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)7H3[Co4(H2O)2(PW9O34)2] (TBA-Co4(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 Co4(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 Co4(PW9)2@SWCNT, Co4(PW9)2@GF, and Co4(PW9)2@N-FLG, respectively). All the Co4(PW9)2@CM ECs showed selectivity toward direct O2 reduction to water with the exception of Co4(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). Co4(PW9)2@CM ECs also showed excellent tolerance to methanol with no significant changes in current density, in contrast to Pt/C (decrease of ≈59% after methanol addition) and good long-term electrochemical stability with current retentions between 75 and 84%.