Direct Conversion of Natural Date Pulp to Hierarchically Porous Carbon Spheres As Efficient and Stable Oxygen-Reduction Electrocatalysts

Abstract

The impending global energy crisis and environmental concerns associated with the excessive use of fossil fuels have prompted us to explore highly sustainable alternative energy sources as well as highly efficient eco-friendly energy conversion and storage technologies. Fuel cells are electrochemical energy conversion devices with energy conversion efficiencies two times greater than those of conventional fire powered plants. Such electrochemical devices normally involve an oxygen reduction reaction (ORR) over their positive electrode; unfortunately, the sluggish ORR kinetics at room temperature significantly limit their efficiency and performance, and thus have become a major obstacle towards their practical applications. The development of efficient electrocatalysts for promoting the ORR is the key to realize the practical use of such advanced technologie. Carbon materials are promising alternatives to noble metal catalysts for oxygen reduction reactions (ORR) in energy conversion devices. Here we report hierarchically porous carbon spheres with a trace amount of encapsulated cobalt as highly active and stable electrocatalysts for the ORR under both alkaline and acidic conditions. We use renewable natural date pulp as the carbon precursor and a facile hydrothermal carbonization method with in situ formed cobalt as a template for the synthesis. These catalysts yield competitive catalytic activity (a small Tafel slope of 53 mV dec-1) and superb durability and methanol tolerance compared to the benchmark Pt/C catalyst in alkaline electrolyte. Even under harsh acidic conditions, the catalysts deliver a satisfactory catalytic performance and good stability, indicating their extensive applicability. This performance is mainly attributed to the rich surface defects and the hierarchically porous structure that provide abundant active sites for the ORR. In situ formed cobalt nanoparticles are critical to the creation of the abundant mesopores, high specific surface area, and catalytically active defect sites over the carbon material. The encapsulated cobalt residual further enhances the ORR activity of the catalysts, while having a negligible effect on the cost due to its trace level (0.2 at.%).