Solar Optofluidics (SOLO): Water Splitting beyond the 1.23 eV Thermodynamic Constraints

Renewable hydrogen will play an important role in the UK's energy future for low carbon transport, heating, grid-scale energy storage and CO2 capture/utilisation. The UK's hydrogen demand would reach 143~860 TWh/year by 2050, while the current production capacity is only 27 TWh/year. Conversion of abundant sunlight to produce H2 is one of attractive approach to meet the demand. Among various solar H2 technology, photoelectrochemical (PEC) water splitting has gained much attention due to its operational flexibility, reduced electron-hole recombination and natural separation of H2 and O2 in two electrodes.

Learning from the historic trajectory of solar PV commercialisation, the key to deliver market acceptable PEC hydrogen production will be (1) enabling the use of much cheaper materials (such as silicon) and (2) significantly increasing the STF efficiency to at least 20%.

SOLO aims to remove the 1.23 eV thermodynamic restraints from the PEC water splitting system, by developing a pH-differential strategy to alter the individual equilibrium potentials of anodic (OER) and cathodic (HER) half reactions, thus reducing the energy barrier. A novel membraneless optofluidic platform is proposed to accommodate the pH-differential design, where acid and alkaline electrolyte will be able to co-exist in a single cell. Promising low bandgap materials will be demonstrated in the SOLO platform to achieve cost effectiveness and high STF efficiency.

This project will conduct transformative research on a novel pH-differential optofluidics engineering platform, SOLO, to reduce the cost and enhance the energy efficiency of solar fuel production from photoelectrochemical (PEC) water splitting. It is believed to boost international and inter-sectoral knowledge transfer with diverse interests, thereby benefit the industrial advancement and economy growth of UK and beyond, particularly solar energy, hydrogen and energy materials.

The UK has growing needs for alternative energy vectors such as hydrogen to support its low-carbon economy and there is significant potential to develop solar fuel technology for H2 production to meet this demand. This project will deliver technological innovation on eco-attractive solar hydrogen production to support the UK's transition to a low carbon future, and consequently, increase the robustness of the UK economy. The project outcomes will contribute to achieving the UK's ambitious and legislated targets of 100 % renewable energy and 80 % carbon emission reduction by 2050. The SOLO platform developed in this project are critical to meet the UK energy and environmental demands, as well as ensuring security of supply.

The government-led Technology Innovation Needs Assessment (TINA) 2014 revealed that the creation of a UK hydrogen industry has the potential to contribute £19-50 billion to the economy by 2050. The barrier to apply solar hydrogen production in the UK is its high cost (currently $10/kg, compared to $3-4/kg from wind and biomass). The SOLO project aims to develop an enabling technology, i.e., pH-differential optofluidic platform, to remove the 1.23 eV thermodynamic constraints from current PEC water splitting systems, allowing (1) an increase in the solar-to-fuel (STF) efficiency and (2) the use of low bandgap inexpensive materials for PEC devices, which are known as the two most important factors to drive down the solar H2 costs. Increasing the STF efficiency from 5% to 20% could cut 2/3 of PEC H2 production costs, while reducing the cell cost has the potential to half the H2 production costs (Energy Environ Sci, 2013, 6, 1983). If the above targets can be met, as to be addressed in the proposed work, the H2 production costs can reach the U.S. Department of Energy's targeted threshold of $2-4/kg H2. This project offers the chance to make PEC water splitting a viable route for H2 production.