Effect of Organic Cations on Hydrogen Oxidation Reaction of Carbon Supported Platinum

Abstract

Effect of organic cations on hydrogen oxidation reaction (HOR) of carbon supported platinum (Pt/C) is investigated using three 0.1 M alkaline electrolytes, tetramethylammonium hydroxide (TMAOH), tetrabutylammonium hydroxide (TBAOH) and tetrabutylphosphonium hydroxide (TBPOH). Rotating disk electrode experiments indicate that the HOR of Pt/C is adversely impacted by time-dependent and potential-driven chemisorption of organic cations. In-situ infrared reflection adsorption spectroscopy experiments indicated that the specific chemisorption of organic cations drives the hydroxide co-adsorption on Pt surface. The co-adsorption of TMA + and hydroxide at 0.1 V vs. reversible hydrogen electrode is the strongest; consequently, complete removal of the co-adsorbed layer from Pt surface is difficult even after exposure the Pt surface to 1.2 V. Conversely, the chemisorption of TBP + is the weakest, yet notable decrease of HOR current density is still observed. The adsorption energies, E, for TMA + , TBA + , and TBP + on Pt (111) surface from density functional theory are computed to be −2.79, −2.42 and −2.00 eV, respectively. The relatively low adsorption energy of TBP + is explained by the steric hindrance and electronic effect. This study emphasizes the importance of cationic group on HOR activity of alkaline anion exchange membrane fuel cells. Alkaline anion exchange membrane fuel cells (AAEMFCs) are drawing increasing interest due to the fact that non-precious group metal (non-PGM) catalysts perform well in alkaline environment. 1Over the last decade, numerous non-PGM catalysts have demonstrated excellent oxygen reduction reaction (ORR) activity.2,3 However, poor activity for the hydrogen oxidation reaction (HOR) is still a challenge under alkaline conditions even for Pt catalysts. Therefore, development of highly active HOR catalysts remain one of the major technical challenges, hampering the success of AAEMFCs. 4 The reason for the poor HOR activity in alkaline environments is unclear. Markovic et al. suggested that an optimal balance between the adsorption/dissociation of H 2 and the adsorption of hydroxyl species is critical for improving HOR activity.5 Conversely, Gasteiger et al. suggested that the hydrogen binding energy is the relevant descriptor for the HOR in alkaline electrolytes.6 Yan et al. further elucidated that the hydroxide species do not directly participate in the reaction through adsorption, while the alkalinity change induced by the hydroxide ion affects the hydrogen binding energy and in turn influences the HOR activity. 7While aforementioned studies primarily consider the fundamental Heyrovsky and Volmer steps to explain the slow HOR kinetics, recently Janik et al. have proposed that a co-adsorbed alkali metal cation-hydroxide-water layer on the Pt surface may impact the HOR activity as observed from density functional theory (DFT) calculations and electrochemical experiments.8 Substantial Pt HOR inhibition was observed with organic cation solutions, in addition to the alkali metal cation in aqueo...

Durability challenges of anion exchange membrane fuel cells

“…0.1 V vs. RHE was observed by several researchers. 72,[95][96][97][98][99] Surface infrared and neutron reflectometry studies indicated that the chemisorbed layer contains highly concentrated ammonium hydroxide and does not allow fast hydrogen and water transport. 72,100 This means the water distribution in the AEMFC anode is nonuniform and the AEMFC anode requires high porosity for fast water transport, yet is more prone to flooding.…”

Caffeinated Interfaces Enhance Alkaline Hydrogen Electrocatalysis

“…While the complexity of this task is daunting, literature already provides many examples where manipulating solution structure enhances electrocatalytic rates. In alkaline HER/HOR, cation effects provide a simple demonstration of this approach 7,24,25 . More nuanced effects can be realized with ionic liquid electrolytes in the carbon dioxide reduction reaction (CO2RR) and oxygen reduction reaction (ORR) [26][27][28] .…”

Recent Insights on Catalyst Layers for Anion Exchange Membrane Fuel Cells

“…The coadsorption may block some of the active sites of catalyst and increase the barrier for hydrogen access. [ 162–165 ] The group of Kim observed that phenyl groups often present in polyaromatic AEI reduce the HOR activity leading to inferior AEMFC performance. As is shown in Figure a, Pt/C catalyst in phenyl containing benzyltrimethylammonium hydroxide (BTMAOH) organic solution exhibited a clear drop of HOR current density during 0.0 to 0.06 V, compared with phenyl free tetramethylammonium hydroxide (TMAOH) solution.…”

“…The coadsorption may block some of the active sites of catalyst and increase the barrier for hydrogen access. [162][163][164][165] The group of Kim observed that phenyl groups often present [166] Copyright 2018, American Chemical Society. b) Relationship of AEMFC performances of different AEI structures and their phenyl adsorption energies; DFT-optimized geometries of various aromatic compounds and calculated adsorption energies.…”