- Email: [email protected]
H2O2 fuel cell
Journal Pre-proof A free-standing NiCr-CNT@C anode mat by electrospinning for a high-performance urea/H2O2 fuel cell Bohyeon Kim, Gautam Das, Bang Ju Park, Dal Ho Lee, Hyon Hee Yoon PII:
S0013-4686(20)31050-1
DOI:
https://doi.org/10.1016/j.electacta.2020.136657
Reference:
EA 136657
To appear in:
Electrochimica Acta
Received Date: 30 March 2020 Revised Date:
16 June 2020
Accepted Date: 18 June 2020
Please cite this article as: B. Kim, G. Das, B.J. Park, D.H. Lee, H.H. Yoon, A free-standing NiCr-CNT@C anode mat by electrospinning for a high-performance urea/H2O2 fuel cell, Electrochimica Acta, https:// doi.org/10.1016/j.electacta.2020.136657. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
CRediT author statement Bohyeon Kim: Conceptualization, Investigation, Writing- Original draft preparation. Das Gautam: Data curation. Bang Ju Park and Dal Ho lee: Supervision, Writing- Reviewing and Editing. Hyon Hee Yoon: Supervision.
A free-standing NiCr-CNT@C anode mat by electrospinning for a high-performance urea/H2O2 fuel cell Bohyeon Kim1, Gautam Das1, Bang Ju Park2, Dal Ho Lee2, and Hyon Hee Yoon*1 1
Department of Chemical and Biological Engineering, Gachon University, Gyeonggi-Do 461-701,
Republic of Korea 2
Department of Electronic Engineering, Gachon University, Gyeonggi-Do 461-701, Republic of Korea
*Corresponding author: Tel. No. (+82)031-750-5356, email: [email protected] Abstract A highly porous free-standing NiCr-bimetallic catalyst is synthesized by electrospinning and carbonization and is applied as an anode in a direct urea/H2O2 fuel cell for electro-oxidation of urea. It is observed that the morphology of the calcined NiCr catalyst, which has a higher specific surface area, is identical to that of the electrospun NiCr catalyst fiber. Cr-doping at 40 % (Cr/Ni) significantly increases the oxidation peak current of the urea oxidation reaction. The addition of carbon nanotubes also considerably enhances the catalytic activity. A direct urea/H2O2 fuel cell, which utilizes the synthesized catalyst (NiCr-CNT@C) as a free-standing anode, exhibits excellent performance with a maximum power density of 48.1 mW cm-2 and an open-circuit voltage of 0.92 V at 80 °C. Thus, the highly porous free-standing NiCr-CNT@C catalyst mat can be employed as an efficient anode material for urea oxidation in urea fuel cells. Keywords: Urea electrocatalyst; Urea fuel cell; Ni-Cr bimetal; Electrospinning; Free-standing electrode
1
1. Introduction Urea, also known as carbamide, is a non-flammable, non-toxic, and low-cost chemical, which has a high energy density (16.9 MJ L-1). Furthermore, this solid material is easy to transport and store; thus, it can serve as an efficient fuel for fuel cells and as a hydrogen carrier in electrolysis cells [1–3]. A conventional urea fuel cell utilizes O2 oxidant; on the other hand, a direct urea/H2O2 fuel cell (DUHpFC) uses H2O2 as oxidant as shown below [4]: Anode: CO(NH2)2 + 8KOH → N2 + 6H2O + K2CO3 + 6K+ + 6e– –
Cathode: 3H2O2 + 3H2SO4 + 6e → 6H2O +
4SO42-
Overall: CO(NH2)2 + 3H2O2 + 3H2SO4 + 8KOH → K2CO3 + 3K2SO4 +N2 + 12H2O
(1) (2) (3)
The theoretical cell voltage in urea/O2 and urea/H2O2 type fuel cell is 1.147 and 2.509 V, respectively; therefore, a higher cell voltage can be expected in DUHpFC, although the oxidant chemicals (i.e., H2O2 and H2SO4) should require extra cost. DUHpFCs with enhanced cell voltage can be used for a distributed power generator using commercial urea compounds. In a urea electrolytic cell, urea is also oxidized as follows [5]: Anode: CO(NH2)2 + 6OH– → N2 + CO2 + 5H2O + 6e– –
–
(4)
Cathode: 6H2O + 6e → 3H2 + 6OH
(5)
Overall: CO(NH2)2 + H2O → N2 + 3H2 + CO2
(6)
The electro-oxidation of urea is, therefore, an important process in the utilization of urea to produce electricity and hydrogen. Fortunately, the inexpensive Ni-based catalysts exhibit excellent catalytic activity toward urea oxidation (UOR) in alkaline solutions [6–8]. However, high oxidation potentials (0.55 – 0.65 V vs. SHE) for the UOR over Ni-based catalysts have been observed relative to theoretical values (−0.46 V vs. SHE). Furthermore, the power density obtained in DUHpFCs that employ Ni-based catalysts as the anode is still considerably lower (i.e., 10 – 30 mW cm-2) than those obtained in hydrogen and methanol fuel cells [4,9,10], mainly due to the sluggish kinetics of the sixelectron-transfer UOR [11]. The activity of Ni-based catalysts can be improved by conjugating Ni with other metals and by structural modification. For instance, NiCo [6], NiMn [12], NiZn [13], NiWC [14], NiFe [15], and NiCr [16] bimetallic catalysts have been reported to exhibit promising activity and stability toward UOR. A high specific surface area, associated with a high number of active sites for the UOR, and a mesoporous structure for fast mass transport are also critical properties of an excellent catalyst, particularly because urea is a large molecule compared to hydrogen and methanol. Therefore, Ni-based catalysts were nanostructured and/or combined with carbon-based supporting materials for further activity enhancement [17,18]. Most supporting materials have a three-dimensional (3-D) structure such as graphene aerogel [19], carbon sponge [20] and Ni foam [21]. For example, a DUHpFC, which utilizes a 3-D film of Ni-Co on a Ni foam, synthesized by electro-deposition, as the anode, with 0.5 M urea in 7 M KOH at 20 °C, exhibited a high power density of 17.4 mW cm-2. Further, electrospinning is frequently used for the preparation of catalyst matrices [22,23]; it is possible to synthesize evenly dispersed metal
2
nanoparticles with high surface areas by simply adding various metal precursors to the electrospinning solution. The electrospinning technique has been applied in various devices, such as in gas sensors [24], lithium-ion batteries [25], fuel cells [26], and biomedical devices [27], due to its simplicity and uniform properties. Recently, Ni-Mn nanoparticle-decorated carbon nanofibers, synthesized by electrospinning, showed reportedly distinct activity toward UOR [12]. In this study, we developed a novel Ni-based catalyst-modified free-standing electrode for UOR. Ni and Cr-impregnated polyacrylonitrile (PAN) nanofiber mats (NiCr-PAN) were prepared by electrospinning. The electrospun mat was calcined in nitrogen to form a free-standing electrode (NiCr@C) for UOR, which makes the fabrication process of a membrane electrode assembly (MEA) simple and economic. Conventionally, Ni-based catalysts are coated onto a gas-diffusion-layer, such as carbon felt and carbon paper, to fabricate an MEA. However, the presence of organic binders in the catalyst coating can adversely affect the activity of the catalyst at low current density [28]. Furthermore, to improve the mechanical and electrical properties of the NiCr@C electrode, multi-walled carbon nanotubes (MWCNTs) were incorporated (NiCr-CNT@C). The morphological and electrochemical properties of NiCr-CNT@C were rigorously analyzed. The single-cell performance of the DUHpFC with the NiCr-CNT@C as the free-standing anode was also investigated. 2. Experimental Section 2.1. Synthesis of NiCr-CNT@C catalyst mats NiCr-CNT@C electrospinning
catalyst solution
mats was
were
synthesized
prepared by
by
mixing
electrospinning
and
carbonization.
The
3.31 mmol
Ni(OCOCH3)2 4H2O
and
of
(CH3CO2)7Cr3(OH)2 precursors in the desired Ni/Cr ratio, MWCNT (0.5 wt%), if necessary, and PAN (9 wt%) in 5 mL of N, N-dimethylformamide, sequentially. Before adding PAN, MWCNTs were dispersed evenly by ultra-sonication for 30 min. The final solution was stirred for 12 h at 85 °C. The electrospun mat was obtained by the electrospinning solution process, which was operated using the prepared solution at 20 kV with a 15 cm distance between the tip and the collector. The nanofibers, which were synthesized by electrospinning, were collected on a cylindrical stainless drum covered by an aluminium foil. The collected electrospun mats were dried at 80 °C for 12 h to evaporate the remaining solvent, after which the stabilization was conducted in an air atmosphere at 250 °C for 1 h. After the stabilization, carbonization was carried out under a nitrogen atmosphere at 1000 °C for 1 h. For the other ratio of Ni/Cr, the total moles of Ni and Cr were kept constant. 2. 2. Electrochemical measurements and DUHpFC tests. For electrochemical properties of catalysts, cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were measured using potentiostat and galvanostat (SP-240, Bio-Logic). The as-prepared catalysts mats were cut into pieces of dimensions 5 mm x 5 mm; these pieces were directly used as the working electrode for the electrochemical measurements. A conventional three-electrode system was applied using Ag/AgCl (3 M NaCl) and Pt wire as the
3
reference electrode and the counter electrode, respectively. The EIS measurements were performed using an AC amplitude of 10 mV in the frequency range of 0.05 to 100000 Hz. Z-fit software from EClab was utilized to fit a Nyquist plot obtained by EIS. A DUHpFC was prepared using the NiCr-CNT@C catalyst mat as the anode, which had a catalyst loading of 0.28 mg cm–2, and Pt/C (30 wt% Pt) on carbon paper with 2.0 mg Pt cm-2 as the cathode. A cation exchange membrane (Nafion® 115) was used as the polymer electrolyte; the membrane was sandwiched between the anode and cathode to prepare a membrane electrode assembly with an active area of 5.0 cm2, as described in a previous work [29]. Single-cell tests were carried out using a gold-coated stainless current collector and a graphitic plate with serpentine flow channels. An aqueous solution of urea and KOH was fed into the anode side, and 2 M H2O2 in 2 M H2SO4 solution was supplied to the cathode. Polarization and power density curves were obtained by the potentiostatgalvanostat interfaced with EC-lab 11.01 data acquisition software.
