Enhancing Direct Methanol Oxidation with Pt-Pd Alloy Nanoparticles Synthesized By Gas Diffusion Electrocrystallization (GDEx)

Abstract

Direct methanol fuel cells (DMFCs) are a promising sustainable energy solution due to their high energy density, low-temperature operation, and portability. However, sluggish methanol oxidation reaction (MOR) kinetics hinders their successful commercialization.1,2 Pt is a highly active MOR catalyst due to its ability to adsorb and decompose methanol on its surface. However, when pure Pt is used, the adsorption of oxidation intermediaries, i.e., CO, occurs, blocking catalytically-active sites and reducing the catalytic efficiency.2 A promising strategy to improve the catalytic activity and CO tolerance of Pt is by alloying. Among several candidates, Pd stands out due to its identical crystal structure and similar lattice constant to Pt, resulting in single-phase bimetallic materials with strong coupling and improved catalytic performance. In addition, the active surface area is enhanced, and stability is improved by using Pt-Pd alloys.3 We recently reported an electrochemical method for synthesizing platinum group (PGMs, i.e., Pt, Pd, Rh) nanoparticles (NPs) using Gas-Diffusion Electrocrystallization, best known as the GDEx process.4 In GDEx, CO2 and water are reduced using a gas diffusion electrode, producing CO and H2, which in turn reduce PGMs ions in solution, forming metal nanoclusters of 10 nm–40 nm composed of even smaller nanocrystals of 2 nm–4 nm. In this work, we used GDEx to synthesize Pt-Pd alloy NPs with different metal ratios (Pt100, Pt75-Pd25, Pt50-Pd50, Pt25-Pd75 and Pd100). The Pt-Pd alloy NPs were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy. The size distribution of the synthesized nanomaterials was between 15 nm–30 nm, increasing in size with increasing Pd content. The catalytic activity of the Pt-Pd alloy NPs toward methanol oxidation was measured in 0.1 M HClO4. The electrochemical surface area (ECSA), measured by the Cu underpotential deposition method,5 was 37.1 ± 2.1, 36.1 ± 2.7, 42.5 ± 1.7 and 56.1 ± 2.9 m2 gPt -1 for Pt100, Pt75-Pd25, Pt50-Pd50, and Pt25-Pd75 respectively. The mass activity (MA, fig.1a), defined by the current density per unit mass of Pt loading at the forward peak, was 452 ± 10, 345 ± 16, 365 ± 21 and 189 ± 15 for Pt100, Pt75-Pd25, Pt50-Pd50, Pt25-Pd75. Even though the MA for Pt75-Pd25 and Pt50-Pd50 was lower than for Pt100, their onset potential was smaller (0.503 VRHE for Pt75-Pd25 and 0.488 VRHE for Pt50-Pd50) than for Pt100 (0.530 VRHE) indicating that the MOR is more favourable on those alloys. Besides, during the accelerated stability test (ADT, Fig.1b), after 1000 cycles, Pt75-Pd25 and Pt50-Pd50 showed higher stability and resistance towards CO poisoning than Pt100 by holding 80% and 70% of their peak current density. In comparison, Pt100 only kept ∼60% of its peak current density under the same conditions.In summary, the GDEx process allows the synthesis of Pt-Pd alloy NPs with a similar or superior catalytic performance towards MOR than pure Pt NPs, i.e., similar MA but with a better tolerance towards CO poisoning. The selectivity of the GDEx process for PGMs allows the synthesis of Pt-Pd catalysts for DMFCs not only from synthetic solutions but also from complex matrixes that contains a mixture of both PGMs (i.e., leachates from spent automotive catalytic converters).This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreements No. 958302 (PEACOC project) and No. 101091715 (FIREFLY project). Yuda A, Ashok A, Kumar A. Catalysis Reviews. 2022;64(1):126-228.Ren X, Lv Q, Liu L, Liu B, Wang Y, Liu A, Wu G. Sustain. Energy Fuels. 2020;4(1):15-30.Yousaf AB, Imran M, Uwitonze N, Zeb A, et al. J . Phy s. Chem . C. 2017;121(4):2069-79.Martinez-Mora O, Pozo G, Leon-Fernandez LF, Fransar J, Dominguez-Benetton X., RSC Sustain. 2023; DOI: 10.1039/D3SU00046J.Green CL, Kucernak A. J. Phys. Chem. B. 2002;106(5):1036-47. Figure 1