A highly efficient and stable electrode for solar-driven water electrolysis, interrogated by advanced operando and in situ techniques

Solar driven water splitting is an elegant way to address the intermittent nature of sunlight by storing it in the form of chemicals bonds, in this case as renewable hydrogen gas. A key component in a light-driven water electrolysis cell is the anode electrode, which must be activated by sunlight and at the same time release oxygen gas by the oxidation of water. The other electrode, the cathode, is responsible for releasing the hydrogen gas by the reduction of water. The primary losses in such photoelectrochemical (PEC) cells come from the anode electrode, as the oxygen evolution reaction (OER) is a complicated four electron reaction and moreover, the oxidative conditions during operation degrade the anode electrode materials. In SolOPP we study and develop a Ta3N5 photo-anode, which is one of the most efficient materials for PEC cells. However, the material degrades over time in contact during PEC water splitting. We will surface modify Ta3N5 with the main aim to increase its durability but also gain fundamental understanding of the corrosion processes during operation. In order to do so, we will set up experiments at international laboratories such as DTU, Lund and Cambridge where we can study the material properties when in contact with water during light exposure and the OER. These experiments will closely resemble the conditions the materials will be exposed to when in use. We can therefore gain insight and extract important knowledge that can be used to further develop a more stable photoanode material.

The intermittent nature of sunlight necessitates the storage of the solar energy into chemical bonds. Renewable hydrogen fuel from solar-driven water splitting is a goal of great significance, as it will provide a predictable, carbon-neutral, and high-energy density fuel. Photoelectrochemical (PEC) water splitting is an elegant way for solar water electrolysis and hydrogen production, as the light absorbers are also the water electrolyzing electrodes, i.e. photoelectrodes. A good photoelectrode material must possess high water splitting efficiency and exceptional stability against photocorrosion. The goal behind SolOPP is to produce a Ta3N5-based photoelectrode with improved stability above the limit of at least 8 mA/cm2 set by the EU and US Energy Departments for commercial exploitation. This will be an unprecedented achievement within the field, and a significant step towards bringing photoelectrochemical (PEC) water electrolysis to a commercial level. Photocurrent densities almost as high as the theoretical limit of Ta3N5 has recently been achieved within the project consortium, but stability remains a major challenge. In SolOPP, we will develop innovative approaches to improve the stability, based on application of co-catalysts and protective coatings with closely tailored properties. The development will be aided by operando studies of the semiconductor/electrolyte interface using (near)-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), combined with in situ transmission microscopy (TEM) and theoretical calculations. The ambition of the project is to establish a complimentary framework for knowledge-based electrode development, all the way from state-of-the-art fundamental science to working PEC cell, combining expertise in photochemistry, semiconductors physics, advance characterization techniques, nanotechnology and theoretical modeling, in order to address important technological challenges that can form a sustainable future energy landscape.