Roxanne Garland

Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols

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

UV-Vis Spectroscopy

In a UV-Vis (ultraviolet-visible light) spectroscopic measurement, light absorption as a function of wavelength provides information about electronic transitions occurring in the material. For semiconductors, UV-Vis spectroscopy offers a convenient method of estimating the optical band gap, since it probes electronic transitions between the valence band and the conduction band. Transmission UV-Vis, Diffuse Reflectance UV-Vis, and Absorption UV-Vis configurations are discussed.

Flat-Band Potential Techniques

It is important to determine the conductivity and flat-band potential (Efb) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. The three different techniques that can estimate the Efb are: Illuminated OCP, Mott–Schottky and Photocurrent Onset. The Efb should be independent of the technique used to determine it. Due to the inherent shortcomings of each technique, there is often a lack of agreement of the values determined by the various analyses. Researchers should be aware of these limitations in interpreting results.

Efficiency Definitions in the Field of PEC

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.

PEC Characterization Flowchart

The goal of PEC materials development is to design a material system that has the potential to satisfy most, if not all, of the requirements for cost-effective PEC hydrogen production. Figure 1 presents a recommended flowchart for the characterization of candidate PEC materials. The key knowledge gained as well as limitations of the different characterization techniques, highlighted in Table 1, are described in detail in following sections of this document. A material that can survive the rigorous testing set forth in this flowchart will be a particularly promising candidate for incorporation into an industrially deployable device for PEC hydrogen production. This organized approach to PEC characterization is intended to streamline the process for material screening so that discovery of promising candidates occurs at a faster and more orderly pace.

Incident Photon-to-Current Efficiency and Photocurrent Spectroscopy

The incident photon-to-current efficiency (IPCE) is a measure of the ratio of the photocurrent (converted to an electron transfer rate) versus the rate of incident photons (converted from the calibrated power of a light source) as a function of wavelength. Measuring the IPCE is also useful to determine the band gap. The band gap derived from IPCE may be higher than that obtained by optical spectroscopy techniques. Applied bias IPCE and white light bias IPCE experiments are discussed. Photocurrent spectroscopy examines the photocurrent produced by an electrochemical cell as a function of wavelength of the incident light. The optical bulk band gap of the semiconductor electrode can be determined along with information about whether it is a direct or indirect transition.

Hydrogen and Oxygen Detection from Photoelectrodes

The chemical products of PEC water splitting processes are the evolved hydrogen and oxygen gases. Standard experimental methods for detecting and validating the quantity and quality of the product gases are critical. Faradaic efficiencies for the water splitting reaction in the given system can be determined. Three examples of PEC reactors are discussed, including batch, flow, and recirculating batch reactor.

2-Electrode Short Circuit and j–V

Solar-to-hydrogen (STH) conversion efficiency is the most important figure of merit to gage the potential of a semiconductor material to photoelectrochemically split water (see Chapter “ Efficiency Definitions in the Field of PEC”). It is projected that STH conversion efficiencies in excess of 10 % will be needed for practical hydrogen production systems [1]. Taken in conjunction with gas detection measurements (see Chapter “ Stability Testing”), the photocurrent density (j SC) under short-circuited conditions (i.e., zero applied bias) in a 2-electrode measurement is critical in determining the STH conversion efficiency. Moreover, applied bias experiments using the 2-electrode configuration can shed important light on the water splitting capabilities and limits of a PEC material system.