Evidence for Oxygen Reduction Reaction Activity of a Ni(OH)2/Graphene Oxide Catalyst

Effective catalysis of the oxygen reduction reaction in alkaline solution is crucial to a variety of electrochemical energy storage and conversion devices like metal-air batteries and alkaline fuel cells.  While Pt-based catalysts exhibit excellent activity, their high cost and lack of stability motivates the search for alternative electrocatalysts.  Non-noble metal oxides like Mn_xO_y have shown particular promise, especially when supported by carbon nanomaterials like graphene.  Most recently, the performance and stability of a Mn_xO_y has shown additional improvement with the doping of Ni(OH)₂.  The performance improvement is typically ascribed to stabilization of the Mn³⁺/Mn⁴⁺ couple, but it is also possible that Ni(OH)₂ itself displays significant oxygen reduction reaction catalytic activity.  This work provides the first report of the oxygen reduction reaction activity of a Ni(OH)₂nanoparticle catalyst supported by graphene oxide in basic media.

A Ni(OH)₂/graphene oxide material was synthesized using a single step microwave reduction method. Cyclic voltammetry shows that the Ni(OH)₂/graphene oxide material has a peak potential of -310 mV vs. Ag/AgCl, a shift of + 110 mV as compared to the unsupported Ni(OH)₂ nanoparticles and + 90 mV as compared to the graphene oxide support alone.  Rotating disk electrode studies confirm that the activity of the Ni(OH)₂/graphene oxide catalyst is greater than either the graphene oxide support or the Ni(OH)₂ nanoparticles alone.  Significantly, the limiting current density of the oxygen reduction reaction on the Ni(OH)₂/graphene oxide catalyst is 1.3 mA/cm² (as normalized by electrochemically active surface area) and the electron transfer number is 3.5.  Chronoamperometry demonstrates that the Ni(OH)₂/graphene oxide material is relatively stable, with the oxygen reduction current density sustaining a steady state value of 60 % of its initial value. Electrochemical impedance spectroscopy shows that the charge transfer resistance of the Ni(OH)₂/graphene oxide catalyst is lower than either the Ni(OH)₂ nanoparticles or the graphene oxide support.  Taken together, these data indicate that the Ni(OH)₂/graphene oxide catalyst exhibits significant oxygen reduction reaction activity which may be related to the interaction of the support and the nanoparticles.