Sci Adv. 2023 Jul; 9(27): eadh2885.

Published online 2023 Jul 5. doi: 10.1126/sciadv.adh2885

PMCID: PMC10321749

PMID: 37406120

Efficient acidic hydrogen evolution in proton exchange membrane electrolyzers over a sulfur-doped marcasite-type electrocatalyst

Xiao-Long Zhang, Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing,1 ,† Peng-Cheng Yu, Data curation, Formal analysis, Investigation, Resources, Validation,1 ,† Xiao-Zhi Su, Data curation, Formal analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing - original draft, Writing - review & editing,2 ,† Shao-Jin Hu, Formal analysis, Validation, Writing - review & editing,3 Lei Shi, Investigation, Resources,1 Ye-Hua Wang, Investigation,1 Peng-Peng Yang, Formal analysis, Resources, Validation,1 Fei-Yue Gao, Conceptualization, Methodology, Visualization,1 Zhi-Zheng Wu, Investigation,1 Li-Ping Chi, Investigation,1 Ya-Rong Zheng, Formal analysis, Investigation,4 and Min-Rui Gao, Conceptualization, Data curation, Funding acquisition, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing📷1 ,*

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Abstract

Large-scale deployment of proton exchange membrane (PEM) water electrolyzers has to overcome a cost barrier resulting from the exclusive adoption of platinum group metal (PGM) catalysts. Ideally, carbon-supported platinum used at cathode should be replaced with PGM-free catalysts, but they often undergo insufficient activity and stability subjecting to corrosive acidic conditions. Inspired by marcasite existed under acidic environments in nature, we report a sulfur doping–driven structural transformation from pyrite-type cobalt diselenide to pure marcasite counterpart. The resultant catalyst drives hydrogen evolution reaction with low overpotential of 67 millivolts at 10 milliamperes per square centimeter and exhibits no degradation after 1000 hours of testing in acid. Moreover, a PEM electrolyzer with this catalyst as cathode runs stably over 410 hours at 1 ampere per square centimeter and 60°C. The marked properties arise from sulfur doping that not only triggers formation of acid-resistant marcasite structure but also tailors electronic states (e.g., work function) for improved hydrogen diffusion and electrocatalysis.

A marcasite-type cobalt diselenide exhibits promise as an efficient cathode in proton exchange membrane water electrolyzers.

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INTRODUCTION

Electrochemical splitting of water into green hydrogen (H2) fuels, especially driven by renewable electricity, offers an elegant path toward carbon-neutral energy society (1, 2). Since proposed in 1789, alkaline water electrolysis has been progressively developed as a matured technology for industrial H2 production, but its limited current density (high ohmic resistance), low partial load range, and low operating pressure are often drawbacks (3, 4). Proton exchange membrane (PEM) water electrolysis that relies on proton transfers can effectively surmount these issues in alkali, but the corrosive acidic environments require the use of expensive platinum group metal (PGM) catalysts, raising the stack cost (4–6). In present-day PEM electrolyzers, carbon-supported platinum (Pt/C) remains the catalyst of choice for the cathodic hydrogen evolution reaction (HER). Thanks to the fast HER kinetics in acid, the Pt loading at the cathode is typical 0.5 to 1.0 mgPt cm−2 per PEM electrolyzer (5), whose cost is lower than that of IrOx catalyst (~2 mgIr cm−2) used at the anode (4); thus, less previous research efforts have been made on the cost reduction at the cathode. While the Pt/C catalyst contributes small to the system cost nowadays, future large-scale H2 production at terawatt range would demand a substantial amount of Pt, which is not sustainable and hampers commercialization of industrial-grade PEM electrolyzers (6, 7).

To enable widespread prevalence of PEM electrolyzers, the Pt loading at the cathode should be substantially reduced or ideally the Pt catalyst should be replaced with PGM-free materials (6, 7). Over the past decade, PGM-free catalysts for the acidic HER have been well explored (6, 8–21), exemplified by the molybdenum disulfide (MoS2) catalyst inspired by nitrogenase enzyme (8–10). Subsequently, numerous transition metal dichalcogenides (11–15) and phosphides (16–20) were designed and synthesized to display decent activities in rotating disk electrode (RDE) measurements. Recently, King et al. (17) have demonstrated a very promising PEM electrolyzer using cobalt phosphide (CoP) as the HER catalyst, illustrating the practical relevance of PGM-free cathode (17). Despite big advances, under acidic environments, PGM-free materials generally show high propensity to chemical and structural changes, and even component dissolution, leading to decreased catalytic performances (4–6). To date, the design of PGM-free HER catalysts with exceptional activity and stability, particularly under realistic PEM electrolysis condition, remains a daunting challenge.

Besides MoS2, transition metals and chalcogens can form many other important groups of minerals in nature, for example, pyrite (11, 12, 22, 23) and marcasite (24, 25). We (22) and others (11, 23) have reported pyrite-type cobalt diselenide (CoSe2) to be promising catalyst for a variety of reactions with notable performances. Furthermore, we recently found that partial transformation of pyrite CoSe2 into marcasite structure enables enhanced HER stability in acidic electrolyte (13). Unfortunately, this pyrite-marcasite mixed CoSe2 catalyst still falls short of the requirement of practical PEM electrolysis. Yet, in nature, marcasite minerals often form under very acidic environments (26); moreover, the dissolution rate was experimentally observed to be more than 10-fold lower for marcasite than for pyrite (27). We therefore reason that marcasite-type CoSe2 might show the potential as cathode for PEM electrolyzers, given its corrosion-resistant feature in acid.

Here, we report a complete structural transformation from pyrite- to marcasite-type CoSe2 driven by sulfur doping. Not only creating marcasite structure with improved acid stability, but also the doping effect caused by partial substitution of Se with S also tailors electronic structures of the catalyst for more favorable hydrogen diffusion in the electrical double layer (EDL) and adsorption on the catalytic surface. The resultant sulfur-doped marcasite CoSe2 (M-CoSe2) as cathode was demonstrated to be active and stable in a practical PEM electrolyzer, highlighting potential of PGM-free catalysts for commercial-scale water electrolysis.