Synthesis-Sequence-Controlled Surface and Electronic Structure in Pd–Ag–Ni(OH) ₂ /C Electrocatalysts for Efficient Formic Acid Oxidation

The development of efficient and durable electrocatalysts for the formic acid oxidation reaction (FAOR) is central to the progress of direct formic acid fuel cells (DFAFCs). Here, we investigate how the synthesis sequence and reduction pathway influence the surface and electronic structure of PdAgNi(OH)2/C nanocomposites and, consequently, their FAOR performance in acidic media. Binary Pd/Ni(OH)2 catalysts with Pd/Ni(OH)2 mass ratios of 30:70, 50:50, and 70:30 were first screened, revealing 50:50 as the optimal composition. Partial substitution of Pd by Ag (Pd40Ag10 and Pd30Ag20 on Ni(OH)2(50)/C) was then combined with either sequential or simultaneous NaBH4-assisted reduction. Structural characterization by XRD, TEM, and XANES/EXAFS show that simultaneous coreduction tightens the Pd–Ag–Ni interfacial coupling, enhances Pd dispersion, and increases the contribution of Pd–O and Pd–Ni scattering paths, indicative of strong metal–oxide interactions. Electrochemical measurements demonstrate that the Pd30Ag20Ni(OH)2(50)/C catalyst prepared by simultaneous reduction exhibits the highest mass activity toward FAOR (6164 mA mgPd–1), a ca. 23-fold enhancement over commercial Pd/C, together with improved stability under potential cycling. These results demonstrate that controlling the synthesis sequence is an effective method for tuning the interfacial electronic structure of multicomponent Pd-based catalysts, providing practical guidelines for designing FAOR electrocatalysts for DFAFCs and related liquid-fuel energy conversion devices.