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Photoelectrochemical solar-to-hydrogen conversion efficiencies as high as 7.8% (based on the lower heating value of hydrogen) have been demonstrated in outdoor testing using a photocathode fabricated from triple junction amorphous silicon-solar cells and a separate catalytic anode. The tests were conducted in a specially designed Teflon-sealed reactor in 1 N KOH with a photoactive area of 0.27 cm2 and anode and cathode areas of 1 cm2. The hydrogen production rates, inferred from direct measurement of the anodic/cathodic currents and confirmed by independent volumetric and gas chromatographic measurements of the evolved hydrogen, were in excellent agreement with the rates expected from the measured solid-state JV behavior of the solar cell and the overpotentials of the thin-film catalysts. The thin-film catalysts, CoMo hydrogen catalysts deposited by sputtering from a compound target and NiFeyOx oxygen catalysts deposited from nickel−iron Permalloy target by reactive sputtering, have, in separate tests, shown no degradation after over 7200 h of operation in 1 N KOH electrolyte. During outdoor testing, the solar-to-hydrogen conversion efficiency decreased in the late afternoon as the blue portion of the spectrum decreased, a result of the spectral sensitivity of the solar cell used to construct the photoelectrode. Detailed modeling of the multijunction amorphous silicon cells is being conducted to identify structures that are better load-matched to the catalyst performance and that could yield higher hydrogen production efficiencies. Future work to advance this technology includes development of improved thin-film catalysts and development of transparent protective coatings that will allow complete immersion of the active electrode into the electrolyte.
Rocheleau et al. (Thu,) studied this question.