min. catalysts. Its characteristics are the efficient transportation The illustration of the process and a plasma-etching image of gases and liquids, good thermal, and electrical conduc- can be found in Fig. 1. After plasma-etching, a small piece tion in high corrosive environments and high-temperature of the CP was tested by dropping water onto the surface. conditions [24–26]. For these reasons, CP is a suitable The treated CP was then cut into pieces with a dimension option as support for Ni nanoparticles for electrocatalysis of 2 cm × 0.5 cm as support of Ni nanoparticles. The hydro- applications. philic area of each sample was 1 cm × 0.5 cm. There are several techniques for the production of Ni nanoparticles [15], which may include synthesis by ther- Nanoparticles production mal decomposition [27], chemical reduction [28], microe- mulsion [29], sol–gel method [30], sonochemical synthesis The hydrophobic part of the final CP support was covered [31], Pulsed Laser Deposition (PLD) [32], etc. Among the by a piece of kapton tape and was mounted onto the sub- mentioned techniques, PLD is known for its good control strate holder of PLD system for Ni deposition. The back- of the distribution and particle size during its production −6 ground pressure of the PLD system was ~ 10 Torr. The Ni [33]. This technique is based on the ablation of a Ni tar- nanoparticles were deposited from pure Ni (99.99%) target get induced by a high power pulsed laser inside the PLD onto the hydrophilic part of the CP in the presence of an Ar chamber, that can occur in vacuum conditions or in the background atmosphere (1.0 Torr) at room temperature. The 1 3 Materials for Renewable and Sustainable Energy (2020) 9:20 Page 3 of 11 20 Fig. 1 Left: Illustration of the electrode preparation. Upper right: carbon papers under plasma etching. Lower right: Wetting test for the etched and non-etched CP surfaces by drooping water on −1 target ablation was done using the first harmonic (1064 nm recorded from 0.0 V and 0.75 V vs Hg/HgO in 1 mol L 2 −1 wavelength) of a pulsed Nd:YAG laser with 0.12 J/mm KOH + 0.33  mol  L urea solution. The concentration of energy density, 7 ns pulse duration, and 10 Hz repetition 0.33 M urea was used during the experiment to simulate the rate. The laser beam was focused onto the Ni target with an concentration of urea in urine waste and it is the urea con- incidence angle of 45° and the ablated Ni atoms expanded centration usually studied [2–4, 10, 36–39]. In both cases, in the direction of the substrate, with a fixed 3.5 cm distance the last cycle is shown. Chronoamperometric experiments from the target. Samples with deposition time of 3, 6 and were carried out at 0.55 V vs Hg/HgO for 60 min. The cur- 12 min were prepared to obtain the different amount of Ni rent from the urea electro-oxidation process was normalized nanoparticles on the CP support and were named as 3-Ni/ by the electroactive surface areas (ESA), estimated accord- CP, 6-Ni/CP, and 12-Ni/CP, respectively. ing Eq. 4. ESA = Q∕q (4) Characterization 3+ 2+ where Q is the charge required to reduce Ni to Ni The morphology and thickness of the samples were investi- [NiOOH to Ni(OH) ], which can be calculated from the gated by using field-emission scanning electron microscope cyclic voltammograms in electrolyte support. In Eq.  4, q (SEM: JEOL JSM 7100F) and field-emission high-resolution is the charge related to the formation of a monolayer of transmission electron microscopy (HRTEM. JEOL JEM Ni(OH) from NiOOH that involves one electron transfer −2 2100F). The chemical composition and electronic structure and the value can be taken as 257 μC cm [10, 40, 41]. of the products were characterized by Raman spectroscopy The procedure to determine the ESA was similar to that one (Witec alpha 300R Raman microscope, with 532 nm wave- reported elsewhere [10, 36]. However, the difference is that length excitation) and energy-dispersive X-ray spectroscopy the obtained ESA was not normalized by nickel loading in (EDS). the electrode like in the literature [10, 36]. The results were also normalized by electrode geometric area (GSA) of the