The study focuses on the influence of Ni and Bi on alkaline ethanol oxidation reaction (EOR) activities, stabilities and structure characteristics of carbon supported Pd-based nanocatalysts (Pd/C, Pd₆₀Ni₄₀/C, Pd₆₀Bi₄₀/C, Pd₆₀Ni₂₀Bi₂₀/C) by cyclic voltammetry/chronoamperometry using rotating disk electrode and various physico-chemical methods such as X-ray powder diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and inductively coupled plasma optical emission spectrometry. Nickel generates more adsorbed OH on the Pd catalyst surface than Bi and promotes the oxidation of adsorbed ethanol species. This results in a low onset potential toward ethanol oxidation with high current density. The presence of Bi facilitates high tolerance toward various reaction intermediates resulting from the incomplete ethanol oxidation, but might also initiate the agglomeration of Pd nanoparticles. The novel Pd₆₀Ni₂₀Bi₂₀/C nanocatalyst displays exceptional byproduct tolerance, but only satisfying catalytic activity toward ethanol oxidation in an alkaline medium. Therefore, the EOR performance of the novel carbon supported ternary Pd_xNi_yBi_z anode catalyst with various atomic variations (Pd₇₀Ni₂₅Bi₅/C, Pd₇₀Ni₂₀Bi₁₀/C, Pd₈₀Ni₁₀Bi₁₀/C and Pd₄₀Ni₂₀Bi₄₀/C) using the common instant reduction synthesis method was further optimized for the alkaline direct ethanol fuel cell. The carbon supported Pd:Ni:Bi nanocatalyst with atomic ratio of 70:20:10 displays outstanding catalytic activity for the alkaline EOR compared to the other Pd_xNi_yBi_z/C nanocatalysts as well as to the benchmarks Pd/C, Pd₆₀Ni₄₀/C and Pd₆₀Bi₄₀/C. The synergy and the optimal content in consideration of the oxide species of Pd, Ni and Bi are crucial for the EOR kinetic enhancement in alkaline medium. Open image in new window

Direct ethanol fuel cells (DEFC) still lack active and efficient electrocatalysts for the alkaline ethanol oxidation reaction (EOR). In this work, a new instant reduction synthesis method was developed to prepare carbon supported ternary PdNiBi nanocatalysts with improved EOR activity. Synthesized catalysts were characterized with a variety of structural and compositional analysis techniques in order to correlate their morphology and surface chemistry with electrochemical performance. The modified instant reduction synthesis results in well-dispersed, spherical Pd₈₅Ni₁₀Bi₅ nanoparticles on Vulcan XC72R support (Pd₈₅Ni₁₀Bi₅/C^(II-III)), with sizes ranging from 3.7 ± 0.8 to 4.7 ± 0.7 nm. On the other hand, the common instant reduction synthesis method leads to significantly agglomerated nanoparticles (Pd₈₅Ni₁₀Bi₅/C^(I)). EOR activity and stability of these three different carbon supported PdNiBi anode catalysts with a nominal atomic ratio of 85:10:5 were probed via cyclic voltammetry and chronoamperometry using the rotating disk electrode method. Pd₈₅Ni₁₀Bi₅/C^(II) showed the highest electrocatalytic activity (150 mA⋅cm⁻²; 2678 mA⋅mg⁻¹) with low onset potential (0.207 V) for EOR in alkaline medium, as compared to a commercial Pd/C and to the other synthesized ternary nanocatalysts Pd₈₅Ni₁₀Bi₅/C^(I) and Pd₈₅Ni₁₀Bi₅/C^(III). This new synthesis approach provides a new avenue to developing efficient, carbon supported ternary nanocatalysts for future energy conversion devices.

Membrane degradation is a severe factor limiting the lifetime of polymer electrolyte fuel cells. Therefore, obtaining a deeper knowledge is fundamental in order to establish fuel cells as competitive product. A segmented single cell was operated under open circuit voltage with alternating relative humidity. The influence of the catalyst layer on membrane degradation was evaluated by measuring a membrane without electrodes and a membrane-electrode-assembly under identical conditions. After 100 h of accelerated stress testing the proton conductivity of membrane samples near the anode and cathode was investigated by means of ex situ electrochemical impedance spectroscopy. The membrane sample near the cathode inlet exhibited twofold lower membrane resistance and a resulting twofold higher proton conductivity than the membrane sample near the anode inlet. The results from the fluoride ion analysis have shown that the presence of platinum reduces the fluoride emission rate; which supports conclusions drawn from the literature.