PtCo Alloy Catalysts Research Articles

One-Dimensional Numerical Simulation of Pt-Co Alloy Catalyst Aging for Proton Exchange Membrane Fuel Cells

The service life of catalysts is a key aspect limiting the commercial development of proton exchange membrane fuel cells (PEMFCs). In this paper, a one-dimensional degradation model of a Pt-Co alloy catalyst in the cathode catalytic layer (CCL) of a PEMFC is proposed, which can track the catalyst size evolution in real time and demonstrate the catalyst degradation during operation. The results show that severe dissolution of particles near the CCL/membrane leads to uneven aging of the Pt-Co alloy catalyst along the CCL thickness direction. When the upper potential limit (UPL) is less than 0.95 V, it does not affect the catalyst significantly; however, a slight change may cause great harm to the catalyst performance and service life after UPL > 0.95 V. In addition, it is found that operating temperature increases the Pt mass loss on the carbon support near the CCL/membrane side, while it has little effect on the remaining Pt mass on the carbon support near the CCL/GDL side. These uncovered degradation mechanisms of Pt-Co alloy provide guidance for its application in PEMFCs.

Electron-enriched Pt induced by CoSe2 promoted bifunctional catalytic ability for low carbon alcohol fuel electro-reforming of hydrogen evolution

Alcohol fuel electro-reforming is promising for green hydrogen generation while developing efficient bifunctional catalysts for alcohol fuel electrolysis is still very tricky. Herein, we for the first time proposed the electron-enriched Pt induced by CoSe2 has an efficient bi-functional catalytic ability for alcohol fuels electro-reforming of hydrogen in acid electrolytes. The theoretical calculation revealed the advantages of electron-enriched Pt surface for the adsorption of intermediate, which is well supported by spectroscopic analysis and CO-stripping techniques. Largely improved catalytic performances of activity, durability, and kinetics are demonstrated compared to the conventional alloy system and commercial Pt/C catalyst, due to the efficient synergism of Pt and CoSe2; the peak current density of Pt/CoSe2 for methanol (ethanol) oxidation is 87.61 (48.27) mA cm−2, which is about 3.3 (2.0) times higher than that of Pt/C catalyst and 2.0 (1.5) times that of the traditional PtCo alloy catalysts. Impressively, about 80% of the initial current was found after 1000 cycles of stability test for alcohol fuel oxidation of Pt/CoSe2 catalyst, higher than that of Pt/C (ca. 50%) and PtCo catalyst (65%). When Pt/CoSe2 catalyst serviced as bi-functional catalysts for electrolyzer, a low cell potential of 0.65 (0.78) V for methanol (ethanol) electrolysis was required to reach 10 mA cm−2, which was about 1030 (900) mV less than that of conventional water electrolysis using Pt/C as the catalyst. The current result is instructive for the design of novel bifunctional catalyst and the understanding of hydrogen generation via alcohol fuel electro-reforming.

Mitigation of PtCo/C Cathode Catalyst Degradation via Control of Relative Humidity

Maintaining the high performance of proton-exchange membrane fuel cells (PEMFC) over the course of its lifetime is a key enabling factor for its successful commercialization as a primary power source in zero-emission transportation applications. In this context, it is important to mitigate the degradation of PtCo-alloy based cathode catalysts used for oxygen reduction reaction (ORR). PtCo-alloy catalysts exhibit high activity at beginning-of-life (BOL) which tends to decrease during operation due to loss of electrochemical surface area (ECSA) and dissolution-contamination related effects of the Co-alloying component. Here, we demonstrate the use of relative humidity (RH) of the inlet gases as a controllable parameter to mitigate the degradation of PtCo-alloy catalyst degradation. We employ a catalyst-specific voltage cycling accelerated stress test (AST) durability protocol as a function of inlet RH to degrade PtCo catalysts. A series of in situ electrochemical diagnostics and ex situ characterizations have been carried out to investigate the catalyst layer characteristics at end-of-test (EOT). Our results show that at sub-saturated conditions of durability protocol operation, PtCo catalyst sustains higher EOT H2/air performance due to better retention of ECSA and smaller impact of Co2+ dissolution/contamination.

