Enhancing Solar Fuel Production Performance through Catalyst Design and Device Engineering

Overview

Directly converting renewable solar energy into solar fuels presents a promising strategy to reduce reliance on fossil fuels and contribute to sustainable development. Solar hydrogen production via light-driven water splitting attracts significant research interest for solar energy conversion and storage toward a clean energy economy with carbon-zero emissions. Among the available technologies, photovoltaic-driven electrolysis (PV-EC) is the closest to commercialization due to the maturity of the solar cell industry. However, the high cost of the PV-EC system limits its large-scale application for solar hydrogen production. While photoelectrochemical (PEC) water splitting provides a low-cost alternative for direct solar-to-hydrogen conversion, the low efficiency and poor device stability, particularly at the photoanode, currently hinder the road from reaching the maturity of real-world implementations. Developing new strategies to address these challenges represents great research opportunities for advancing solar fuel production technologies.

First, we developed a fluorine (F) and sulfur (S) dual-doped metal-free carbon nanofiber catalyst for highly active and selective hydrogen peroxide (H2O2) pro- duction for the PV-EC system. The optimized catalyst reported a high onset potential of 0.814 V vs. the reversible hydrogen electrode (RHE) and an almost ideal 2e− pathway selectivity of 99.1%. By combining first principle theoretical calculations and structural characterizations, we investigated the mechanism of F/S dual-doping in enhancing the catalytic activity and selectivity of H2O2 production.

Continuing with this, we engineered a nickel-iron (NiFe) oxyhydroxide-alloy hybrid co-catalyst layer for photoanodes, reporting a state-of-the-art applied bias photon-to-current efficiency of 4.24% and highly enhanced stability of over 250 hours in water oxidation. By employing advanced microscopic and spectroscopic characterization techniques, we studied the light-induced atomic reconfiguration process that induces the dynamic evolution of a NiFe oxyhydroxide-alloy catalytic-protective layer based on the NiFe alloy nanofilm, thereby improving the efficiency and durability of the photoanodes.

Following this work, we further improved photoanode performance by replacing the conventional oxygen evolution reaction (OER) with the thermodynamically favored urea oxidation reaction (UOR) and decoupling photon management from catalysis on bifacial microstructured-black Si photoanodes. The engineered photoanodes reported the lowest onset potential of 0.782 V vs. RHE among Si-based systems so far and a saturated current density near 93% of the theoretical limit of Si. We employed in-situ spectroscopic and electrochemical characterizations to investigate the self-reconfiguration behavior of the NiFe alloy co-catalyst layer for UOR when competing with OER. Finally, we implemented a bias-free solar hydrogen production system coupled with urea oxidation by integrating three engineered Si substrates in series, achieving a solar-to-hydrogen efficiency of 4.10% in the HER-UOR system.