Cobalt Phosphotungstate-Based Composites as Bifunctional Electrocatalysts for Oxygen Reactions

Abstract

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key reactions in energy-converting systems, such as fuel cells (FCs) and water-splitting (WS) devices. However, the current use of expensive Pt-based electrocatalysts for ORR and IrO2 and RuO2 for OER is still a major drawback for the economic viability of these clean energy technologies. Thus, there is an incessant search for low-cost and efficient electrocatalysts (ECs). Hence, herein, we report the preparation, characterization (Raman, XPS, and SEM), and application of four composites based on doped-carbon materials (CM) and cobalt phosphotungstate (MWCNT_N8_Co4, GF_N8_Co4, GF_ND8_Co4, and GF_NS8_Co4) as ORR and OER electrocatalysts in alkaline medium (pH = 13). Structural characterization confirmed the successful carbon materials doping with N and/or N, S, and the incorporation of the cobalt phosphotungstate. Overall, all composites showed good ORR performance with onset potentials ranging from 0.83 to 0.85 V vs. RHE, excellent tolerance to methanol crossover with current retentions between 88 and 90%, and good stability after 20,000 s at E = 0.55 V vs. RHE (73% to 82% of initial current). In addition, the number of electrons transferred per O2 molecule was close to four, suggesting selectivity to the direct process. Moreover, these composites also presented excellent OER performance with GF_N8_Co4 showing an overpotential of 0.34 V vs. RHE (for j = 10 mA cm−2) and jmax close to 70 mA cm−2. More importantly, this electrocatalyst outperformed state-of-the-art IrO2 electrocatalyst. Thus, this work represents a step forward toward bifunctional electrocatalysts using less expensive materials.

Keywords

• oxygen reduction
• water oxidation
• carbon materials
• N
• S-doping
• polyoxometalates