In photoelectrochemical (PEC) water splitting, the photocurrent looks like a complete performance record, but it is only half the story. Current measures charge; it says nothing about how much of that charge became hydrogen versus side reactions, corrosion or capacitive charging. The missing half is actual gas production, measured online.
A three-electrode configuration, working photoelectrode, reference and counter, yields onset potential, photocurrent density and stability. These define the electrochemical behavior of the electrode. But faradaic efficiency is an assumption until gas analysis confirms it.
Photocurrent includes contributions from surface recombination, sacrificial oxidation of the electrode itself and double-layer charging. At low efficiency, the gas actually produced can be far below what the integrated charge implies. Reporting a solar-to-hydrogen efficiency from current alone can therefore overstate performance by a large factor.
Sealing the PEC cell and sweeping the headspace to an online H2 analyzer lets you compare the gas evolution rate against the charge passed in real time. The faradaic efficiency falls out of the comparison. Discrepancies between the two traces are diagnostic: gas lagging current suggests recombination or leakage; gas exceeding current suggests chemical contributions.
The cell must be gas-tight with electrode feedthroughs that do not leak; the carrier must not disturb the electrochemical measurement; and the analyzer's detection limit must match micromole-scale production. Both measurements should share one time base for direct comparison.
Perfectlight pairs photoelectrocatalysis systems with online gas analysis so that current and product data arrive as one closed evidence chain.
