Catalyst Support Based on Oxygen-Deficient Titanium Suboxide Nanosheets with Pt Nanoparticles for Oxygen Reduction Reaction in Proton-Exchange Membrane Fuel Cell Application

Abstract

Developing stable nanocatalysts with high activity for oxygen reduction reaction (ORR) remains a great challenge in the field of clean energy. As one of the most important means of electrochemical energy conversion, proton-exchange membrane fuel cells (PEMFCs) have attracted much attention in the scientific and industrial communities [1]. There has been increasing interest in the development of advanced electrode materials for the next generation of high-performance, durable PEMFCs. Importantly, the practical use of conventional carbon supported platinum (Pt) catalyst (i.e., Pt/C) materials is limited by their low electrochemical oxidation resistance and the consequent structural collapse of the carbon-based support material in electrodes, especially under dynamic operating conditions [2,3]. As such, there are still some limitations to the performance of PEMFCs with these carbon-based hybrid electrodes, and further improvement or optimization of their electrochemical performance remains challenging. Additionally, the electronic paths in these surfaces functionalized carbon supports are not effectively shortened, because electrons move less freely between them, via direct contact or by jumping from one particle to another as the conductive carbon particles are passivated [4]. Therefore, the development of conductive and corrosion-resistant non-carbon nanostructured support materials for catalysis, to replace carbon black particle aggregates, is the ultimate route to eliminating electrochemical instability [5]. Herein, we developed two-dimensional oxygen-deficient Titania nanosheets (TiO2-x NSs) as electrocatalyst support by simple hydrothermal reaction followed by NaBH4 reduction in ambient atmosphere. Electrochemical accelerated degradation tests (ADTs) and post-test elemental analysis revealed that the higher surface area and oxygen deficiency play a key role in the high-performance corrosion-resistant TiO2-x NSs electrocatalyst supports, as they serve as a means for improving electrical conductivity. The density of states of the valence band (VB) for TiO2-x NSs was measured by X-ray photoelectron spectroscopy (XPS), and exhibited a remarkable band gap narrowing (~ 2.7 eV) based on the introduced surface defect (oxygen vacancy). After the deposition of Pt nanoparticles (NPs), the TiO2-x NS catalysts exhibited both higher electrocatalytic activity and outstanding electrochemical stability compared to commercial Pt/C. From the cyclic voltammogramms, it was observed that the Pt-TiO2-x NSs displayed positive shift of an oxidation wave assigned to the adsorption of oxygenated species (e.g., OHad, at 0.7 - 0.9 V) compared to Pt/C. These observations suggest that the surface of catalyst is less oxophilic and has weaker binding to the oxygenated species than the surface of pure Pt, which is desired for lowering the kinetic barrier of the ORR. The calculated Tafel slope was 68 mVs-1 at low current density region, indicating that the transfer of the first electron on the catalyst is the rate determining step for the adsorption of intermediates. The mass activity (MA) of the catalyst is 314 Agpt -1, which is 1.5 times higher than that of Pt/C. The enhanced activity for ORR of Pt-TiO2-x NSs was attributable to the strong metal-support interaction (SMSI) effect. Due to the spillover effect, charge transfer occurred from the TiO2-x NSs to the Pt NPs and caused the electron density of the Pt NPs to increase. Accordingly, the obtained catalyst delivers a high power density of 958 mWcm-2 in H2-O2 PEMFC at 80 °C in ambient pressure as cathode. Moreover, no decay was observed during continuous operation at 1.5 Acm-2 for 100 h from the projected catalyst support based fuel cell. Benefiting from the unique structural functionality, interaction and electronic effects, the Pt-TiO2-x NSs displayed a negligible performance deterioration overlong-term cycling, demonstrating cell performance comparable to that of state-of-the-art commercial Pt/C during practical PEMFC operation. This strategy offers a new concept for designing an ultra-low Pt loading yet highly active and durable catalytic support for fuel cell applications and beyond. ACKNOWLEDGMENTS We acknowledge the financial support from the (Grants in Aid for scientific research (KAKENHI)) Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the ‘Multifunctional durable nanostructured redox electrocatalysts on conducting titanium oxide support’ project (Project Number: 18F18372). Keerti M. Naik is thankful to the Japan Society for the Promotion of Sciences for a JSPS Postdoctoral Fellowship (Standard).