Pt Catalyst Degradation in Aqueous and Fuel Cell Environments studied via In-Operando Anomalous Small-Angle X-ray Scattering

Self Cite

Platinum-cobalt alloy nanoparticles are of great interest as cathode catalysts for PEMFCs as they have been shown to have enhanced activity versus platinum. However, their relative stability against loss of electrochemically-active surface area in relation to Pt catalysts is still debated. In this study, the evolutions of Pt 3 Co particle size distributions (PSDs) in fuel cell and aqueous environments were followed during accelerated stress tests (ASTs) using in-operando ASAXS. The measured evolutions showed a degradation mechanism dominated by loss of particles smaller than the critical particle diameter (<5.2 to 6.1 nm, CPD), which depended on environment and the AST. These evolutions were compared to that of a Pt catalyst with a similar initial PSD, which was found to have a lower degradation rate than Pt 3 Co. The ASAXS data, as well as data from aqueous dissolution, X-ray absorption spectroscopy, individual particle energy-dispersive spectroscopy, X-ray fluorescence spectrometry, and kinetic Monte Carlo calculations support a loss mechanism of increased Pt dissolution from Pt 3 Co versus Pt due to destabilization caused by extensive dealloying of particles <∼5 nm during ASTs. Polymer electrolyte fuel cells (PEFCs) are a promising highefficiency energy conversion technology suitable for mobile and stationary applications. Two of the major issues still needing to be addressed for large-scale adoption are cost and durability. The major contributing factor to these issues is the electrocatalyst needed for the cathodic oxygen reduction reaction (ORR). Pt has been shown to be the best monometallic catalyst material for ORR 1 with high activity and good stability in the acidic environment of the PEFC.2,3 However, there is still a need for a more active, more stable, and less expensive cathode electrocatalyst to meet the demanding cost and lifetime requirements for many applications, especially for automotive propulsion power.Pt alloys are of great interest as ORR electrocatalysts as they are generally less expensive and have been shown to have enhanced activity versus Pt. [4][5][6] This enhancement in activity was found to be directly correlated with the Pt-Pt interatomic distance 7,8 and to the Pt d-band occupancy in the alloy. [8][9][10] However, there are concerns that the use of base metal alloying elements, which are generally less stable than Pt in acidic environments, could be accompanied by a loss in particle stability and corresponding loss in ORR activity. Conflicting reports exist about the overall stability of Pt alloy catalysts in relation to pure Pt catalysts. 5,6,[11][12][13] Some studies suggest that alloying has a positive effect while others show no significant impact on stability. It is known that relative stability is dependent on particle size, 14-16 the larger the initial particle size the more stable, and the majority of Pt alloy synthesis techniques are based on high temperature annealing processes which result in larger initial particle sizes. Therefore any comparisons between larger annea...

Section: Resultsmentioning

confidence: 99%

“…14,[18][19][20][21][22] A comprehensive discussion of the ASAXS method and its use in characterizing carbon-supported metal catalysts can be found in several publications and textbooks. [23][24][25][26][27][28][29] One advantage this technique has is its ability to obtain element specific PSDs.…”

mentioning

confidence: 99%

In-Operando Anomalous Small-Angle X-Ray Scattering Investigation of Pt₃Co Catalyst Degradation in Aqueous and Fuel Cell Environments

Gilbert

Kropf

Kariuki

et al. 2015

Self Cite

“…Equations 1 to 2 are discretized by the finite-volume method and solved numerically to obtain species, potential, and current distributions with the following boundary conditions: dc H2 dx = 0 at x = 0 and x = x 2 [4] dc O2 dx = 0 at x = 0 and x = x 2 [5] dc H2 dy = 0 at x < x 1 c H2 = const at x ≥ x 1 at y = 0 [ 6 ] dc O2 dy = 0 at x < x 1 c O2 = const at x ≥ x 1 at y = y 3 [7] dφ e dx = 0 at x = 0 and x = x 2 [8] dφ e dy = 0 at y = 0 and y = y 3 [9] It is assumed that gas flow through the electrodes is fully developed at x = 0 and symmetry applies at x = x 2 . The gasket is an insulating layer, impermeable to gas.…”

