Abstract
High-temperature tolerant enzymes offer multiple advantages over enzymes from mesophilic organisms for the industrial production of sustainable chemicals due to high specific activities and stabilities towards fluctuations in pH, heat, and organic solvents. The production of molecular hydrogen (H2) is of particular interest because of the multiple uses of hydrogen in energy and chemicals applications, and the ability of hydrogenase enzymes to reduce protons to H2 at a cathode. We examined the activity of Hydrogen-Dependent CO2 Reductase (HDCR) from the thermophilic bacterium Thermoanaerobacter kivui when immobilized in a redox polymer, cobaltocene-functionalized polyallylamine (Cc-PAA), on a cathode for enzyme-mediated H2 formation from electricity. The presence of Cc-PAA increased reductive current density 340-fold when used on an electrode with HDCR at 40 °C, reaching unprecedented current densities of up to 3 mA·cm−2 with minimal overpotential and high faradaic efficiency. In contrast to other hydrogenases, T. kivui HDCR showed substantial reversibility of CO-dependent inactivation, revealing an opportunity for usage in gas mixtures containing CO, such as syngas. This study highlights the important potential of combining redox polymers with novel enzymes from thermophiles for enhanced electrosynthesis.
Highlights
Over the time span of the experiment, faradaic efficiency of cobaltocene-mediated hydrogen evolution converged at approximately 90%, showing that reductive waves observed during CV and reductive currents observed in amperometric i-t are useful indicators of hydrogen production
cobaltocene-functionalized polyallylamine (Cc-PAA) was previously found to enhance hydrogen electrosynthesis by hydrogenases, with 20- to 40-fold increase in reductive current densities compared to the absence of the redox polymer [18]
We showed that Cc-PAA, paired with Hydrogen-Dependent CO2 Reductase (HDCR) of thermophilic T. kivui, increased current density 340-fold in an RDE system
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Hydrogen is interesting due to its high energy content, low redox potential, and usage in a wide variety of industrial processes [4,6] Enzymes, such as hydrogenases and formate dehydrogenases, are promising catalysts for conversion of electrical energy to storable chemical energy, due to their high selectivities and activities [3,7]. Given the high specific activity of thermophilic enzymes at elevated temperatures, polymers capable of increasing electron flux to the enzyme at a cathode would further increase activity compared to current electrochemical enzymatic hydrogen evolution reaction (HER) systems. We demonstrate the hydrogen evolution activity of T. kivui HDCR via direct electron transfer when immobilized at an electrode. Cc-PAA is a promising redox polymer for enzyme-mediated, enhanced hydrogen production even at elevated temperatures, making it an auspicious component of new technology for use in chemical storage for renewable energy. All subsequent electrochemical tests were carried out in the optimal buffer: 2 M sodium citrate, 400 mM potassium phosphate, pH 5.0
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