Using excess solar power to produce hydrogen from water could be a good way to store solar energy. Photo-electrochemical cells (PECs) have received much attention for this, but the challenge for PECs is finding a combination of materials that absorbs light, minimizes losses and is stable. This project sought to begin development of solid-state PECs that capture thermal and photon energy from concentrated sunlight at very high temperatures. By utilizing thermal energy that is discarded in conventional PECs, record efficiency without utilizing precious catalysts could be possible.
This project confirmed the importance of thermal energy for maximizing the sunlight-to-fuel conversion, which lays out a clear path towards a prototype. The research team demonstrated that two important parameters describing the performance of solar PECs—fill factor and saturation photocurrent density—improved significantly with high temperature, contrary to the view that solar cell efficiency decreases with temperature. Also, the researchers identified important materials that could allow them to fabricate such solid-state PECs.
This work developed a PEC design with a demonstrated water-to-hydrogen conversion efficiency exceeding 30 percent. The new design uses a solid-state oxide material, in contrast to typical liquid or polymer based PECs, the operation temperature of which is limited by the boiling point of water. A semi-conductor/mixed ionic and electronic conductor heterojunction, reported for the first time, provides a potentially efficient mechanism for generating, separating and emitting electron-hole pairs. The researchers also identified a suitable material for a proof-of-concept solar fuels generator: hematite, an iron oxide and one of the most common solids in nature.
The team fabricated a proof-of-concept device incorporating iron oxide. The current density, which gives the rate of hydrogen generation, was two to three orders of magnitude greater than conventional PECs. The researchers then commissioned a high throughput thin-film deposition system to screen 50 additional materials. They have secured follow-on funding from the Global Climate & Energy Program and the National Science Foundation.