Semiconductor Systems for Reduction

More recently, Rudolph et al. was able to reduce C02 to oxalate with faradaic efficiencies approaching 100% with their most active and stable complex [102]. These authors examined a variety of macrocyclic nickel chelate complexes with various substituent groups on the ring in acetonitrile solution. Whilst it is interesting that the group was able to produce oxalate catalyzed by a metal complex, the potentials required for reduction were —1.9 to —2.2 V (versus SCE), similar to the potential required for the direct reduction of C02 in aprotic solvent (—2.21 V versus SCE). The very negative potentials in this reaction highlight the overall theme of the electrochemical reduction of C02. 

Recently, interest has been expressed in natural enzymes that effect the reduction of C02 to various products (see Section 11.4). For example, Reda et al. used a tungsten-containing formate dehydrogenase 1 enzyme derived from Syntropho-bacterjumaroxidans in the mediated electroreduction of C02 to formate [103]. The enzyme, which is either adsorbed onto the graphite electrode surface or is free in solution, was observed to reduce C02 to formate with near-100% faradaic efficiency. Although a minimal overpotential for the process was required (-0.4V of applied bias), the current densities were rather low. 

Similar to the molecular photosensitizers described above, solid semiconductor materials can absorb photons and convert light into electrical energy capable of reducing C02. In solution, a semiconductor will absorb light, and the electric field created at the solid-liquid interface effects the separation of photo-excited electron-hole pairs. The electrons can then carry out an interfacial reduction reaction at one site, while the holes can perform an interfacial oxidation at a separate site. In the following sections, details will be provided of the reduction of C02 at both bulk semiconductor electrodes that resemble their metal electrode counterparts, and semiconductor powders and colloids that approach the molecular length scale. Further information on semiconductor systems for C02 reduction is available in several excellent reviews [8, 44, 104, 105], 

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