Enhancing MEA Performance of PEFCs with Ionic Liquid Impregnated ORR Catalysts

For polymer electrode fuel cells (PEFCs) vehicles, low precious metal (PGM) loading is desired to reduce the cost of stacks for future mass applications. In order to improve catalyst activity and reduce PGM loading, intensive research activities were conducted on the development of novel electrocatalysts. Among them, bimetallic alloy catalysts such as PtCo and PtNi received great attention.1 Along with alloy catalysts, surface modifications were also studied to promote catalyst activity.2 Among them, ionic liquids have been used and studied to promote activity of oxygen reduction (ORR) catalysts.3 We recently developed two unique ionic liquids aiming to improve ORR catalyst activity by mitigating anion poisoning on platinum surface. The effects of the ionic liquids were studied in membrane electrode assembly (MEA) on catalyst activity and durability with PtCo alloy catalyst. Polarization curves at different relative humidity, electrochemical active surface area (ECSA), mass activity (MA), electrochemical impedance spectroscopy and gas transport resistance were compared with in-house catalyst to confirm the advantages and study the mechanism of the improvements. Accelerated durability test with potential cycling on cathode was performed to study the catalyst durability after impregnation of ionic liquids. The enhanced activity and durability in MEAs provide opportunity to reduce PGM loading while maintaining high performance.References Kodama, T. Nagai, A. Kuwaki et al., Nat. Nanotechnology, 2021 , 16, 140–147.Gui-Rong, W. Thomas, J.S. Daniel. et al., ACS Catalysis, 2018, 8 (9), 8244-8254.Huang, T. Song, O. Morales-Collazo, H. Jia, J.F. Brennecke, 16TJ. Electrochem. Soc., 2017, 16T 164, F1448-F1459. Figure 1

Preparation of PEFC Electrocatalysts Using SnO2 Thin Layer Support

Pt/C is widely used as polymer electrolyte fuel cell (PEFC) electrocatalyst. However, under high potential at the cathode, Pt detachment and aggregation due to carbon corrosion can occur. Therefore, conventional Pt/C electrocatalysts have a difficulty in durability, for e.g. heavy-duty applications. Our research group has been preparing electrocatalysts with both high activity and high durability (1-3). However, their electrocatalytic activity has to be further improved. For this reason, we are further improving catalytic activity by alloying platinum with cobalt on such SnO2-based catalyst support. Cobalt is one of the relatively stable metals under the PEFC electrocatalyst condition, and PtxCoy alloy catalysts are known to be capable of exhibiting higher activity over Pt catalysts (4). In addition, the doping of SnO2 with 2 mol% Nb is expected to increase electronic conductivity of SnO2 (5,6). Experimental Sn0.98Nb0.02O2 thin layer was deposited on the GCB (denoted as Sn0.98Nb0.02O2/GCB) by the ammonia coprecipitation method. Pt(acac)2 and Co(acac)2 were applied to simultaneously impregnate Pt and Co to obtain Pt-Co alloy catalysts (acac method). The Pt : Co ratio was adjusted to prepare Pt3Co alloy to obtain Pt3Co/Sn0.98Nb0.02O2/GCB electrocatalyst. The preparation procedure is schematically described in Figure.1. Results and discussion In order to evaluate Pt and Co loadings, ICP analysis was performed. We then made half-cell tests to evaluate electrochemical activities of these electrocatalysts. Electrochemical surface area (ECSA) was measured by cyclic voltammetry (CV), and oxygen reduction reaction (ORR) activity was derived from kinetically controlled current density (ik ) in the rotating disk electrode (RDE) measurement. We prepared MEA (Membrane Electrode Assembly) using this electrocatalyst (Pt3Co/Sn0.98Nb0.02O2/GCB) on the cathode side. We then made full-cell tests to evaluate cell performance such as current-voltage characteristicsFrom the ICP results, we confirmed that we could impregnate both Pt and Co close to the designated ratio in the acac method. Microstructures and EDS maps are shown in Figure. 2, confirming that Pt and Co were uniformly supported on the SnO2 surface layer on GCB. Electrochemical results and detailed microstructural analysis will be reported and discussed. Acknowledgement Financial support from New Energy and Industrial Technology Development Organization (NEDO) is gratefully acknowledged (Contract No. 20001214-0). References S. Matsumoto, M. Nagamine, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), F1164 (2018).Y. Nakazato, D. Kawachino, Z. Noda, J. Matsuda, S. M. Lyth, A. Hayashi, and K. Sasaki, J. Electrochem. Soc., 165 (14), F1154 (2018). T. Yoshizumi, M. Nagamine, Z. Noda, J. Matsuda, A. Hayashi, and K. Sasaki, ECS Trans., 92 (8), 479 (2019). T. Tada, Y. Yamamoto, K. Matsutani, K. Hayakawa, and T. Namai, ECS Trans., 16 (2), 215 (2008). F. Takasaki, S. Matsuie, Y. Takabatake, Z. Noda, A. Hayashi, Y. Shiratori, K. Ito, and K. Sasaki, J. Electrochem. Soc., 158, B1270 (2011). K. Sasaki, F. Takasaki, Z. Noda, S. Hayashi, Y. Shiratori, and K. Ito, ECS Trans., 33 (1), 473 (2010). Figure 1

