Enzymatic biofuel cells: 30 years of critical advancements
Introduction
One common example of a bioelectronic device is an enzymatic biofuel cell, as shown in Fig. 1. Enzymatic biofuel cells are a type of fuel cell that utilizes enzymes as the electrocatalysts to catalyze the oxidation of fuel and/or the reduction of oxygen or peroxide for energy conversion to electricity. Most traditional fuel cell electrocatalysts are conductive-metal nanoparticle catalysts that operate at temperatures ranging from 45 °C to 150 °C. These catalysts have the advantage of high stability and high activity in highly acidic and/or basic environment. However, they are plagued with passivation issues that require simple and high purity fuels (i.e. hydrogen and methanol). On the other hand, living organisms can consume and metabolize complex fuels (i.e. sucrose, fructose, etc.) and fuel mixtures and their catalysts do not have the same issues. Therefore, the field of enzymatic biofuel cells has expanded over the last 3 decades, and it is now possible to consider applications (i.e. implantable fuel cells, JP-8 fuel cells, etc.) where traditional metal nanoparticle electrocatalysts have limitations. As shown in Fig. 1, enzymatic biofuel cells are more complex than traditional fuel cells, because typically open circuit potentials are significantly lower than theoretical due to cofactor redox potentials, enzyme redox potentials, and mediator redox potentials. This review article will detail the advancements in the field of enzymatic biofuel cells over the last 30 years and discuss the challenges that will face enzymatic biofuel cell development in the next 30 years.
Section snippets
History before the debut of Biosensors & Bioelectronics
Although microbial biofuel cells have been studied for over a century, enzymatic biofuel cell history only goes back to the early 1960s. In 1964, Yahiro et al. (1964) invented the concept of enzymatic biofuel cells with a glucose oxidase bioanode and a Pt cathode. This system had very low open circuit potentials (175–350 mV), but showed the proof of concept that an oxidoreductase enzyme could catalyze a fuel cell half-reaction, in this case, the oxidation of glucose to gluconolactone, and
Cofactor regeneration
Oxidoreductase enzymes contain or require redox cofactors that change oxidation state during substrate catalysis. There are a variety of natural oxidoreductase organic and inorganic cofactors including: nicotinamide adenine dinucleotide (NAD+), nicotinamide adenine dinucleotide phosphate (NADP+), flavin adenine dinucleotide (FAD), pyrolloquinoline quinone (PQQ), hemes, iron–sulfur clusters, coenzyme Q, coenzyme F420, flavin mononucleotide (FMN) and ascorbic acid. Many of these cofactors are
Applications of enzymatic biofuel cells
There are a variety of applications for enzymatic biofuel cells. When researchers consider the low current/power density of enzymatic biofuel cells, the first application that comes to mind is powering sensors. In 2001, Katz and Willner invented the concept of self-powered biosensors that utilized a biofuel cell as a biosensor for the fuel (Katz et al., 2001). This was later translated to include inhibition and activation based self-powered biosensors (Meredith and Minteer, 2011, Wen et al.,
Challenges
Significant progress has been made in the field of biofuel cells in the last 30 years, but several issues still need to be addressed for the commercial viability of enzymatic biofuel cells for the applications discussed above. Currently, enzymatic biofuel cells are still plagued with lower than desirable stability and electrochemical performance. Performance is a vague term that can mean lower current density, power density, volumetric catalytic activity, efficiency, energy density, or open
Conclusions
Although enzymatic fuel cells predate the first issue of Biosensors & Bioelectronics, the field was very young and focused toward only mediated electron transfer. The last three decades have seen major advancements in the field to address low open circuit potentials, low current and power densities, low fuel efficiency, and low stability. These advances have resulted in open circuit potentials increasing from 175 mV to almost 1 V and current densities increasing from nA/cm2 to mA/cm2, as shown in
Acknowledgments
The authors would like to thank the National Science Foundation for funding (Grants nos.1158943 and 1057597).
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