The dinuclear Ru diazadiene olefin complex, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2], is an active catalyst for hydrogen evolution in a Polymer Exchange Membrane (PEM) water electrolyser. When supported on high surface area carbon black and at 80 °C, [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2]@C evolves hydrogen at the cathode of a PEM electrolysis cell (400 mA cm-2, 1.9 V). A remarkable turn over frequency (TOF) of 7800 molH2 molcatalyst -1 h-1 is maintained over 7 days of operation. A series of model reactions in homogeneous media and in electrochemical half cells, combined with DFT calculations, are used to rationalize the hydrogen evolution mechanism promoted by [Ru2(OTf)(μ-H)(Me2dad)(dbcot)2].
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