PtCo Catalyst Dissolution and Oxidation Modeling for Durability Improvement of Automotive Fuel Cell

Polymer electrolyte fuel cell (PEFC) is one of the candidates to replace automotive power sources using fossil fuels. For this purpose, we need to overcome three major criteria: cost, performance, and durability. One of the important topics of PEFC durability is how to keep its catalyst activity for oxygen reduction reaction (ORR). The catalyst activity is product of the electrochemical surface area (ECSA) and activity per surface area. A central issue of catalyst durability is the loss of ECSA over time. This ECSA is approximately the same as the effective surface area of the catalyst. To maximize the ECSA, the radius of catalyst particles is in the order of nano meters.In this study we focus on the ECSA loss of PtCo alloy catalysts on carbon support. These alloy catalysts have much higher activity for ORR than traditionally used Pt1. Our model of ECSA loss combines the following two mechanism:(i) Co dissolution from the surface of the alloy catalyst,(ii) Pt dissolution from small particles and precipitation onto large (Ostwald ripening).The mechanism (i) changes the distribution of the alloy ratio in particle. Because of the standard dissolution potential of Co is -0.28 V vs NHE at 25◦C, Co is easy to dissolve in the aqueous media especially under low pH condition such as PEFC, and the Co ratio near surface is smaller than that of the center 2. In the case of dissolution under high potential, the amount of its depends on the Co ratio.In the mechanism (ii), the balance between Pt and its oxides on particle surface has an important role to simulate ECSA change. While Pt is known to form stable oxide species as well as hydroxide intermediates, the mechanism and specification still remain the subject of research 1. In the previous papers, this oxidation step is described by the nonlinear reaction rate coefficient called Temkin term1, 3, 4. Though this nonlinear term is able to describe the generation of the higher oxidation such as multi-layer of PtO and PtOx (x ≥2) by only one equation3. This term is one of the reasons for extended computing time, and we cannot derive the term consistently from (non-equilibrium) thermodynamics. Here we consider the kinetics of PtCo catalyst dissolution under 0.9V/cell, which is a typical condition in vehicle usage for improving the durability5. Under this condition we need not to consider the higher oxidation process, the following simple two independent fundamental reactions1 is considered:Pt + H2O = PtOH + H+ + e−, (1)PtOH = PtO + H+ + e−. (2)These consecutive reactions are based on spectroscopic results by Ishiguro et al .6 The reaction rates are determined by the ordinary Butler-Volmer equations with some parameters which depend on the temperature and the humidity, without Temkin term. Using the above model, we can calculate the evolution of ECSA loss for the long period and the results are in good agreement with experimental data [Fig. 1].Another application of our model is to predict the necessary conditions in order to achieve both initial performance and durability. For example, if the mean radius of the catalyst is 4 nm, the deviation of this size distribution function must have about 0.7 nm or than less. The results are very severe conditions but needed to overcome for widely spread of PEFC vehicles.Figure 1.Experiment (symbols) and modeled (solid line) ECSA change as function of square-wave potential cycle (0.1 V (3 sec) <-> 0.80, 0.85, and 0.90 V (3 sec))1. R. K. Ahluwalia, X. Wang, J.-K. Peng, N. N. Kariuki, D. J. Myers, S. Rasouli, P. J. Ferreira, Z. Yang, A. Martinez-Bonastre, D. Fongalland and J. Sharman, J. Electrochem. Soc. , 165, 6, p. F3316 (2018).2. M. Watanabe, H. Yano, D. A. Tryk and H. Uchida, J. Electrochem. Soc. , 163 , 6, p. F455 (2016).3. R. M. Darling and J. P. Meyers, J Electrochem. Soc. , 150 , 11, p. A1523 (2003).4. P. Schneider, C. Sadeler, A.-C. Scherzer, N. Zamel and D. Gerteisen, J. Electrochem. Soc. , 166 , 4, p. F322 (2019).5. S. Stariha, N. Macauley, B. T. Sneed, D. Langlois, K. L. More, R. Mukundan and R. L. Borup, J. Electrochem. Soc. , 165 , 7, p. F492 (2018).6. N. Ishiguro, S. Kityakarn, O. Sekizawa, T. Uruga, H. Matsui, M. Taguchi, K. Nagasawa, T. Yokoyama and M. Tada, J. Phys. Chem. C , 120 , 35, p. 19642 (2016). Figure 1

