Molecular hydrogen is a major high‐energy carrier for future energy technologies, if produced from renewable electrical energy. Hydrogenase enzymes offer a pathway for bioelectrochemically producing hydrogen that is advantageous over traditional platforms for hydrogen production because of low overpotentials and ambient operating temperature and pressure. However, electron delivery from the electrode surface to the enzyme's active site is often rate‐limiting. Here, it is shown that three different hydrogenases from Clostridium pasteurianum and Methanococcus maripaludis, when immobilized at a cathode in a cobaltocene‐functionalized polyallylamine (Cc‐PAA) redox polymer, mediate rapid and efficient hydrogen evolution. Furthermore, it is shown that Cc‐PAA‐mediated hydrogenases can operate at high faradaic efficiency (80–100 %) and low apparent overpotential (−0.578 to −0.593 V vs. SHE). Specific activities of these hydrogenases in the electrosynthetic Cc‐PAA assay were comparable to their respective activities in traditional methyl viologen assays, indicating that Cc‐PAA mediates electron transfer at high rates, to most of the embedded enzymes.
While enzymes catalyze reactions with high selectivity and specificity at ambient temperature and pressure, electroactive enzymes retain such remarkable catalytic properties for catalyzing redox reactions on the basis of direct or mediated electron transfer with an electrode. They offer the possibility for alternative, environmentally friendly production for a variety of industrially relevant chemicals, such as hydrogen gas, ammonia, and methanol. This Perspective summarizes the recent progress in electrochemistry involving hydrogenases, nitrogenases, and enzymes involved in the chemistry of one-carbon compounds. We discuss the current challenges of achieving high catalytic rates and stability on the basis of two promising improvements to existing enzymatic electrochemical systemsredox polymers and gasdiffusion electrodes.
Hydrogenotrophic methanogens oxidize molecular hydrogen to reduce carbon dioxide to methane. In methanogens without cytochromes, the initial endergonic reduction of CO to formylmethanofuran with H-derived electrons is coupled to the exergonic reduction of a heterodisulfide of coenzymes B and M by flavin-based electron bifurcation (FBEB). In Methanococcus maripaludis, FBEB is performed by a heterodisulfide reductase (Hdr) enzyme complex that involves hydrogenase (Vhu), although formate dehydrogenase (Fdh) has been proposed as an alternative to Vhu. We have identified and purified three Hdr complexes of M. maripaludis, where homodimeric Hdr complexes containing (Vhu) or (Fdh) were found, in addition to a heterocomplex that contains both Vhu and Fdh. Formate was found in in vitro assays using the purified Hdr complex to act directly as the electron donor for FBEB via the associated Fdh. Furthermore, while ferredoxin was slowly reduced to 30% [-360 mV vs the standard hydrogen electrode (SHE)] by H and formate (0.8 atm and 30 mM, according to thermodynamics), the addition of CoB-S-S-CoM as the high-potential electron acceptor ( E°' = -140 mV vs SHE; to induce FBEB) resulted in the rapid and more complete reduction of Fd to 94% (-455 mV vs SHE).
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.