Polyoxometalate-graphene Electrocatalysts for the Hydrogen Evolution Reaction

Electrocatalytic reduction of water, also known as the hydrogen evolution reaction (HER), may be one of the solutions for a sustainable energy supply in the future, but the current use of precious platinum electrocatalysts is a major drawback, owing to the high cost. So, it is urgent to discover cost-effective and earth-abundant materials to replace precious-metal electrocatalysts in order to make this technology economically viable. Herein, we report the preparation and application of three new nanocomposites based on polyoxometalates (POMs) and reduced oxidized-graphene flakes (rGF_ox) as HER electrocatalysts (ECs). In highly acidic medium (pH=0.3), the POM@rGF_ox ECs showed similar overpotentials (0.033-0.044 V vs. RHE) for a current density of 10 mAcm(-2), and these are very close to the state-of-the-art HER EC, 20 wt% Pt/C (0.024 V). The POM-based nanocomposites also exhibited excellent exchange current densities (0.65-1.02 mAcm(-2)) and very low Tafel slopes (0.033-0.041 Vdec(-1)). P5W30@rGF_ox exhibited the best HER metrics and also showed long-term electrochemical stability, as the overpotential only changed 0.009 V for j=10 mAcm(-2) after 1000 linear sweep voltammetry cycles. Besides highlighting POM-oxidized-graphene flakes composites as a new class of nanomaterial with several important electrochemical applications, this work also points out that the POM shape, size, charge, and composition (W/P ratio) may play an important role in fine-tuning the HER activity, and opens new avenues for designing a new set of highly efficient, cost-effective, and durable non-precious-metal HER ECs in acidic medium.