Electrochemical measurements work electrode. Electrochemical measurements were carried out at room temperature using a µStat400 bipotentiostat/galvanostat (Metrohm DropSens), by using a three-electrode electro- Results and discussion chemical cell, with a platinum foil as the counter electrode, Hg/HgO as reference electrode and the CP with nickel As can be seen from Fig. 1, the CP surface has turned from particles as the work electrode. CV experiments were hydrophobic to hydrophilic by the NH plasma-etching and −1 −1 performed in 1 mol L KOH at a scan rate of 50 mV s then became suitable as catalyst support for electrocatalysis from 0.0 to 0.65 V vs Hg/HgO. Prior each experiment N applications. To investigate the surface modification, the was bubbled for 15 min, and ten consecutive cycles were original and plasma-treated CP was characterized by using −1 recorded for each material. Five cycles (10 mV s ) were Raman spectroscopy, with the results being shown in Fig. 2. 1 3 20 Page 4 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 The spectrum of the original CP showed clearly the most −1 prominent features of graphite: the G band at 1582 cm , D' G' −1 −1 the D band at ~ 1350 cm , the D′ band at about 1620 cm −1 and the G′ band at ~ 2700 cm [42]. The G band is associ- ated with a doubly degenerate phonon mode for sp2 carbon Plasma-treated networks. The D and D′ bands are related to defects, and thus being usually absent in the spectrum of highly crystalline graphite. The G′ band is a feature of D band overtone in dif- ferent kinds of graphitic materials. The integrated intensity ratio I /I for the D band and G band is usually applied for D G analyzing the number of defects in graphitic materials [43]. Non-treated A relatively weak D band is clearly observed for the original CP in Fig. 2, which indicates that the CP is constructed by graphite fibers with a small number of defects. After plasma- 1000 1500 200025003000 3500 etching, the intensity of D band significantly increased, sug- -1 Raman shift (cm ) gesting a large quantity of defect generation; other bands were not obviously modified. Fig. 2 Raman spectra of the original and plasma-etched carbon paper, Detailed microstructure studies were performed for the showing the main Raman features of graphite: the D, G, D′ and G′ original and plasma-etched CP. As can be seen from Fig. 3, bands the CP is constructed by carbon fibers with diameters in the range of 5–10 μm and possesses plenty of pores and voids. From the back-scattered electron image of the original CP Fig. 3 a Low magnification secondary electron image, b low mag- the plasma-treated CP. g A low-mag back scattered SEM image of the nification back-scattered electron image and c high resolution SEM original CP and the elemental mapping with EDS for h carbon and i images of the original CP. d, e and f are the corresponding images of fluorine 1 3 Intensity (a.u.) Materials for Renewable and Sustainable Energy (2020) 9:20 Page 5 of 11 20 (Fig. 3b), a layer with a higher brightness than the carbon plasma-etching is the roughness increase on the surface of fibers appeared. Since the back-scattered electron image each carbon fiber. From Fig.  3c we can see that the carbon gives atomic mass contrast and the surface of the CP is fiber has a very flat surface. After plasma-etching, however, hydrophobic, the brighter layer might be fluorine rich and abundant nano-sized points appeared and were homogene- is responsible for the hydrophobic property. The EDS anal- ously distributed on the surface of the carbon fibers. Higher ysis is shown in Fig. 3g–i confirmed that the original CP resolution images can be found in the supplementary infor- fibers are composted by purely carbon and the bright layer mation to show it more clearly. The result is in well agree- is rich of fluorine. No other elements with a concentration ment with the conclusion of Raman spectra, which suggested higher than the level of contamination were observed. The a large number of defect generation due to plasma-etching. first effect of plasma-etching process is fluorine removal, Both transformations from a hydrophobic