The “Tandem Photoelectrolysis Plant” Concept: A Strategy for Fuel Production via Biomass Conversion Wastes

Abstract

One often-overlooked difficulty associated with the solar photoelectrolysis of water is the potential explosion hazard posed by the simultaneous evolution of hydrogen and oxygen. This problem can be addressed, in principle, by the addition of suitably-chosen redox electrolytes to the photoelectrolysis cell. We have found, as have A. J. Bard and his coworkers, that the evolution of oxygen can be suppressed completely in certain aqueous photoelectrolysis cells, with no adverse effect on either the photocurrent or the rate of cathodic hydrogen evolution, by simple carboxylate ions such as acetate or butyrate. In basic solutions, water-soluble products derived from the carboxylate ions are formed at the photoanode, but in acidic solutions, hydrocarbons such as ethane or hexane are formed (along with CO₂) via a “photo-Kolbe” reaction. Such cells can therefore produce a valuable fuel mixture, consisting of hydrogen (from the cathode) and hydrocarbons (from the photoanode). Because both acetic acid and butyric acid are abundantly available in the aqueous effluent from the enzymatic digestion of biomass material, these observations lead to the concept of a “tandem photoelectrolysis plant”, in which a photoelectrolysis device would derive its input water supply from a biomass conversion device. While the problems of electrode stability and light absorption still remain to be solved, the approach described here completely eliminates the necessity of separating the anodic and cathodic gases produced in photoelectrolysis. This, in turn, can pave the way for the design of simple and less expensive photoelectrolysis plants than would otherwise be possible.