Synergistic Interfaces of Pd Nanocrystals and Metal Hydroxides for Ethanol Oxidation Reaction
Authors: Kitano Sho , Motohashi Hiroya , Iwai Mana , Fushimi Koji , Habazaki Hiroki 
Published: December 2024

Introduction

As one of the environmentally friendly power generation methods, alcohol fuel cells using bioalcohols have gained attention. Ethanol offers advantages such as low toxicity and an existing distribution network. However, its power generation efficiency is limited by the ethanol oxidation reaction (EOR) at the anode. In an alkaline environment, Pd-based catalysts are regarded as promising candidates due to high intrinsic catalytic activities. Thus, improving the catalytic activity of Pd-based catalysts is a challenging issue to meet the requirements of commercial applications. Several metal hydroxides achieved to improve catalytic activities of Pd nanoparticles for EOR, however, previous studies have not systematically investigated the effects of the kinds of metal hydroxide, structure of the contacting interface, and facets of the materials on catalytic activities of composite catalysts, lacking effective design guide of highly active electrocatalysts. In this study, we synthesize composite electrocatalysts of cubic and octahedral Pd nanoparticles (Pd-NPs) with well-defined facets and monolayer metal hydroxide nanosheets with containing Ni, Fe and Mn, and systematically elucidate highly active interfacial sites between Pd and metal hydroxides for EOR.

Experimental

The cubic and octahedral Pd (c-Pd, o-Pd) were synthesized by liquid phase synthesis using shape-controlling agents. The multi-metal hydroxide nanosheets were synthesized by exfoliating layered double hydroxides, resulting in FeNi hydroxide nanosheets (FeNi-NS) and MnNi hydroxide nanosheets (MnNi-NS). The suspensions of Pd-NPs and metal hydroxide nanosheets were mixed to synthesize the composite catalysts (Pd/NS). The samples were coated onto a glassy carbon rotating electrode and used as a working electrode. The ethanol oxidation reaction (EOR) was conducted using three electrodes system with a Hg/HgO reference electrode and a Pt wire counter electrode. The catalytic performances were evaluated by a cyclic voltammetry measurement in an Ar-saturated 1.0 mol dm-3 KOH aqueous solution containing 1.0 mol dm-3 ethanol.

Results and Discussion

SEM and TEM observations confirmed that c-Pd and o-Pd nanoparticles exposed (100) and (111) facets on their surfaces, respectively. Crystallographic diffraction peaks specific to these facets were observed in electron diffraction patterns. STEM observations of Pd/NS revealed that the faceted surface of Pd-NPs was modified with FeNi-NS and MnNi-NS (Fig. 1). The valence states of Pd in the composite catalysts changed to the oxidative states compared to that of the corresponding pristine Pd-NPs due to modification of nanosheets. All catalysts showed anodic peaks originated from ethanol oxidation in the rage of 0.5 and 0.9 V vs RHE. The composite catalysts showed higher current density than that of the pristine Pd-NPs, indicating that nanosheet modification enhanced the catalytic activities of Pd by change in valence states. The Pd/MnNi-NS composite catalysts showed higher activity than Pd/FeNi-NS composite catalysts. This suggests that higher hydroxide ion affinity of MnNi-NS facilitated the dehydrogenation reaction in EOR. While the c-Pd was superior for EOR than the o-Pd in the pristine Pd-NPs, the o-Pd/NS showed higher EOR performances than those of c-Pd/NS. The results suggest that the combination of metal hydroxides and the Pd (111) facet provides highly active interfacial sites for EOR.
Fig. 1. The composite catalysts of FeNi-NS and (a)c-Pd and (b)o-Pd.

Figure 1