to a hydrophilic which is clearly shown in Fig. 3d, e. Most of the fluorine surface and surface roughness increase greatly benefit the has been removed by NH plasma and only few of them is catalytic behavior, while the former increases the contact of left in the sharp corners, where was more difficult for plasma the CP surface with a solution and the latter enhances the generation. Quantitative analyses of the average EDS spec- specific surface area. tra obtained from the elemental mapping results for origi- A detailed microstructure study for the Ni deposited CP nal and plasma-treated CP clearly demonstrated a decrease has been done with SEM and the images are shown in Fig. 4. of fluorine from ~ 30 wt% to ~ 9 wt% after plasma etching The 3-Ni/CP sample showed that many nano-sized particles (as shown in the supplementary information). The absence appeared on the surface of each nanofiber. The high-resolu- of nitrogen signal on the EDS spectrum for the treated CP tion image (Fig. 4b) reviled that the bright nanoparticles in suggested the nonexistence of residual ammonia on the CP Fig. 4a are non-regular and are constructed by smaller sized surface. The significant decrease of fluorine concentration particles (< 100  nm). The back-scattered electron image on the surface resulted in the transformation of the CP sur- (Fig.  4c) clearly shows atomic mass contrast, indicating face from hydrophobic to hydrophilic. Another effect of the that the nanoparticles are composted by heavier element Fig. 4 a Low magnification secondary electron image, b high magnification secondary electron image and c high magnification back-scattered electron image of the 3-Ni/CP. d–f are the corresponding images of the 6-Ni/CP and g–i, of 12-Ni/CP 1 3 20 Page 6 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 than C and in this work, it is apparently Ni. With 3 min perfectly correspondents to the structure and morphology of Ni deposition, the Ni nanoparticles were well dispersed of the CP as shown in Fig. 5c. The result indicates that Ni on CP surface without large agglomerations. After 6 min nanoparticles are deposited homogeneously on the surface. of Ni deposition, more agglomeration of Ni nanoparticles The black areas in Fig. 5d do not mean the absence of Ni, occurred. Moreover, the nanoparticles started to connect but less detection of Ni X-ray signal. At those places, the Ni with each other and formed bigger patterns. With a back- nanoparticles were deposited to the deeper part of the CP scattered electron image, one can still see the low-density and due to the shape effect, X-ray photons from Ni exited by morphology and each of the bigger patterns was in fact an the electron beam hardly reach the detector. The high resolu- assembly of many smaller nanoparticles. The mean size of tion mapping of Ni element in Fig. 5f shows clearly that the the agglomerated nanoparticles is higher than of the materi- agglomerations of the particles are Ni. als prepared with 3 min of Ni deposition, however, is still The SEM images suggest that the Ni layer deposited on less than 100 nm, as can be seen from Fig. 4e. With even CPs is constructed by nanoparticles with size much smaller more Ni nanoparticles deposited (12 min), the size of the than that of the patterns. With only SEM, one cannot get agglomerations significantly increased, with a mean value detailed size and microstructure of the Ni nanoparticles. In of about 200 nm. The small-sized nanoparticles in Fig. 4c, order to obtain this information, one sample has been pre- f are hardly seen and instead, much bigger and condensed pared by depositing Ni nanoparticles onto a Cu grid with C particles appeared in Fig. 4i. support for less than 1 min and studied by using HRTEM Figure 5a, b show a side-view of the deposited Ni layer with results shown in Fig. 6. From the low magnification on top of one fiber for sample 12-Ni/CP. One can see that the image, we can see that the Ni nanoparticles have spherical- agglomerated nanoparticles have columnar structure with shape and were homogeneously deposited on the surface. the height of several hundred nanometers. Almost all