Section: Resultsmentioning

confidence: 99%

“…In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells. [1][2][3][4][5][6][7] While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to membrane-electrode assembly (MEA) performance degradation requires looking beyond wellresearched catalyst nanoparticle degradation processes.…”

mentioning

confidence: 99%

Electrode Edge Cobalt Cation Migration in an Operating Fuel Cell: An In Situ Micro-X-ray Fluorescence Study

Cai

Ziegelbauer

Baker

et al. 2018

PtCo-alloy cathode electrocatalysts release Co cations under operation, and the presence of these cations in the membrane electrode assembly (MEA) can result in large performance losses. It is unlikely that these cations are static, but change positions depending on operating conditions. A thorough accounting of these Co cation positions and concentrations has been impossible to obtain owing to the inability to monitor these processes in operando. Indeed, the environment (water and ion content, potential, and temperature) within a fuel cell varies widely from inlet to outlet, from anode to cathode, and from active to inactive area. Synchrotron micro-X-ray fluorescence (μ-XRF) was leveraged to directly monitor Co 2+ transport in an operating H 2 /air MEA for the first time. A Nafion membrane was exchanged to a known Co cation capacity, and standard Pt/C electrocatalysts were utilized for both electrodes. Co Kα 1 XRF maps revealed through-plane transient Co transport responses driven by cell potential and current density. Because of the cell design and imaging geometry, the distributions were strongly impacted by the MEA edge configuration. These findings will drive future imaging cell designs to allow for quantitative mapping of cation through-plane distributions during operation. In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells. [1][2][3][4][5][6][7] While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to membrane-electrode assembly (MEA) performance degradation requires looking beyond wellresearched catalyst nanoparticle degradation processes.2,8 Indeed, the performance of a PEM fuel cell is affected by many internal and external factors, such as fuel cell design and assembly, material degradation, operational conditions, and impurities or contaminants.9 The above-mentioned factors interact, and they are closely related to the cation activities in the fuel cell. In the current PEM fuel cell system, in addition to protons, various other cations are introduced from a variety of sources. For example, cerium ions are intentionally introduced to the MEA to improve membrane durability by neutralizing radical species before they attack the ionomer. 10,11 In contrast, when Pt-alloy catalysts are used, cobalt, nickel, or other 3d transition metal cations can leach out during fuel cell operation.1,3,5 Finally, cell component corrosion or impurities in the reactant/fuel flows may serve as additional cation sources. 12,13 In general, these cations exhibit greater affinities for the sulfonic acid groups in the ionomers than protons. Proton flux to the cathode is reduced not only due to proton site occupancy but also decreased proton mobility caused by cationic interaction.14-16 Strong interactions between the cations and sulfonic acid sites may also induce io...

“…12 Operando XAS of fuel cells is now widely practiced. [13][14][15][16][17] A hydrated membrane is essential for fuel cell operation. Operando IR spectroscopy 7,[18][19][20][21][22][23][24][25][26][27] requires stringent control of reactant stream humidification, flow rate, and cell temperature to prevent condensed water from overwhelming the IR spectra.…”

mentioning

confidence: 99%

Operando Raman Micro-Spectroscopy of Polymer Electrolyte Fuel Cells

Kendrick

Fore

Doan

et al. 2016

Operando Raman micro-spectroscopy of the membrane electrode assembly (MEA) of a fully operating hydrogen/oxygen Nafion electrolyte fuel cell is described. Coarse depth profiling of the fuel cell system enabled appropriate positioning of the microspectroscopy laser focal point for MEA catalytic layer spectroscopy. An increase in the ionomer state-of-hydration, from oxygen reduction at the cathode, transitions ion exchange sites from the sulfonic acid to the dissociated sulfonate form. Visualization of density functional theory calculated normal mode eigenvector animations enabled assignments of Nafion side-chain vibrational bands in terms of the exchange site local symmetry: C 1 and C 3V modes correlate to the sulfonic acid and sulfonate forms respectively. The gradual transition of the MEA spectra from C 1 to C 3V modes, from the fuel cell open circuit voltage to the short circuit current respectively, demonstrate the utility of vibrational group mode assignments in terms of exchange site local symmetry.