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.