When sunlight strikes the MnOₓ(OH)y/GaN nanowire photocatalyst, electron-hole pairs are generated. Without assistance, these charges recombine quickly, limiting efficiency. The MnOₓ(OH)y cocatalyst acts as a hole extractor, drawing photogenerated holes to the oxidation sites and leaving electrons in the GaN framework for hydrogen reduction. But the real surprise came when the researchers looked at seawater's ionic content.
"Chloride has long been viewed as a major obstacle in photocatalytic seawater splitting," said Prof. Baowen Zhou, the corresponding author of the study and professor at Shanghai Jiao Tong University. "but in our system, we found it actually becomes an ally — promoting charge separation while simultaneously lowering the energy barrier for water oxidation."
Using density functional theory calculations and in-situ spectroscopic techniques — including irradiation XPS, electron paramagnetic resonance, and operando infrared spectroscopy — the team traced the mechanism step by step. Chloride ions adsorb onto the electron-deficient MnOₓ(OH)y surface, creating a localized electric field that further reduces the Coulomb attraction between photogenerated carriers from 4.10 to 3.19 eV, and simultaneously lowers the energy barrier for water oxidation. In parallel, the hydroxyl groups within MnOₓ(OH)y undergo dynamic exchange with water molecules — an intrinsic property of the cocatalyst — accelerating the formation of *OH and *OOH intermediates, the key steps in the oxygen evolution reaction. This synergy between chloride-ion-assisted charge separation and the inherent hydroxyl-exchange mechanism of MnOₓ(OH)y underpins the exceptional performance of the system in natural seawater.
The GaN nanowire array, grown on a 4-inch silicon wafer by molecular beam epitaxy, provides a high-surface-area architecture that enhances light absorption and shortens charge carrier diffusion pathways. The silicon substrate also offers industry-ready scalability, a critical advantage for practical deployment.
The system also benefits from photothermal effects under concentrated illumination. Infrared thermography showed that the catalyst surface temperature rises to 88–100 °C under 2.7–5.5 W cm−2 light intensity. External cooling experiments confirmed that photothermal heating contributes approximately 56% of the observed activity enhancement, revealing a synergy between photogenerated charge carriers and thermal effects.
Importantly, the photocatalyst produced no detectable chlorine (Cl2), hypochlorite (Cl−), or hydrogen peroxide (H2O2) byproducts — confirming that water oxidation proceeds selectively over chloride oxidation. Isotope-labeling experiments using D2O and H218O confirmed that both hydrogen and oxygen evolved from water splitting. The catalyst achieved a turnover number of 44,278 mol H2 per mol of Mn species over the 360-min stability test.
In outdoor field tests conducted on the campus of Shanghai Jiao Tong University using a Fresnel lens to concentrate natural sunlight, the system showed dynamic solar-responsive behavior. Hydrogen evolution rates closely followed solar irradiance variations from 10:00 to 16:00 local time, peaking at 1.31 mmol H2 cm−2 h−1 during maximum insolation.
The findings clarify the distinct roles of chloride ions — reducing both the Coulomb attraction energy and the water oxidation barrier — while revealing the intrinsic contribution of the hydroxyl-exchange mechanism of MnOₓ(OH)y. This dual synergy provides a new theoretical foundation for designing more efficient and durable catalysts for marine hydrogen production. Since seawater is abundant and sunlight is free, this study opens a practical pathway toward coastal and offshore hydrogen production without competing for freshwater resources.
The study was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Shanghai Pilot Program for Basic Research, and the State Key Laboratory of Photoelectric Conversion and Utilization of Solar Energy.
