Introduction
Hydrogen is seen as a key element of a future carbon-free economy. The International Energy Agency forecasts that the hydrogen demand will increase from 90 Mt (2020) to 260 Mt by 2050 if the currently existing government programs are implemented.[1] Yousif et al . calculated a demand of 568 Mt for a scenario which meets the goals of the Paris Agreement (2015). [2]
Currently, hydrogen is produced primarily from fossil resources (e.g ., steam reforming, coal gasification). Carbon neutral production is possible through water electrolysis with electricity from renewable resources, which is expected to play a major role in the coming years even through its present contribution to the global hydrogen production is negligible.[1] However, scaling-up electrolysis will require an corresponding expansion of the energy infrastructure, which adds to the investment costs. For example, an expansion of the power grid will be necessary in central Europe, because wind and solar plants are placed decentral and are spatially separated from large-scale electrolyzers. A solution to this problem could come from approaches which combine energy uptake and hydrogen production in a single system (»artificial photosynthesis«) and offer a cost-effective alternative for low production volumes and off-grid applications.[3]
Photoelectrochemical (PEC) water splitting is a method of artificial photosynthesis, in which illuminated semiconductors (e.g. , WO3, Fe2O3) in direct contact with an aqueous electrolyte cause the splitting of water molecules to hydrogen and oxygen without the need for traditional photovoltaics or electrolyzers. Selected systems achieve high efficiencies of up to 19%, and a photoelectrochemical solar park with an active area of 100 m2 was constructed and operated for several months.[4, 5] However, semiconductors that offer a balanced set of properties (efficiency, stability, cost and scalability) are lacking.[6] Therefore, large parts of the current research still focuses on material development on the laboratory scale. Among the common measurements are methods for electrode characterization (e.g., voltammetry, stability measurements, impedance spectroscopy), and methods for determining device metrics (e.g. the solar-to-hydrogen efficiency, STH).[7]
It has been pointed out that measurements on similar materials in different labs lead to different results, which was attributed to a lack of accepted standards.[8] A number of recent reviews addresses this issue, provides guidelines for measurement routines and gives best-practice examples, e.g. for the solar-to-hydrogen (STH) efficiency.[7,8,9,10] A second reason for insufficient comparability is that photoelectrochemical measurements require special equipment that is not common in chemical laboratories and is challenging to install and operate (e.g., solar simulators, gas chromatographs). It has been shown that standard measurement routines are prone to systematic errors induced by the equipment (e.g ., solar simulators), which can result in an overestimation of the efficiency.[11]
In this Application Note, we present a versatile and reliable measurement system for PEC water splitting which allows to obtain high quality electrochemical data, including the STH efficiency. It addresses three major issues which, to our experience, pose difficulties in many labs and can hamper the accuracy and reproducibility of measurement data: (1) the implementation of large area light sources with high spectral quality, (2) the stabilization of the reaction temperature under irradiation, and (3) the quantitative determination of hydrogen.