the The mean diameter of the nanoparticles is about 3.5 nm with surface of the CP is covered by Ni. Smaller sized nanopar- a very narrow size distribution (width at half maximum: ticles still can be seen at the border of the Ni layer and C ~ 1.5 nm). The high-resolution image in Fig.  6b demon- fiber. Although from the back-scattered electron images we strates a crystalline microstructure for the particles. During can conclude that the deposited nanoparticles might be Ni, it PLD process, the nanoparticles were formed before they needs to be proved by EDS. Figure 5c–f show the elemental were deposited onto the substrate, and their size depends mapping results of Ni by EDS for the sample 12-Ni/CP. The only on the gas pressure in the chamber. Since the same Ni distribution in the low-magnification mapping (Fig.  5d) pressure and distance from the target to the substrate were Fig. 5 a Side-view secondary electron image and b back-scattered EDS for d Ni. e High magnification back-scattered electron image electron image of the deposited Ni nanoparticles for 12-Ni/CP. c Low and elemental mapping with EDS for f Ni magnification secondary electron image and elemental mapping with 1 3 Materials for Renewable and Sustainable Energy (2020) 9:20 Page 7 of 11 20 Fig. 6 a Low magnification image of Ni nanoparticles. Inset shows the size distribution of the particles, b HRTEM image of Ni nanoparticles used for all the three samples in this work, the Ni nanoparti- cles in the three samples are then similar. The Ar molecules in the chamber play a role not only in the formation of Ni nanoparticles but also in decreasing their velocity. The Ni 1.0 3-Ni/CP nanoparticles with less kinetic energy deposited on the CP surface have much lower impact and hence, resulted in the 0.5 formation of less dense material, namely, columnar struc- ture. In contrast, it forms a thin film in a vacuum (much 0.0 lower pressure) and extremely low-density nanofoam in much higher pressure [44]. X-ray photoelectron spectros- -0.5 copy (XPS) experiments are of high interest to exam the possible oxidation of Ni nanoparticles after the sample being 2.4 6-Ni/CP removed from the vacuum chamber. The CV curves of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP elec- 1.2 −1 trocatalysts in 1  mol  L KOH in the potential range of 0.0 V to 0.65 V are shown in Fig. 7. The shape of the CVs 0.0 is in agreement with nickel catalysts behavior reported in the same experimental condition of the current study [2, -1.2 45]. The characteristic redox feature of nickel can be seen 3.6 in CVs. The peak in the forward scan corresponds to the 12-Ni/CP 2+ 3+ Ni oxidation to Ni [Ni(OH) to NiOOH] and the peak 2.4 3+ 2+ in the backward scan is related to the Ni reduction to Ni 1.2 [2, 36]. It is important to point out that NiOOH is the phase responsible for urea electro-oxidation, and not Ni(OH) [2, 0.0 37]. As can be seen, the current related to the nickel oxi- -1.2 dation and reduction process increases from the material 3-Ni/CF to the 12-Ni/CF, which might be related to a higher -2.4 0.00.2 0.40.6 amount of nickel deposited on the CP due to longer time deposition. The calculated ESA was 3.81 cm for the 3-Ni/ E / V vs Hg/HgO 2 2 CF, 6.92 cm for 6-Ni/CF and 7.86 cm for 12-Ni/CF. As can be seen, the ESA increases almost twice (82%) from 3 to Fig. 7 Cyclic voltammograms of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP −1 −1 6 min of nickel deposition, which is reasonable. On the other electrocatalysts in 1 mol L KOH. Scan rate of 50 mV s 1 3 I / mA 20 Page 8 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 hand, from 6 to 12 min of deposition the ESA increases only onset potential, the best results were obtained using 3-Ni/CP 13.6%. As shown in Fig. 4, the mean agglomerated particle catalyst. However, the maximum current density on 3-Ni/CP size of the materials increased as the time of nickel deposi- was only about 8% higher than on 6-Ni/CP. As shown in the tion increases, suggesting a smaller specific surface area for SEM images (Fig. 4), the agglomerated nickel particles of the material prepared with longer time deposition. However, 