We discuss recent developments in nanostructured molybdenum sulfide catalysts for the electrochemical hydrogen evolution reaction. To develop a framework for performing consistent and meaningful comparisons between catalysts, we review standard experimental methodologies for measuring catalyst performance and define two metrics used in this perspective for comparing catalyst activity: the turnover frequency, an intrinsic activity metric, and the total electrode activity, a device-oriented activity metric. We discuss general strategies for synthesizing catalysts with improved activity, namely, increasing the number of electrically accessible active sites or increasing the turnover frequency of each site. Then we consider a number of state-of-the-art molybdenum sulfide catalysts, including crystalline MoS 2 , amorphous MoS x , and molecular cluster materials, to highlight these strategies in practice. Comparing these catalysts reveals that most of the molybdenum sulfide catalysts have similar active site turnover frequencies, so the total electrode activity is primarily determined by the number of accessible active sites per geometric electrode area. Emerging strategies to overcome current catalyst limitations and potential applications for molybdenum sulfide catalysts including photoelectrochemical water splitting devices and electrolyzers are also considered.

We demonstrate the translation of a low cost, non-precious metal cobalt phosphide (CoP) catalyst from 1 cm 2 lab-scale experiments to a commercial-scale 86 cm 2 polymer electrolyte membrane (PEM) electrolyser. A 2-step bulk synthesis was adopted to produce CoP on a high surface area carbon support that was readily integrated into an industrial PEM electrolyser fabrication process. The performance of the CoP was compared head-to-head with a platinum-based PEM under the same operating conditions (400 psi, 50 °C). CoP was found to be active and stable, operating at 1.86 A.cm-2 for >1700 hours of continuous hydrogen production while providing substantial material cost savings relative to platinum. This work illustrates a potential pathway for non-precious hydrogen evolution catalysts developed in past decades to translate to commercial applications.

Alloying of Au and Pd leads to new synergistic catalytic effects, providing insights into electrocatalytic CO2 reduction on alloy surfaces.

Transition metal phosphide catalysts have recently emerged as active, earth abundant alternatives to precious metals for the hydrogen evolution reaction in acid. High performance, scalable catalysts are necessary for the successful implementation of photoelectrochemical water splitting devices, which have the potential to generate hydrogen in a sustainable manner. Herein, a general synthetic route is reported to produce transition metal phosphide thin films, which is used to fabricate cobalt phosphide (CoP) catalysts with high average turnover frequency (TOFavg), 0.48 H2 s−1 and 1.0 H2 s−1 at 100 and 120 mV overpotential, respectively. Furthermore, it is shown that CoP thin films can be applied to silicon photoabsorbers to generate one of the most active precious metal‐free crystalline silicon photocathodes to date, achieving −10 mA cm−2 at +0.345 V vs. reversible hydrogen electrode. The synthesis route presented here provides a platform for both fundamental studies of well‐defined electrocatalysts and the fabrication of high‐performance photoelectrodes.

Developing materials, interfaces, and devices with improved stability remains one of the key challenges in the field of photoelectrochemical water splitting. As a barrier to corrosion, molybdenum disulfide is a particularly attractive protection layer for photocathodes due to its inherent stability in acid, the low permeability of its basal planes, and the excellent hydrogen evolution reaction (HER) activity the MoS edge. Here, we demonstrate a stable silicon photocathode containing a protecting layer consisting of molybdenum disulfide, molybdenum silicide, and silicon oxide which operates continuously for two months. We make comparisons between this system and another molybdenum sulfide-silicon photocathode embodiment, taking both systems to catastrophic failure during photoelectrochemical stability measurements and exploring mechanisms of degradation. X-ray photoelectron spectroscopy and transmission electron microscopy provide key insights into the origins of stability.

Molybdenum sulfides have been identified as promising materials for catalyzing the hydrogen evolution reaction (HER) in acid, with active edge sites that exhibit some of the highest turnover frequencies among non-precious metal catalysts. The thiomolybdate [Mo3S13] 2− nanocluster catalyst contains a structural motif that resembles the active site of MoS2 and has been reported to be among the most active forms of molybdenum sulfide. Herein, we improve the activity of the [Mo3S13] 2− catalysts through catalyst-support interactions. We synthesize [Mo3S13] 2− on gold, silver, glassy carbon, and copper supports to demonstrate the ability to tune the hydrogen binding energy of [Mo3S13] 2− using catalyst-support electronic interactions and optimize HER activity.

Tandem photoelectrochemical water splitting cells utilizing crystalline Si and metal oxide photoabsorbers are promising for low-cost solar hydrogen production. We present a device design and a scalable fabrication scheme for a tandem heterostructure photoanode: p + n black-Si / SnO 2 interface / W-doped BiVO 4 / CoPi catalyst. The black-Si not only provides a substantial photovoltage of 550 mV, it also serves as a conductive scaffold to decrease charge transport path lengths within the Wdoped BiVO 4 shell. When coupled with CoP NPs as hydrogen evolution catalysts, the device demonstrates spontaneous water splitting without employing any precious metals, achieving an This article is protected by copyright. All rights reserved. 2average solar-to-hydrogen (STH) efficiency of 0.45% over the course of an hour at pH 7. Our fabrication scheme offers the modularity to optimize individual cell components, e.g. Si nanowire dimensions and metal oxide film thickness, involving steps that are compatible with fabricating monolithic devices. This design we present is general in nature and can be readily adapted to novel, higher performance semiconducting materials beyond BiVO 4 as they become available, which will accelerate the process of device realization.

The development of low-cost hydrogen evolution reaction (HER) catalysts that can be readily integrated into electrolyzers is critical if H2 from renewable electricity-powered electrolysis is to compete cost effectively with steam reforming. Herein, we report three distinct earth-abundant Mo-based catalysts, namely those based on MoSx , [Mo3 S13 ](2-) nanoclusters, and sulfur-doped Mo phosphide (MoP|S), loaded onto carbon supports. The catalysts were synthesized through facile impregnation-sulfidization routes specifically designed for catalyst-device compatibility. Fundamental electrochemical studies demonstrate the excellent HER activity and stability of the Mo-sulfide based catalysts in an acidic environment, and the resulting polymer electrolyte membrane (PEM) electrolyzers that integrate these catalysts exhibit high efficiency and durability. This work is an important step towards the goal of replacing Pt with earth-abundant catalysts for the HER in commercial PEM electrolyzers.