Fabrication of the pyrolyzing carbon-supported cobalt–dicyandiamide electrocatalysts and study on the active sites and mechanism for oxygen reduction in alkaline electrolyte

Abstract

In this work, carbon-supported cobalt–dicyandiamide (Co–DCD) was synthesized by a simple chemical method followed heat-treated at 600,700, 800, 900, and 1000 °C to acquire electrocatalysts with excellent activity for the oxygen reduction reaction (ORR). The resulting catalysts Co–N/C–T pyrolyzed at 600–1000 °C all showed substantial activities to ORR, and the catalyst heat-treated at 800 °C exhibited the best ORR activity. The catalytic performance of Co _y –N _x /C _0.25–T catalysts synthesized with different amounts of DCD as nitrogen source and Co(OAc)₂⋅4H₂O as metal precursor were measured by cyclic voltammetry in alkaline electrolyte. The onset potential for oxygen reduction on the optimum catalyst was approximately 0.938 V (vs. RHE) in 0.1 M NaOH solution, higher by 83 mV than that on the commercial catalyst of 20 % JM Pt/C. Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy were used to characterize the catalyst in terms of the structure and composition in order to sketch the contours of the catalytic active sites of the catalysts. The characterization studies indicate that pyridinic N–C was the most important of the catalytic active sites and responsible for the ORR catalytic activity of Co–N/C–T in alkaline electrolyte. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) was also used to obtain the overall ORR electron transfer number and electron transfer coefficiency. The overall electron transfer number for ORR catalyzed by the optimum catalyst Co_0.2–N_2.35/C_0.25–800 was determined to be 3.47 by CV and 3.89–3.96 by LSV,respectively, suggesting that the ORR was a mixture of two- and four-electron transfer pathways, but dominated by a four-electron transfer process. Based on these measurements and other references, an ORR mechanism was proposed to facilitate further investigation. The results also show that this novel catalyst with ORR excellent activity would have outstanding methanol tolerance and potential application as a kind of nonprecious metal cathodic ORR catalyst for direct methanol fuel cell.