Over the past decades, there has been a wealth of interest in electrofuels, where electricity is used to generate fuel electrochemically. As an important chemical in modern society, ammonia production from dinitrogen have gained particular attention. Although a lot of molecular and heterogeneous electrocatalysts have been studied, reducing dinitrogen to ammonia still represents a technological challenge. Therefore, nitrogenase, as the only enzyme known to reduce molecular nitrogen to ammonia, become an oxidoreductase of great interests. Nitrogenase enzymes are a bienzyme cascade, where the catalytic protein (MoFe) gain electrons from Fe protein, which delivered electrons and ATP to nitrogenase. Nitrogenase bioelectrocatalysis has been first realized using mediated electron transfer systems. It was found that cobaltocene/cobaltocenium is an effective mediator for MoFe nitrogenase, and a variety of substrate including proton, azide and nitrite have been studied. However, there is no mediated bioelectrocatalysis of N2 been observed with MoFe alone. Therefore, we combined MoFe protein with Fe protein into a bienzyme cascade. Using methyl viologen as a mediator, N2 can be reduced to NH3 with relatively high faradic efficiency (59 ± 6%). Coupling of this nitrogenase cathode with a bioanode that utilizes the enzyme hydrogenase to oxidize molecular hydrogen (H2) results in an enzymatic fuel cell (EFC) able to produce NH3 from H2 and N2 with Faradaic efficiency of 26.4 %, while simultaneously producing an electrical current. Importantly, this EFC produces NH3 and electrical energy in a carbon-neutral manner. To address the oxygen sensitivity issue, a structural protein, Shethna, were introduced to protect nitrogenase from oxygen. Additionally, to eliminate the need for FeP and ATP regeneration system, a direct electron transfer system was set up by immobilization of MoFe in a pyrene-modified hydrogel, which allowed for bioelectrocatalysis happened at the potential of electron relay. Future research efforts should focus on more mechanism and kinetic studies, to help better understanding of nitrogen reduction.

Nitrogenase bioelectrocatalysis: From electron-transfer mechanisms to energy applications

2019 Spring National Meeting

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