2.3. Characterization. The morphology of the samples was investigated using a scanning electron microscope (SEM, A JEOL JSM-6700F) and transmission electron microscope (TEM, Tecnai G2 F30 S-Twin) equipped with an EDS analyzer. The crystal information of the samples was analyzed using a multi-purpose X-ray diffractometer (MPXRD, X’Pert Pro MPD) using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and a scan rate of 2° min-1. The X’pert high score was utilized to identify the peaks in the XRD patterns. XPS (Kalpha) analysis was conducted to obtain the binding energy plot of the samples with monochromated Al Kα as the X-ray source. All the binding energy values were calibrated to C1s (284.6 eV) to define the chemical states. Specific surface area and porosity were determined based on nitrogen adsorptiondesorption measurements performed using a Micromeritics ASAP 2020 apparatus according to the Brunauer-Emmett-Teller (BET) theory. The electrical resistivity of the catalyst mats was determined by four-probe resistivity method (CMT-100S, AIT). 3. Results and Discussion 3.1 Characterization of the NiCr-CNT@C catalysts NiCr@C and NiCr-CNT@C were prepared by electrospinning, followed by calcination. The electrospinning solution was composed of nickel acetate and chromium acetate precursors, MWCNTs (if necessary), and PAN. The electrospun fiber mat was calcined in a nitrogen environment to obtain NiCr-CNT@C, as shown in Suppl. Scheme S1. Figure 1a shows the X-ray diffraction (XRD) patterns of NiCr-CNT@C with different Cr/Ni ratios. The peaks at 2θ = 44°, 52° and 77° were indexed to the (111), (200), and (220) planes, respectively, of pure nickel. All the catalysts containing Ni and Cr showed small peaks indicative of the oxidized metals; NiO peaks were observed at 2θ = 43.7°, 63.7° and 76.3°, which were assigned to the (200), (220) and (311) planes of NiO, respectively; Cr2O3 peaks were observed at 2θ values of 24.6°, 33.7°, 36.3°, 50.3°,
4
55.9°, 63.7° and 65.3°, corresponding to the (012), (104), (110), (024), (116), (214) and (300) planes of Cr2O3, respectively [30,31]. The intensity of the crystalline peaks decreased gradually with the addition of Cr, implying that Cr affected the crystalline structure of Ni and promoted a more amorphous structure, as previously observed for Ni–Cr alloys [16,32,33]. Additionally, the crystalline peaks for Figure 1a shows the X-ray diffraction (XRD) patterns of NiCr-CNT@C with different Cr/Ni ratios. The peaks at 2θ = 44°, 52° and 77° were indexed to the (111), (200), and (220) planes, respectively, of pure nickel. All the catalysts containing Ni and Cr showed small peaks indicative of the oxidized metals; NiO peaks were observed at 2θ = 43.7°, 63.7° and 76.3°, which were assigned to the (200), (220) and (311) planes of NiO, respectively; Cr2O3 peaks were observed at 2θ values of 24.6°, 33.7°, 36.3°, 50.3°, 55.9°, 63.7° and 65.3°, corresponding to the (012), (104), (110), (024), (116), (214) and (300) planes of Cr2O3, respectively [30,31]. The intensity of the crystalline peaks decreased gradually with the addition of Cr, implying that Cr affected the crystalline structure of Ni and promoted a more amorphous structure, as previously observed for Ni–Cr alloys [16,32,33]. Additionally, the crystalline peaks for metallic Cr were not observed in the XRD patterns. These results might indicate the formation of a Ni– Cr alloy. The composition and chemical states of Ni and Cr in the NiCr-CNT@C catalyst were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 1b and 1c show the XPS spectra of NiCr-CNT@C with a Ni:Cr ratio of 6:4, which is denoted as Ni6Cr4-CNT@C. The results revealed that both Ni and Cr were present in metal and oxide forms. The elemental composition analysis results indicated that the NiCrCNT@C catalyst comprised Ni, Cr, C, and O at 2.0, 1.5, 89.0, and 7.5 atomic %, respectively, as determined by the integration of the XPS spectrum of each component (Suppl. Table. S1). The Ni:Cr ratio of 57:43 was close to the theoretical loading ratio of 60:40. The metallic Cr was detected in the XPS analysis, which was not observed in the XRD patterns. Additionally, the peaks indicative of the metallic Cr and Ni in the XPS were shifted, implying the formation of a Ni–Cr alloy [34,35]. The electrospun Ni6Cr4-CNT-PAN precursor exhibited a nanofiber morphology with a smooth and uniform diameter as observed in the scanning electron microscopy (SEM) images (Figure 2a). The Ni6Cr4-CNT-PAN precursor was calcined to decompose the polymer matrix. After the calcination, the morphology of Ni6Cr4-CNT@C remained the same as that of the pristine Ni6Cr4-CNT-PAN, as observed in the SEM image (Figure 2b), owing to the skeletal C–C single bond structure, which was confirmed by a carbon XPS analysis (Suppl. Figure S1). The porous structure of the Ni6Cr4-CNT@C fiber was observed in a focused-ion beam (FIB)-SEM image of the fiber cross-section (Figure 2c). The fiber, shown in the upper right corner of Figure 2c, was previously truncated using an ion beam. White Ni–Cr nanoparticles were distributed throughout the cross-section of the fiber. The uniformly distributed Ni–Cr particles were more clearly observed as dark spots in the transmission electron microscopy (TEM) image (Figure 2d). The structures of NiCr-CNT@C were further characterized by high-resolution (HR)-TEM. The NiCr particles were uniformly embedded through the carbon matrix, as shown in the TEM image (Figure 2e). The presence of NiO and Cr2O3 was identified by the lattice spacings that appeared in the highly magnified TEM images (insets of Figure 2e), in line with the XRD results. The
5
lattice spacings of 0.203, 0.217, and 0.265 nm were ascribed to the (202), (113), and (104) planes of Cr2O3, respectively, and the d-spacing of 207 nm was matched with the (200) plane of NiO. However, the crystalline structures of metallic Cr and Ni were not identified by HR-TEM, implying the formation of an amorphous NixCry alloy in line with the XPS results. Figures 2f – 4k indicate the TEM–energy dispersive X-ray spectroscopy (EDS) mappings for C, Ni, and Cr, respectively. The Ni and Cr mappings overlapped, indicating that both Ni and Cr were detected in each particle, and thereby further confirming the presence of the NixCry alloy. The pore formation by calcination evidently provided a high specific surface area for the catalyst particles, as evidenced by Brunauer–Emmett–Teller (BET) measurements. The BET and average pore sizes of different catalysts are summarized in Table 1. The BET value increased considerably upon the calcination of the NiCr-PAN precursor and further to approximately 300 – 380 m2 g-1 by the addition of MWCNT, which is higher than the previously reported values (i.e., 50 - 200 m2 g-1 [36–39]). The average pore size of the Ni6Cr4-CNT@C catalyst was approximately 6.0 nm as calculated from the Barrett–Joyner–Halenda (BJH) pore size distribution curve (Suppl. Figure S2), which was indicative of the mesoporosity as commonly required for electrochemical applications. Table 1. BET specific surface area and average pore size of different catalyst. Surface area Average pore Sample name [m2 g-1] size [nm] @C
504.15
5.31
CNT@C
599.60
3.81
Ni6Cr4@C
296.16
5.58
Ni-CNT@C
350.70
5.43
Ni8Cr2-CNT@C
382.21
5.8
Ni6Cr4-CNT@C
354.73
5.95
Ni4Cr6-CNT@C
348.44
5.42
3.2 Electrochemical properties of NiCr-CNT@C The electrochemical property of NiCr-CNT@C with different Ni/Cr ratios was investigated by cyclic voltammetry (CV) to assess the effect of Cr doping and CNT addition. The CV experiments were conducted in 1 M KOH with and without 0.33 M urea in a voltage window of 0 – 0.8 V. In the CV curves (Figure 3) of all the NiCr-based catalysts in 1 M KOH, a pair of redox peaks was observed at 0.4 – 0.5 and 0.2 – 0.3 V vs. Ag/AgCl, corresponding to the anodic and cathodic peak potentials, respectively, due to the inter-conversion of Ni2+(OH)2/Ni3+OOH, as previously reported [1,2]. From the CV curves, the electrochemical active surface area (ECSA) value was calculated to be 202 m2 g-1 (337 m2 gNi-1) for Ni6Cr4-CNT@C, which was higher than that of Ni-CNT@C [174 m2 g-1 (290 m2 gNi-1)], indicating better charge transfer-kinetics (Suppl. Table. S2). With 0.33 M urea in 1 M KOH, the NiCr-based catalysts
6
also exhibited redox peaks at potentials similar to those observed in 1 M KOH (Figure 3), implying that the Ni3+OOH catalyzed UOR [1,6,40] occurring via Ni2+/Ni3+ redox pathway. In the presence of urea, all the catalysts showed an increase in current density in the forward scan, indicating their activity for UOR. by doping Cr at 40%, the anodic peak current increased considerably, as observed previously [16]. However, the onset potential remained approximately constant upon the addition of Cr up to 40%, after which it decreased once the Cr content exceeded 40%. The peak current for the UOR of Ni6Cr4@C (145.5 mA cm-2) was further enhanced by the addition of the MWCNTs (Ni6Cr4-CNT@C, 223 mA cm-2). Therefore, Ni6Cr4-CNT@C was chosen as the best catalyst and further electrochemical experiments were conducted using it. Figure 4 shows the chronoamperometric (CA) plots of different catalysts for the UOR. All the catalysts exhibited a stable current density during the measurement in 0.33 M urea and 1 M KOH for 2000 s. The highest current density was observed for Ni6Cr4-CNT@C, which was in line with the CV results. Electrochemical impedance spectroscopy(EIS) was carried out to further investigate the UOR over the NiCr catalysts, as shown in Figure 5. The charge-transfer resistance (Rct), which refers to the resistance due to the accumulation of charges at the solution-electrode interface [41], was determined from the diameter of the semicircle of the associated Nyquist plot. The Rct decreased considerably to 1.45 from 2.2 Ω cm-2 due to the doping of Cr at 40% and from 2.1 Ω cm-2 upon the addition of MWCNT, indicating an enhanced charge transfer kinetics of Ni6Cr4-CNT@C toward UOR, in line with the above CV results. The effect of the scan rate on the CV results was examined to investigate the electrochemical reaction mechanism of the UOR over the Ni6Cr4-CNT@C catalyst. Figure 6a shows the CV curves in 0.33 M urea and 1 M KOH at different scan rates ranging from 10 to 120 mV s-1. As shown in the inset of Figure 6a, the anodic peak current (Epa) increased linearly with the square root of the scan rate, suggesting that the UOR on the free-standing Ni6Cr4-CNT@C catalyst surface was a diffusioncontrolled process that occurred according to the Randles-Sevcik model [17,42]. Additionally, the potential (Epa) increased linearly with the logarithm of the scan rate, indicating the kinetic limitations of the UOR, and thus the mixed control reactions of the UOR over the Ni6Cr4-CNT@C catalyst [17,42]. The diffusion coefficient (D) of urea in the free-standing Ni6Cr4-CNT@C electrode was estimated from the CV results. The D was calculated to be 7.86×10-5 cm2 s-1, which is higher than the reported values for Ni foam [15], MWCNTs [17] and graphene [43], mainly because of the porous 3D network structure. The detailed calculation procedure for D is described in Suppl. Table S3. Figure 6b presents the CA plot at different applied potentials, which indicates that the current output at a certain voltage is the sum of those from the UOR and other side reactions, such as the oxygen evolution reaction (OER). The net current from the UOR was calculated by subtracting the baseline current, which was the current generated in the KOH solution at the corresponding voltage. The percentage of the net current from the UOR in the total current (denoted as efficiency) is shown in the inset of Figure 6b Ni6Cr4-CNT@C showed the highest efficiency of 86.4 % at 0.5 V.
