Improved Oxygen Reduction Activity and Durability of Dealloyed PtCo_x Catalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects

Abstract

The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo3/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo3/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co. In situ X-ray absorption spectroscopy (XAS) reveals that these two catalysts possess similar Pt–Pt and Pt–Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The similar activity of the two catalysts is thus ascribed to their comparable strain, ligand, and particle size effects. Ex situ XAS performed on D-PtCo3/HSC under different voltage cycling stage shows that the continuous dissolution of Co leaves behind the NPs with a Pt-like structure after 30k cycles. The attenuated strain and/or ligand effects caused by Co dissolution are presumably counterbalanced by the particle size effects with particle growth, which likely accounts for the constant specific activity of the catalysts along with voltage cycling.

Pt3M (M: Co, Ni and Fe) Bimetallic Alloy Nanoclusters as Support-Free Electrocatalysts with Improved Activity and Durability for Dioxygen Reduction in PEM Fuel Cells

“…The calculated lattice parameter and the percentage of compressive strain in Pt-M catalysts were determined and summarized in Table 1. Among the Pt-M catalysts, Pt-Co shows higher compressive strain (0.84 %) which could play a vital role for enhanced ORR activity and it was well acknowledged by many researchers [34,35]. The average crystallite size was found to be 6.5, 6.2 and 6.3 nm for Pt-Co, Pt-Ni and Pt-Fe, respectively.…”

Linking morphology with activity through the lifetime of pretreated PtNi nanostructured thin film catalysts

“…Electrodes were assembled into CCMs, and were then submitted to a conditioning process to activate the fuel cell, after which electrochemical measurements were recorded and cyclical voltage testing was performed. 13 32 consisting of 30,000 50 mV/s cycles between 0.6-1.0 V at 80 o C, 100 kPa H 2 /N 2 , 800/1800SCCM, 100% RH. ORR activity, specific surface area, and H 2 /Air performance were evaluated at beginning of life and after 10, 20, and 30 thousand cycles.…”

“…12 Such treatments can be used to form and control nanoporosity, the formation of which is highly dependent on particle size and composition. 3,12,13 While these pretreatments clearly contribute to increases in fuel cell performance, drawing structure-property relationships based on the characterization of the pretreated catalyst alone can be misleading due to the often overlooked impact of fuel cell conditioning. In order to reach peak performance, a newly assembled fuel cell must typically undergo what is known as a conditioning or break-in process.…”

Actualizing In Situ X-ray Absorption Spectroscopy Characterization of PEMFC-Cycled Pt-Electrodes

“…The ORR activity of platinum can be significantly improved by alloying Pt with a wide range of transition metals (denoted as M) such as Co, Ni, Y, and the activity improvement has been attributed to the strain and/or ligand effects induced by M via optimization of the Pt-O binding energy and surface coordinate configuration. [1][2][3][4][5] However, the activity gain induced by M is generally not sustainable owing primarily to the dissolution of M during long-term PEMFC operation, which leads to the attenuation of strain/ligand effects, particle growth, and the loss of the favorable surface coordinate configuration. 6 Huang et al 7 recently reported that both the activity and durability of the PtNi/C octahedral NPs can be markedly improved by doping with a transition metal such as Mo on the surface.…”

“…In addition, XAS is extremely suitable for understanding the structural and mechanistic basis of the ORR activity and durability of PtM/C electrocatalysts since it allows for quantitative evaluation of the strain, ligand, particle size, and site-blocking effects on their catalytic performance. 5,6,[9][10][11] Therefore, substantial efforts have been devoted to actualizing in situ/operando XAS characterization of PtM/C NPs cycled in PEMFCs to obtain the structure-activity-durability correlations. 12,13 An operando XAS fuel cell was built wherein the MEA was composed of a cathode electrode of PtNi/C NPs subject to XAS characterization, and an anode electrode of Pd/C NPs (the Pt/C NPs cannot be used due to the convolution of the Pt signals from the Pt/C and the PtNi/C).…”