Enhancement of the Catalytic Activity and Load Cycle Durability of a PtCo Alloy Cathode Catalyst Supported on Ta-Doped SnO2 with a Unique Fused Aggregated Network Microstructure for Polymer Electrolyte Fuel Cells

We succeeded in the synthesis of two types of PtCo alloy catalysts supported on Ta-doped SnO2 without using carbon black additives. The PtCo disordered alloy supported on Ta-SnO2 (BET surface area,...

PtCo/N-doped carbon sheets derived from a simple pyrolysis of graphene oxide/ZIF-67/H2PtCl6 composites as an efficient catalyst for methanol electro-oxidation

Many alloy catalysts have been developed for methanol electro-oxidation, but most synthetic methods are complicated. Herein, PtCo alloy catalysts supported on N-doped carbon sheets (PtCo/NCS) are successfully prepared by a simple pyrolysis of graphene oxide/ZIF-67/H2PtCl6 composites at different temperatures (700, 800, 900 °C) under a gas flow of H2/Ar, in which ZIF-67 is served as Co and N sources. SEM, TEM, XRD, XPS and electrochemical characterization are employed to study as-prepared catalysts. In acidic methanol solution, the area-specific activity (1.25 mA cm−2 Pt) of PtCo alloy catalyst obtained at 800 °C (PtCo/NCS-800) is 2.6 times of commercial Pt/C (0.48 mA cm−2 Pt), and the area-specific activity of PtCo/NCS-800 is 3.5 times of Pt/C after 1000 cycles. Furthermore, an improved CO-tolerance of Pt is confirmed. The electronic effect and synergistic effect of metallic elements are responsible for outstanding performance of as-prepared catalysts. This work provides a simple approach to obtain high performance alloy catalysts.

Highly stable and ordered intermetallic PtCo alloy catalyst supported on graphitized carbon containing Co@CN for oxygen reduction reaction

Ordered intermetallic PtCo synthesized from N-doped graphitic carbon-containing Co presents the high durability with low Pt loading after 30 000 load-simulated cycles in proton exchange membrane fuel cells.