7
3.3 Urea/H2O2 fuel cell performance The performance of the DUHpFC with Ni6Cr4-CNT@C as the anode was evaluated. A single-cell MEA was prepared using the free-standing Ni6Cr4-CNT@C catalyst mat without a gas-diffusion-layer. Figures 7a, 7b, and 7c show the IV and power density curves obtained at different pH, urea concentrations, and temperatures. It was clearly observed that the maximum power density (MPD) and open-circuit voltage (OCV) increased with the KOH concentration up to 7 M, as shown in Figure 7a, because OH- was required for the reaction with urea and the formation of NiOOH catalysts [1]; consequently, the onset potential for the UOR was reduced (Suppl. Figure S3). However, above 7 M KOH, a transition zone, where the voltage and power density decreased rapidly, was observed in the polarization curve, indicating a high overpotential owing to the catalyst site blockage. The MPD and OCV increased also with the urea concentration up to 1 M, implying substrate saturation kinetics, as shown in Figure 7b. The cell performance considerably increased with the increase in the operating temperature. The MPD increased from 18.1 mW cm-2 to 48.1 mW cm-2 as the temperature increased from 20 °C to 80 °C (Figure 7c), mainly because of the concomitant increase in the electrochemical redox reaction rates and the ion conductivity of the membrane with temperature. Within the scope of this study, the highest MPD of 48.1 mW cm-2 was obtained when a solution of 1 M urea and 5 M KOH was used as the anolyte at 80 °C and 2 M H2O2 and 2 M H2SO4 as the catholyte, which is the highest value among those in the literature to the best of our knowledge [4,9,10]. A stability test was conducted at 80 °C. As shown in Figure 7d, the cell voltage at a discharge current density of 20 mA cm-2 decreased to 0.34 V until 12 h, after which it remained stable for 20 h. The electrochemical behavior of the DUHpFC was further characterized by EIS. As shown in Figure 8. the impedance was measured at different urea concentrations and temperatures under opencircuit conditions, As shown in Figure 8a, the Rct in the 3 M urea solution (0.054 Ω) increased considerably compared to that in the 1 M (0.033 Ω). A high urea concentration has been reported to induce blockage of used active sites which can be restored by the OH- ion [44]. Additionally, the ohmic resistance (Ro) of the cell, which was estimated from the high-frequency intercept, in the 3 M urea solution increased by 20% compared to that in the 1 M urea solution, mainly because the 3 M urea solution exhibited higher series resistance than the 1 M urea solution, as shown in Suppl. Figure S4. These results are in line with the lower power density observed earlier for the 3 M urea solution than that for 1 M urea solution. Figure 8b shows the effect of temperature on the EIS. Both Ro and Rct decreased as temperature increased in the range 20 – 80 °C (Suppl. Table 4), because of the relatively high ionic conductivity of the membrane and high UOR rate at high temperatures. Thus, the highest MPD was obtained at 80 °C as observed above. 4. Conclusions NiCr bimetallic catalysts embedded in a carbon skeleton structure were synthesized by electrospinning and carbonization (NiCr-CNT@C). The synthesized NiCr-CNT@C catalyst had a highly porous structure with uniformly distributed NiCr nanoparticles. The Cr doping of the Ni catalyst considerably
8
improved its electro-catalytic activity. Furthermore, the addition of MWCNTs also remarkably enhanced the catalytic activity by providing a high surface area and enhancing electrical conductivity. A DUHpFC with NiCr-CNT@C as the free-standing anode achieved an excellent MPD of 48.1 mW cm-2 with an OCV of 0.92 V at 80 °C, outperforming previously reported urea fuel cells. The result suggested that the NiCr-CNT@C catalyst mat is a promising free-standing anode for urea fuel cells.
Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2017R1AB4002083) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20194030202290). References [1]
A.N. Rollinson, J. Jones, V. Dupont, M. V. Twigg, Urea as a hydrogen carrier: A perspective on its potential for safe, sustainable and long-term energy supply, Energy Environ. Sci. 4 (2011) 1216–1224. https://doi.org/10.1039/c0ee00705f.
[2]
R. Lan, S. Tao, J.T.S. Irvine, A direct urea fuel cell - Power from fertiliser and waste, Energy Environ. Sci. 3 (2010) 438–441. https://doi.org/10.1039/b924786f.
[3]
W. Xu, Z. Wu, S. Tao, Urea-Based Fuel Cells and Electrocatalysts for Urea Oxidation, Energy Technol. 4 (2016) 1329–1337. https://doi.org/10.1002/ente.201600185.
[4]
F. Guo, D. Cao, M. Du, K. Ye, G. Wang, W. Zhang, Y. Gao, K. Cheng, Enhancement of direct urea-hydrogen peroxide fuel cell performance by three-dimensional porous nickel-cobalt anode, J. Power Sources. 307 (2016) 697–704. https://doi.org/10.1016/j.jpowsour.2016.01.042.
[5]
D. Wang, W. Yan, S.H. Vijapur, G.G. Botte, Enhanced electrocatalytic oxidation of urea based on nickel hydroxide nanoribbons, J. Power Sources. 217 (2012) 498–502. https://doi.org/10.1016/j.jpowsour.2012.06.029.
[6]
W. Xu, H. Zhang, G. Li, Z. Wu, Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell, Sci. Rep. 4 (2014). https://doi.org/10.1038/srep05863.
[7]
W. Yan, D. Wang, G.G. Botte, Nickel and cobalt bimetallic hydroxide catalysts for urea electrooxidation, Electrochim. Acta. 61 (2012) 25–30. https://doi.org/10.1016/j.electacta.2011.11.044.
9
[8]
R. Ding, J. Liu, J. Jiang, F. Wu, J. Zhu, X. Huang, Tailored Ni-Cu alloy hierarchical porous nanowire as a potential efficient catalyst for DMFCs, Catal. Sci. Technol. 1 (2011) 1406–1411. https://doi.org/10.1039/c1cy00164g.
[9]
B. Li, C. Song, J. yin, J. Yan, K. Ye, K. Cheng, K. zhu, D. Cao, G. Wang, Effect of graphene on the performance of nickel foam-based CoNi nanosheet anode catalyzed direct urea-hydrogen peroxide fuel cell, Int. J. Hydrogen Energy. (2019). https://doi.org/10.1016/j.ijhydene.2019.05.158.
[10]
F. Guo, K. Cheng, K. Ye, G. Wang, D. Cao, Preparation of nickel-cobalt nanowire arrays anode electro-catalyst and its application in direct urea/hydrogen peroxide fuel cell, Electrochim. Acta. 199 (2016) 290–296. https://doi.org/10.1016/j.electacta.2016.01.215.
[11]
C. Li, Y. Liu, Z. Zhuo, H. Ju, D. Li, Y. Guo, X. Wu, H. Li, T. Zhai, Local Charge Distribution Engineered by Schottky Heterojunctions toward Urea Electrolysis, Adv. Energy Mater. 8 (2018) 1–8. https://doi.org/10.1002/aenm.201801775.
[12]
N.A.M. Barakat, M.H. El-Newehy, A.S. Yasin, Z.K. Ghouri, S.S. Al-Deyab, Ni&Mn nanoparticlesdecorated carbon nanofibers as effective electrocatalyst for urea oxidation, Appl. Catal. A Gen. 510 (2016) 180–188. https://doi.org/10.1016/j.apcata.2015.11.015.
[13]
W. Yan, D. Wang, G.G. Botte, Electrochemical decomposition of urea with Ni-based catalysts, Appl. Catal. B Environ. 127 (2012) 221–226. https://doi.org/10.1016/j.apcatb.2012.08.022.
[14]
L. Wang, M. Li, Z. Huang, Y. Li, S. Qi, C. Yi, B. Yang, Ni-WC/C nanocluster catalysts for urea electrooxidation, J. Power Sources. 264 (2014) 282–289. https://doi.org/10.1016/j.jpowsour.2014.04.104.
[15]
W. Xu, D. Du, R. Lan, J. Humphreys, Z. Wu, S. Tao, Highly active Ni-Fe double hydroxides as anode catalysts for electrooxidation of urea, New J. Chem. 41 (2017) 4190–4196. https://doi.org/10.1039/c6nj04060h.
[16]
R.K. Singh, A. Schechter, Electroactivity of NiCr Catalysts for Urea Oxidation in Alkaline Electrolyte, ChemCatChem. 9 (2017) 3374–3379. https://doi.org/10.1002/cctc.201700451.
[17]
R.M. Tesfaye, G. Das, B.J. Park, J. Kim, H.H. Yoon, Ni-Co bimetal decorated carbon nanotube aerogel as an efficient anode catalyst in urea fuel cells, Sci. Rep. 9 (2019) 1–9. https://doi.org/10.1038/s41598-018-37011-w.
10
[18]
T.Q.N. Tran, G. Das, H.H. Yoon, Nickel-metal organic framework/MWCNT composite electrode for non-enzymatic urea detection, Sensors Actuators, B Chem. 243 (2017) 78–83. https://doi.org/10.1016/j.snb.2016.11.126.
