Impact of controlling the crystallinity on bifunctional electrocatalytic performances toward methanol oxidation and oxygen reduction in binary Pd–Cr solid solution

Abstract

Understanding the relationship between crystallographic structure and electrocatalytic performance is important to successfully design an efficient electrocatalyst. With finely controlled thermal H₂-reduction condition, herein, binary Pd–Cr nanofibers were fabricated as a bifunctional electrocatalyst toward both methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) for direct methanol fuel cells (DMFCs), retaining the distinct crystalline characters. The Pd–Cr nanofiber series was synthesized through the thermal reduction of single-phase Pd_xCr_1−xO_y nanofibers in the presence of a hydrogen gas flow. The resulting nanofibers exhibited different levels of crystallinity, which were significantly influenced by the reduction temperature. At a temperature of 350 °C, the Pd–Cr nanofibers were synthesized in an amorphous state, while the nanofibers reduced at temperatures above 500 °C gradually crystallized into a face-centered cubic (fcc) structure. Notably, the amorphous Pd–Cr nanofibers exhibited superior alkaline MOR performance, including high mass activity and a small Tafel slope, compared to the other crystalline counterparts in the Pd–Cr series. In situ Raman spectroscopy and CO stripping measurements further confirmed the remarkable catalytic activity and stability of the amorphous nanofibers, outperforming commercial Pd/C catalysts. Similarly, for alkaline ORR, the amorphous Pd–Cr nanofibers demonstrated superior catalytic performance, with a higher onset potential and positive half-wave potential, compared to the crystalline counterparts. Additionally, the amorphous catalyst exhibited improved resistance against agglomeration and methanol crossover issues, which are commonly observed with commercial Pt/C catalysts, serving as a benchmark for alkaline ORR. Therefore, this study highlights the facile strategy of designing optimal electrocatalysts for DMFCs by controlling the novel crystallographic structure within the binary Pd–Cr solid solution.