Graphene Oxide-Supported Cobalt Phthalocyanine As Highly Efficient Electrocatalysts for the Oxygen Reduction Reaction

Abstract

The rapid depletion of oil reserves and the resulting environmental pollution have stimulated the development of clean and renewable alternative energy sources. Fuel cells, which own high efficiency and environmental advantages, have received great attention. 1 The main bottleneck of fuel cells lies in the sluggish kinetics of oxygen reduction reaction (ORR), which is the key process in the cathode. 2 High performance ORR catalysts are urgently desired. In order to reduce the cost of ORR catalyst and to promote the commercialization of fuel cells, many researchers have been exploring alternative low-cost non-precious metal catalysts (NPMCs) with attractive performance for ORR. Among these NPMCs materials, metal phthalocyanines (MPcs) have received a great deal of interest due to highly conjugated structure, chemical and thermal stability as well as low cost of preparation. 3 In addition, the catalytic activity and durability of MPcs can be improved by employing suitable electrocatalyst supports. In this work Cobalt Phthalocyanine (CoPc) was immobilized between layers of graphene oxide (GO) through non-covalent functionalization approach. Briefly, the catalyst ink was prepared using CoPc/GO, water, ethanol and Nafion. Electrodes were prepared by drop-casting over a Rotating Ring-Disc Electrode (RRDE electrode, glassy carbon disk with diameter of 5.61 mm and surface area of 0.2475 cm2 surrounded by a Pt ring with a surface area 0.1866 cm2). The RRDE technique with a three-electrode electrochemical cell was employed to study the ORR activity, using a Pine electrochemical system. A Ag/AgCl (3 M KCl) electrode, a Pt wire and the catalyst coated RRDE were used as reference electrode, counter electrode and working electrode respectively. All electrochemical tests were performed at room temperature in 0.1 M KNO3 solution. Cyclic voltammetry curves (CV) were recorded from 0.2 to -0.8 V until a steady state situation was reached in Ar-saturated and O2-saturated electrolyte. The scan rate was set at 20 mVs−1. For the ORR test, linear sweep voltammetry (LSV) curves were recorded at different electrode rotation speeds (100, 225, 400, 625, 900, 1225, 1600 rpm) in the oxygen saturated 0.1 M KNO3 at the same potential range with a scan rate of 5 mVs-1. The formation of a composite structure was revealed by the appearance of characteristics graphene oxide and phthalocyanine bands in the Raman spectra, as shown in Fig. 1a. The cyclic voltammograms (CVs) of CoPc/GO in Ar and O2-saturated 0.1 M KNO3 solution are shown in Fig. 1b. It can be seen that the composite exhibited only a double-layer capacitive behavior in Ar-saturated KNO3. In contrast, a cathodic peak emerged at -0.15 V for CoPc/GO in O2-Saturated electrolyte. This value is close to related for Pt/C and more positive than that observed for CoPc/3D-G in the literature, 4 indicating the good electrocatalytic abilities of CoPc/GO toward ORR. The polarization curves on CoPc/GO at different rotation speeds are shown in Fig. 1c. The disk current density increased with the increasing rotation rate, from 100 to 1600 rpm, due to the shortened diffusion distance at high speeds, and all reached well-defined diffusion limiting currents. The transferred electron number n of CoPc/GO at various potentials was further calculated by Koutecky-Levich (K-L) equation, as shown in Fig. 1d. The K-L plots have good linearity with a rather consistent slope, implying a similar n on CoPc/GO at different electrode potentials. The average number for n regarding the O2 reduction at CoPc/GO obtained from K-L plots is ∼3.7, indicating the near four-electron pathway during the ORR. The Eonset of CoPc/GO was + 0.02 V (at 1600 rpm). An enhancement in the catalytic activity of CoPc/GO was noticed by using GO as the support, demonstrating that the structure of GO and the strong π-π interaction between GO and CoPc was beneficial. Detailed characterizations of the composite are in progress to correlate its nanostructure with the superior electrochemical performance. T. Shimura, Z. Jiao, and N. Shikazono, J. Electrochem. Soc., 164, F1158 (2017). K. A. Kuttiyiel, Y. Choi, S.-M. Hwang, G.-G. Park, T.-H. Yang, D. Su, K. Sasaki, P. Liu, and R. R. Adzic, Nano Energy, 13, 442 (2015). A. B. Sorokin, Chemical Reviews, 113, 8152-8191 (2013). C. Sun, Z. Li, X. Zhong, S. Wang, X. Yin, and L. Wang, J. Electrochem. Soc., 165 (2), F24-F31 (2018) Figure 1