[19]
L. Ren, K.S. Hui, K.N. Hui, Self-assembled free-standing three-dimensional nickel nanoparticle/graphene aerogel for direct ethanol fuel cells, J. Mater. Chem. A. 1 (2013) 5689– 5694. https://doi.org/10.1039/c3ta10657h.
[20]
K. Ye, D. Zhang, F. Guo, K. Cheng, G. Wang, D. Cao, Highly porous nickel@carbon sponge as a novel type of three-dimensional anode with low cost for high catalytic performance of urea electro-oxidation in alkaline medium, J. Power Sources. 283 (2015) 408–415. https://doi.org/10.1016/j.jpowsour.2015.02.149.
[21]
M. Shalom, D. Ressnig, X. Yang, G. Clavel, T.P. Fellinger, M. Antonietti, Nickel nitride as an efficient electrocatalyst for water splitting, J. Mater. Chem. A. 3 (2015) 8171–8177. https://doi.org/10.1039/c5ta00078e.
[22]
B. Saha, G.C. Schatz, Carbonization in polyacrylonitrile (PAN) based carbon fibers studied by reaxff molecular dynamics simulations, J. Phys. Chem. B. 116 (2012) 4684–4692. https://doi.org/10.1021/jp300581b.
[23]
Q. Lin, Y. Li, M. Yang, Polyaniline nanofiber humidity sensor prepared by electrospinning, Sensors Actuators, B Chem. 161 (2012) 967–972. https://doi.org/10.1016/j.snb.2011.11.074.
[24]
P.M. Bulemo, H.-J. Cho, N.-H. Kim, I.-D. Kim, Mesoporous SnO 2 Nanotubes via Electrospinning–Etching Route: Highly Sensitive and Selective Detection of H 2 S Molecule, ACS Appl. Mater. Interfaces. 9 (2017) 26304–26313. https://doi.org/10.1021/acsami.7b05241.
[25]
L. Wang, Y. Yu, P.C. Chen, D.W. Zhang, C.H. Chen, Electrospinning synthesis of C/Fe3O4 composite nanofibers and their application for high performance lithium-ion batteries, J. Power Sources. 183 (2008) 717–723. https://doi.org/10.1016/j.jpowsour.2008.05.079.
[26]
T. Tamura, H. Kawakami, Aligned electrospun nanofiber composite membranes for fuel cell electrolytes, Nano Lett. 10 (2010) 1324–1328. https://doi.org/10.1021/nl1007079.
[27]
S. Agarwal, J.H. Wendorff, A. Greiner, Use of electrospinning technique for biomedical applications, Polymer (Guildf). 49 (2008) 5603–5621. https://doi.org/10.1016/j.polymer.2008.09.014.
11
[28]
Y.S. Li, T.S. Zhao, Z.X. Liang, Effect of polymer binders in anode catalyst layer on performance of alkaline direct ethanol fuel cells, J. Power Sources. 190 (2009) 223–229. https://doi.org/10.1016/J.JPOWSOUR.2009.01.055.
[29]
M.H. Tran, B.J. Park, B.H. Kim, H.H. Yoon, Mesoporous silica template-derived nickel-cobalt bimetallic catalyst for urea oxidation and its application in a direct urea/H2O2 fuel cell, Int. J. Hydrogen Energy. 45 (2020) 1784–1792. https://doi.org/10.1016/j.ijhydene.2019.11.073.
[30]
B.T. Sone, E. Manikandan, A. Gurib-Fakim, M. Maaza, Single-phase α-Cr2O3nanoparticles’ green synthesis using Callistemon viminalis’ red flower extract, Green Chem. Lett. Rev. 9 (2016) 85–90. https://doi.org/10.1080/17518253.2016.1151083.
[31]
N. Srivastava, P.C. Srivastava, Realizing NiO nanocrystals from a simple chemical method, Bull. Mater. Sci. 33 (2010) 653–656. https://doi.org/10.1007/s12034-011-0142-0.
[32]
T. Ishimasa, H.U. Nissen, Y. Fukano, New ordered state between crystalline and amorphous in Ni-Cr particles, Phys. Rev. Lett. 55 (1985) 511–513. https://doi.org/10.1103/PhysRevLett.55.511.
[33]
A.R. Sethuraman, R.J. De Angelis, P.J. Reucroft, Diffraction studies on Ni-Co and Ni-Cr alloy thin films, J. Mater. Res. 6 (1991) 749–754. https://doi.org/10.1557/JMR.1991.0749.
[34]
M. Hassel, I. Hemmerich, H. Kuhlenbeck, H.-J. Freund, High Resolution XPS Study of a Thin Cr 2 O 3 (111) Film Grown on Cr(110) , Surf. Sci. Spectra. 4 (1996) 246–252. https://doi.org/10.1116/1.1247795.
[35]
K. Yang, Q. Yao, W. Huang, X. Chen, Z.H. Lu, Enhanced catalytic activity of NiM (M = Cr, Mo, W) nanoparticles for hydrogen evolution from ammonia borane and hydrazine borane, Int. J. Hydrogen Energy. 42 (2017) 6840–6850. https://doi.org/10.1016/j.ijhydene.2016.12.029.
[36]
W. Shi, R. Ding, X. Li, Q. Xu, E. Liu, Enhanced performance and electrocatalytic kinetics of NiMo/graphene nanocatalysts towards alkaline urea oxidation reaction, Electrochim. Acta. 242 (2017) 247–259. https://doi.org/10.1016/j.electacta.2017.05.002.
[37]
D. Wang, S.H. Vijapur, Y. Wang, G.G. Botte, NiCo2O4 nanosheets grown on current collectors as binder-free electrodes for hydrogen production via urea electrolysis, Int. J. Hydrogen Energy. 42 (2017) 3987–3993. https://doi.org/10.1016/j.ijhydene.2016.11.048.
[38]
Y. Liang, Q. Liu, A.M. Asiri, X. Sun, Enhanced electrooxidation of urea using NiMoO4·xH2O nanosheet arrays on Ni foam as anode, Electrochim. Acta. 153 (2015) 456–460.
12
https://doi.org/10.1016/j.electacta.2014.11.193. [39]
R. Ding, L. Qi, M. Jia, H. Wang, Facile synthesis of mesoporous spinel NiCo2O4 nanostructures as highly efficient electrocatalysts for urea electro-oxidation, Nanoscale. 6 (2014) 1369–1376. https://doi.org/10.1039/c3nr05359h.
[40]
E.T. Sayed, T. Eisa, H.O. Mohamed, M.A. Abdelkareem, A. Allagui, H. Alawadhi, K.J. Chae, Direct urea fuel cells: Challenges and opportunities, J. Power Sources. 417 (2019) 159–175. https://doi.org/10.1016/j.jpowsour.2018.12.024.
[41]
J.B. Aellen, R.F. Larry, Electrochemical methods fundamentals and applications, 2001. https://doi.org/10.1016/B978-0-12-381373-2.00056-9.
[42]
G. Das, R.M. Tesfaye, Y. Won, H.H. Yoon, NiO-Fe2O3 based graphene aerogel as urea electrooxidation catalyst, Electrochim. Acta. 237 (2017) 171–176. https://doi.org/10.1016/j.electacta.2017.03.197.
[43]
R.M. Abdel Hameed, S.S. Medany, Influence of support material on the electrocatalytic activity of nickel oxide nanoparticles for urea electro-oxidation reaction, J. Colloid Interface Sci. 513 (2018) 536–548. https://doi.org/10.1016/j.jcis.2017.11.032.
[44]
V. Vedharathinam, G.G. Botte, Understanding the electro-catalytic oxidation mechanism of urea on nickel electrodes in alkaline medium, Electrochim. Acta. 81 (2012) 292–300. https://doi.org/10.1016/j.electacta.2012.07.007.
13
Figure 1. (a) XRD patterns of NiCr-CNT@C catalysts with different Ni/Cr ratio and XPS spectra of Ni6Cr4-CNT@C sample showing (b) Cr 2p region and (c) Ni 2p region.
14
Figure 2. SEM image of Ni6Cr4-CNT-PAN precursor (a); SEM (b), a cross-sectional FIB SEM (c), TEM (d), HR-TEM (e) image of Ni6Cr4-CNT@C; EDS elemental mapping of Ni6Cr4-CNT@C, corresponding TEM image (f), for C (g), Ni (h), and Cr (k).
15
Figure 3. CV plots of NixCry-CNT@C samples in 1 M KOH with/without 0.33 M urea at a scan rate of 20 mV s-1 (a-e) and corresponding onset potentials and peak current densities for UOR (at the 0.62 V with 0.33 M urea) as a function of Cr content.
16
Figure 4. CA plot of NiCr-CNT@C samples in 1 M KOH and 0.33 M urea at 0.4 V.
Figure 5. Nyquist plots of NiCr-CNT@Cs at 0.4 V in 1 M KOH and 0.33 M urea solution with a frequency range of 0.05 Hz to 1 MHz.
17
Figure 6. (a) CV plots of Ni6Cr4-CNT@C in 1 M KOH and 0.33 M urea with different scan rates of 10 to 120 mV s-1 and (b) CA plots of Ni6Cr4-CNT@C at different potentials from 0.35 to 0.70 V in 1 M KOH with/without urea (0.33 M).
Figure 7. Performance of a DUHpFC using Ni6Cr4-CNT@C as anode; (a) effect of KOH concentration (0.33 M urea) at 20 °C, (b) effect of urea concentration (5 M KOH) at 20 °C, (b) effect of temperature (1 M urea, 5 M KOH); (d) stability test of a DUHpFC at a discharge current density of 20 mA cm-2 at 80 °C with 1 M urea in 5 M KOH solution as fuel.
18
Figure 8. Nyquist plots of a DUHpFC with Ni6Cr4-CNT@C as anode using (a) 1 M and 3 M urea solution in 5 M KOH as anolytes at 20 °C, and (b) 1 M urea in 5 M KOH at different temperatures, in the frequency range of 0.01 Hz to 1 MHz at open circuit voltage. Each Inset figure shows an enlarged Nyquiet plots in the high frequency range.
