Water oxidation remains the principal half-reaction of water splitting to produce clean chemical fuels (e.g., hydrogen) due to inherently high reaction barrier, multiple electron transfer, and proton-coupled electron transfer (PCET). The development of efficient, sustainable water oxidation catalysts based on earth-abundant transition metals has been pursued for the past decades. Electrodeposited cobalt-based oxygen evolving catalyst (Co-OEC) is known to show high activity for water oxidation under neutral conditions. However, the applied potentials can lead to the dissolution of Co ions from Co-OEC, causing the physical delamination of the catalysts from the electrode. Here we show a simple method to stabilize the interface between Co-OEC and electrode using polydopamine (pD) as a redox-active, adhesive layer. In natural photosystem II, a tyrosine Z, which is well oriented with the oxygen evolving manganese oxo-cluster, facilitates PCET through redox-mediation. Similar as the role of tyrosine Z, we speculated that the pD layer can be redox coupled with Co-OEC to mediate PCET at the interface with electrode (e.g., ITO) through the redox reaction of catechol and (hydro)quinone. X-ray adsorption near-edge structure (XANES) analysis shows that Co-OEC integrated with the pD layer has higher oxidation states compared to the catalyst without pD at the same water oxidation potential. Tafel slope of Co-OEC/pD/ITO (185 mV decade-1) is much lower than that of Co-OEC/ITO (261 mV decade-1). In addition, Co-OEC/pD/ITO exhibits the lower overpotential (297 mV) compared to Co-OEC/ITO (354 mV), indicating that the redox-active pD interlayer promotes water oxidation through PCET. Interestingly, the Co-OEC is easily delaminated from the electrode under 0.6 V (vs. NHE) due to the decreased oxidation state of the cobalt (from Co3+ to Co2+). The pD interlayer prevents the delamination of the Co-OEC by reducing the pD itself instead of Co-OEC. The adhesive property of pD also lead to form mechanically stable contact between the catalyst and the electrode, sustaining the water oxidation reaction even under vigorous working conditions. We expect that the redox-active pD interlayer can be utilized to a variety of electrochemical catalysts for the enhanced catalytic efficiency and sustainable catalytic activity.

Polydopamine coatings for stabilizing the interface between cobalt-based water oxidation catalysts and electrode

253rd National Meeting (2017)

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