(621ar) Transition Metal Phosphide Thin Films: Applications As Catalysts and Protecting Layers in Photoelectrochemical Water Splitting Devices

Photoelectrochemical (PEC) water splitting is a promising route for synthesizing hydrogen using solar energy without direct CO₂ emissions.¹ PEC water splitting devices require highly active hydrogen evolution reaction (HER) catalysts to efficiently produce hydrogen at the photocathode.² Platinum and its alloys are the most active HER catalysts but platinum is an expensive precious metal.³ There is a need to replace platinum with a non-precious metal earth-abundant alternative to enable cost effective, scalable devices.⁴

Recently, transition metal phosphide catalysts have been shown to be among the most active nonprecious HER catalysts on a total electrode activity and turnover frequency (TOF) basis.⁵Though extremely active, the high surface area morphologies of these previously synthesized electrodes present additional challenges for integration onto PEC water splitting electrodes. As compared to nanostructured catalysts, very thin, conformal catalysts can enhance PEC water splitting device performance by forming superior interfaces and allowing a larger portion of the incident light to be absorbed.

We discuss our progress in synthesizing transition phosphide catalysts in thin, conformal geometries. In so doing, we are able to quantify the true TOF of the transition metal phosphide catalysts, and compare their activity with other highly active non-precious metal HER catalysts such as molybdenum sulfide (MoS₂). We leverage these synthesis techniques to efficiently catalyze the HER and protect a variety of different semiconductors. We compare the device performance of these thin film transition metal phosphide containing photocathodes with other state of the art transition metal phosphide containing photocathodes and suggest general strategies for maximizing device performance.

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