10.1117/2.1201704.006793 Hydrogen production with holes: what we learn from operando studies Artur Braun, Kelebogile Maabong, Mmantsae M. Diale, and Rita Toth Using x-ray spectroscopy to analyze a photoelectrochemical cell during water oxidation and hydrogen formation sheds light on the physics and chemistry of photoelectrodes. As we become more aware of the limited amount of energy available from traditional sources, we are increasingly turning to solar power as a viable alternative. 1, 2 Of the total worldwide energy consumption, 20% is electrical, with an increasing share being produced by photovoltaics. Scientists, engineers, technolo- gists, and investors are now working towards a renewable alter- native for the remaining 80%, which is currently obtained from fossil fuels, nuclear fuels, and biomass. 3–5 Photoelectrochemical cells (PECs), which use sunlight to convert water into solar-hydrogen fuel, represent one route to achieving a renewable energy source. PECs are based on semiconductor photoelectrodes, 6 but their principles of energy conversion and storage are analogous to photosynthesis. The photoelectrodes within PECs are comprised of two electrodes. At least one contains a light absorber (which is applied as a coating on a transparent conducting oxide, TCO) and one has an electrocatalytic surface (e.g., an aqueous-electrolyte coating). When light strikes the absorber, photoelectrons and holes are created. The electrons then migrate through the TCO, which acts as a current collector, and enter the electric circuit. The holes dif- fuse to the electrode surface, where they chemically react with water molecules and cause them to electrochemically split into oxygen gas. This gas evolves at the photoanode and can be col- lected in a container for any potential further use. Protons mi- grate through the electrolyte to the counter electrode, where they combine with electrons to form hydrogen gas, which is collected as fuel. We have designed a PEC reactor (a prototype of which is shown in Figure 1) that has a large (10  10cm) iron oxide Figure 1. The photoelectrochemical cell (PEC) reactor prototype. The device has an active area of 100cm 2 and is comprised of glass coated with an iron-oxide photoelectrode. The design incorporates an oxygen gas outlet (top left). The white compartment on the right of the device holds the platinum counter electrode for hydrogen gas evolution and collection. One molar mass of potassium hydroxide, acting as the elec- trolyte, is supplied continuously. photoanode (applied as a coating on glass, i.e., the TCO) in a sealed steel frame filled with an electrolyte (1 mole of potas- sium hydroxide). The design of our units means they can be arranged in large clusters, like photovoltaic panels, and could therefore enable the development of a decentralized, scalable fuel-production facility (i.e., which could improve the resilience of human settlements). 7 Moreover, by using a form of x-ray spec- troscopy, we can measure processes that occur in our device as they happen (i.e., ‘operando’ measurements). In this way, we are able to unveil processes and characteristics that are otherwise extremely difficult to investigate. 8 Continued on next page