19
Highlights A free standing NiCr catalyst mat was synthesized by electrospinning technique. NiCr bimetal nanoparticles were uniformly embedded in the carbon nanofibers. The NiCr catalyst featured excellent electro-catalytic activity for urea oxidation. A urea fuel cell with NiCr catalyst exhibited a power density of 48.1 mW cm–2.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(20)31050-1
DOI:
https://doi.org/10.1016/j.electacta.2020.136657
Reference:
EA 136657
To appear in:
Electrochimica Acta
Received Date: 30 March 2020 Revised Date:
16 June 2020
Accepted Date: 18 June 2020
Please cite this article as: B. Kim, G. Das, B.J. Park, D.H. Lee, H.H. Yoon, A free-standing NiCr-CNT@C anode mat by electrospinning for a high-performance urea/H2O2 fuel cell, Electrochimica Acta, https:// doi.org/10.1016/j.electacta.2020.136657. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier Ltd. All rights reserved.
CRediT author statement Bohyeon Kim: Conceptualization, Investigation, Writing- Original draft preparation. Das Gautam: Data curation. Bang Ju Park and Dal Ho lee: Supervision, Writing- Reviewing and Editing. Hyon Hee Yoon: Supervision.
A free-standing NiCr-CNT@C anode mat by electrospinning for a high-performance urea/H2O2 fuel cell Bohyeon Kim1, Gautam Das1, Bang Ju Park2, Dal Ho Lee2, and Hyon Hee Yoon*1 1
Department of Chemical and Biological Engineering, Gachon University, Gyeonggi-Do 461-701,
Republic of Korea 2
Department of Electronic Engineering, Gachon University, Gyeonggi-Do 461-701, Republic of Korea
*Corresponding author: Tel. No. (+82)031-750-5356, email: [email protected] Abstract A highly porous free-standing NiCr-bimetallic catalyst is synthesized by electrospinning and carbonization and is applied as an anode in a direct urea/H2O2 fuel cell for electro-oxidation of urea. It is observed that the morphology of the calcined NiCr catalyst, which has a higher specific surface area, is identical to that of the electrospun NiCr catalyst fiber. Cr-doping at 40 % (Cr/Ni) significantly increases the oxidation peak current of the urea oxidation reaction. The addition of carbon nanotubes also considerably enhances the catalytic activity. A direct urea/H2O2 fuel cell, which utilizes the synthesized catalyst (NiCr-CNT@C) as a free-standing anode, exhibits excellent performance with a maximum power density of 48.1 mW cm-2 and an open-circuit voltage of 0.92 V at 80 °C. Thus, the highly porous free-standing NiCr-CNT@C catalyst mat can be employed as an efficient anode material for urea oxidation in urea fuel cells. Keywords: Urea electrocatalyst; Urea fuel cell; Ni-Cr bimetal; Electrospinning; Free-standing electrode
1
1. Introduction Urea, also known as carbamide, is a non-flammable, non-toxic, and low-cost chemical, which has a high energy density (16.9 MJ L-1). Furthermore, this solid material is easy to transport and store; thus, it can serve as an efficient fuel for fuel cells and as a hydrogen carrier in electrolysis cells [1–3]. A conventional urea fuel cell utilizes O2 oxidant; on the other hand, a direct urea/H2O2 fuel cell (DUHpFC) uses H2O2 as oxidant as shown below [4]: Anode: CO(NH2)2 + 8KOH → N2 + 6H2O + K2CO3 + 6K+ + 6e– –
Cathode: 3H2O2 + 3H2SO4 + 6e → 6H2O +
4SO42-
Overall: CO(NH2)2 + 3H2O2 + 3H2SO4 + 8KOH → K2CO3 + 3K2SO4 +N2 + 12H2O
(1) (2) (3)
The theoretical cell voltage in urea/O2 and urea/H2O2 type fuel cell is 1.147 and 2.509 V, respectively; therefore, a higher cell voltage can be expected in DUHpFC, although the oxidant chemicals (i.e., H2O2 and H2SO4) should require extra cost. DUHpFCs with enhanced cell voltage can be used for a distributed power generator using commercial urea compounds. In a urea electrolytic cell, urea is also oxidized as follows [5]: Anode: CO(NH2)2 + 6OH– → N2 + CO2 + 5H2O + 6e– –
–
(4)
Cathode: 6H2O + 6e → 3H2 + 6OH
(5)
Overall: CO(NH2)2 + H2O → N2 + 3H2 + CO2
(6)
The electro-oxidation of urea is, therefore, an important process in the utilization of urea to produce electricity and hydrogen. Fortunately, the inexpensive Ni-based catalysts exhibit excellent catalytic activity toward urea oxidation (UOR) in alkaline solutions [6–8]. However, high oxidation potentials (0.55 – 0.65 V vs. SHE) for the UOR over Ni-based catalysts have been observed relative to theoretical values (−0.46 V vs. SHE). Furthermore, the power density obtained in DUHpFCs that employ Ni-based catalysts as the anode is still considerably lower (i.e., 10 – 30 mW cm-2) than those obtained in hydrogen and methanol fuel cells [4,9,10], mainly due to the sluggish kinetics of the sixelectron-transfer UOR [11]. The activity of Ni-based catalysts can be improved by conjugating Ni with other metals and by structural modification. For instance, NiCo [6], NiMn [12], NiZn [13], NiWC [14], NiFe [15], and NiCr [16] bimetallic catalysts have been reported to exhibit promising activity and stability toward UOR. A high specific surface area, associated with a high number of active sites for the UOR, and a mesoporous structure for fast mass transport are also critical properties of an excellent catalyst, particularly because urea is a large molecule compared to hydrogen and methanol. Therefore, Ni-based catalysts were nanostructured and/or combined with carbon-based supporting materials for further activity enhancement [17,18]. Most supporting materials have a three-dimensional (3-D) structure such as graphene aerogel [19], carbon sponge [20] and Ni foam [21]. For example, a DUHpFC, which utilizes a 3-D film of Ni-Co on a Ni foam, synthesized by electro-deposition, as the anode, with 0.5 M urea in 7 M KOH at 20 °C, exhibited a high power density of 17.4 mW cm-2. Further, electrospinning is frequently used for the preparation of catalyst matrices [22,23]; it is possible to synthesize evenly dispersed metal
2
nanoparticles with high surface areas by simply adding various metal precursors to the electrospinning solution. The electrospinning technique has been applied in various devices, such as in gas sensors [24], lithium-ion batteries [25], fuel cells [26], and biomedical devices [27], due to its simplicity and uniform properties. Recently, Ni-Mn nanoparticle-decorated carbon nanofibers, synthesized by electrospinning, showed reportedly distinct activity toward UOR [12]. In this study, we developed a novel Ni-based catalyst-modified free-standing electrode for UOR. Ni and Cr-impregnated polyacrylonitrile (PAN) nanofiber mats (NiCr-PAN) were prepared by electrospinning. The electrospun mat was calcined in nitrogen to form a free-standing electrode (NiCr@C) for UOR, which makes the fabrication process of a membrane electrode assembly (MEA) simple and economic. Conventionally, Ni-based catalysts are coated onto a gas-diffusion-layer, such as carbon felt and carbon paper, to fabricate an MEA. However, the presence of organic binders in the catalyst coating can adversely affect the activity of the catalyst at low current density [28]. Furthermore, to improve the mechanical and electrical properties of the NiCr@C electrode, multi-walled carbon nanotubes (MWCNTs) were incorporated (NiCr-CNT@C). The morphological and electrochemical properties of NiCr-CNT@C were rigorously analyzed. The single-cell performance of the DUHpFC with the NiCr-CNT@C as the free-standing anode was also investigated. 2. Experimental Section 2.1. Synthesis of NiCr-CNT@C catalyst mats NiCr-CNT@C electrospinning
catalyst solution
mats was
were
synthesized
prepared by
by
mixing
electrospinning
and
carbonization.
The
3.31 mmol
Ni(OCOCH3)2 4H2O
and
of
(CH3CO2)7Cr3(OH)2 precursors in the desired Ni/Cr ratio, MWCNT (0.5 wt%), if necessary, and PAN (9 wt%) in 5 mL of N, N-dimethylformamide, sequentially. Before adding PAN, MWCNTs were dispersed evenly by ultra-sonication for 30 min. The final solution was stirred for 12 h at 85 °C. The electrospun mat was obtained by the electrospinning solution process, which was operated using the prepared solution at 20 kV with a 15 cm distance between the tip and the collector. The nanofibers, which were synthesized by electrospinning, were collected on a cylindrical stainless drum covered by an aluminium foil. The collected electrospun mats were dried at 80 °C for 12 h to evaporate the remaining solvent, after which the stabilization was conducted in an air atmosphere at 250 °C for 1 h. After the stabilization, carbonization was carried out under a nitrogen atmosphere at 1000 °C for 1 h. For the other ratio of Ni/Cr, the total moles of Ni and Cr were kept constant. 2. 2. Electrochemical measurements and DUHpFC tests. For electrochemical properties of catalysts, cyclic voltammetry (CV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) were measured using potentiostat and galvanostat (SP-240, Bio-Logic). The as-prepared catalysts mats were cut into pieces of dimensions 5 mm x 5 mm; these pieces were directly used as the working electrode for the electrochemical measurements. A conventional three-electrode system was applied using Ag/AgCl (3 M NaCl) and Pt wire as the
3
reference electrode and the counter electrode, respectively. The EIS measurements were performed using an AC amplitude of 10 mV in the frequency range of 0.05 to 100000 Hz. Z-fit software from EClab was utilized to fit a Nyquist plot obtained by EIS. A DUHpFC was prepared using the NiCr-CNT@C catalyst mat as the anode, which had a catalyst loading of 0.28 mg cm–2, and Pt/C (30 wt% Pt) on carbon paper with 2.0 mg Pt cm-2 as the cathode. A cation exchange membrane (Nafion® 115) was used as the polymer electrolyte; the membrane was sandwiched between the anode and cathode to prepare a membrane electrode assembly with an active area of 5.0 cm2, as described in a previous work [29]. Single-cell tests were carried out using a gold-coated stainless current collector and a graphitic plate with serpentine flow channels. An aqueous solution of urea and KOH was fed into the anode side, and 2 M H2O2 in 2 M H2SO4 solution was supplied to the cathode. Polarization and power density curves were obtained by the potentiostatgalvanostat interfaced with EC-lab 11.01 data acquisition software.
