Oxygen reduction reaction (ORR) is one of the fundamental reactions that occur on the cathode of fuel cells. The sluggish kinetics of ORR and the high cost of the conventionally used ORR electrocatalyst mandate extensive research towards the development of inexpensive, high-performance and stable electrocatalysts. Various supported nanostructures of pristine noble metals and its alloys, transition metals/metal oxides (MOs), chalcogens/pnictogens doped metals/MOs, etc. have attracted great attention towards ORR in both acidic and alkaline electrolytes. This review primarily documents the recent advancements and challenges of supported nanomaterials with fascinating properties towards ORR. This study focuses on the use of carbon-based materials as support. The significant factors like size, morphology, hetero-atom doping, defects and interfaces (metal/metal, metal/oxide and metal/hydroxide) that can tailor ORR activity are portrayed elaborately. The electrochemical techniques and parameters that are adopted mainly during data collection and evaluation of ORR are also discussed.

We report a controllable synthesis of Co 3 O 4 nanooctahedron anchored on Vulcan XC 72R (Oct-Co 3 O 4 /C) to explore the crystal symmetry effects on the bifunctional electrocatalysis of oxygen reduction and oxygen evolution reaction (ORR/OER) in alkaline electrolyte. Systematic electrochemical measurements reveal that Oct-Co 3 O 4 /C exhibits remarkable ORR/OER activity with a higher ORR onset potential (E onset = 0.89 V), half-wave potential (E 1/2 = 0.68 V), large limiting current density (j = −6.38 mA cm −2 ), and significantly lower overpotential for 10% energy conversion (η 10 = 0.45 V). The superior bifunctional activity can be attributed to the synergistic contribution of high specific surface area (131.3 m 2 g −1 ), electrochemically active surface area (ECSA = 35.83 m 2 g −1 ), and rich oxygen vacancies in the well-defined octahedral crystal symmetry. Furthermore, the chronoamperometric and accelerated durability test demonstrated superior stability and durability for the catalytic process. This study underscores the significance of morphology control in the design of an advanced ORR/OER electrocatalyst in alkaline medium.

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Pd-based electrocatalysts are considered as suitable replacement for Pt-based ones owing to the similar electronic structure of Pd, relatively low cost, and superior catalytic performance. The activity and utilization efficiency of Pd in electrochemical reactions can be increased by alloying with transition metals, like Cu and Co. Herein, we report the synthesis of a homogeneously embedded Pd 2 CuCo alloy in the carbon matrix. Compared with the benchmark Pt/C, the as-synthesized Pd alloy electrocatalyst exhibited an improved limiting current density (J m ; −5.65 mA cm −2 ), a smaller Tafel slope (58.7 mV dec −1 ), and a comparable onset potential (E onset ; 0.91 V vs RHE). Comparatively, the optimized Pd 2 CuCo showed a current retention of ∼96% after a 6 h stability test and only a 4 mV loss in the half-wave potential (E 1/2 ) after 10,000 redox cycles, while Pt/C showed a current retention of 72.5% and a loss of 5 mV in E 1/2 . Meanwhile, the Pd 2 CuCo alloy electrocatalyst also demonstrated excellent methanol tolerance compared to benchmark Pt/C. In addition, the electrocatalyst showed higher activity and long-term stability toward the formic acid oxidation reaction than Pd/C, signifying the potential for wide applicability.