Polyoxotungstate@Carbon Nanocomposites As Oxygen Reduction Reaction (ORR) Electrocatalysts

The oxygen reduction reaction (ORR) has a crucial function as the cathode reaction in energy-converting systems, such as fuel cells (FCs), which contributes to a sustainable energy supply. However, the current use of precious Pt-based electrocatalysts (ECs) is a major drawback for the economic viability of fuel cells. Hence, it is urgent to develop cost-effective and efficient electrocatalysts (ECs) without noble metals to substitute the Pt-based ECs. Herein, we report the preparation and application as ORR electrocatalysts of four new nanocomposites based on sandwich-type phosphotungstate (TBA)(7)H-3[Co-4(H2O)(2)(PW9O34)(2)] (TBA-Co-4(PW9)(2)) immobilized onto different carbon nanomaterials [single-walled carbon nanotubes (SWCNT), graphene flakes (GF), carbon nanotubes doped with nitrogen (N-CNT), and nitrogen-doped few layer graphene (N-FLG)]. In alkaline medium, the four nanocomposites studied presented comparable onset potentials (0.77-0.90 V vs RHE), which are similar to that observed for Pt/C (0.91 V vs RHE). Higher diffusion-limiting current densities (j(L,0.26V,1600 rpm) = -168.3 mA cm(-2) mg(-1)) were obtained for Co-4(PW9)(2)@N-CNT, as compared to Pt/C electrode -130.0 mA cm(-2) mg(-1)) and the other ECs (-45.0, -50.7, and -87.5 mA cm(-2) mg(-1) for Co-4(PW9)(2)@SWCNT, Co-4(PW9)(2)@ GF, and Co-4(PW9)(2)@N-FLG, respectively). All the Co-4(PW9)(2)@CM ECs showed selectivity toward direct O-2 reduction to water with the exception of Co-4(PW9)(2)@GF where a mixture of the 2- and 4-electron mechanisms is observed. Furthermore, low Tafel slopes were obtained for all the nanocomposites (68-96 mV dec(-1)). Co-4(PW9)(2)@CM ECs also showed excellent tolerance to methanol with no significant changes in current density, in contrast to Pt/C (decrease of approximate to 59% after methanol addition) and good long-term electrochemical stability with current retentions between 75 and 84%.