Understanding and mitigating performance-loss and degradation pathways in protected organic semiconductor photoelectrodes for solar water splitting

The production of sustainable fuels such as green hydrogen will help us to achieve Net Zero ambitions by powering sectors that are difficult to electrify, such as shipping, aviation and concrete and steel manufacture. Currently less than 4% of UK hydrogen production comes from renewable electricity powered water electrolysis and relies on the use of toxic and scarce catalysts. The production of truly green fuels requires technological innovation based on earth-abundant and low-cost materials. Photoelectrochemical cells offer a transformative pathway towards sustainable fuel production, but so-far high (>15%) solar-energy-to-fuel conversion efficiency has only been demonstrated using photoelectrodes based on expensive inorganic III-V semiconductors. Photoelectrodes based on next-generation solution processable semiconductors, including perovskites, organics, and metal-oxides, promise to be less resource intensive, support more environmentally friendly fabrication methods, and bring greater variation in optical and electronic properties. However, solution processed photoelectrochemical technology has failed to meet the complex optical, electrical and chemical requirements for achieving both high efficiency and long-term stability. In recent work, I addressed some of these challenges in photoanodes for solar water splitting that comprise organic-semiconductor light absorbing layers protected by inorganic carbon sheets functionalised with earth-abundant metal-oxide electrocatalysts. The resulting photoanodes demonstrated record photocurrent for solution-processed photoelectrodes, and promising stability over several days. Despite this progress, significant challenges still need to be overcome in terms of decreasing photovoltage losses and drastically improving operational stability to bring these devices closer towards real-world application. In this project, I will address these challenges through generating new fundamental understanding of the underlying physical processes underpinning the operation and degradation of these multi-layered photoanodes. This will be aided by a unique characterisation tool that I have recently developed (photoelectrochemical mass spectrometry) that allows for unprecedent sensitivity and time-resolution of measuring reaction products, as well as through operando spectroscopy and opto-electronic characterisations. Through careful characterisation of how the properties of the organic photoactive layer, charge-transport interlayers and electrocatalysts affect photoelectrochemical performance and degradation pathways, I will develop design-rules for how to further improve the efficiency and stability of these devices and use these to fabricate optimised photoanodes. Finally, I will design and fabricate photoanodes based on tandem organic photo-absorbing layers which can generate sufficient photovoltage to split water without the need for any external power source. I will investigate the degradation mechanisms and photoelectrochemical water splitting reaction pathways in these tandem photoanodes and use the generated understanding to optimise the efficiency and stability of these photoelectrochemical cells towards metric relevant for cost and energy-return competitive sustainable hydrogen production. The work will provide insight into photoelectrochemical devices comprising multiple types of earth-abundant semiconductors and develop new fundamental understanding to help drive forward photoelectrochemical, photocatalytic and electrocatalytic research into generating solar-driven sustainable fuels and chemicals, cementing the UK’s position as a global leader in this research field. Ultimately, success in this project will help realise devices and understanding that bring us a step-closer to realising sustainable hydrogen production technology to help reducing the worst effects of climate change.