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Exploring the Composition-Activity Relation of Ni-Cu Binary Alloy Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Media Gongwei Wang, Wenzheng Li, Bing Huang, Li Xiao, Juntao Lu, and Lin Zhuang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02206 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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ACS Applied Energy Materials

Exploring the Composition-Activity Relation of Ni-Cu Binary Alloy Electrocatalysts for Hydrogen Oxidation Reaction in Alkaline Media

Gongwei Wanga, Wenzheng Lia, Bing Huanga, Li Xiaoa*, Juntao Lua, Lin Zhuangab

aCollege

of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China

bInstitute

for Advanced Studies, Wuhan University, Wuhan 430072, China *Corresponding

authors. E-mail: [email protected]

Abstract The development of non-precious electrocatalysts for hydrogen oxidation reaction (HOR) in alkaline is becoming one of the major obstacles to alkaline polymer electrolyte fuel cells (APEFCs). In this work, a series of Ni-Cu binary alloy films are high-throughput prepared using a combinatorial magnetron co-sputtering method, and their corresponding electrocatalytic performance towards HOR in alkaline media are systemically studied. Both the intrinsic activity and anti-oxidation property of Ni are remarkably enhanced after Cu doping. A volcanotype relation between HOR activity and Cu doping content is revealed. The maximum activity is found at ca. 40 at. % Cu, with the exchange current density 4 times higher than that of pure Ni. This study gives us a deeper understanding of the impact of Cu addition on the HOR catalytic activity of Ni.

Keywords: Hydrogen oxidation reaction; Alkaline polymer electrolyte fuel cells; Ni-Cu; Magnetron sputtering; Volcano-shape relationship.

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1. Introduction Over the past ten years, alkaline polymer electrolyte fuel cells (APEFCs) have attracted great attention due to their potential of using non-precious catalysts in a less corrosive environment than proton-exchange membrane fuel cells (PEMFCs).1-5 Tremendous efforts have been devoted to search for non-precious catalysts with high activity and durability towards the cathodic oxygen reduction reaction (ORR) in alkaline media,6-9 and various earth-abundant materials with comparable or even superior ORR electrocatalytic performance to Pt have been discovered, such as nitrogen doped carbon composites10 and transition metallic oxides11. In contrast, studies on the anodic hydrogen oxidation reaction (HOR) in alkaline media are still relatively rare, to say nothing of non-precious HOR catalysts. The corresponding mechanism is also not well understood.12 Davydova et al. recently made a comprehensive review on the alkaline HOR electrocatalysis.13 Ni is regarded as the most active non-precious monometallic HOR catalyst. Nevertheless, its electrocatalytic activity is significantly inferior relative to that of Pt, and meanwhile it can hardly maintain activity at a relatively high anode overpotential (> 0.1 V vs. reversible hydrogen electrode, RHE) owing to surface oxidation, limiting durability and achievable power density of APEFCs.14-16 A feasible way to overcome these problems is to develop Ni-based composite catalysts. For example, several Ni-based binary and ternary catalysts contained Cr, Ti, La, Cu, Mo, Fe, Ag and Co were found to process superior HOR catalytic activity to pure Ni.17-22 Zhuang et al. reported that the exchange current density of nitrogen-doped carbon nanotubes supported Ni nanoparticles (Ni/N-CNT) is 21 times higher than that of pure Ni nanoparticles.23 Davydova et al. systematically studied carbon supported nanoparticles of binary Ni - first row transition metal (from Sc to Zn) composities towards alkaline HOR by a combined computational and experimental approach. They found that the Ni3Fe1/C exhibited a record-high in exchange current density of 0.06 mA cm-2Ni.24 Despite these progresses, their performance are still far from satisfactory. Previous studies revealed a volcano-type relation between hydrogen binding energy and HOR activity of various metal catalysts, in which Ni and Cu locate two sides of the apex separately.25-27 Alloying Ni with Cu is expected to optimize the hydrogen binding energy and further enhance the HOR catalytic activity, and the stable nature of Cu is also expected to improve the anti-oxidation property of Ni. However, the preparation of Ni-Cu alloys is


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challenging, because of the immiscible nature of Ni and Cu in almost whole compositional range making Ni-Cu alloys thermodynamically unstable. For example, Oshchepkov et al. reported carbon supported Ni1-xCux binary catalysts with various Ni/Cu atomic ratios as HOR catalysts.28 The phase segregation could be clearly observed according to the X-ray diffraction (XRD) patterns of as-prepared samples, indicating the Ni-Cu catalysts are not completely alloyed. Also, size or morphology effect induced by the differences between each powder sample is unable to be excluded. To study the intrinsic HOR electrocatalytic performance on Ni-Cu binary alloy, as well as exclude the size and morphology effects to the catalyst performance which commonly exist in nano-catalyst, a series of well-defined planar electrodes with similar morphology will be an optimal choice. Magnetron co-sputtering enables the uniform mixing of metal atoms (or clusters) at a low temperature, facilitating the formation of thermodynamically unstable phases. Moreover, the amount of deposition is usually inversely proportional to the target-to-substrate distance. It’s thus possible to obtain binary alloy with different composition by adjusting the relative position of the substrate and the two targets. In this study, a set of Ni-Cu binary alloy planar electrodes with various compositions were high-throughput prepared by combinatorial co-sputtering deposition of Ni and Cu. The composition-activity relation of Ni-Cu binary alloy towards HOR was systematically conducted and revealed for the first time under alkaline media. 2. Experimental Section Preparation of Ni-Cu binary alloy, pure Ni and Cu electrodes. Ni-Cu binary alloy planar electrodes were prepared by a combinatorial direct current (DC) magnetron sputtering system as illustrated in Scheme S1,29 similar to the co-sputtering mode as reported by König et al.30 The sputtering chamber was firstly evacuated for 2 hours until reaching a base pressure of 4×10-4 Pa. When sputtering deposition, it was fed with high purity argon gas at a flow rate of 50 sccm and the chamber pressure was adjusted to be 1 Pa. The sample holder installed with substrates was placed ~ 12 cm away from targets to avoid samples overheating. In that case, the temperature of sample holder could keep below 30 oC throughout sputtering and facilitate the formation of thermodynamically unstable Ni-Cu alloys. Meanwhile, the sputtering efficiency is still acceptable at this distance. Pure Ni (99.95%) and Cu (99.95%) disks were used as targets to co-sputter an upper bimetallic film. The holder remains stationary during the



sputtering deposition. Since the deposited amount decreases monotonously with the increase of the distance between substrate and target, Ni-Cu alloys with various continuous compositions along the two targets were obtained. The compositional range depends on relative sputtering powers of the two targets. In order to facilitate the following electrochemical test, seven interchangeable gold rotating disk electrode (φ = 5 mm, Au iRDE) were chose as substrates and evenly arranged on the sample holder. All these iRDE were polished with an alumina suspension (0.05 mm) and repeatedly washed with water and ethanol in advance. The Ni-Cu samples prepared by two independent sputtering were denoted as Ax and Bx (x = 1, 2, 3, 4, 5, 6, 7) respectively. Pure Ni and Cu electrodes were separately deposited on Au iRDE in a similar way. The details of sputtering currents, voltage and time are provided in Table S1. Materials Characterizations. X-ray diffraction (XRD) was carried out using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation source (λ = 1.5406 Å). Seven glass substrates were placed adjacent to the Au iRDEs during sputtering deposition. The corresponding films on glass substrates were characterized by XRD in order to avoid the interference of diffraction peaks from Au substrate. X-ray fluorescence (XRF) was analyzed with a Shimadzu EDX-720 instrument using Rh X-ray tube operated at 50 kV and 30 μA. X-ray photoelectron spectroscopy (XPS) was collected with a Thermo Scientific Escalab 250Xi using Al Kα X-ray source. Atomic force microscopy (AFM) analysis was carried out using a Bruker MultiMode8 system with SCANASYST-AIR probe (f0 = 70 kHz, k = 0.4 N/m). Electrochemical Measurements. Electrochemical experiments were carried out in a threeelectrode system using a CHI-600 potentiostat equipped with a rotating disk electrode (RDE) system (Pine Research Instruments). The Au iRDE can be assembled to a standard RDE and directly utilized as the working electrode. The counter electrode and reference electrode were a sheet of graphite paper (Toray) and a homemade reversible hydrogen electrode (RHE), respectively. Each electrode successively underwent the following electrochemical measurements: (a) hydrogen evolution / oxidation reaction (HER/HOR) polarization in H2saturated 0.1 M KOH within the potential range of -0.1 V and 0.1 V vs. RHE; (b) HOR/HER polarization in H2-saturated 0.1 M KOH within the potential range of -0.1 V and 0.5 V vs. RHE; (c) CV in Ar-deaerated 0.1 M KOH within the potential range of -0.1 V and 0.5 V vs. RHE. The scan rate was 10 mV/s and the rotation speed was 2500 rpm (rotation per minute). All


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solutions were prepared in ultrapure water (> 18 MΩ·cm), and all measurements were carried out at 25 oC. 3. Results and Discussion The crystalline structure of as-sputtered pure Ni, pure Cu and Ni-Cu bimetallic films (Ax and Bx) were characterized by XRD (Figure 1a). Since both Ni and Cu have a face centered cubic (f.c.c) structure, Ni-Cu binary alloys are expected to have an f.c.c structure as well. The diffraction peaks between 42o and 46o for all samples can be ascribed to (111) plane of f.c.c structure. All samples exhibit only the peaks of (111) plane, indicating a highly oriented growth of as-deposited films perpendicular to the substrates. Given that Cu is larger in atomic size than Ni, the doping of Cu will induce an increase in lattice parameter and thus a shift towards lower diffraction angle. As expected, the peaks of Ni-Cu samples shift monotonously between those of pure Ni and Cu (Figure 1b), suggesting the formation of Ni-Cu binary alloy. No recognizable peaks assigned to any impurities or phase-separated structures were detected. Yet it is necessary but not sufficient to determine the complete alloying of Ni and Cu, such as the possible existence of either very small crystallite or amorphous phase with no diffraction peak. The compositions of as-sputtered Ni-Cu alloys could be obtained according to their lattice parameter and Vegard's law (Eq. 1, Table S2). a = (1 ― 𝑥𝐶𝑢) ∙ a𝑁𝑖 + 𝑥𝐶𝑢 ∙ a𝐶𝑢

(1)

Where a is the lattice parameter of Ni-Cu alloy which can be estimated from the (111) peak position. aNi and aCu are the lattice parameters of pure Ni and Cu, respectively. xCu is the atomic fraction of Cu in Ni-Cu alloy. The bulk and surface composition of as-deposited Ni-Cu films were also determined by XRF and XPS analysis (Figure S1, S2 and Table S3), respectively. As shown in Figure 1c and 1d, the bulk and surface compositions are very close to each other, suggesting no surface segregation of Ni or Cu. Moreover, the resulting alloy compositions are also in line with the corresponding bulk or surface compositions, confirming the complete alloying of Ni and Cu with no phase separation in all nominal compositions. SEM (Figure 2) and AFM (Figure S3) were used to observe the morphology of as-sputtered films. The surface roughness values (Ra and Rq) extracted from AFM analysis are presented in Figure S4. It’s clearly that all samples are flat with similar surface morphology. A rotating disk electrode (RDE) method was applied to study the HOR electrocatalytic



performance. Figure 3a shows the HER/HOR polarization curves for as-sputtered Ni, Cu and Ni-Cu alloys after iR-correction in H2-saturated 0.1 M KOH. The electrocatalytic activities of a catalyst is often normalized to its electrochemical surface area (ESA), which represents the intrinsic activity. The ESA of Ni can be estimated according to the formation of α-Ni(OH)2 using a charge density of 514 μC·cm−2.31 However, it is hardly possible to determine the ESA of Ni-Cu alloy using normal electrochemical approach. Given that all deposited electrodes are flat and the difference in surface roughness is neglectable (Figure 2, S3 and S4), the HOR electrocatalytic activities of Ni, Cu and Ni-Cu alloys were compared in terms of exchange current densities (i0), which were calculated by normalizing the exchange currents directly to their geometric areas. The exchange currents were determined from the linearization of the HOR/HER micro-polarization region between -10 mV and +10 mV.23 As illustrated in Figure 3b, a volcano-type relation between HOR activities and Cu atomic fraction (xCu) is clearly observed. Maximum activity is found at ca. 40% content of Cu (electrode No. Ni-Cu B1), by which 4 times larger i0 than that of pure Ni is attained. The ESA of Ni (ESANi) for Ni-Cu B1 was further estimated to be 3.3 cm2 (Figure S5). Considering the low activity of Cu in HOR, the exchange current density on Ni sites (i0’, normalized to ESANi) was determined to be 0.034 mA/cm2Ni, which is close to that of state-of-the-art non-precious catalysts.23, 28 Recently, some Ni-Cu binary nanocatalysts with incomplete alloying (for example, Ni3Cu1/C reported Davydova et al.24) were reported to have a little bit higher i0’ relative to Ni-Cu B1. It could be ascribed to the size or morphology effect. Therefore, we believe that the activity of completely alloyed Ni-Cu (with 40 at. % Cu) could be further enhanced by preparing highly-dispersed NiCu alloy nanocatalyst with optimized size or morphology. We further increased the upper potential limit to 0.5 V to evaluate the anti-oxidation properties of as-sputtered films. Take the most active electrode (Ni-Cu B1) for instance, Figure S6 compares the polarization curves in Ar- and H2-saturated 0.1 M KOH. The anodic current was clearly increased in H2 compared to Ar atmosphere, indicating that H2 is a reactant for the reaction. The net HOR current could be obtained by subtracting the polarization curve in Ar from that of in H2 atmosphere (Figure 4a). The Ni-Cu B1 displayed a higher net current than other electrodes, and it could remain a catalytic effect until the potential over 0.35 V (vs. RHE). In contrast, the net current for pure Ni was very low in the whole potential range. Noticeably,


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almost all the net current curves for Ni-Cu alloys exhibited two anodic peaks. The peak 1 can be attributed to the HOR and Ni-OHad formation, while the peak 2 can be attributed to the HOR and Ni–(OH)2,ad formation.16 When the polarization potential is high enough (e.g. > 0.35 V vs. RHE for Ni-Cu B1), the surface Ni active sites will be completely blocked by (OH)2,ad and thus the HOR net current tends to be zero. The peak potential was utilized as a descriptor of surface passivation. Figure 4b shows the relation between peak potential and Cu content. Interestingly, a significant upshift of the peak potential can be observed along with increasing Cu content, suggesting that adding Cu into Ni catalysts improves their tolerance to oxidation. 4. Conclusions In summary, a series of complete alloyed Ni-Cu films were high-throughput prepared by a combinatorial magnetron co-sputtering method and well characterized by structural, compositional and morphological analysis. The composition-activity relation of Ni-Cu binary alloy towards HOR in alkaline media was explored and a volcano-type relation between HOR exchange current density (i0, normalized to geometric area) and Cu atomic fraction (xCu) was obtained. A maximum activity was observed when the Cu atomic fraction is ca. 40 at. %. Moreover, the tolerance to oxidation at relatively high overpotential was clearly enhanced after Cu doping. In future research efforts, highly-dispersed Ni-Cu nanocatalyst with well-defined structure and composition (40 at. % Cu) will be synthesized and applied as anodic catalyst of APEFCs.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Scheme of the combinatorial sputtering of Ni and Cu; Detailed sputtering conditions; XRF and XPS spectra; AFM images and roughness factor values; CV of Ni-Cu B1; ESANi; Volcano-type relation between i0’ and Cu atomic fraction.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21872108, 21633008, 21573167 and 91545205), Wuhan University Innovation Team (2042017kf0232), the National Key Research and Development Program (2016YFB0101203), the Fundamental



Research Funds for the Central Universities (2014203020207).


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Figure 1. (a) XRD patterns of pure Ni, pure Cu and Ni-Cu binary alloy with different composition (A1 to A7, B1 to B7). (b) Normalized fitting diffraction peaks between 42o and 46o marked with dashed frame in (a). (c, d) Corresponding composition of each sample obtained from XRF, XPS and XRD analysis.



Figure 2. SEM images of pure Ni, pure Cu and Ni-Cu binary alloys with different composition (A1 to A7, B1 to B7).


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Figure 3. (a) The polarization curves of HER/HOR on pure Ni, pure Cu and Ni-Cu binary alloys after iR-correction in H2-saturated 0.1 M KOH collected at a sweep rate of 10 mV/s and a rotation rate of 2500 rpm (positive-going sweep). (b) The relation between HER/HOR exchange current densities (i0) calculated from (a) and Cu atomic fraction analyzed by XRF.



Figure 4. (a) The net HOR currents of Ni, Cu and Ni-Cu binary alloys. The relation between anodic peak potential and Cu atomic fraction (b).


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