Enzyme bioelectrochemistry deals with the integration of oxidoreductase enzymes into electrodes to enable and study electron transfer between the surface of the solid material and the biological catalyst. High surface area enzyme immobilization strategies have been employed to achieve higher enzyme loading and thus higher current densities, but these porous electrode architectures enhance the formation of local chemical gradients. The selectivity and activity of the enzyme is strongly dependent on such changes in the local environment as e.g. B. the substrate concentration, the pH and the concentration of the electrolyte species. Here, electrochemistry and computational techniques are applied to explore and thus optimize the local environment of the fuel-producing oxidoreductases, hydrogenase and formate dehydrogenase, within porous electrodes.
Bioelectrochemistry uses a range of high surface area meso- and macroporous electrode architectures to increase protein loading and electrochemical current response. While the local chemical environment has been studied in small molecule and heterogeneous electrocatalysis, the conditions in enzyme electrochemistry are still often set based on bulk solution assays without adequately considering the non-equilibrium constraints of the limited electrode space. Here we apply electrochemical and computational techniques to probe the local environment of fuel-producing oxidoreductases within porous electrode architectures. This improved understanding of the local environment allowed easy manipulation of the electrolyte solution by adjusting the pH and buffer pKone to achieve an optimal local pH for maximum activity of the immobilized enzyme. Application to macroporous inverse opal electrodes took advantage of higher loading and increased mass transport, and consequently the electrolyte was adjusted to achieve −8.0 mA ⋅ cm−2 for the h2 evolutionary response and −3.6 mA ⋅ cm−2 for the CO2 Reduction reaction (CO2RR), showing an 18-fold improvement over previously reported enzymatic CO2RR systems. This research emphasizes the critical importance of understanding the limited enzymatic chemical environment, thereby expanding the known possibilities of enzyme bioelectrocatalysis. These considerations and insights can be directly applied to bio(photo)electrochemical fuels and chemical synthesis as well as to enzymatic fuel cells to greatly improve the fundamental understanding of the enzyme-electrode interface as well as device performance.
- Accepted November 16, 2021.
Author contributions: EEM and ER designed research; EEM conducted experimental research; SJC performed computer modeling; AMC, ARO, and IACP provided the enzymes and performed dissolution studies; EEM, SJC and ER analyzed data; and EEM and ER wrote the paper.
The authors declare no competing interests.
This article is a PNAS Direct Submission. NP is an Editorial Board invited guest editor.
This article has supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2114097119/-/DCSupplemental.
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