The architecture of the system is deceptively simple in concept but demanding in execution. A dense array of gallium nitride nanowires is grown on a four-inch silicon wafer by molecular beam epitaxy, producing a high-surface-area forest of semiconductor needles that harvests light efficiently and shortens the distance photogenerated charge carriers must travel before reaching the surface. Silicon was chosen deliberately as the substrate: beyond its role as a mechanical and electrical support, it is the workhorse material of the semiconductor industry, which means the fabrication route is compatible with the large-area, wafer-scale processing that any commercially meaningful hydrogen technology will eventually require. On top of the GaN nanowires, the team deposited a cocatalyst of amorphous manganese oxhydroxide, denoted MnOₓ(OH)ᵧ, which performs the critical task of managing the charges that sunlight generates.

The fundamental problem in any photocatalytic water-splitting scheme is charge recombination. When a photon is absorbed, it excites an electron from the valence band to the conduction band, leaving behind a hole. In an ideal catalyst, the electron migrates to one site to reduce protons into hydrogen gas while the hole migrates elsewhere to oxidize water into oxygen. In reality, the oppositely charged particles attract each other through Coulomb forces and frequently annihilate before they can do useful work, dissipating the absorbed energy as heat. The MnOₓ(OH)ᵧ layer functions as a hole extractor: it draws photogenerated holes toward the oxidation sites while leaving the electrons within the GaN framework, where they are available for the hydrogen evolution reaction. This spatial separation of charges is the first pillar of the system’s performance.

The second and most surprising pillar emerged when the researchers examined what happens in the ionic environment of actual seawater. Rather than hindering the reaction, chloride ions were found to assist it in two distinct ways. According to density functional theory calculations and a battery of in-situ spectroscopic measurements — including irradiation-mode X-ray photoelectron spectroscopy, electron paramagnetic resonance, and operando infrared spectroscopy — chloride ions adsorb onto the electron-deficient surface of the MnOₓ(OH)ᵧ cocatalyst. This adsorption creates a localized electric field that reduces the Coulomb attraction between photogenerated electron-hole pairs from 4.10 to 3.19 electron volts, measurably loosening the grip that keeps carriers bound together and thereby improving charge separation efficiency.

At the same time, the adsorbed chloride lowers the energetic barrier for the water oxidation half-reaction, the thermodynamically and kinetically demanding step that has historically limited the overall rate of artificial photosynthesis. ‘Chloride has long been viewed as a major obstacle in photocatalytic seawater splitting,’ said Prof. Baowen Zhou, the corresponding author of the study. ‘But in our system, we found it actually becomes an ally — promoting charge separation while simultaneously lowering the energy barrier for water oxidation.’ The observation inverts a widely held assumption in the field, where seawater splitting efforts have typically focused on designing catalysts and membranes that shield active sites from chloride rather than on harnessing it.

The cocatalyst itself contributes a third mechanism, one intrinsic to its chemistry. The hydroxyl groups embedded within the MnOₓ(OH)ᵧ structure undergo dynamic exchange with surrounding water molecules, a property the researchers identified as fundamental to the material’s behavior. This continuous turnover accelerates the formation of the *OH and *OOH intermediates that constitute the key elementary steps of the oxygen evolution reaction, smoothing the multi-proton, multi-electron pathway that water must traverse on its way to becoming molecular oxygen. The synergy between chloride-ion-assisted charge separation and the hydroxyl-exchange mechanism underpins the exceptional performance the team observed in natural seawater, a performance that neither effect could deliver alone.

Concentrated sunlight adds yet another dimension to the system’s efficiency: heat. Under illumination intensities ranging from 2.7 to 5.5 watts per square centimeter, infrared thermography revealed that the catalyst surface warms to between 88 and 100 degrees Celsius. Cooling experiments conducted from the outside confirmed that this photothermal heating accounts for approximately 56 percent of the observed activity enhancement, demonstrating a genuine synergy between photogenerated charge carriers and thermal energy. Water oxidation is an activated process that benefits from elevated temperature, and the photocatalyst effectively recycles a fraction of the incident solar energy that would otherwise be wasted, converting it into the thermal push that helps drive the rate-limiting step.

Perhaps the most consequential finding for practical deployment concerns selectivity. In most seawater-splitting schemes, chloride ions are oxidized at the anode to form chlorine gas, hypochlorite, or related reactive chlorine species — chemicals that are hazardous, corrosive, and represent wasted input energy. The MnOₓ(OH)ᵧ/GaN system produced no detectable chlorine (Cl₂), hypochlorite, or hydrogen peroxide byproducts, confirming that water oxidation proceeds selectively even in the presence of abundant chloride. Isotope-labeling experiments using D₂O and H₂¹⁸O provided definitive proof that both the hydrogen and the oxygen gases evolved from water molecules rather than from any side chemistry of the dissolved salts or the catalyst itself.

Durability and real-world performance were addressed with equal rigor. Over a 360-minute stability test, the catalyst achieved a turnover number of 44,278 moles of hydrogen per mole of manganese species, an indication that the active sites remain intact and productive through extended operation. The team then moved the experiment out of the laboratory entirely. In outdoor field tests on the campus of Shanghai Jiao Tong University, a Fresnel lens concentrated natural sunlight onto the photocatalyst, and the system displayed dynamic solar-responsive behavior: hydrogen evolution rates tracked the day’s irradiance from 10:00 to 16:00 local time, peaking at 1.31 millimoles of hydrogen per square centimeter per hour during maximum insolation. That responsiveness to real, fluctuating sunlight — rather than the steady output of a laboratory lamp — is precisely the behavior a practical solar fuels technology must exhibit.

The implications extend beyond the immediate efficiency numbers. By clarifying the distinct roles of chloride ions in reducing both the Coulomb attraction energy and the water oxidation barrier, and by revealing the intrinsic contribution of the hydroxyl-exchange mechanism, the study provides a new theoretical foundation for designing catalysts tailored to marine environments. Because seawater is abundant and sunlight is free, the approach points toward coastal and offshore hydrogen production facilities that do not compete with agriculture or drinking water supplies for freshwater — a constraint that has shadowed every large-scale electrolysis scenario to date. With a noble-metal-free composition, a silicon-based fabrication platform, and demonstrated stability in unprocessed seawater, the MnOₓ(OH)ᵧ/GaN nanowire system represents a credible step from laboratory demonstration toward the scalable green hydrogen infrastructure that decarbonization roadmaps increasingly demand. 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.

Subject of Research: Photocatalytic seawater splitting for hydrogen production using a chloride-assisted noble-metal-free nanowire photocatalyst

Article Title: Chloride ions turn from foe to friend in seawater-to-hydrogen breakthrough

Article References: Chloride ions turn from foe to friend in seawater-to-hydrogen breakthrough. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: photocatalysis, seawater splitting, green hydrogen, chloride ions, gallium nitride nanowires, manganese oxhydroxide, solar-to-hydrogen efficiency, charge separation, oxygen evolution reaction, photothermal effect, water oxidation, marine hydrogen production