Fabrication of a biocathode for formic acid production upon the immobilization of formate dehydrogenase from Candida boidinii on a nanoporous carbon

Abstract

The immobilization of the non-metallic enzyme formate dehydrogenase from Candida boidinii (CbFDH) into a nanoporous carbon with appropriate pore structure was explored for the bioelectrochemical conversion of CO₂ to formic acid (FA). Higher FA production rates were obtained upon immobilization of CbFDH compared to the performance of the enzyme in solution, despite the lower nominal CbFDH to NADH (β-nicotinamide adenine dinucleotide reduced) cofactor ratio and the lower amount of enzyme immobilized. The co-immobilization of the enzyme and a rhodium complex as mediator in the nanoporous carbon allowed the electrochemical regeneration of the cofactor. Preparative electrosynthesis of FA carried out on biocathodes of relatively large dimensions (ca. 3 cm × 2 cm) confirmed the higher production rate of FA for the immobilized enzyme. Furthermore, the incorporation of a Nafion binder in the biocathodes did not modify the immobilization extent of the CbFDH in the carbon support. Coulombic efficiencies close to 46% were obtained for the electrosynthesis carried out at -0.8 V for the biocathodes prepared using the lowest Nafion binder content and the co-immobilized enzyme and rhodium redox mediator. Although these values may yet be improved, they confirm the feasibility of these biocathodes in larger scales (6 cm²) beyond most common electrode dimensions reported in the literature (ca. a few mm²).

Keywords: Biocathode; Formate dehydrogenase from Candida boidinii; Formic acid, NADH-regeneration; Mesoporous carbon.