2.3. Characterization. The morphology of the samples was investigated using a scanning electron microscope (SEM, A JEOL JSM-6700F) and transmission electron microscope (TEM, Tecnai G2 F30 S-Twin) equipped with an EDS analyzer. The crystal information of the samples was analyzed using a multi-purpose X-ray diffractometer (MPXRD, X’Pert Pro MPD) using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and a scan rate of 2° min-1. The X’pert high score was utilized to identify the peaks in the XRD patterns. XPS (Kalpha) analysis was conducted to obtain the binding energy plot of the samples with monochromated Al Kα as the X-ray source. All the binding energy values were calibrated to C1s (284.6 eV) to define the chemical states. Specific surface area and porosity were determined based on nitrogen adsorptiondesorption measurements performed using a Micromeritics ASAP 2020 apparatus according to the Brunauer-Emmett-Teller (BET) theory. The electrical resistivity of the catalyst mats was determined by four-probe resistivity method (CMT-100S, AIT). 3. Results and Discussion 3.1 Characterization of the NiCr-CNT@C catalysts NiCr@C and NiCr-CNT@C were prepared by electrospinning, followed by calcination. The electrospinning solution was composed of nickel acetate and chromium acetate precursors, MWCNTs (if necessary), and PAN. The electrospun fiber mat was calcined in a nitrogen environment to obtain NiCr-CNT@C, as shown in Suppl. Scheme S1. Figure 1a shows the X-ray diffraction (XRD) patterns of NiCr-CNT@C with different Cr/Ni ratios. The peaks at 2θ = 44°, 52° and 77° were indexed to the (111), (200), and (220) planes, respectively, of pure nickel. All the catalysts containing Ni and Cr showed small peaks indicative of the oxidized metals; NiO peaks were observed at 2θ = 43.7°, 63.7° and 76.3°, which were assigned to the (200), (220) and (311) planes of NiO, respectively; Cr2O3 peaks were observed at 2θ values of 24.6°, 33.7°, 36.3°, 50.3°,
4
55.9°, 63.7° and 65.3°, corresponding to the (012), (104), (110), (024), (116), (214) and (300) planes of Cr2O3, respectively [30,31]. The intensity of the crystalline peaks decreased gradually with the addition of Cr, implying that Cr affected the crystalline structure of Ni and promoted a more amorphous structure, as previously observed for Ni–Cr alloys [16,32,33]. Additionally, the crystalline peaks for Figure 1a shows the X-ray diffraction (XRD) patterns of NiCr-CNT@C with different Cr/Ni ratios. The peaks at 2θ = 44°, 52° and 77° were indexed to the (111), (200), and (220) planes, respectively, of pure nickel. All the catalysts containing Ni and Cr showed small peaks indicative of the oxidized metals; NiO peaks were observed at 2θ = 43.7°, 63.7° and 76.3°, which were assigned to the (200), (220) and (311) planes of NiO, respectively; Cr2O3 peaks were observed at 2θ values of 24.6°, 33.7°, 36.3°, 50.3°, 55.9°, 63.7° and 65.3°, corresponding to the (012), (104), (110), (024), (116), (214) and (300) planes of Cr2O3, respectively [30,31]. The intensity of the crystalline peaks decreased gradually with the addition of Cr, implying that Cr affected the crystalline structure of Ni and promoted a more amorphous structure, as previously observed for Ni–Cr alloys [16,32,33]. Additionally, the crystalline peaks for metallic Cr were not observed in the XRD patterns. These results might indicate the formation of a Ni– Cr alloy. The composition and chemical states of Ni and Cr in the NiCr-CNT@C catalyst were analyzed by X-ray photoelectron spectroscopy (XPS). Figure 1b and 1c show the XPS spectra of NiCr-CNT@C with a Ni:Cr ratio of 6:4, which is denoted as Ni6Cr4-CNT@C. The results revealed that both Ni and Cr were present in metal and oxide forms. The elemental composition analysis results indicated that the NiCrCNT@C catalyst comprised Ni, Cr, C, and O at 2.0, 1.5, 89.0, and 7.5 atomic %, respectively, as determined by the integration of the XPS spectrum of each component (Suppl. Table. S1). The Ni:Cr ratio of 57:43 was close to the theoretical loading ratio of 60:40. The metallic Cr was detected in the XPS analysis, which was not observed in the XRD patterns. Additionally, the peaks indicative of the metallic Cr and Ni in the XPS were shifted, implying the formation of a Ni–Cr alloy [34,35]. The electrospun Ni6Cr4-CNT-PAN precursor exhibited a nanofiber morphology with a smooth and uniform diameter as observed in the scanning electron microscopy (SEM) images (Figure 2a). The Ni6Cr4-CNT-PAN precursor was calcined to decompose the polymer matrix. After the calcination, the morphology of Ni6Cr4-CNT@C remained the same as that of the pristine Ni6Cr4-CNT-PAN, as observed in the SEM image (Figure 2b), owing to the skeletal C–C single bond structure, which was confirmed by a carbon XPS analysis (Suppl. Figure S1). The porous structure of the Ni6Cr4-CNT@C fiber was observed in a focused-ion beam (FIB)-SEM image of the fiber cross-section (Figure 2c). The fiber, shown in the upper right corner of Figure 2c, was previously truncated using an ion beam. White Ni–Cr nanoparticles were distributed throughout the cross-section of the fiber. The uniformly distributed Ni–Cr particles were more clearly observed as dark spots in the transmission electron microscopy (TEM) image (Figure 2d). The structures of NiCr-CNT@C were further characterized by high-resolution (HR)-TEM. The NiCr particles were uniformly embedded through the carbon matrix, as shown in the TEM image (Figure 2e). The presence of NiO and Cr2O3 was identified by the lattice spacings that appeared in the highly magnified TEM images (insets of Figure 2e), in line with the XRD results. The
5
lattice spacings of 0.203, 0.217, and 0.265 nm were ascribed to the (202), (113), and (104) planes of Cr2O3, respectively, and the d-spacing of 207 nm was matched with the (200) plane of NiO. However, the crystalline structures of metallic Cr and Ni were not identified by HR-TEM, implying the formation of an amorphous NixCry alloy in line with the XPS results. Figures 2f – 4k indicate the TEM–energy dispersive X-ray spectroscopy (EDS) mappings for C, Ni, and Cr, respectively. The Ni and Cr mappings overlapped, indicating that both Ni and Cr were detected in each particle, and thereby further confirming the presence of the NixCry alloy. The pore formation by calcination evidently provided a high specific surface area for the catalyst particles, as evidenced by Brunauer–Emmett–Teller (BET) measurements. The BET and average pore sizes of different catalysts are summarized in Table 1. The BET value increased considerably upon the calcination of the NiCr-PAN precursor and further to approximately 300 – 380 m2 g-1 by the addition of MWCNT, which is higher than the previously reported values (i.e., 50 - 200 m2 g-1 [36–39]). The average pore size of the Ni6Cr4-CNT@C catalyst was approximately 6.0 nm as calculated from the Barrett–Joyner–Halenda (BJH) pore size distribution curve (Suppl. Figure S2), which was indicative of the mesoporosity as commonly required for electrochemical applications. Table 1. BET specific surface area and average pore size of different catalyst. Surface area Average pore Sample name [m2 g-1] size [nm] @C
504.15
5.31
CNT@C
599.60
3.81
Ni6Cr4@C
296.16
5.58
Ni-CNT@C
350.70
5.43
Ni8Cr2-CNT@C
382.21
5.8
Ni6Cr4-CNT@C
354.73
5.95
Ni4Cr6-CNT@C
348.44
5.42
3.2 Electrochemical properties of NiCr-CNT@C The electrochemical property of NiCr-CNT@C with different Ni/Cr ratios was investigated by cyclic voltammetry (CV) to assess the effect of Cr doping and CNT addition. The CV experiments were conducted in 1 M KOH with and without 0.33 M urea in a voltage window of 0 – 0.8 V. In the CV curves (Figure 3) of all the NiCr-based catalysts in 1 M KOH, a pair of redox peaks was observed at 0.4 – 0.5 and 0.2 – 0.3 V vs. Ag/AgCl, corresponding to the anodic and cathodic peak potentials, respectively, due to the inter-conversion of Ni2+(OH)2/Ni3+OOH, as previously reported [1,2]. From the CV curves, the electrochemical active surface area (ECSA) value was calculated to be 202 m2 g-1 (337 m2 gNi-1) for Ni6Cr4-CNT@C, which was higher than that of Ni-CNT@C [174 m2 g-1 (290 m2 gNi-1)], indicating better charge transfer-kinetics (Suppl. Table. S2). With 0.33 M urea in 1 M KOH, the NiCr-based catalysts
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also exhibited redox peaks at potentials similar to those observed in 1 M KOH (Figure 3), implying that the Ni3+OOH catalyzed UOR [1,6,40] occurring via Ni2+/Ni3+ redox pathway. In the presence of urea, all the catalysts showed an increase in current density in the forward scan, indicating their activity for UOR. by doping Cr at 40%, the anodic peak current increased considerably, as observed previously [16]. However, the onset potential remained approximately constant upon the addition of Cr up to 40%, after which it decreased once the Cr content exceeded 40%. The peak current for the UOR of Ni6Cr4@C (145.5 mA cm-2) was further enhanced by the addition of the MWCNTs (Ni6Cr4-CNT@C, 223 mA cm-2). Therefore, Ni6Cr4-CNT@C was chosen as the best catalyst and further electrochemical experiments were conducted using it. Figure 4 shows the chronoamperometric (CA) plots of different catalysts for the UOR. All the catalysts exhibited a stable current density during the measurement in 0.33 M urea and 1 M KOH for 2000 s. The highest current density was observed for Ni6Cr4-CNT@C, which was in line with the CV results. Electrochemical impedance spectroscopy(EIS) was carried out to further investigate the UOR over the NiCr catalysts, as shown in Figure 5. The charge-transfer resistance (Rct), which refers to the resistance due to the accumulation of charges at the solution-electrode interface [41], was determined from the diameter of the semicircle of the associated Nyquist plot. The Rct decreased considerably to 1.45 from 2.2 Ω cm-2 due to the doping of Cr at 40% and from 2.1 Ω cm-2 upon the addition of MWCNT, indicating an enhanced charge transfer kinetics of Ni6Cr4-CNT@C toward UOR, in line with the above CV results. The effect of the scan rate on the CV results was examined to investigate the electrochemical reaction mechanism of the UOR over the Ni6Cr4-CNT@C catalyst. Figure 6a shows the CV curves in 0.33 M urea and 1 M KOH at different scan rates ranging from 10 to 120 mV s-1. As shown in the inset of Figure 6a, the anodic peak current (Epa) increased linearly with the square root of the scan rate, suggesting that the UOR on the free-standing Ni6Cr4-CNT@C catalyst surface was a diffusioncontrolled process that occurred according to the Randles-Sevcik model [17,42]. Additionally, the potential (Epa) increased linearly with the logarithm of the scan rate, indicating the kinetic limitations of the UOR, and thus the mixed control reactions of the UOR over the Ni6Cr4-CNT@C catalyst [17,42]. The diffusion coefficient (D) of urea in the free-standing Ni6Cr4-CNT@C electrode was estimated from the CV results. The D was calculated to be 7.86×10-5 cm2 s-1, which is higher than the reported values for Ni foam [15], MWCNTs [17] and graphene [43], mainly because of the porous 3D network structure. The detailed calculation procedure for D is described in Suppl. Table S3. Figure 6b presents the CA plot at different applied potentials, which indicates that the current output at a certain voltage is the sum of those from the UOR and other side reactions, such as the oxygen evolution reaction (OER). The net current from the UOR was calculated by subtracting the baseline current, which was the current generated in the KOH solution at the corresponding voltage. The percentage of the net current from the UOR in the total current (denoted as efficiency) is shown in the inset of Figure 6b Ni6Cr4-CNT@C showed the highest efficiency of 86.4 % at 0.5 V.
