Development of highly active and robust earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) is of great significance for the broad utilization of alkaline electrolyzers.

Metrics & MoreArticle Recommendations CONSPECTUS: Catalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under neutral-to-alkaline conditions, first-row transitionmetal oxides/(oxy)hydroxides are the fastest-known OER catalysts and have been the subject of intense study for the past decade. Critical to their high performance is the intentional or accidental addition of Fe to Ni/Co oxides that convert to layered (oxy)hydroxide structures during the OER. Unraveling the role that Fe plays in the catalysis and the molecular identity of the true "active site" has proved challenging, however, due to the dynamics of the host structure and absorbed Fe sites as well as the diversity of local structures in these disordered active phases.In this Account, we highlight our work to understand the role of Fe in Ni/Co (oxy)hydroxide OER catalysts. We first discuss how we characterize the intrinsic activity of the first-row transition-metal (oxy)hydroxide catalysts as thin films by accounting for the contributions of the catalyst-layer thickness (mass loading) and electrical conductivity as well as the underlying substrate's chemical interactions with the catalyst and the presence of Fe species in the electrolyte. We show how Fe-doped Ni/Co (oxy)hydroxides restructure during catalysis, absorb/desorb Fe, and in some cases degrade or regenerate their activity during electrochemical testing. We highlight the relevant techniques and procedures that allowed us to better understand the role of Fe in activating other first-row transition metals for OER. We find several modes of Fe incorporation in Ni/Co (oxy)hydroxides and show how those modes correlate with activity and durability. We also discuss how this understanding informs the incorporation of earthabundant transition-metal OER catalysts in anion-exchange-membrane water electrolyzers (AEMWE) that provide a locally basic anode environment but run on pure water and have advantages over the more-developed proton-exchange-membrane water electrolyzers (PEMWE) that use platinum-group-metal (PGM) catalysts. We outline the key issues of introducing Fe-doped Ni/Co (oxy)hydroxide catalysts at the anode of the AEMWE, such as the oxidative processes triggered by Fe species traveling through the polymer membrane, pH-gradient effects on the catalyst stability, and possibly limited catalyst utilization in the compressed stack configuration. We also suggest possible mitigation strategies for these issues. Finally, we summarize remaining challenges including the long-term stability of Fe-doped Ni/Co (oxy)hydroxides under OER conditions and the lack of accurate models of the dynamic active surface that hinder our understanding of, and thus ability to design, these catalysts.

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Efficient and stable bifunctional electrocatalysts composed of Earth-abundant elements are crucial to the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Herein, FeCo 2 S 4 nanosheet arrays loaded on Ni foam (NF) are synthesized and first employed as a bifunctional electrocatalyst for full water-splitting. Remarkably, the self-assembled, binder-free, and cost-effective FeCo 2 S 4 /NF electrode shows high OER catalytic activity, which only requires an overpotential of 270 and 290 mV to achieve current densities of 50 and 100 mA cm −2 , respectively. Moreover, the FeCo 2 S 4 /NF electrode exhibits considerable OER stability over 20 h at a static current density of 50 mA cm −2 , with negligible potential change in alkaline electrolyte. Meanwhile, when serving as a catalyst for the HER under alkaline conditions, an overpotential of just 132 mV is required to deliver the current density of 10 mA cm −2 . The structural investigation demonstrates the formation of a Co(Fe)-(oxy)hydroxides layer on the catalyst surface during the OER test, which could be the real active species. Furthermore, because of the high catalytic activity and stability of this bifunctional electrocatalyst, we prepared a high-performance overall water electrolyzer that could achieve a current density of 10 mA cm −2 at a cell voltage of 1.56 V.

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The development of efficient and stable non-noble-metal electrocatalytic materials for the oxygen evolution reaction (OER) is a huge and important challenge at present.

Layered NbS 2 , a member of group-V transition metal dichalcogenides, was synthesized via a colloidal synthesis method and employed as a negative material for a supercapacitor. The morphologies of NbS 2 can be tuned from ultrathin nanosheets to hierarchical structures through dynamics controls based on growth mechanisms. Electrochemical energy storage measurements present that the ultrathin NbS 2 electrode exhibits the highest rate capability due to having the largest electrochemical surface area and its efficient ion diffusion. Meanwhile, the hierarchical NbS 2 shows the highest specific capacitance at low current densities for small charge transfer resistance, displays 221.4 F g −1 at 1 A g −1 and 117.1 F g −1 at 10 A g −1 , and cycling stability with 78.9% of the initial specific capacitance after 10,000 cycles. The aggregate or stacking of nanosheets can be suppressed effectively by constructing hierarchical structure NbS 2 nanosheets.