Facile Synthesis and Performances of a CoSe2/C Catalyst for the Oxygen Reduction Reaction

In recent years, cobalt-based chalcogenides are widely investigated as promising non-platinum catalysts, due to low cost and high catalytic activity towards the oxygen reduction reaction (ORR) in acid medium [1-3]. For example, the CoSe₂/C nanoparticles synthesized by Feng et al. [4] displayed an open circuitry potential of 0.81V (vs. NHE) and have higher methanol tolerance as compared to Pt/C catalyst. At present, the cobalt-based chalcogenides catalysts were usually prepared by the thermal decomposition of cobalt carbonyl in xylene under refluxing conditions [4, 5]. Xylene is toxic, and ingestion of xylene will cause severe gastrointestinal distress and toxic hepatitis. In this work, carbon-supported CoSe₂ catalysts with different nominal loading from 10 wt% to 30 wt% were synthesized by low temperature refluxing method using Co₄(CO)₁₂, Se powder and Vulcan XC-72 carbon as precursors in 1, 6-hexanediol solvent. The ORR activity and stability of the catalyst were evaluated by using the rotating disk electrode (RDE) and cyclic voltammetry technique in 0.5 mol dm^-3 H₂SO₄ solution at 25^oC. The glassy carbon (GC) electrode, platinum wire and Hg/Hg₂SO₄electrode was used as the work, counter and reference electrode, respectively.

    The XRD patterns of CoSe₂/C catalysts with different nominal loading are shown in Fig. 1 (inset). All samples displayed the crystalline characteristics of orthorhombic CoSe₂ (JCPDS-PDF 10-0408). Moreover, the un-reacted Se phase (relative to JCPDS-PDF 27-0601) is also found. The (1 1 1) crystal plane near 2θ=34.4^◦has the highest intensity among the diffraction peaks of the orthorhombic CoSe₂, and the intensity was increased with rising catalyst loading.

    The catalytic activity of the CoSe₂/C catalysts for the ORR changes with increasing CoSe₂ loading rate on the carbon substrate, and the 20 wt% CoSe₂/C catalyst presents higher catalytic activity with an open circuit potential of 0.80 V (vs. NHE). Polarization curves of the ORR on the catalyst were recorded by the LSV technique with a scanning rate of 10 mV s^-1 at rotating speeds of 100, 200, 400, 900, 1600 and 2500 r min^-1 in O₂-saturated 0.5 mol dm^-3 H₂SO₄ solution, as shown in Fig. 1. It can also be seen that the current density of the ORR on the catalyst increases with rising rotation speeds due to the increase of oxygen molecule diffusion through the GC electrode surface. The ORR curves showed 2.5 times higher current density than reported in our previous work with non-carbon CoSe₂catalyst at the same rotating speed [6].  

    The 20 wt% CoSe₂/C catalyst shows better kinetic properties towards the ORR and the experimental average Levich slope of 10.03×10^-2 mA cm^-2 r^-1/2 min^1/2 in the potential region of 0.30-0.45 V (vs. NHE) is closer to that of the theoretical value of 9.41×10^-2 mA cm^-2 r^-1/2 min^1/2 for four electrons transfer during the reduction per oxygen molecule. This suggests that the ORR is a nearly complete reduction of O₂ to H₂O on the CoSe₂/C catalyst. The kinetic parameters, such as transfer coefficient, Tafel slope and exchange current density of the catalyst are determined to be 0.50, -0.119 V and 3.85×10^-6 mA cm^-2 from the mass-transfer corrected Tafel plot, respectively. To investigate the electrochemical stability of the CoSe₂/C catalysts in the acid solution during oxygen reduction, chronoamperometry was carried out during 1000 s at fixed potential of -0.2 V. It was also clear that the decay rate of catalytic activity was calculated to be 26.7%, 36.5% and 45.7% for the 10 wt%, 20 wt% and 30 wt% CoSe₂/C catalysts, respectively. The results suggested that the electrochemical stability of the CoSe₂/C catalysts should be further improved.

Fig. 1 ORR polarization curves of the 20 wt% CoSe₂/C catalyst and XRD patterns for CoSe₂/C catalysts with various loading (inset on the bottom). The currents were recorded at a scan rate of 10 mV s^-1.

Acknowledgement

This work is financially supported by Natural Science Foundation of Heilongjiang Province of China (No. B201423), Science and Technology Project of Suihua City of China (No. KJZD20130086), and Doctor Scientific Research Fund of Suihua University (No. SD13001).

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