III-V Semiconductor Alloys and Earth-Abundant Cocatalyst Foils for Immersed Solar Water Splitting Devices

Abstract

As the world transitions away from fossil fuels, the reliance on intermittent renewable energy technologies such as wind and solar power grows. With this comes the demand for new energy storage methods, allowing renewable energy to be harvested more effectively and ensuring a stable power supply. One such proposed scheme is the green hydrogen economy, in which renewable energy is used to convert water into molecular hydrogen H2. A highly energy-dense chemical fuel, H2 can be stored and used to power fuel cells as needed. The only by-product from fuel cells is water, thus completing the carbon-free cycle. However, while solar-driven H2 production will be critical for a green hydrogen economy, there remain key challenges. Specifically, solar water splitting efficiencies remain substantially lower than theoretically possible, while the most efficient semiconductors for light-harvesting are highly susceptible to corrosion. Additionally, solar water splitting devices are currently far too costly for large-scale commercialisation. This thesis aims to address these issues, firstly by investigating new III-V semiconductor alloys and secondly by advancing the design and fabrication of immersed solar water splitting devices.

The III-V alloys InGaAsP and AlGaAs have great potential as narrow-gap and wide gap materials for tandem cells, respectively. Their photoelectrochemical (PEC) properties were therefore thoroughly investigated for the first time. Both materials generated good photocurrent densities under 1 sun, with reflection accounting for most losses, while also providing photovoltages approaching their theoretical limits. A TiO2-coated InGaAsP photocathode with a band gap of 0.92 eV generated a photocurrent density of 30 mA/cm2, with onset and saturation potentials of 0.48 and 0.20 V vs RHE, respectively, equating to a half-cell solar-to-chemical (HC-STC) efficiency of 7.1%. It was found that TiO2 forms an electron-selective type II heterojunction with InGaAsP, greatly enhancing the PEC performance. A TiO2-coated buried-junction AlGaAs photocathode with a band gap of 1.64 eV generated a photocurrent density of over 15 mA/cm2, with an excellent onset potential of 1.02 V vs RHE. By adding a 5 nm n-GaAs passivation layer, the onset potential improved even further to 1.11 V vs RHE, equating to an HC-STC efficiency of 9.6%. These results show that both InGaAsP and AlGaAs have highly efficient PEC properties, although much care will be needed in the future to ensure their stability in aqueous electrolyte.

Stability is also an issue for photoabsorbers during device fabrication, particularly when depositing earth-abundant cocatalysts, for which solution-based methods are commonly employed. To address this, earth-abundant cocatalysts can instead be deposited on metal foil before being combined with photoabsorbers. This approach was used to fabricate fully decoupled Si and GaAs artificial leaves, which attained excellent solar-to-hydrogen (STH) efficiencies of up to 14% under 1 sun. Both devices also exhibited remarkable stability, with the GaAs artificial leaf maintaining an STH efficiency of over 10% for longer than 9 days. As well as being highly efficient and stable, cocatalyst foils permit the use of earth-abundant cocatalysts in place of noble metal cocatalysts, greatly reducing material costs.

Finally, solar water splitting with triple-junction cells was investigated. Triple-junction cells provide a much larger photovoltage than is necessary for water splitting, hence power is wasted. However, by adjusting the ratio of triple-junction cells to electrochemical cells, the excess photovoltage can be utilised. An immersed triple-junction device with multiple electrochemical cells was constructed to demonstrate this concept. Triple-junction cells were combined with earth-abundant cocatalyst foils to create photoanodes, with three photoanodes capable of driving four electrochemical cells. The combined excess photovoltage from Show more