Impact of Operating Conditions on PEMFC Durability

PtCo-alloy catalysts have achieved mainstream application approximately 20 years after they were first reported. The current state of art PEM fuel cell cathode catalyst is represented by Pt-alloy nanoparticles dispersed on high surface area carbon (HSC) supports. [1] Mass activity > 0.6 A/mgPt has been reported consistently for such catalysts. Unfortunately, the high BOL activity of the Pt-alloy nanoparticle catalysts can suffer fairly rapid degradation in mass activity (MA) over time on exposure to fuel cell operating conditions, especially under voltage cycling (V-cycling). The prevalent mechanism for loss of specific activity is due to dissolution of the non-noble alloying element and thickening of Pt shell during voltage cycling. [2] Not withstanding the advances and numerous impactful studies, the degradation of Pt-alloy nanoparticles and its impact on electrode performance is poorly understood. For example, a recent article reviews the conflicting results on the impact of alloying on Pt nanoparticle stability, from positive, to neutral, to negative. [3] Additionally, the existing literature on durability of PtCo alloys are focused on the activity and nanoparticle characterization rather than on associated V-loss in the composite electrode. Given the criticality of this issue, a careful systematic study of this phenomenon with best-in-class materials in a state of art (SOA) membrane electrode assembly (MEA) is desired. In this present work, the use of operating conditions as a key factor to mitigate electrode degradation is explored. This was accomplished by conducting voltage cycling tests in H2/N2 environment at different operating conditions up to 60,000 voltage cycles. To identify the factors affecting the degradation, fractional factorial design of experiments was implemented. The fractional design allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. Factors in the test matrix include temperature, relative humidity, upper potential limit and upper potential hold time etc. In-situ electrochemical diagnostics such as electrochemical surface area (ECSA), mass activity (MA), specific activity (SA) was measured to understand the kinetics and degradation mechanism. Transport properties such as proton transport resistance and oxygen transport resistance was measured using H2-N2 impedance [4] and oxygen limiting current measurements [5]. The end of test (EOT) MEAs were characterized by ex-situ characterization measurements such as TEM for particle size distribution, electron micro probe analysis (EPMA) and EDS measurements to quantify the cobalt composition and loss up on voltage cycling at different operating conditions. In addition to multi-factor design of experiments, few key factors like RH and temperature were studied independently. Figure 1 shows the impact of relative humidity (RH) on H2-air performance and measured electrochemical surface area. Up to 100 mV loss at high current density (2.0 A/cm2) and ~30% ECSA loss observed for MEAs cycled up to 30,000 voltage cycles. Both performance and ECSA loss decreases with decreasing relative humidity. Almost zero ECSA loss after 30,000 voltage cycles at 25% RH observed. This example indicates that PtCo alloy catalyst degradation can be mitigated by carefully identifying the operating condition. The effect of various such operating condition and degradation mechanisms will be discussed. References S. Kumaraguru, 2018 Annual Progress Report, https://www.hydrogen.energy.gov/pdfs/progress18/fc_kumaraguru_2018.pdf C.E. Carlton, S. Chen, P.J. Ferreira, L.F. Allard, Y. Shao-Horn, J. Phys. Chem.Lett. 3, 161 (2012)J.A. Gilbert, A.J. Kropf, N.N. Kariuki, S. DeCrane, X. Wang, S. Rasouli, K. Yu, P.J. Ferreira, D. Morgan, D.J. Meyers, J. Electrochem. Soc., 162 (14), F1487 (2015)R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5), A970 (2005)T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012) Figure 1

Investigation of Membrane Chemical Degradation As a Function of Catalyst Platinum Loading

Membrane chemical degradation is one of many factors that can impact fuel cell durability. The fuel cell’s lifetime heavily depends on the membrane and its ability to maintain chemical and mechanical integrity. Studies indicate degradation is due to the formation of hydroxyl radicals that attack the polymer structure resulting in membrane thinning, and the release of fluoride and sulfate ions1. This consequently causes mechanical failures to transpire, e.g. cracks, tears, perforations and pinhole formation, leading to an increase in membrane degradation rate. Previous research employed supported and non-supported platinum (Pt) catalysts and PtCo alloy catalysts in their studies. Their findings showed a lower membrane degradation rate when using the alloy catalyst at low RH indicating that catalyst activity may be an important factor in membrane degradation.2 In this study, we investigated how varying the amount of Pt-supported catalyst affects membrane degradation. In order to expedite degradation, we used the DOE’s accelerated stress test (AST) for chemical stability to induce the production of free radicals.3 Free radicals are known to degrade membranes and other membrane electrode assembly (MEA) components. The focus of this research is to probe membrane degradation for MEAs with ultra-low Pt loadings (i.e. ≤ 0.1 mgPt cm-2) at each electrode to contribute to existing knowledge of membrane degradation mechanisms. Twenty-five cm2 catalyst coated membranes (CCMs) were made by spraying catalyst inks on to Nafion® membranes (NR211, Ion Power, Inc.) using a Sono-Tek Exactacoat spray coater. The catalyst inks were comprised of 50% Pt/C (TKK) catalyst mixed with 5% ionomer solution (Dupont D521). Fuel Cell Technologies current collection endplates were used to assemble the cell, which utilized SGL’s Sigracet 25 BC gas diffusion layers and two graphite plates with triple serpentine flow channels sealed by Furon® gaskets. The AST test conditions require the fuel cell to be held at open circuit voltage while at 90oC, 30% relative humidity and a back pressure of 150kPa. H2 and air flows were set at 10 stoics at 0.2 A/cm2. We evaluated the fuel cell performance every 12 hours using various diagnostic tests to probe the membrane degradation. Polarization (VI) curves were measured to analyze overall fuel cell performance; electrochemical impedance spectroscopy (EIS) provided useful information on the membrane resistance, electrode kinetics and transport properties, and cyclic voltammetry allowed tracking electrochemical active surface area (ECSA). We also assessed the hydrogen permeation through the membrane using linear sweep voltammetry. In addition, a performance recovery technique similar to Zhang et al. was applied to the fuel cell to mitigate impacts of recoverable catalyst poisoning4. We also collected the effluent water for further scrutiny of membrane degradation by probing for fluoride and sulfate ions using Ion Chromatography (IC). Fluoride emission rates (FER) and sulfate emission rates (SER) were used to gauge the condition of the MEA during its lifetime. Figure 1A shows VI curves in 24 hour increments for a fuel cell exposed to AST conditions with 0.1 mgPtcm-2 at each electrode. The results show recoverable voltage losses within the first 48 hours. This indicates that the performance losses were due to catalyst poisoning rather than membrane pinhole formation. IC results for FER are shown in Figure 1B. Although FER increased during AST conditions, fluoride loss is not impactful on performance in the initial 48 hours of testing. After 72 hours, the voltage losses are irreversible which suggest the onset of membrane failure. SER results also increased during recovery, and degradation is also evident by irreversible open circuit voltage losses and a lower ECSA. EIS results are consistent with the polarization curve data in that the charge transfer resistances remained constant over the initial 48 hours after recovery. The ECSA gradually decays during the testing with only partial surface area restoration after the recovery protocol was applied. H2 crossover results showed a constant increase during fuel cell testing further signifying membrane failure. This talk will summarize the results of these tests at various catalyst loadings and illustrate the influence of catalyst loading on membrane degradation. Acknowledgements: The authors wish to acknowledge the financial support of the LANL African American Partnership Program and NNSA Minority Serving Institutes Program Managers: David Canty, Jennifer Kline and Dave Rude.

