The synthesis of fuels from sunlight offers a promising sustainable solution for chemical energy storage. However, the inefficient utilisation of the solar spectrum limits its commercial viability. Requiring high energy photons to drive the reaction, a large portion of the solar spectrum, particularly visible (Vis) and infrared (IR) energy, could not be converted for most solar fuel synthesis, and thus lost in the form of wasted heat. Apart from fundamental improvements to the (photo)catalyst materials and device architectures, solar fuel production systems can also be designed to improve their solar energy utilisation and their overall functionality by integrating complementary technologies.

In this PhD thesis, three distinct complementary approaches were explored to tackle two main challenges in the solar fuel research community; (i) the intermittent nature of renewable energy sources which prevent a sustained and predictable fuel production output and (ii) thermal-related energy losses due to inefficient utilisation of the solar spectrum. The first approach describes an integrated device that in addition to possessing photoelectrochemical (PEC) water splitting functionality, also operates as an electrolyser in the nocturnal period to achieve a steady round-the-clock fuel production. The device is based on a perovskite-BiVO4 tandem PEC architecture. The light-absorbers are interfaced with a H2 evolution catalyst and a Co-based water oxidation catalyst, respectively, which can also be directly driven by electricity. The system provides a solar-to-hydrogen (STH) efficiency of 1.3% under simulated solar irradiation and an onset voltage for water electrolysis at 1.8 V.

The second approach integrates a thermoelectric (TE) with a PEC device, where TE converts wasted heat into additional electricity. A range of integrated TE-PEC systems, which benefit from the reactor heating to bolster water splitting under concentrated light irradiation, were demonstrated. Unassisted water splitting can already be achieved under 2 sun irradiation by wiring a TE element to a BiVO4 photoanode, whereas the photocurrent is further increased by introducing a perovskite solar cell.

The third approach describes a hybrid solar vapour generator (SVG) and a photocatalytic (PC) device, where the full solar spectrum is harvested via a light management strategy. By floating the device on aqueous feedstock, it absorbs UV light via the PC top layer, and the SVG synergistically utilises the remaining Vis-IR light. The SVG evaporates aqueous feedstock generating distilled water vapour, which is absorbed by the PC layer to drive the overall solar-driven water splitting with STH of 0.11-0.14%. Because the PC layer does not make direct contact with the aqueous feedstock which may contain contaminants, the device confers over 150 hours stability for operation in seawater and other waste streams while providing 0.95 Kg m−2 h−1 of purified water as a secondary product. This work provides an example of how two solar driven processes could be coupled to provide a more versatile approach toward practical solar fuel synthesis, especially in an area where clean water is scarce.

These strategies can improve the efficiency, production rates, and practicality of existing solar fuel systems, and therefore the overall economics of solar fuel production. More broadly, the approaches highlight the necessary collaboration between materials science and engineering to help drive the adoption of a sustainable energy economy using existing technologies.