3-Ni/CP are smaller (< 100 nm) than of the other materials longer time deposition also results in a higher amount of and did not cover the CP surface completely. On the other nickel and larger CP surface coverage. Comparing the two hand, for 12-Ni/CP catalyst, the nickel particle mean size is electrodes 3-Ni/CP and 6-Ni/CP, it is expected that 6-Ni/CP about 200 nm which forms dense agglomerations, covering to have a twice amount of nickel loading than that in 3-Ni/ almost completely the surface of CP. The nickel particles CP since the deposition time doubled, the same comparison of 6-Ni/CP are also more agglomerated than on 3-Ni/CP, can be made for 6-Ni/CP and 12-Ni/CP. Thus, although the but the agglomerated nickel particles are also smaller than nickel deposition time of 12-Ni/CP is twice longer than that 100 nm. It is reported that the materials with at least one of 6-Ni/CP, it results in only 13.6% specific surface area dimension smaller than 100 nm show different properties increase, which is related to the larger agglomerated particle from the bulk ones, such as, quantum confinement, electri- sizes observed for 12-Ni/CP catalyst. cal, magnetic [36]. Nanostructured nickel shows improved The CV curves of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP elec- catalytic activity towards UER (urea electro-oxidation reac- −1 −1 trocatalysts in 1 mol L KOH + 0.33  mol  L urea in the tion) if compared with bulk nickel [2, 47]. In this sense, potential range of 0.0 V to 0.75 V are shown in Fig. 8. As the higher catalytic activity of 3-Ni/CP followed by 6-Ni/ can be seen, the onset potential, an important parameter to CP could be related to the smaller particles agglomeration be extracted from CV experiments was about 30 mV less of nickel and better dispersion on the CP support than of positive on 3-Ni/CF (0.47 V) than on 6-Ni/CP and 12-Ni/CP. 12-Ni/CP. The decrease of the onset potential is related to a change in Figure  9 shows the chronoamperometric curves in −1 −1 the required activation energy for the process, and a suitable 1 mol L KOH + 0.33 mol L urea on 3-Ni/CP, 6-Ni/CP catalyst enhances the reaction rate by decreasing the activa- and 12-Ni/CP electrocatalysts at 0.55 V during 60 min. As tion energy [46]. It is important to point out that the onset in CV experiments, better result was obtained on 3-Ni/CP potential starts at the similar potential of the conversion from catalyst. The current density from urea electro-oxidation 2+ 3+ Ni to Ni , confirmed that NiOOH is the active phase for on 3-Ni/CP at the end of CA experiment was 44% higher the urea electro-oxidation reaction [37, 38]. than on 6-Ni/CP and 133% on 12-Ni/CP. The slow decay Another import data from the CV experiments is the peak of the current density over time is induced either by the current density. The maximum current density on 3-Ni/CP gas bubbles (i.e., N, CO ) produced from urea electro- 2 2 was about 8% higher than on 6-Ni/CP and 99% on 12-Ni/CP. oxidation or by the blockage of the active site due to the Thus, considering both of the peak current density and the intermediate products [37, 48], e.g. CO can cause the deactivation of NiOOH which is actually the active phase 3-Ni/CP 1.2 6-Ni/CP 12-Ni/CP 0.9 3-Ni/CP 0.6 6-Ni/CP 0.3 12-Ni/CP 0.0 0.00.1 0.20.3 0.40.5 0.60.7 E/V vs Hg/HgO Time (min) Fig. 8 Cyclic voltammograms of 3-Ni/CP, 6-Ni/CP and 12-Ni/CP Fig. 9 Chronoamperometric measurements at 0.55  V vs Hg/HgO −1 −1 −1 electrocatalysts in 1 mol L KOH + 0.33  mol  L urea. Scan rate of on 3-Ni/CP, 6-Ni/CP and 12-Ni/CP electrocatalysts in 1  mol  L −1 −1 10 mV s KOH + 0.33 mol L urea 1 3 -2 J / mA cm -2 J / mA cm Materials for Renewable and Sustainable Energy (2020) 9:20 Page 9 of 11 20 6-Ni/CP (a) (b) 6-Ni/CP 12-Ni/CP 40 3-Ni/CP 4 3-Ni/CP 12-Ni/CP 0 0 Fig. 10 a Peak current density from urea electrooxidation (from CV experiments), and b current density from CA at 60 min of experiments to promote urea electro-oxidation [49]. The urea consump- Conclusions tion in the electro-oxidation process also influences the drop of current over time [4, 10]. As can be seen, the cur- In summary, we employed commercial CP