Photoelectrochemical (PEC) water splitting ends in a number, solar-to-hydrogen efficiency (STH), and that number is only as good as the chain of measurements behind it. In flow operation, the chain runs from photon flux to photocurrent to gas production, and every link must be calibrated.
Light source calibrated at the electrode plane; flow cell or immersed panel with defined geometry; potentiostatic control or unbiased two-electrode operation; and a sealed gas path to online hydrogen analysis. Every interface, window, membrane, seal, is part of the measurement and must be characterized.
Measure photocurrent and hydrogen simultaneously on one time base. Faradaic efficiency, gas produced over charge passed, is the honesty check: below 100% indicates recombination, crossover or leakage. STH computed from current alone assumes perfect efficiency and overstates real devices; STH from measured hydrogen is the number that matters.
Flow keeps products separated and the interface fresh, but adds questions: does the electrolyte absorb light on its path? Do bubbles generated in the channel shadow the electrode? Verify bubble management optically, and confirm the flow rate does not strip volatile products before measurement.
STH at hour zero is a snapshot; the degradation curve is the story. Run hold tests at operating point with continuous gas logging, and report efficiency retention over a stated duration, that is the metric device research actually needs.
Perfectlight pairs PEC systems with online gas analysis for closed-loop evaluation; see solutions.