Investigation of Membrane Chemical Degradation as a Function of Catalyst Platinum Loading

Membrane chemical degradation is one of many factors that can impact fuel cell durability. The fuel cell’s lifetime heavily depends on the membrane and its ability to maintain chemical and mechanical integrity. Studies indicate the chemical degradation is due to the formation of hydroxyl radicals that attack the polymer structure resulting in membrane thinning, pinhole formation, and the release of fluoride and sulfate ions1. In addition, mechanical failures can transpire, e.g. cracks, tears, perforations and pinhole formation, due to membrane swelling/de-swelling leading to an increase in membrane degradation rate. Previous research employed supported and non-supported platinum (Pt) catalysts and PtCo alloy catalysts in their studies. In this study, we investigated how varying the amount of Pt-supported catalyst affects membrane degradation.

Effect of Varying Co Content in PtCo Nanoparticles on the Performance of PEM Fuel Cells

PtCo alloy catalysts are leading cathode candidates for PEMFC because of their relatively high activity and durability. Although it is known that during fuel cell operation the dissolution of Co results in loss of oxygen reduction reaction activity and that the migration of Co2+ into the membrane can have a negative impact on fuel cell high-current-density performance, the detailed mechanisms are unclear. Thus there are opportunities for PtCo alloy optimization to minimize these losses. Here, we focused on dealloyed PtCo catalysts with varying degrees of acid leaching in an attempt to vary the Pt-rich shell content while maintaining the Pt-to-Co core composition. While the specific activity of the catalyst decreased with reducing Co content, as expected, the high current fuel cell performance improved. Post-mortem analyses were done to quantify the amount of Co2+ in the membrane. Mathematical modelling was used to help interpret the voltage losses at high current density. We also employed newly developed nanobeam electron diffraction techniques to measure local crystal structure across the PtCo core-shell nanoparticles at subnanometer resolution. This allows us to correlate lattice strain relaxation and defect sites to the apparent catalyst kinetic activity. This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Effect of Cobalt Ion Exchange and Thermal Pretreatment of Ionomer Thin Films on Conductivity, Water Uptake and Swelling

