Noble Metal-Added Titanate Nanosheets for PEFC Cathode

Abstract

A large overpotential of oxygen reduction reaction (ORR) at a cathode in polymer electrolyte fuel cells (PEFCs) is one of the serious problems to decrease the energy conversion efficiency. Nørskov’s group reported that even Pt which is currently used as an ORR catalyst cannot reach the theoretical oxygen electrode potential of 1.23 V at 298 K [1]. On the other hand, Yamamoto et al. recently suggested that noble metal doped titanium dioxides (TiO2) can reach the theoretical potential by changing the absorption energies of the ORR intermediates according to the first-principle calculations [2]. However, the first-principle calculations did not consider the continuous electron supply to proceed the ORR. It is difficult to form the sufficient electron conduction path from supports to active sites on the TiO2 because TiO2 has low conductivity due to a large band gap. Therefore, we focused on the formation of the active sites on the TiO2 surface by doping noble metal such as Rh and Pd and the electron conduction path from carbon supports to the active sites by preparing the titanate nanosheets.Na2Ti3O7 was synthesized as precursor of anionic titanate nanosheets([Ti3O7]2- ;TiNS) according to the previous paper [3]. Na2Ti3O7 was treated through ion-exchange reaction with hydrochloric acid and tetramethylammonium aqueous solution, finally TiNS were obtained. A TEM observation revealed that the formation of very large sheet-like materials with a size of approximately upwards of ten micrometers, resulting that TiNS were completely synthesized.Next, we focused on the formation of the electron conduction path through nanosheets from supports using the electron tunneling effect. When the nanosheets were directly supported on the glassy carbon electrode, the ORR activity was the same as that of GC, and almost no ORR current flowed even low potential region. Then, vapor grown carbon fibers (VGCFs) as purchased, with immersion of mixed acid (MA; H2SO4 and HNO3) at 170 oC for 24 h and PDDA (Poly(diallyl dimethylammonium chloride)) as polycation were attempted to use as supports. The TiNS were immersed in the distilled water dispersed with the treated VGCFs to be supported by them. These catalysts were designated as TiNS/VGCF, TiNS/VGCF(MA), and TiNS/VGCF(MA-PDDA).The ORR current of TiNS/VGCF(MA-PDDA) was larger than that of TiNS/VGCF, TiNS/VGCF(MA). In addition, the ORR onset potential of the TiNS/VGCF(MA-PDDA) increased by 0.3 V compared to that of TiNS/VGCF. These results suggest that surface treatment of VGCFs was effective to form the electron conduction path from VGCF to active sites of nanosheet surface. The electrostatic interaction between VGCF surface positively charged by PDDA and anionic TiNS contributed the formation of the electron conduction path. However, according to the TEM observation, TiNS were partially contacted with VGCF and some TiNS were agglomerated. It is necessary to obtain higher ORR activity that a control of size of TiNS and contact with supports such as VGCF.On the other hand, we attempted to dope Rh and Pd to TiNS through solid-state reaction. Rh-,Pd-doped TiNS (designated as Rh-TiNS and Pd-TiNS) were synthesized the same way as previous paper [3]. XRD patterns of the precursors revealed that Rh2O3 or PdO were existed with Na2Ti3O7 above the addition of 5 atomic %. Rh-TiNS and Pd-TiNS were supported by VGCF(MA-PDDA) after Rh-TiNS and Pd-TiNS were treated TMAOH aqueous solution. The ORR activities of the samples with the addition of 5at% Rh or 10at% Pd were higher than those of the samples which Rh2O3 or PdO were not observed in XRD patterns. The ORR active sites were estimated by Rh2O3 or PdO deposited on the surface of the TiNS. This result showed that an amount of doped Rh or Pd was limited to Na2Ti3O7. The limitation of doped Rh or Pd was 5at% or 10at%, respectively. Therefore, the doped amounts of noble metals such as Rh or Pd to TiNS should be appropriately controlled.AcknowledgementThe authors thank the New Energy and Industrial Technology Development Organization (NEDO) for financial support. This work was partly supported by TOKYO CITY UNIVERSITY Interdisciplinary Research Center for Nano Science and Technology for instrumental analysis.Reference(1) A. Kulkarni et al., Chem. Rev., 118, 2302 (2018).(2) Y. Yamamoto et al., J. Phys. Chem. C., 123, 19486 (2019).(3) W. Soontornchaiyakul et al., RSC Adv., 7, 21790 (2017). Figure 1