Biological Fuel Cells: Cardinal Advances and Critical Challenges

Abstract

The always-young field of bioelectrochemistry has seen an unusual development: a move away from its comfortable analytical roots toward energy harvesting and power generation, as manifested by an explosion of projects, symposia and published papers. Wiley(-VCH) publications have provided a platform for this emerging trend by not only being the home for many of those individual breakthrough and developmental papers, but also by dedicating the pages of its well-established Electroanalysis for the first special issue on biological fuel cells (or biofuel cells) [1] in 2010, a special issue on microbial fuel cells in ChemSusChem in 2012, [2] and in 2014 publishing a book that is a compendium of the enzymatic fuel cell effort to date and another dedicated to implanting these biofuel cells in living organisms. [3, 4] Now, this growing, exciting and captivating field is presenting itself on the pages of the special issue of one of the newest members of the Wiley family, ChemElectroChem. The guest-editors are among the most engaged contributors to this field, and you will find an authorship list that will both convince you of the depth of the questions asked and the breadth of the solutions suggested. We are very happy to present these 30 + original contributions and focused topical reviews that provide a snapshot of a burgeoning field so dynamic and explosive in ideas and, sometimes, controversy, that the only problem the guest-editors had, was how not to lose the momentum. We thank all the contributors for trusting this issue with their best work of 2014. It was an amazing experience to not hear a single “no” in the process of recruiting this “by invitation only” volume, but rather to only receive suggestions on who else should be invited as well. This speaks volumes of the sense of camaraderie, mutual respect and promotion of the best science in the field of electrochemistry of biological systems for energy conversion, harvesting and power generation. Biological fuel cells (BFCs) are usually understood as devices that utilize biological systems (enzymes, organelles, microorganisms or tissues) and their intrinsic catalytic activity for energy transformation reactions. As those processes are usually associated with ubiquitous fuels, available in the environment, both natural and anthropogenic, the practical application of the BFCs is often associated with energy harvesting. The two main classes of these devices are the enzymatic fuel cells (EFCs) and microbial fuel cells (MFCs) divided by the primary use of either isolated enzymes or living microorganisms as catalysts. A broader definition also includes an abiotic fuel cell (or specific electrodes) that are designed to function by converting biologically available fuel (biomass, components in cellular or intercellular fluids of a living organism), regardless of the active catalytic components. Such abiotic electrodes are usually combined in a hybrid device with a bio-electrode (enzymatic or microbial), or an abiotic cell is used for energy harvesting while implanted in living tissue. The full diversity of these approaches is captured in this special issue. The EFCs are predominantly associated with small/ miniature devices with relatively high power density output that can be designed as a bio-battery, either containing all their fuel in a compartment of the cell or as a flow-through, open system that utilizes an external fuel stream. Often EFCs share design concepts with electrochemical biosensors, with the principal difference being the spontaneity of the electrochemical reactions happening at the electrodes. EFCs, however, are used as self-powered sensors and/or logical devices that can be integrated as electrical actuators in complex systems. The critical drawback of the EFCs has always been the timespan of their operation, which is critically limited by the environmental stability of immobilized enzymes while operating in abiotic environments. This special issue addresses the issues of integration of enzymes and nanomaterials, both carbonaceous (carbon nanotubes, their composites or synthetic hierarchically structured carbons) and metallic nanoparticles. Special attention is paid to the state and modification of the electrode interface to facilitate the charge transfer between the enzyme and the electrode for co-factors, among them PQQ, FAD and NADH. The works presented here address the issues of fuel flexibility and design for utilization of complex fuels, such as monosaccharides, sugars, starches and others. The design solutions vary from laboratory test units and well-defined electrodes to practical prototypes suitable for in vivo implantation or environmental testing.