It is known that Cobalt (Co) can leach out from the Pt-Co alloy catalysts causing degradation of long-term fuel cell performance. Although the fundamental mechanism of performance loss is not fully understood, it has been hypothesized that the Co ion interacts with the nanothin ionomer films in the fuel cell catalyst layer by exchanging with the protons and thereby adversely affecting the performance. However, the effect of Cobalt ion, specifically and the transport properties of nanothin fluorinated ionomers films, in general is not fully known. We have studied the effect of cation(Cobalt) on spin-coated nanothin ionomers on its key properties – (i) proton conduction and water uptake which directly affects the electrochemical performance of fuel cell catalyst layers, and (ii) swelling behavior, which is an indicator of mechanical strength and thereby the durability of the ionomer. Electrochemical impedance spectroscopy (EIS) of films on interdigitated elecrodes for conductivity measurements, quartz crystal microbalance (QCM) for water uptake, and environmental ellipsometry for swelling measurement was employed, Quantification of exchanged Cobalt in nanothin films is non-trivial problem and we have used a set of x-ray techniques (EDX, XPS) for determination of cobalt in the cation exchanged films. In this work conductivity of the different ionomer thin film (~ 30 nm) were tested before and after Cobalt exchange at different temperatures (30-80 °C) and humidity (40-90 % RH) conditions. It was seen that the conductivity of the ionomer thin films decreased after Cobalt exchanged but the proton conduction of some of the non-commercial ionomers is not as strongly affected by Co ion exchange as that of the state-of-the-art Nafion ionomer. We also investigated the effect of ionomer thin film pretreatment on the conductivity of the cobalt exchanged ionomer. It was found that the the samples annealed at 160 °C and then cation exchanged by exposure to cobalt solution showed less reduction in conductivity than unannealed cation exchanged ionomer films. The presentation will share the details of the film preparation and characterization as well as the results for the conductivity, water uptake and swelling of the ionomer films. Figure 1

Impact of Co-Alloy Leaching and Cation in Ionomer Thin-Films

Strong R&D progress and increased demand for hydrogen powered fuel-cell cars, is shifting Polymer Electrolyte Fuel Cells (PEFCs) to lower Platinum electrocatalyst loading and to Platinum-cobalt (Pt-Co) alloys that have demonstrated double the kinetic activity improvement [1]. Implementation of these catalysts can lower catalyst cost to $7/kW (from $18/kW of standard Pt/C catalyst), surpassing the automotive requirement target of $10/kW and expediting commercialization [2]. Key challenge in integrating Pt-Co alloy catalyst as well as other Pt-Metal alloyed catalysts into the electrode assembly of PEFC has been durability and stability in the highly acidic environment. Co-alloy dissolution, leaching, surface oxide over layer formation upon heating, hollowing and particle facet loss have been demonstrated for Pt-Metal alloys via short and long lifetime tests and imaging techniques [3]. Amplifying this detrimental impact is the contamination of the membrane and the ionomer thin-film in the catalyst layer distressing conductivity, water uptake, and even gas transport [4]. In this talk, the direct impact of co-alloy and cation contamination in thin ionomer films of thickness < 200 nm is investigated in an effort to illuminate the direct effect of cations on structure and performance. Different levels of neutralization and leaching is mimicked by varying cation concentration and different counter ion types (Co2+, Ni2+, Na+, Cs+), which can elucidate impact of strength of ionic crosslinking and interactions on the polymer film’s nano-structure and water uptake behavior, characterized by grazing-incidence X-ray scattering and ellipsometry, respectively. Significant impact of confinement on structure and property will also be explored as a function of ionomer thickness to understand the performance of ionomer thin-films employed in the electrode assembly. AcknowledgementThis work made use of facilities at the Joint Center for Artificial Photosynthesis at Lawrence Berkeley National Laboratory. This work was funded in part by the National Science Foundation Graduate Fellowship and the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy (under Contract No. DE-AC02-05CH11231).

(Invited) In Situ Structural Analysis of PtCo Alloy Catalysts with Stabilized Platinum-Skin Layers and Durability/Activity Towards Oxygen Reduction Reaction

