Significant Efficiency Increase in Self‐Driven Photoelectrochemical Cell for Water Photoelectrolysis

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Abstract
A theory relating the electrochemical and solid‐state properties of semiconductors to their photoelectrochemical behavior has been used to predict the electrodes that, when combined, will give the optimum efficiencies for the splitting of water by means of solar light to hydrogen and oxygen. The present paper represents the first application of this theory. A p‐type indium phosphide electrode has been decorated with platinum and combined with an n‐type gallium arsenide electrode which has been protected against photocorrosion by depositing a thin film of Mn‐oxide on it. Examination has been made of the individual photoelectrochemical behavior of these electrodes in aqueous solution. The I‐V curves of these electrodes indicated that, when placed together in a cell, they would spontaneously give rise to hydrogen and oxygen when photoirradiated. X‐ray photoelectron spectroscopic examination of the protected gallium arsenide electrode showed no indication of the substrate, i.e., the Mn‐oxide completely covered the gallium arsenide. Ellipsometric examination showed the thickness of manganese oxide to be ∼200Å. When these two electrodes were used in a photoelectrochemical cell and irradiated at an intensity of ∼1 sun, photoelectrolysis of water took place. Evaluations of the solar conversion efficiency for the production of hydrogen and electricity showed a maximum total conversion efficiency of 8.2%, the conversion being predominantly to hydrogen. The efficiency as a function of time undergoes a decrease of about 10% during the first hour of operation, but then remains constant for at least a further ten hours.

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