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3.3 Urea/H2O2 fuel cell performance The performance of the DUHpFC with Ni6Cr4-CNT@C as the anode was evaluated. A single-cell MEA was prepared using the free-standing Ni6Cr4-CNT@C catalyst mat without a gas-diffusion-layer. Figures 7a, 7b, and 7c show the IV and power density curves obtained at different pH, urea concentrations, and temperatures. It was clearly observed that the maximum power density (MPD) and open-circuit voltage (OCV) increased with the KOH concentration up to 7 M, as shown in Figure 7a, because OH- was required for the reaction with urea and the formation of NiOOH catalysts [1]; consequently, the onset potential for the UOR was reduced (Suppl. Figure S3). However, above 7 M KOH, a transition zone, where the voltage and power density decreased rapidly, was observed in the polarization curve, indicating a high overpotential owing to the catalyst site blockage. The MPD and OCV increased also with the urea concentration up to 1 M, implying substrate saturation kinetics, as shown in Figure 7b. The cell performance considerably increased with the increase in the operating temperature. The MPD increased from 18.1 mW cm-2 to 48.1 mW cm-2 as the temperature increased from 20 °C to 80 °C (Figure 7c), mainly because of the concomitant increase in the electrochemical redox reaction rates and the ion conductivity of the membrane with temperature. Within the scope of this study, the highest MPD of 48.1 mW cm-2 was obtained when a solution of 1 M urea and 5 M KOH was used as the anolyte at 80 °C and 2 M H2O2 and 2 M H2SO4 as the catholyte, which is the highest value among those in the literature to the best of our knowledge [4,9,10]. A stability test was conducted at 80 °C. As shown in Figure 7d, the cell voltage at a discharge current density of 20 mA cm-2 decreased to 0.34 V until 12 h, after which it remained stable for 20 h. The electrochemical behavior of the DUHpFC was further characterized by EIS. As shown in Figure 8. the impedance was measured at different urea concentrations and temperatures under opencircuit conditions, As shown in Figure 8a, the Rct in the 3 M urea solution (0.054 Ω) increased considerably compared to that in the 1 M (0.033 Ω). A high urea concentration has been reported to induce blockage of used active sites which can be restored by the OH- ion [44]. Additionally, the ohmic resistance (Ro) of the cell, which was estimated from the high-frequency intercept, in the 3 M urea solution increased by 20% compared to that in the 1 M urea solution, mainly because the 3 M urea solution exhibited higher series resistance than the 1 M urea solution, as shown in Suppl. Figure S4. These results are in line with the lower power density observed earlier for the 3 M urea solution than that for 1 M urea solution. Figure 8b shows the effect of temperature on the EIS. Both Ro and Rct decreased as temperature increased in the range 20 – 80 °C (Suppl. Table 4), because of the relatively high ionic conductivity of the membrane and high UOR rate at high temperatures. Thus, the highest MPD was obtained at 80 °C as observed above. 4. Conclusions NiCr bimetallic catalysts embedded in a carbon skeleton structure were synthesized by electrospinning and carbonization (NiCr-CNT@C). The synthesized NiCr-CNT@C catalyst had a highly porous structure with uniformly distributed NiCr nanoparticles. The Cr doping of the Ni catalyst considerably
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improved its electro-catalytic activity. Furthermore, the addition of MWCNTs also remarkably enhanced the catalytic activity by providing a high surface area and enhancing electrical conductivity. A DUHpFC with NiCr-CNT@C as the free-standing anode achieved an excellent MPD of 48.1 mW cm-2 with an OCV of 0.92 V at 80 °C, outperforming previously reported urea fuel cells. The result suggested that the NiCr-CNT@C catalyst mat is a promising free-standing anode for urea fuel cells.
Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2017R1AB4002083) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20194030202290). References [1]
A.N. Rollinson, J. Jones, V. Dupont, M. V. Twigg, Urea as a hydrogen carrier: A perspective on its potential for safe, sustainable and long-term energy supply, Energy Environ. Sci. 4 (2011) 1216–1224. https://doi.org/10.1039/c0ee00705f.
[2]
R. Lan, S. Tao, J.T.S. Irvine, A direct urea fuel cell - Power from fertiliser and waste, Energy Environ. Sci. 3 (2010) 438–441. https://doi.org/10.1039/b924786f.
[3]
W. Xu, Z. Wu, S. Tao, Urea-Based Fuel Cells and Electrocatalysts for Urea Oxidation, Energy Technol. 4 (2016) 1329–1337. https://doi.org/10.1002/ente.201600185.
[4]
F. Guo, D. Cao, M. Du, K. Ye, G. Wang, W. Zhang, Y. Gao, K. Cheng, Enhancement of direct urea-hydrogen peroxide fuel cell performance by three-dimensional porous nickel-cobalt anode, J. Power Sources. 307 (2016) 697–704. https://doi.org/10.1016/j.jpowsour.2016.01.042.
[5]
D. Wang, W. Yan, S.H. Vijapur, G.G. Botte, Enhanced electrocatalytic oxidation of urea based on nickel hydroxide nanoribbons, J. Power Sources. 217 (2012) 498–502. https://doi.org/10.1016/j.jpowsour.2012.06.029.
[6]
W. Xu, H. Zhang, G. Li, Z. Wu, Nickel-cobalt bimetallic anode catalysts for direct urea fuel cell, Sci. Rep. 4 (2014). https://doi.org/10.1038/srep05863.
[7]
W. Yan, D. Wang, G.G. Botte, Nickel and cobalt bimetallic hydroxide catalysts for urea electrooxidation, Electrochim. Acta. 61 (2012) 25–30. https://doi.org/10.1016/j.electacta.2011.11.044.
9
[8]
R. Ding, J. Liu, J. Jiang, F. Wu, J. Zhu, X. Huang, Tailored Ni-Cu alloy hierarchical porous nanowire as a potential efficient catalyst for DMFCs, Catal. Sci. Technol. 1 (2011) 1406–1411. https://doi.org/10.1039/c1cy00164g.
[9]
B. Li, C. Song, J. yin, J. Yan, K. Ye, K. Cheng, K. zhu, D. Cao, G. Wang, Effect of graphene on the performance of nickel foam-based CoNi nanosheet anode catalyzed direct urea-hydrogen peroxide fuel cell, Int. J. Hydrogen Energy. (2019). https://doi.org/10.1016/j.ijhydene.2019.05.158.
[10]
F. Guo, K. Cheng, K. Ye, G. Wang, D. Cao, Preparation of nickel-cobalt nanowire arrays anode electro-catalyst and its application in direct urea/hydrogen peroxide fuel cell, Electrochim. Acta. 199 (2016) 290–296. https://doi.org/10.1016/j.electacta.2016.01.215.
[11]
C. Li, Y. Liu, Z. Zhuo, H. Ju, D. Li, Y. Guo, X. Wu, H. Li, T. Zhai, Local Charge Distribution Engineered by Schottky Heterojunctions toward Urea Electrolysis, Adv. Energy Mater. 8 (2018) 1–8. https://doi.org/10.1002/aenm.201801775.
[12]
N.A.M. Barakat, M.H. El-Newehy, A.S. Yasin, Z.K. Ghouri, S.S. Al-Deyab, Ni&Mn nanoparticlesdecorated carbon nanofibers as effective electrocatalyst for urea oxidation, Appl. Catal. A Gen. 510 (2016) 180–188. https://doi.org/10.1016/j.apcata.2015.11.015.
[13]
W. Yan, D. Wang, G.G. Botte, Electrochemical decomposition of urea with Ni-based catalysts, Appl. Catal. B Environ. 127 (2012) 221–226. https://doi.org/10.1016/j.apcatb.2012.08.022.
[14]
L. Wang, M. Li, Z. Huang, Y. Li, S. Qi, C. Yi, B. Yang, Ni-WC/C nanocluster catalysts for urea electrooxidation, J. Power Sources. 264 (2014) 282–289. https://doi.org/10.1016/j.jpowsour.2014.04.104.
[15]
W. Xu, D. Du, R. Lan, J. Humphreys, Z. Wu, S. Tao, Highly active Ni-Fe double hydroxides as anode catalysts for electrooxidation of urea, New J. Chem. 41 (2017) 4190–4196. https://doi.org/10.1039/c6nj04060h.
[16]
R.K. Singh, A. Schechter, Electroactivity of NiCr Catalysts for Urea Oxidation in Alkaline Electrolyte, ChemCatChem. 9 (2017) 3374–3379. https://doi.org/10.1002/cctc.201700451.
[17]
R.M. Tesfaye, G. Das, B.J. Park, J. Kim, H.H. Yoon, Ni-Co bimetal decorated carbon nanotube aerogel as an efficient anode catalyst in urea fuel cells, Sci. Rep. 9 (2019) 1–9. https://doi.org/10.1038/s41598-018-37011-w.
10
[18]
T.Q.N. Tran, G. Das, H.H. Yoon, Nickel-metal organic framework/MWCNT composite electrode for non-enzymatic urea detection, Sensors Actuators, B Chem. 243 (2017) 78–83. https://doi.org/10.1016/j.snb.2016.11.126.
