Bioelectrochemical systems for energy and materials conversion

Abstract

In bioelectrochemical devices, enzymes, as catalytic parts, are smartly combined with electroconductive surfaces. In this way electroconductive materials might replace one of the natural enzyme substrates in a so-called direct electron transfer mechanism (DET). Alternatively, an artificial enzyme substrate, a so-called mediator, might be used and regenerated electrochemically via a mediated electron transfer mechanism (MET). Currently, there is a high level of interest in the use of enzymes in technical systems, which is mainly triggered by their high selectivity and excellent catalytic activity under mild conditions (neutral pH, low temperature). These features favor applications where technical systems respond selectively to components of complex mixtures as expected in biosensors. Similarly, in enzymatic fuel cells high selectivity enables a simplified fuel cell design where classical fuel cell components, such as separator, fuel cell tank and even housing, can be avoided. This design is beneficial for so-called “energy harvesting” where the fuel/oxidant is directly extracted out of the environment and enables miniaturization beyond the level possible for other electrochemical devices (e.g. conventional batteries or fuel cells). For these reasons enzymatic fuel cells are considered to be promising implantable miniaturized power sources for medical electronic devices such as pacemakers, medical pumps, sensors etc. In addition to sensing and energy conversion applications, electroenzymatic systems offer good prospects for chemical production. Chemicals can be produced in both enzymatic fuel cells and electroenzymatic reactors. In the former case, partial oxidation products of typical “fuels” such as glucose, methanol and ethanol, can be obtained. In the latter case selective reduction processes are targeted where, for example, CO2 can be converted into methanol (or formic acid or formaldehyde). The level of the oxidation/reduction process depends on the length of the enzymatic cascade. The high enzyme selectivity is theoretically reflected in greater product purity than is the case in conventional processes, which further simplifies downstream processing in the chemical industry. This work focuses on enzymatic fuel cells and their applications in energy and material conversions. The performance of these devices is mainly limited by obstacles in electron transfer between enzymes and electrodes. Therefore, most of my research activities were directed towards understanding the limiting processes at the level of enzymatic electrodes. Taking a systematic approach, we studied the main factors influencing the performance of enzymatic electrodes with direct and mediated electron transfer mechanisms. We have shown how the electrode architecture influences the electrode performance. Two different electrode architectures were studied: layered and intermixed. Additionally, the influence of the binder was checked. We have shown that the binder dramatically influences the performance of DET electrodes, while the performance of MET electrodes is affected only moderately. To clarify these issues, the influence of the binder on enzyme agglomeration and the organization on conductive surfaces was studied. Finally, a generic procedure for the preparation of DET and MET electrodes was suggested. We have shown that the choice of mediator is not always simple. Initially two different mediators for glucose oxidase (GOx) were tested. An artificial electron transport chain enabling electron transport between FAD redox center and electrode surface was assembled. Although the results showed a glucose oxidation current, it was not possible to ascribe the electrocatalytic activity to GOx solely but rather to the underlying gold surface. CNT and TTF are promising mediators for further applications of enzymatic electrodes. Our experimental studies have shown that activities of enzymatic electrodes for the same enzyme loading vary within 2 orders of magnitude. The preparation procedure has an impact on the enzyme orientation on the conductive surface as well as its distribution and arrangement inside the porous matrix. Additionally, it impacts the structure of the catalyst layer in terms of porosity, electrode thickness and available surface area; this goes on to affect the enzyme kinetics as well as the charge and mass transport inside of the catalyst layer. Since these cross-correlations cannot always be understood intuitively, in addition to the experimental approaches we also used mathematical modeling as an important tool in accelerating/evaluating the processes towards real applications. In consideration of the fact that the bottleneck of bioelectrochemical systems is still at the level of understanding and improving enzymatic electrodes, we placed a great deal of emphasis on understanding and quantifying the porous enzymatic electrode responses. We were able to demonstrate that electrochemical methods combined with the proper mathematical descriptions can bring significant insights into the reasons limiting the electrode behavior as a whole. Ultimately this led to a better system design. This model-based analysis resulted in a significant improvement in the electroenzymatic fuel cell performance. Between 2011 and 2016 the power output of the same type of enzymatic catalysts improved by a factor of 40. We have also shown that enzymatic fuel cells can be used for gluconic acid conversion. Very high space- time yields (STY) and selectivity can be achieved in such a system. Yet, a preliminary sustainability analysis highlighted some weak points in current electroenzymatic processes, such as the low product titer and cycle times. Both effects impact the E-factor of enzymatic processes. It will be necessary to achieve further improvements to the enzyme utilization and an increase in cycle times. For the development of sustainable processes, the issues relating to the separation (recycling) and toxicity of selected mediators must be carefully considered. Finally, in order to push forward new exciting bioelectrochemical applications, closer interaction between different disciplines (electrochemistry, biology, bioelectrochemistry, material science and reaction engineering) is strongly advised.