Activating Nickel-Molybdenum Electrocatalyst Via Copper-Element Modification Toward 1 A cm−2 Hydrogen Evolution in Non-Extreme pH Carbonate Buffer Electrolyte
Authors: Nishimoto Takeshi , Obata Keisuke , Takanabe Kazuhiro 
Published: September 2023

Water electrolysis driven by renewable electricity produces green hydrogen, which will play a pivotal role in realizing a sustainable society. However, the production cost of green hydrogen is currently $5.3/kg, nearly four times higher than $1.4/kg of grey hydrogen,[1] which necessitates the reduction of green hydrogen cost toward its large-scale deployment. While reducing electrolyzer cost is one of the key interventions,[2] extremely acidic or alkaline conditions in currently commercialized electrolyzers require expensive corrosion-tolerant materials as the components, which hinders further reduction of electrolyzer cost. In this sense, non-extreme pH water electrolyzer can be innovative technology because its milder conditions than those in currently commercialized electrolyzers potentially enable the use of cheap and abundant materials (such as Fe or stainless steel) as electrolyzer components (such as frame or bipolar plate),[3] which can reduce the electrolyzer cost and, in the end, production cost of green hydrogen. Non-extreme pH electrolysis was reported to suffer inferior performance compared to those in extreme pH conditions.[4] However, our previous study using nickel and iron-based anode in potassium carbonate buffer solution at pH 10.5 achieved high oxygen evolution reaction (OER) performances comparable to those of highly alkaline OER,[5] which should be followed by the development of highly effective cathode for hydrogen evolution reaction (HER) to realize the highly efficient non-extreme pH electrolyzer.
The present study reports on our discovery of the nickel and molybdenum-based electrocatalysts that efficiently catalyze HER in carbonate buffer electrolyte at pH 10.5. First of all, visibly flat films of nickel (Ni), molybdenum oxide (MoOx), and nickel molybdenum oxide (NiMoOx) were prepared by cathodic electrodeposition on rotating disk electrodes. Their electrocatalytic testing revealed that, as in alkaline conditions, the coexistence of Ni and Mo is required to achieve high catalytic activity even in K-carbonate solution at pH 10.5. Subsequently, a variety of transition metal elements (manganese, cobalt, copper, and tungsten) were introduced into NiMoOx via co-electrodeposition onto Ni foam substrate to further enhance the HER performance. Among the developed electrodes, NiMoOx modified with copper element (NiMoOx(Cu)) was the best and achieved 1 A cm−2 at an overpotential of ca. 0.2 V, which is comparable to those of previously reported HER in highly alkaline conditions (Figure 1). Our characterization revealed that Cu addition into NiMoOx made the surface structure to be rougher and led to the largest double-layer capacitance, both of which indicate the enlarged active surface area. Besides, the presence of Cu element decreased the apparent Tafel slope, suggesting the tailored nature of active site, which was consistent with the shift of Ni and Mo peaks in ex situ X-ray photoelectron spectroscopy (XPS) analysis. Finally, combined with previously developed nickel and iron-based anode[5] and carefully selected polyether sulfone (PES) diaphragm, overall cell performance was elevated in 3.0 mol kg−1 K-carbonate solution at pH 10.5 and 353 K. The resultant cell performance including the series resistance achieved 1 A cm−2 at a voltage of merely ca. 2.0 V, competitive to those of top-class commercial alkaline electrolyzers. These findings demonstrate the potential of non-extreme pH electrochemical electrolyzers in industrial applications.
Reference
[1] “Finding the sea of green”, EDISON, 2021.
[2] M. Chatenet, et al., Chem. Soc. Rev.
2022, 51, 4583.
[3] M. Pourbaix, “Atlas of Electrochemical Equilibria in Aqueous solutions”,
1974.
[4] T. Naito, et al., ChemSusChem
2022, 8, e202102294.
[5] T. Nishimoto, et al., ChemSusChem
2022, e202201808.

Figure 1