Engineering enzyme-electrode interactions for bioenergy applications
Keywords: Chemical Engineering.

Rising greenhouse gas levels from non-renewable carbon usage has led to a subsequent rise in global temperatures which have many detrimental environmental effects. Rapidly expanding interest in renewable technologies has led to new forms of energy production and chemical processes. However, these new technologies need efficient energy storage solutions and highly effective catalysts with low levels of rare-earth materials. Oxidoreductase enzymes offer a promising solution in both areas with high specificity and versatility. When used in an electrochemical system, this sub-class of enzymes allows for conversion of electrical energy into stable chemical energy with only a low concentration of rare elements in their active sites. This thesis focuses on the use of an important class of oxidoreductase enzymes -- hydrogenases -- for the design of enzymatic hydrogen evolution catalysts. In this dissertation, enzymatic activity of the heterodisulfide reductase super complex (Hdr-SC) of Methanococcus maripaludis is first characterized. This work focuses on understanding three different complexes in which the Hdr-SC can exist, and the kinetics of each complex. This work sheds light on electron bifurcation, an important phenomenon in the methanogenesis pathway which allows for the reduction of a low potential ferredoxin by a thermodynamically unfavorable electron donor. Electrochemical activity of Hdr-SC was also shown, offering this enzyme complex as an option for enzymatic electrochemical catalytic systems. Once activity of an enzyme is well-characterized, combining the enzyme with redox mediators in electrochemical systems allows for further increase in activity and stability with the electrode acting as an electron donor or acceptor. This work demonstrates that the redox polymer cobaltocene-functionalized poly(allylamine) (Cc-PAA) enhances electrochemical activity of the Hdr-SC hydrogenases, among other classes of hydrogenases. Cc-PAA immobilizes enzymes at the electrode surface and enables low-potential electron mediation by a covalently bound mediator which cannot diffuse away over extended times of operation. Further, the ability of Cc-PAA to mediate electron transfer to thermophilic enzymes, which tend to have higher activities at higher temperatures, is demonstrated. The redox polymer was shown to greatly enhance both current consumption and hydrogen formation rates and attained high levels of faradaic efficiency. Sensitivity of the polymer to degradation at high temperatures is investigated and discussed. This work contributed important advances in the usage of hydrogenases in electrochemical catalytic designs and offers insights that can be broadly applied to improve enzymatic electrochemistry for renewable energy storage and conversion. To further this impact, recent advances in industrially relevant enzyme classes are evaluated, with two technologies identified for focus in future work: advanced redox polymers and gas diffusion electrodes. Combining these with the described work will improve renewable energy technologies to meet energy and chemical needs