Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized

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SUMMARYFor several decades, the main body of research in photoelectrochemical (PEC) hydrogen production has centered on a small number of semiconductor materials classes, including stable but inefficient metal-oxides, as well as some more efficient materials such as III-V compounds which suffer from high cost and poor stability. While demonstrating some limited success in meeting the rigorous PEC demands in terms of bandgap, optical absorption, band-edge alignment, surface energetics, surface kinetics, stability, manufacturability and cost, none of the 'traditional' PEC semiconductors are adequate for application in water-splitting devices with high performance (greater than 15% solar-to-hydrogen conversion) and long durability (greater than 200 h life). As a result, it is widely held that new semiconductor classes and configurations need to be identified and developed specifically for practical implementation of solar water-splitting. Examples include ternary and quaternary metal-oxide compounds, as well as non-oxide semiconductor materials, such as silicon-carbide and the copper-chalcopyrites. This paper describes recent progress at the University of Hawaii to develop improved semiconductor absorbers and interfaces for solar photoelectrolysis based on polycrystalline tungsten trioxide and polycrystalline copper-gallium-diselenide. Specific advantages and disadvantages of both materials classes in terms of meeting long-term PEC hydrogen production goals are detailed.

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Overall solar-to-hydrogen (STH) efficiency is the most important parameter to characterize a PEC device. In fact, materials systems themselves are effectively defined by their highest-recorded STH efficiency; it is the single value by which all PEC devices can be reliably ranked against one another [1]. Unfortunately, published literature in the area of PEC sometimes contains confusing information regarding efficiency including invalid mathematical expressions for device efficiency, improper experimental methods for obtaining efficiency values, and/or wide-scale reporting of efficiencies other than STH without clear distinction. The first goal of this document is to establish proper definitions and mathematical expressions for device efficiencies. Among these definitions, we identify those that are acceptable for wide-scale benchmarking and reporting (for instance in the form of press releases to mainstream media) as well as those definitions which are helpful for their scientific value in material characterization and diagnostic testing (and suitable for journal publications). Later in this document, we overview the proper experimental procedures as well as common pitfalls that concern each type of efficiency measurement.One of the reasons why so much pluralism exists in describing PEC efficiency is that several different measures of efficiency (g) exist; each has a different place in PEC research. Four primary measures of efficiency will be discussed here, which can be split into the two main categories:• Benchmark efficiency (suitable for mainstream reporting of stand-alone water splitting capability) -solar-to-hydrogen conversion efficiency (STH)