Following Electroenzymatic Hydrogen Production by Rotating Ring–Disk Electrochemistry and Mass Spectrometry**

Gas-processing metalloenzymes are of interest to future bio- and bioinspired technologies. Of particular importance hydrogenases nitrogenases, which both produce molecular hydrogen (H2) from proton (H+) reduction. Herein, we report on the use rotating ring–disk electrochemistry (RRDE) mass spectrometry (MS) follow production H2 isotopes produced deuteron (D+) reduction (HD D2) using [FeFe]-hydrogenase Clostridium pasteurianum, a model hydrogen-evolving metalloenzyme. This facilitates enzymology studies independent non-innocent chemical reductants. We anticipate that these approaches will be value in resolving catalytic mechanisms H2-producing design catalysts for N2 fixation.