23. Energy Conversion Based on Bio(electro)catalysts

Redox enzymes can be efficiently coupled with an electrode surface giving prospect of highly efficient and selective bio(electrochemical) transformations for energy conversion and/or production of commodities or fine chemicals. One example is glucose oxidase that immobilized on the electrode surface and in the presence of glucose and oxygen reduction cathode generates electricity and D-glucono-1,5-lactone with applications in different industries. Other examples might comprise whole enzymatic cascades performing complex sequences of biochemical reactions, turning, for example, such inert and environmentally polluting substances (like CO₂) into useful commodities (e. g., methanol). These processes have a significant potential for development of new enzyme-based production systems, with electrochemistry playing an important role, especially regarding electrochemical regeneration of redox enzymes (redox cofactors). Although the electrochemical regeneration is feasible, its efficiency is still too low to be considered competitive for industrial applications. In this contribution we consider some important aspects of electrochemical regeneration of enzymes and common co-factors. At first, working principles of two typical representatives of bioelectrochemical systems will be described, followed by a short discussion of so-called cell free systems and their relationship to bioelectrochemical systems. For practical development of bioelectrochemical systems, the thermodynamics of related processes as well as kinetics are important. We give some examples of enzymes showing reversible electrode behavior, as an inspiration. Mathematical modeling will play a significant role in the design and optimization of bioelectrochemical systems. For this reason, we show how nonlinear mathematical models for studying the kinetics ofenzymatic processes can be developed. Finally, we discuss some practical aspects of biotransformation with redox enzymes, including examples of electron transfer mechanisms, enzyme adaptation on process conditions, development of electrodes etc.