CO-Reductive and O2-Oxidative Annealing Assisted Surface Restructure and Corresponding Formic Acid Oxidation Performance of PdPt and PdRuPt Nanocatalysts

Dinesh Bhalothia,Po-Chun Chen,Pai-Hung Chou,Kuan-Wen Wang,Tzu-Hsi Huang,Tsan-Yao Chen

doi:10.1038/s41598-020-65393-3

Abstract

Formic acid oxidation reaction (FAOR) at anode counterpart incurs at substantial high overpotential, limiting the power output efficiency of direct formic acid fuel cells (DFAFCs). Despite intense research, the lack of high-performance nanocatalysts (NCs) for FAOR remains a challenge in realizing DFAFC technologies. To surmount the overpotential losses, it is desirable to have NCs to trigger the FAOR as close to the reversible conditions (i.e. with over-potential loss as close to zero as possible). Herein, Pd-based binary and ternary NCs consisting of PdPt and PdRuPt have been synthesized via the polyol reduction method on the carbon support. As prepared PdPt and PdRuPt NCs were further subjected to heat treatment (annealed) in CO (namely PdPt-CO and PdRuPt-CO) and O2 (namely PdPt-O2 and PdRuPt-O2) atmosphere at 473 K temperature. By cross-referencing results of electron microscopy and X-ray spectroscopy together with electrochemical analysis, the effects of heat treatment under CO-reductive and O2-oxidative conditions towards FAOR were schematically elucidated. Of special relevance, the mass activity (MA) of PdPt-CO, PdPt-O2, PdRuPt-CO, and PdRuPt-O2 NCs is 1.7/2.0, 1.3/2.2, 1.1/5.5, and 0.9/4.7 Amg−1 in the anodic/cathodic scan, respectively, which is 2~4-folds improved comparative to of as-prepared PdPt (1.0/1.9 Amg−1 in anodic/cathodic scan, respectively) and PdRuPt (0.9/1.4 Amg−1 in anodic/cathodic scan, respectively) NCs. Meanwhile, after chronoamperometric (CA) stability test up to 2000 s, PdPt-CO (72 mAmg−1) and PdRuPt-CO (213 mAmg−1) NCs exhibit higher MA compared to as-prepared PdPt (54 mAmg−1) and PdRuPt (62 mAmg−1) NCs, which is attributed to the increase of surface Pt composition, especially for PdRuPt-CO NC. Besides, the stability of PdPt-O2 (15 mAmg−1) and PdRuPt-O2 (22 mAmg−1) NCs is deteriorated as compared to that of as-prepared NCs due to severe oxidation in O2 atmosphere. Of utmost importance, we developed a ternary PdRuPt catalyst with ultra-low Pt content (~2 wt.%) and significantly improved FAOR performance than pure Pt catalysts. Moreover, we demonstrated that the FAOR performance can be further enhanced by more than 30% via a unique CO annealing treatment.

Highlights

Severe global energy crisis coupled with adverse climatic issues have ignited great interest in the implementation of the sustainable energy economy
Pd has been confirmed to be a potential alternative to catalyze formic acid oxidation reaction (FAOR) and can greatly enhance the FAOR performance when decorated with Pt-atoms, CO tolerance and high cost of Pd are still major obstacles for their commercial viability
We develop a ternary PdRuPt catalyst with ultra-low Pt content (~2 at%) and a FAOR performance higher than pure Pt catalysts

Summary

Introduction

Severe global energy crisis coupled with adverse climatic issues have ignited great interest in the implementation of the sustainable energy economy. Direct formic acid fuel cells (DFAFCs) have gained more attention owing to their promising properties such as high open-circuit potential (1.45 V) and energy conversion efficiency (theoretical conversion efficiency of DFAFCs is reached to 106%), limited fuel crossover effects and lower toxicity[1,2,3,4] Despite their great merits, the commercial exploitation of DFAFCs www.nature.com/scientificreports is strongly hampered by intrinsically sluggish kinetics of formic acid oxidation reaction (FAOR) at the anode[5]. Pd has been confirmed to be a potential alternative to catalyze FAOR and can greatly enhance the FAOR performance when decorated with Pt-atoms, CO tolerance and high cost of Pd are still major obstacles for their commercial viability To overcome these existing issues, decorating Ru at interface and surface of Pd or Pt NCs offers electronic, geometric and compositional effects that can be used to tune catalytic active sites, leads improved activity and cost reduction but better CO tolerance than commercial catalysts toward FAOR35.

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Conclusion