Parameters affecting the chemical work output of a hybrid photoelectrochemical biofuel cell

A hybrid photoelectrochemical biofuel cell employing the photoanode architecture of a dye-sensitized solar cell has been assembled. A porphyrin dye sensitizes a TiO₂ semiconductor over the visible range to beyond 650 nm. Photoinduced charge separation at the dye–TiO₂ interface results in electron migration to a cathode, and the holes generated on surface bound dyes oxidize soluble electron mediators. The increased [Ox] : [Red] ratio of the mediator drives the solution-based enzymatic oxidation of appropriate substrates. In this report we investigate how the accumulation of anodic and cathodic products limits cell performance. The NAD⁺/NADH and benzoquinone/hydroquinone redox couples were studied as sacrificial electron donors in the absence of appropriate enzymes or substrates. Comparatively poor cell performance was observed using the benzoquinone/hydroquinone couple. This effect is explained in terms of rapid charge recombination by electron donation from the electrode to benzoquinone in solution, as compared to much less recombination with NAD⁺. With the NAD⁺/NADH couple the cell performance is relatively independent of the redox poise of the anode solution, but limited by accumulation of reduction products in the cathodic compartment. Using the NAD⁺/NADH couple, the photochemical reforming of ethanol to hydrogen was demonstrated under conditions where the process would be endergonic in the dark.