CO-Reductive and O ₂-Oxidative Annealing Assisted Surface Restructure and Corresponding Formic Acid Oxidation Performance of PdPt and PdRuPt Nanocatalysts

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 O ₂ (namely PdPt-O ₂ and PdRuPt-O ₂) 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 O ₂-oxidative conditions towards FAOR were schematically elucidated. Of special relevance, the mass activity (MA) of PdPt-CO, PdPt-O ₂, PdRuPt-CO, and PdRuPt-O ₂ 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-O ₂ (15 mAmg ⁻¹) and PdRuPt-O ₂ (22 mAmg ⁻¹) NCs is deteriorated as compared to that of as-prepared NCs due to severe oxidation in O ₂ 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.