Oxygen Reduction Activity of Nb-Doped Titanate Nanosheets in an Acidic Electrolyte

Abstract

Introduction Polymer electrolyte fuel cells (PEFCs) are of great importance as consumer use energy devices, such as fuel cell vehicles and residential micro-cogeneration systems. To reduce usage of platinum in PEFCs, non-noble metal oxide catalysts for oxygen reduction reaction (ORR) have extensively been investigated to date. From the viewpoint of stability, titanium oxides are promising candidates, though conductive path between support and active site should properly be designed because of their low conductivity. In this study, we focus on the use of titanate nanosheets as a cathode catalyst. Because titanate nanosheets are very thin, they can reduce conductive length between support and active sites. In addition, they are advantageous to form uniform composite with support materials because titanate nanosheets can be handled as a colloidal solution. From our previous study on titanium oxide nanoparticles,1) hetero-atom doping is expected to enhance the ORR activity. Here, we report the ORR activity of TNS and Nb-doped TNS (Nb-TNS) supported on carbon nanotube in an acidic electrolyte. Experimental TNS was synthesized according to the literature.2) Titanium isopropoxide (TIP) was refluxed in an aqueous solution of tetramethylammonium hydroxide for 30 min to provide titanate nanosheets with small lateral size. To synthesize Nb-TNS, niobium ethoxide was used instead of 10 mol% of TIP. Electrode materials were prepared as follows. A carbon nanotube (Flotube® 9110, denoted as CNT) was dispersed in the nanosheet dispersion, followed by stirring, sonication, and filtration with a membrane (pore size: 0.20 μm). The obtained composite was again dispersed in 1-hexanol containing Nafion® to prepare a catalyst ink. The catalyst ink was dropped onto a glassy carbon (GC) rod and dried at 60 °C for 1 h. The GC rod was used as a working electrode without rotation. The samples without titanate nanosheets, with TNS, and with Nb-TNS were named as CNT, TNS/CNT, and Nb-TNS/CNT, respectively. Electrochemical measurement was performed in a 3-electrode cell at 30 °C. 0.1 mol L–1 H2SO4 was used as an electrolyte. A reversible hydrogen electrode (RHE) and a GC plate were used as reference and counter electrodes, respectively. As a pre-treatment, 300 cyclic voltammetry (CV) cycles (0.05–1.2 V vs. RHE, 200 mV s–1) were performed under N2 atmosphere. CV (0.2–1.2 V vs. RHE, 50 mV s–1) was performed under N2 atmosphere to evaluate electric double layer capacitance (C dl) from the charges in 0.8–1.0 V vs. RHE. CV (0.2–1.2 V, 5 mV s–1) under N2 and O2 atmospheres were performed to evaluate ORR current (i ORR) which is calculated by subtracting the currents under N2 atmosphere from those under O2 atmosphere. Results and discussion The XRD pattern of the powdery samples showed diffraction peaks assignable to layered titanate (ICSD79800) without any peaks related to impurity. The d 020 value of Nb-TNS was larger than that of TNS by 0.5%. Because the d 020 reflects only in-plane crystal structure of a titanate sheet, these results imply the lattice expansion due to substitution of Ti atom with Nb. The Nb/Ti ratio of Nb-TNS was 13 mol% by XRF, which is consistent with the incorporation of Nb in Nb-TNS. Consequently, Nb-doped TNS was successfully synthesized. Fig. 1 shows the polarization curves of CNT, TNS/CNT, and Nb-TNS/CNT, normalized by the weight of CNT. The onset potential of TNS/CNT and Nb-TNS/CNT were 0.45 and 0.51 V, respectively. These were higher than that of CNT, suggesting the ORR activity due to titanate nanosheets. To compare ORR activities of TNS and Nb-TNS, the polarization curves normalized by the weight of titanate nanosheets are also shown in Fig. 2. Nb-TNS showed higher onset potential than that of TNS by 0.09 V. The Tafel plots whose current density is normalized by C dl is shown in Fig. 3. The activity of TNS/CNT was similar to that of CNT only; thus, the increased ORR activity of TNS/CNT was possibly due to increased C dl. On the other hand, the ORR activity of Nb-TNS was obviously higher than the others. Therefore, the hetero-atom doping is shown to be a useful way to improve the activity of titanate nanosheets as cathode catalyst for PEFCs. Conclusion In conclusion, TNS are advantageous to form powdery electrode composites because they are obtained as dispersions. TNS was shown to have ORR activity and the activity is enhanced by the hetero-atom doping. References 1) A. Ishihara et al, Electrochim. Acta, 283, 1779–1788 (2018). 2) E. L. Tae et al, J. Am. Chem. Soc., 130, 6534–6543 (2008). Figure 1