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

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.