Link: https://bioengineer.org/chloride-ions-become-unexpected-allies-in-solar-powered-seawater-hydrogen-production/

For decades, the salt in seawater has been the villain of solar hydrogen research. Chloride ions corrode electrodes, compete with water molecules at catalyst surfaces, and generate unwanted chlorine chemistry that poisons reaction pathways. Now, a research team led by Prof. Baowen Zhou of Shanghai Jiao Tong University has reported a photocatalytic system that not only tolerates the chloride content of natural seawater but actively exploits it, converting a long-standing contaminant into a functional component of the hydrogen production process. Writing in Science Bulletin, the researchers describe a noble-metal-free photocatalyst built from manganese oxhydroxide grown on gallium nitride nanowires that achieves a solar-to-hydrogen conversion efficiency of 6.15 percent using real seawater, sunlight, and nothing else — no sacrificial chemical agents, no freshwater dilution, and no precious metal catalysts.

For context, I see the following solar to hydrogen conversion ratios.

31% https://www.nature.com/articles/s44172-026-00610-x

9% https://www.nature.com/articles/s41586-022-05399-1

Definition,

Solar-to-Hydrogen Conversion Efficiency
STH efficiency refers to the solar-to-hydrogen conversion efficiency of a photoelectrochemical (PEC) water splitting device, defined as the ratio of chemical energy produced from hydrogen generation to the incident solar energy input, measured under standard solar conditions.

I don't have other context on why they differ so much

The first one uses active energy in the form of PV cells and concentrators, and batteries (?), I can’t access the second one, seems to be a dead page to me, but I’m going to guess, based on the thing above being passive energy (direct conversion), that the 9% one is also passive energy. They are close in efficiency percentages so that seems likely.

So the difference Is probably whether or not it has extra steps/materials/demand for using more power.