Pt Supported on Titanium Nitride/Graphene Nanocomposite As a Beneficial Catalyst for Oxygen Reduction Reaction in Acidic and Alkaline Solutions

Abstract

In this study, titanium nitride/graphene (TiN/G) and supported 2.5 wt.% Pt on TiN/G nanocomposites are synthesized and tested as electrocatalysts for oxygen reduction reaction (ORR). The kinetics of ORR on the synthesized catalysts are measured in 0.5 M H2SO4 and 0.1 NaOH solutions using electrochemical methods, including the cyclic voltammetry (CV), linear sweep voltammetry (LSV) and rotating disk electrode (RDE). Moreover, ORR activity of smooth Pt electrode is compared with the synthesized samples. It is found that the ORR activity of TiN/G and Pt/TiN/G are a strong function of pH of the solution. Fig. 1 shows the cathodic linear sweep voltammograms for different electrodes with catalyst loading of 0.46 mg cm-2 in air saturated in 0.5 M H2SO4 and 0.1 NaOH electrolytes. For the acidic solution (Fig. 1(a)), the onset potential on Pt/TiN/G for ORR, comparing with smooth Pt, shifts to the positive potentials for about 180 mV and the limiting current enhances up to 20%. Moreover, the number of transferred electrons calculated as 3.94 with 4% H2O2 production and ORR current experiences 15% of decay after 1200 cycles. Meanwhile, in alkaline media (Fig. 1(b)), the onset potential shifts to more negative potentials for about 150 mV compare with Pt electrode and ORR proceeds via transferring of 3.65 electrons with 17% H2O2 production. Consequently, the kinetics of ORR on 2.5 wt.% Pt/TiN/G are much higher in acid than in alkaline solution. The pH effect is associated with the pH competitive adsorption of oxygenated species with anions from supporting electrolytes. Unique Tafel slope of 120 mV dec-1 characterized in acidic solution (Fig. 2(a)), shows that the active surface of the Pt/TiN/G is not blocked by OH species and indicates that the rate determining step is the exchange of the first electron. However, the adsorption of OH species in alkaline solution and their coverage variation with potential leads to lower slope value and transition from the low slope (Temkin adsorption condition) to the high Tafel slope (Langmuir adsorption condition) (Fig. 2(b)). The change of the Tafel slopes with potential in alkaline media implies the change in reaction pathway from acidic to alkaline solutions. CV results reveal that in acidic solution the potential of PtOH formation and Pt oxidation shifts to higher overpotentials compared with alkaline media because of the presence of HSO4ˉ/SO42ˉ anions in double layer instead of adsorption of OHˉ anions on the electrode surface. The absence of OH on the surface facilitates the interaction of O2 with Pt/TiN/G catalyst and increases the ORR activity. As a result of the change in oxygen adsorption conditions for TiN/G that arise from a change in the electronic structure of Pt/TiN/G catalyst connected to the electronic interactions between Pt and TiN/G as the support material, TiN/G shows a reverse trend. The presence of OHˉ anions in alkaline media helps the ORR proceeds via a two stages pathway with 17% H2P2 production and the onset potential shifts to negative potentials for about 60 mV compare with Pt electrode. However, the alterations in surface protonation conditions and oxidation of TiN/G in acidic media leads to reduction of ORR activity in such a way that the produced H2O2increases up to 52% and the onset potential shifts to negative potentials for about 230 mV toward Pt electrode and the electrode stability decreases up to 51% after 1200 cycles. The high ORR activity and stability of the TiN/G in the alkaline electrolyte are mainly due to (i) N-doped graphene structure that combines through a hybrid state with TiN nanoparticles, (ii) unstoichiometric nitrogen and oxygen doped into TiN lattice, and (iii) high electrochemical surface area. By modifying catalyst surface with Pt nanoparticles, the ORR activity remarkably promotes in both acidic and alkaline solutions as a result of modification of electronic structure and increasing electrochemical surface area. Finally, the Pt/TiN/G catalyst shows a superior performance in acidic media than does in alkaline media because of changing the chemical in the surface at different pH values. Figure captions: Fig. 1. Voltammograms for ORR of GC electrode coated with TiN/G and 2.5 wt.% Pt/TiN/G in (a) 0.5 M H2SO4and (b) 0.1 M NaOH. The vertical axis shows difference in current between air and Ar atmospheres. The sweep rate was 5 mV/s (cathodic scan) and RDE at 1600 rpm. Fig. 2. Tafel plots for the O2 reduction on different electrodes in (a) 0.5 M H2SO4 and (b) 0.1 M NaOH constructed from the data in Fig. 1. Figure 1