as support of rent density from urea electro-oxidation on 3-Ni/CP at the Ni nanoparticles as catalyst for urea electro-oxidation. The beginning of the process is much higher than on the other Ni nanoparticles have been prepared by PLD with 3, 6 and two catalysts materials. The higher current density causes 12 min of deposition time in order to get different Ni load- faster consumption of urea near to the electrode surface, ing quantity. The TEM study shows that the Ni nanopar- resulting in a more abrupt decrease of current density in ticles have an average size of 3.5 nm with a narrow size the first minutes of the experiment. Similar behaviours distribution. The SEM results reveal that agglomeration of have been reported in the materials showing higher current Ni nanoparticles occurred in all the samples and they were density from the urea electro-oxidation process [4, 10]. more agglomerated with longer deposition time. The cyclic As discussed in the results from VC (Fig. 8) the smaller voltammograms show that the 3-Ni/CP sample shows better size and better dispersion of nickel nanoparticles of 3-Ni/ catalytic activity due to less agglomeration and thus more CP might be the reason for its superior catalytic activity active sites on the surface of Ni nanoparticles. Lower onset for urea electro-oxidation compared to the 6-Ni/CP and potential has been observed in this sample as well. For real 12-Ni/CP. applications, the current normalized by the electrode area is The results shown in the FigS. 8 and 9 are normalized then become important and the experimental results show by ESA. However, considering real applications it would that the 6-Ni/CP has the best performance, with current den- be interesting to normalize the results for the electrode sity ~ 60% higher than the other two samples. The study here geometric area. The Fig. 10 shows the results of peak cur- shows that CP can be used as low cost and environmentally rent density from urea electro-oxidation on CV experi- friendly catalyst support and electrodes with intermediate Ni ments (Fig. 10a) and the current density at the end of CA loading quantity might be of interest for real applications of experiments (Fig. 10b). As can be seen, better results were urea electro-oxidation. obtained using 6-Ni/CP, considering this normalization. Acknowledgements The authors gratefully acknowledge FAPERJ From the results shown in Fig. 10 it is seen that the current for the support with grant numbers of E-26/010.000978/2019, density from urea electro-oxidation on 6-Ni/CP is about E-26/010.001550/2019, E-26/211371/2019 and E-26/200153/2020, and 70% higher than on 12-Ni/CP and 80% on 3-Ni/CP. Con- CNPQ for the support with grant number 422614/2018-1. This study sidering the twice more nickel on 6-Ni/CP than in 3-Ni/ was also supported, in part, by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. CP due to doubled deposition time, it would be expected the current density twice higher. However, this relation Compliance with ethical standards would be true only if both materials have the same nickel agglomerated nanoparticle sizes but they do not. The SEM Conflict of Interest The authors declare that they have no conflict of analysis and the ESA results suggest that the nickel nano- interest. particle sizes of 6-Ni/CP are slightly larger. The current density from urea electro-oxidation on 12-Ni/CP is almost Open Access This article is licensed under a Creative Commons Attri- the same with that on 3-Ni/CP (Fig. 10), which is mainly bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long due to the larger (> 200 nm) aggregated nickel nanoparti- as you give appropriate credit to the original author(s) and the source, cles in 12-Ni/CP. 1 3 -2 J / mA cm -2 J / mA cm 20 Page 10 of 11 Materials for Renewable and Sustainable Energy (2020) 9:20 provide a link to the Creative Commons licence, and indicate if changes 13. Boggs, B.K., King, R.L., Botte, G.G.: Urea electrolysis: Direct were made. The images or other third party material in this article are hydrogen production from urine. Chem. Commun. 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