Importance of Chemical Activation and the Effect of Low Operation Voltage on the Performance of Pt-Alloy Fuel Cell Electrocatalysts

Abstract

Pt-alloy (Pt–M) nanoparticles (NPs) with less-expensive 3d transition metals (M = Ni, Cu, Co) supported on high-surface-area carbon supports are currently the state-of-the-art (SoA) solution to reach the production phase in proton exchange membrane fuel cells (PEMFCs). However, while Pt–M electrocatalysts show promise in terms of increased activity for oxygen reduction reaction (ORR) and, thus, cost reductions from the significantly lower use of expensive and rare Pt, key challenges in terms of synthesis, activation, and stability remain to unlock their true potential. This work systematically tackles them with a combination of electrocatalyst synthesis and characterization methodologies including thin-film rotating disc electrodes (TF-RDEs), an electrochemical flow cell linked to an inductively coupled plasma mass spectrometer (EFC-ICP-MS), and testing in 50 cm 2 membrane electrode assemblies (MEAs). In the first part of the present work, we highlight the crucial importance of the chemical activation (dealloying) step on the performance of Pt–M electrocatalysts in the MEA at high current densities (HCDs). In addition, we provide the scientific community with a preliminary and facile method of distinguishing between a “poorly” and “adequately” dealloyed (activated) Pt-alloy electrocatalyst using a much simpler and affordable TF-RDE methodology using the well-known CO-stripping process. Since the transition-metal cations can also be introduced in a PEMFC due to the degradation of the Pt–M NPs, the second part of the work focuses on presenting clear evidence on the direct impact of the lower voltage limit (LVL) on the stability of Pt–M electrocatalysts. The data suggests that in addition to intrinsic improvements in stability, significant improvements in the PEMFC lifetime can also be obtained via the correct MEA design and applied limits of operation, namely, restricting not just the upper but equally important also the lower operation voltage.

Stability challenges of carbon-supported Pt-nanoalloys as fuel cell oxygen reduction reaction electrocatalysts

“…On the other hand, this Cu is detected with a noticeable increase in the subsequent Cu UPD dissolution peak (A2′) in the following cycle. 18,62,64 In contrast, the other three Pt–M electrocatalytic systems do not experience relevant UPD dissolution. Despite that, a significantly less intense A2′ peak is nevertheless still present in the case of the other three alloys.…”

“…64,93 However, further continuous depletion/dissolution of the M, namely during the operation of the PEMFC, is an undesired and very detrimental process 69,94 as it will always result in voltage losses and consequent decline in PEMFC performance. 62,95 When it comes to the Pt-nanoalloy electrocatalysts, depletion of the M from the core of NPs leads to decay in ligand and/or strain effects, which in consequence results in a decrease in intrinsic ORR activity of the electrocatalyst. 57,96 In specific cases when the chemical composition, as well as particle size, are above a certain critical threshold ( e.g.…”

“…Since the SFC-ICP-MS and SPRDE techniques have already been widely addressed and described elsewhere, 128,130 the present manuscript will focus on the EFC-ICP-MS methodology for time-and-potential resolved ( in situ ) determination of Pt-alloys dissolution, using a modified commercially available cell from BASi. 18,54,62,64,81,82,131 The stationary EFC, which is coupled with an ICP (Fig. 3a), 77,132 has a three-electrode system: working electrode (WE) and counter electrode (CE), which are two glassy carbon discs ( d = 3 mm) embedded into polyether ether ketone (PEEK) material, and reference Ag/AgCl electrode (RE).…”

“…during the start-up/shutdown), not only metal dissolution but also the kinetics of carbon oxidation reaction (COR), responsible for carbon corrosion, increases dramatically. 61 Furthermore, recent research provides strong evidence that not only does carbon corrosion increases also with increasing temperature 58 but so does the dissolution of Pt and consequently, also the dissolution of M. 54,62 However, what is common with all the degradation mechanisms is that they lead to the loss of the active Pt surface area – ECSA, as well as a decline in the ORR mass activity. Furthermore, especially in the case of Pt-alloys, due to the additional dissolution of M in the PEMFC, not only the performance of the electrocatalyst itself but the overall performance and longevity of the PEMFC are affected.…”

“…However, in practice, the understanding of how electrocatalyst degradation affects the fuel cell operation is important for both theoretical and practical reasons since there are no catalysts that do not degrade over a prolonged time or upon exposure to extreme conditions. 8,18,54,56,58,62,69…”