How to Reliably Report the Overpotential of an Electrocatalyst

Catalysts for oxygen reduction and evolution reactions are at the heart of key renewable-energy technologies including fuel cells and water splitting. Despite tremendous efforts, developing oxygen electrode catalysts with high activity at low cost remains a great challenge. Here, we report a hybrid material consisting of Co₃O₄ nanocrystals grown on reduced graphene oxide as a high-performance bi-functional catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Although Co₃O₄ or graphene oxide alone has little catalytic activity, their hybrid exhibits an unexpected, surprisingly high ORR activity that is further enhanced by nitrogen doping of graphene. The Co₃O₄/N-doped graphene hybrid exhibits similar catalytic activity but superior stability to Pt in alkaline solutions. The same hybrid is also highly active for OER, making it a high-performance non-precious metal-based bi-catalyst for both ORR and OER. The unusual catalytic activity arises from synergetic chemical coupling effects between Co₃O₄ and graphene.

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are traditionally carried out with noble metals (such as Pt) and metal oxides (such as RuO₂ and MnO₂) as catalysts, respectively. However, these metal-based catalysts often suffer from multiple disadvantages, including high cost, low selectivity, poor stability and detrimental environmental effects. Here, we describe a mesoporous carbon foam co-doped with nitrogen and phosphorus that has a large surface area of ∼1,663 m(2) g(-1) and good electrocatalytic properties for both ORR and OER. This material was fabricated using a scalable, one-step process involving the pyrolysis of a polyaniline aerogel synthesized in the presence of phytic acid. We then tested the suitability of this N,P-doped carbon foam as an air electrode for primary and rechargeable Zn-air batteries. Primary batteries demonstrated an open-circuit potential of 1.48 V, a specific capacity of 735 mAh gZn(-1) (corresponding to an energy density of 835 Wh kgZn(-1)), a peak power density of 55 mW cm(-2), and stable operation for 240 h after mechanical recharging. Two-electrode rechargeable batteries could be cycled stably for 180 cycles at 2 mA cm(-2). We also examine the activity of our carbon foam for both OER and ORR independently, in a three-electrode configuration, and discuss ways in which the Zn-air battery can be further improved. Finally, our density functional theory calculations reveal that the N,P co-doping and graphene edge effects are essential for the bifunctional electrocatalytic activity of our material.

This review summarizes recent research progress and perspectives on noble-metal-free bifunctional heterogeneous electrocatalysts towards hydrogen and oxygen evolution reactions in overall water splitting. Production of hydrogen by water splitting is an appealing solution for sustainable energy storage. Development of bifunctional catalysts that are active for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is a key factor in enhancing electrochemical water splitting activity and simplifying the overall system design. Here, recent developments in HER–OER bifunctional catalysts are reviewed. Several main types of bifunctional water splitting catalysts such as cobalt-, nickel- and iron-based materials are discussed in detail. Particular attention is paid to their synthesis, bifunctional catalytic activity and stability, and strategies for activity enhancement. The current challenges faced are also concluded and future perspectives towards bifunctional water splitting electrocatalysts are proposed.