[19]
L. Ren, K.S. Hui, K.N. Hui, Self-assembled free-standing three-dimensional nickel nanoparticle/graphene aerogel for direct ethanol fuel cells, J. Mater. Chem. A. 1 (2013) 5689– 5694. https://doi.org/10.1039/c3ta10657h.
[20]
K. Ye, D. Zhang, F. Guo, K. Cheng, G. Wang, D. Cao, Highly porous nickel@carbon sponge as a novel type of three-dimensional anode with low cost for high catalytic performance of urea electro-oxidation in alkaline medium, J. Power Sources. 283 (2015) 408–415. https://doi.org/10.1016/j.jpowsour.2015.02.149.
[21]
M. Shalom, D. Ressnig, X. Yang, G. Clavel, T.P. Fellinger, M. Antonietti, Nickel nitride as an efficient electrocatalyst for water splitting, J. Mater. Chem. A. 3 (2015) 8171–8177. https://doi.org/10.1039/c5ta00078e.
[22]
B. Saha, G.C. Schatz, Carbonization in polyacrylonitrile (PAN) based carbon fibers studied by reaxff molecular dynamics simulations, J. Phys. Chem. B. 116 (2012) 4684–4692. https://doi.org/10.1021/jp300581b.
[23]
Q. Lin, Y. Li, M. Yang, Polyaniline nanofiber humidity sensor prepared by electrospinning, Sensors Actuators, B Chem. 161 (2012) 967–972. https://doi.org/10.1016/j.snb.2011.11.074.
[24]
P.M. Bulemo, H.-J. Cho, N.-H. Kim, I.-D. Kim, Mesoporous SnO 2 Nanotubes via Electrospinning–Etching Route: Highly Sensitive and Selective Detection of H 2 S Molecule, ACS Appl. Mater. Interfaces. 9 (2017) 26304–26313. https://doi.org/10.1021/acsami.7b05241.
[25]
L. Wang, Y. Yu, P.C. Chen, D.W. Zhang, C.H. Chen, Electrospinning synthesis of C/Fe3O4 composite nanofibers and their application for high performance lithium-ion batteries, J. Power Sources. 183 (2008) 717–723. https://doi.org/10.1016/j.jpowsour.2008.05.079.
[26]
T. Tamura, H. Kawakami, Aligned electrospun nanofiber composite membranes for fuel cell electrolytes, Nano Lett. 10 (2010) 1324–1328. https://doi.org/10.1021/nl1007079.
[27]
S. Agarwal, J.H. Wendorff, A. Greiner, Use of electrospinning technique for biomedical applications, Polymer (Guildf). 49 (2008) 5603–5621. https://doi.org/10.1016/j.polymer.2008.09.014.
11
[28]
Y.S. Li, T.S. Zhao, Z.X. Liang, Effect of polymer binders in anode catalyst layer on performance of alkaline direct ethanol fuel cells, J. Power Sources. 190 (2009) 223–229. https://doi.org/10.1016/J.JPOWSOUR.2009.01.055.
[29]
M.H. Tran, B.J. Park, B.H. Kim, H.H. Yoon, Mesoporous silica template-derived nickel-cobalt bimetallic catalyst for urea oxidation and its application in a direct urea/H2O2 fuel cell, Int. J. Hydrogen Energy. 45 (2020) 1784–1792. https://doi.org/10.1016/j.ijhydene.2019.11.073.
[30]
B.T. Sone, E. Manikandan, A. Gurib-Fakim, M. Maaza, Single-phase α-Cr2O3nanoparticles’ green synthesis using Callistemon viminalis’ red flower extract, Green Chem. Lett. Rev. 9 (2016) 85–90. https://doi.org/10.1080/17518253.2016.1151083.
[31]
N. Srivastava, P.C. Srivastava, Realizing NiO nanocrystals from a simple chemical method, Bull. Mater. Sci. 33 (2010) 653–656. https://doi.org/10.1007/s12034-011-0142-0.
[32]
T. Ishimasa, H.U. Nissen, Y. Fukano, New ordered state between crystalline and amorphous in Ni-Cr particles, Phys. Rev. Lett. 55 (1985) 511–513. https://doi.org/10.1103/PhysRevLett.55.511.
[33]
A.R. Sethuraman, R.J. De Angelis, P.J. Reucroft, Diffraction studies on Ni-Co and Ni-Cr alloy thin films, J. Mater. Res. 6 (1991) 749–754. https://doi.org/10.1557/JMR.1991.0749.
[34]
M. Hassel, I. Hemmerich, H. Kuhlenbeck, H.-J. Freund, High Resolution XPS Study of a Thin Cr 2 O 3 (111) Film Grown on Cr(110) , Surf. Sci. Spectra. 4 (1996) 246–252. https://doi.org/10.1116/1.1247795.
[35]
K. Yang, Q. Yao, W. Huang, X. Chen, Z.H. Lu, Enhanced catalytic activity of NiM (M = Cr, Mo, W) nanoparticles for hydrogen evolution from ammonia borane and hydrazine borane, Int. J. Hydrogen Energy. 42 (2017) 6840–6850. https://doi.org/10.1016/j.ijhydene.2016.12.029.
[36]
W. Shi, R. Ding, X. Li, Q. Xu, E. Liu, Enhanced performance and electrocatalytic kinetics of NiMo/graphene nanocatalysts towards alkaline urea oxidation reaction, Electrochim. Acta. 242 (2017) 247–259. https://doi.org/10.1016/j.electacta.2017.05.002.
[37]
D. Wang, S.H. Vijapur, Y. Wang, G.G. Botte, NiCo2O4 nanosheets grown on current collectors as binder-free electrodes for hydrogen production via urea electrolysis, Int. J. Hydrogen Energy. 42 (2017) 3987–3993. https://doi.org/10.1016/j.ijhydene.2016.11.048.
[38]
Y. Liang, Q. Liu, A.M. Asiri, X. Sun, Enhanced electrooxidation of urea using NiMoO4·xH2O nanosheet arrays on Ni foam as anode, Electrochim. Acta. 153 (2015) 456–460.
12
https://doi.org/10.1016/j.electacta.2014.11.193. [39]
R. Ding, L. Qi, M. Jia, H. Wang, Facile synthesis of mesoporous spinel NiCo2O4 nanostructures as highly efficient electrocatalysts for urea electro-oxidation, Nanoscale. 6 (2014) 1369–1376. https://doi.org/10.1039/c3nr05359h.
[40]
E.T. Sayed, T. Eisa, H.O. Mohamed, M.A. Abdelkareem, A. Allagui, H. Alawadhi, K.J. Chae, Direct urea fuel cells: Challenges and opportunities, J. Power Sources. 417 (2019) 159–175. https://doi.org/10.1016/j.jpowsour.2018.12.024.
[41]
J.B. Aellen, R.F. Larry, Electrochemical methods fundamentals and applications, 2001. https://doi.org/10.1016/B978-0-12-381373-2.00056-9.
[42]
G. Das, R.M. Tesfaye, Y. Won, H.H. Yoon, NiO-Fe2O3 based graphene aerogel as urea electrooxidation catalyst, Electrochim. Acta. 237 (2017) 171–176. https://doi.org/10.1016/j.electacta.2017.03.197.
[43]
R.M. Abdel Hameed, S.S. Medany, Influence of support material on the electrocatalytic activity of nickel oxide nanoparticles for urea electro-oxidation reaction, J. Colloid Interface Sci. 513 (2018) 536–548. https://doi.org/10.1016/j.jcis.2017.11.032.
[44]
V. Vedharathinam, G.G. Botte, Understanding the electro-catalytic oxidation mechanism of urea on nickel electrodes in alkaline medium, Electrochim. Acta. 81 (2012) 292–300. https://doi.org/10.1016/j.electacta.2012.07.007.
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Figure 1. (a) XRD patterns of NiCr-CNT@C catalysts with different Ni/Cr ratio and XPS spectra of Ni6Cr4-CNT@C sample showing (b) Cr 2p region and (c) Ni 2p region.
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Figure 2. SEM image of Ni6Cr4-CNT-PAN precursor (a); SEM (b), a cross-sectional FIB SEM (c), TEM (d), HR-TEM (e) image of Ni6Cr4-CNT@C; EDS elemental mapping of Ni6Cr4-CNT@C, corresponding TEM image (f), for C (g), Ni (h), and Cr (k).
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Figure 3. CV plots of NixCry-CNT@C samples in 1 M KOH with/without 0.33 M urea at a scan rate of 20 mV s-1 (a-e) and corresponding onset potentials and peak current densities for UOR (at the 0.62 V with 0.33 M urea) as a function of Cr content.
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Figure 4. CA plot of NiCr-CNT@C samples in 1 M KOH and 0.33 M urea at 0.4 V.
Figure 5. Nyquist plots of NiCr-CNT@Cs at 0.4 V in 1 M KOH and 0.33 M urea solution with a frequency range of 0.05 Hz to 1 MHz.
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Figure 6. (a) CV plots of Ni6Cr4-CNT@C in 1 M KOH and 0.33 M urea with different scan rates of 10 to 120 mV s-1 and (b) CA plots of Ni6Cr4-CNT@C at different potentials from 0.35 to 0.70 V in 1 M KOH with/without urea (0.33 M).
Figure 7. Performance of a DUHpFC using Ni6Cr4-CNT@C as anode; (a) effect of KOH concentration (0.33 M urea) at 20 °C, (b) effect of urea concentration (5 M KOH) at 20 °C, (b) effect of temperature (1 M urea, 5 M KOH); (d) stability test of a DUHpFC at a discharge current density of 20 mA cm-2 at 80 °C with 1 M urea in 5 M KOH solution as fuel.
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Figure 8. Nyquist plots of a DUHpFC with Ni6Cr4-CNT@C as anode using (a) 1 M and 3 M urea solution in 5 M KOH as anolytes at 20 °C, and (b) 1 M urea in 5 M KOH at different temperatures, in the frequency range of 0.01 Hz to 1 MHz at open circuit voltage. Each Inset figure shows an enlarged Nyquiet plots in the high frequency range.
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Highlights A free standing NiCr catalyst mat was synthesized by electrospinning technique. NiCr bimetal nanoparticles were uniformly embedded in the carbon nanofibers. The NiCr catalyst featured excellent electro-catalytic activity for urea oxidation. A urea fuel cell with NiCr catalyst exhibited a power density of 48.1 mW cm–2.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: