Electrochemically fabricated NiCu alloy catalysts for hydrogen production in alkaline water electrolysis
Introduction
Alkaline water electrolysis is a promising method for the production of clean hydrogen energy because it is environment friendly, leading to zero emission of carbon dioxide when combined with renewable energy sources such as solar and wind energy [1], [2], [3], [4], [5], [6]. The challenges associated with commercialization of alkaline water electrolysis include reducing the cost and increasing the efficiency, durability, and safety of the process. Since non-noble metal catalysts can be used in alkaline media, the alkaline water electrolysis is a competitively priced process to other electrolysis methods such as proton exchange membrane water electrolysis and high-temperature steam electrolysis [7], [8], [9], [10], [11], [12], [13]. However, the cost of hydrogen production via alkaline water electrolysis is still higher than that of conventional carbon-based methods (e.g., hydrocarbon steam reforming, coal gasification, and biomass pyrolysis) [14], [15], [16]. Another problem is that HER at the cathode requires a large overpotential because of the lack of a proton source in the alkaline electrolyte. This leads to stability problems primarily associated with long-term and shut down operations [17]. Although there are many ways to overcome these obstacles, the development of highly active catalysts with durability is one method to realize commercially available hydrogen production via alkaline water electrolysis.
To design a highly active catalyst for HER, the hydrogen adsorption and desorption kinetics should simultaneously be considered, which can be characterized by a single parameter of hydrogen binding energy (BEH) at the surface of each catalyst. Therefore, many research groups have investigated the effect of the BEH on catalytic activity via computational and experimental studies [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], and it was revealed that the relationship between exchange current density and BEH has a volcano behavior [33], [34]. According to the works, Ni has the highest exchange current density among the non-noble metals and is considered as a promising candidate for cheap HER catalyst, even though still noble metals of Pt, Ir, and Pd have higher activities. However, as the BEH of Ni is larger than the optimal BEH for HER [20], the overall HER rate on Ni is limited by the hydrogen desorption step. Therefore, the HER activity of Ni catalyst is expected to be further enhanced by decreasing the BEH.
Recently, Ni-based alloys and complex catalysts have been investigated to achieve high catalytic activity and durability as cathode electrocatalysts in alkaline water electrolysis [26], [27], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46]. There are various methods to modify the BEH on metal catalysts, such as controlling the crystal orientation [35], geometrical structure [38], core–shell [39], [40], [41], and forming alloys with other metals [43], [44], [45], [46]. In our previous study, Ni catalysts with various structures were electrodeposited and their catalytic activities toward HER were investigated [35]. As the result, it was revealed that the catalytic activity was related to the difference in the crystal orientations induced by deposition conditions. The electrodeposited Ni electrocatalysts with dendritic structures demonstrated the highest catalytic activity owing to the electronic and geometric effects induced by the high population of (111) facets. Regarding the alloying effect, Greeley et al. reported the effect of the host and solute elements on BEH of the alloy catalyst materials through computational studies based on density functional theory (DFT) [20]. In addition, for NiM alloys, candidate atoms for alloying, such as Ag, Cd, Bi, Pd, Ir, Ru, and Cu, were suggested to enhance HER activity of Ni by decreasing the BEH. Among them, Cu is a suitable material due to the low cost, high corrosion resistance and environmental friendly properties of NiCu [47]. Furthermore, many literature dealing with NiCuM (M = Co, Zn, and Fe) as HER catalysts have been reported [48], [49], [50].
The application of electrodeposited NiCu alloy catalyst for HER in alkaline condition was previously reported by Solmaz et al. with a fixed Cu content of 85.4% [45], [46]. In their study, the electrodeposited NiCu alloy showed 4.0–34 times higher current densities for HER than Ni at various HER overpotentials. They claimed that the enhanced catalytic activity of NiCu alloy was related to its large roughness factor reflecting the real surface area (roughness factor of NiCu alloy was 16.2 times higher than that of Ni) in addition to the synergistic interaction between Ni and Cu. However, obvious evidence about the synergistic interaction related to its material properties was not fully reported. In addition, it can be expected that some critical properties that may affect the activity of electrodeposited NiCu alloy including crystal structure, composition, loading mass, and surface area are altered with Cu contents, remaining necessity of studies on those properties and their effects on the HER.
In this study, NiCu alloy catalysts were prepared by electrodeposition at various deposition potential, and their catalytic activities to HER in alkaline water electrolysis were evaluated. The variations of bulk compositions of electrodeposited electrocatalysts were analyzed by inductively coupled plasma mass spectroscopy (ICPMS) analysis, while the active surface areas and surface compositions were characterized by electrochemical technique. Then, the catalytic activities were correlated with the compositional and structural characteristics of the electrodeposited NiCu alloys, confirming the Cu alloying effect to enhance the HER activity of Ni catalysts.
Section snippets
Experimental
For NiCu alloy deposition, 0.50 M NiSO4·6H2O (Sigma–Aldrich) and 0.05 M CuSO4·5H2O (Sigma–Aldrich) were used as the metal precursors. As a complexing agent, 0.26 M C6H5Na3O7 (Sigma–Aldrich) was added to the electrolyte to prevent the metal ions from acting as autocatalysts [51]. Alloy electrodeposition was carried out using a conventional three-electrode cell system. The glassy carbon (GC, Hochtemperatur-Werkstoffe GmbH) electrode was used as a working electrode. The surface of the working
Results and discussion
LSV analysis was carried out to determine the proper range of deposition potential on the GC electrode in the electrolyte containing Ni2+ (0.50 M), Cu2+ (0.05 M), and 0.26 M complexing agent (citric acid trisodium salt) [51]. As the electrode potential became more negative, the Cu deposition occurred at about −0.25 V, (Fig. 1). When the more cathodic potential was applied (−0.8 V ∼ −0.6 V), the current density did not increase, indicating that the Cu deposition rate was constant due to the
Conclusions
The alloying effect of electrodeposited NiCu alloy catalysts on HER activity was investigated by characterization of material properties and electrochemical measurement of catalytic activity. The compositional and crystallographic changes of NiCu alloy catalysts can be controlled by deposition potential. Among the electrodeposited NiCu alloys, the Ni51Cu49 catalyst achieved the highest current density for HER based on the geometrical area. Interestingly, after normalization of current divided
Acknowledgment
This work was supported by the Joint Research Project funded by the Korea Research Council of Fundamental Science & Technology (KRCF), Republic of Korea (Seed-10-2), by the “COE (Center of Excellence)” program and Institutional Program (contract number 2E22873-12-020) of the Korea Institute of Science and Technology, and by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (No. 2013038315).
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2022, Electrochimica ActaCitation Excerpt :Second, one can see a change in the shape of NiCu nanoparticles from near-spherical (at ED = -0.32 V vs RHE) to the truncated one (at ED = -0.52 V vs RHE). The latter is likely originating from a cumulative effect of accelerated rate of electrodeposition (cf. larger applied overpotential) and involvement of the hydrogen evolution which proceeds in parallel on the surface of deposited metal clusters [32]. Elemental analysis of NiCu deposits using EDS (Table 1) revealed an increase of the Ni fraction with a decrease of the deposition potential.