For polymer electrolyte fuel cells, Pt-based alloy catalysts have been widely studied. We have developed the nanocapsule method1,2 for the synthesis of PtCo alloy nanoparticles (n-PtCo) with well-controlled size and composition. On n-PtCo subsequently treated in hydrogen gas at high temperature (n-PtCoH2-HT), additional Pt-skin layers were formed (PtxAL-PtCo).3 Figure 1 shows the area-specific activities j k for the oxygen reduction reaction (ORR) at carbon-supported n-PtCoH2-HT and PtxAL-PtCo (n-PtCoH2-HT/C and PtxAL-PtCo/C, respectively) and a commercial carbon-supported Pt catalyst (c-Pt/C) coated with Nafion in O2-saturated 0.1 M HClO4 solution at 65 oC and 0.85 V vs. RHE measured by a multi-channel flow double-electrode cell technique.2 Potential-step cycles were applied to the catalyst electrodes as the accelerated durability tests (ADTs) simulating load-change cycles of fuel cell vehicles4 in N2-saturated solution with no solution flow. Both the initial j k and the durability were the highest at Pt2AL-PtCo/C.3 Figure 2 shows changes in the electrochemically active areas (ECAs) of n-PtCoH2-HT/C and PtxAL-PtCo/C during the ADTs in a constant solution flow at 65 °C so that the re-deposition of Pt became negligible. The ECA values of the catalysts decreased as the number of potential-step cycles increased, and the losses of the ECA of n-PtCo/C and PtxAL-PtCo/C after 5000 cycles were 42% and 26%, respectively. Therefore, the ECA loss was much mitigated by the existence of Pt skins. Figure 3 shows the in-situ X-ray diffraction (XRD) pattern of the (111) peak of Pt-Co alloy of n-PtCo/C and PtxAL-PtCo/C measured during the ADT in a solution flow. For n-PtCo/C, the diffraction peak at ca. 27.6° was close to that for Pt (27.5°), suggesting that Co easily dissolved from n-PtCo into the hot acid electrolyte solution. The peak was small and broad even before the ADT, probably because of a low order of atoms in crystallite. In the XRD pattern of PtxAL-PtCo/C, a significantly large, distinct peak was observed before the potential cycles; the ordering of atoms in PtxAL-PtCo was very high. The peak was almost unchanged after the potential-step cycles, indicating that the crystallite structures of PtxAL-PtCo remained the same. The parameters of the Pt-Co alloy catalysts from the data obtained by the electrochemical measurements, transmission electron microscopy, and in-situ XRD are listed in Table 1. For n-PtCo/C, the crystallite size was approximately 2.3 nm during 5000 cycles. The number of crystallites per area and the metal retention ratio gradually decreased from 14 × 103 to 10 × 103 mm-2 and from 100% to 50%, respectively. Not only the dissolution but the migration/coalescence also played a role. For PtxAL-PtCo/C, the crystallite size was approximately 2.9 nm during 5000 cycles. No change was indicated in the shape and the number of the crystallites per area, as well as the loss of the metals. Interestingly, the ECA of PtxAL-PtCo/C gradually decreased during 5000 cycles by 26%. Therefore, it is assumed that the PtxAL-PtCo crystallites were aggregated into a larger particle consisting of multi-crystallites, while keeping the shape of each crystallite unchanged. The loss of ECA is explained by the loss of surface areas by a contact between crystallites. As shown in Fig. 1, the j k value at PtxAL-PtCo/C remained the same level during the potential-step cycles. The coalescence of alloy crystallites without any dissolution of Pt and Co clearly explains this constant value of j k. Analyzing an EXAFS of Pt in PtxAL-PtCo/C, the Debye-Waller factor for Pt–Pt in PtxAL-PtCo/C was even smaller than that of Pt nanoparticles with a similar size, revealing that the crystallinity of the outer Pt skin was extremely high.6 Thus, the existence of a rigid Pt skin realized the extensively high durability of PtxAL-PtCo/C. Further structural analyses of the Pt-Co catalyst at the atomic level are now in progress. Acknowledgements This work was carried out under the “SPer-FC” project, NEDO, Japan. The synchrotron radiation experiments were performed at BL14B2, BL16B2, BL19B2, and BL46XU of SPring-8 (No. 2015B3388, 2016A1760, 2016A1805, 2016A5390, 2016B1550, and 2016B1856, 2016B5390). References Yano, H.; Kataoka, M.; Yamashita, H.; Uchida, H.; Watanabe M. Langmuir, 2007, 23, 6438.Yano, H.; Song, J. M.; Uchida, H.; Watanabe, M. Phys. Chem. C, 2008, 112, 8372.Watanabe, M.; Yano, H.; Tryk, A.; Uchida, H. J. Electrochem. Soc., 2016, 163, F455.http://fccj.jp/pdf/23_01_kt.pdf in Japanese.Watanabe, M.; Tryk, D. A.; Wakisaka, M.; Yano, H; Uchida, H. Acta, 2012, 84, 187.Shi, G.; Yano, H; Tryk, D. A.; Matsumoto, M.; Tanida, H.; Arao, M.; Imai, H; Inukai, J.; Iiyama A.; Uchida H. Sci. Technol., 2017, 7, 6124. Figure 1

DMF-Coordination Assisted Electrodeposition of Highly Active PtCo Alloy Catalysts for the Oxygen Reduction Reaction

We propose here a facile electrodeposition method that is assisted by N, N-Dimethylformamide (DMF) selective coordination with Pt ions, and succeed in synthesizing highly active PtCo alloy catalysts for the oxygen reduction reaction (ORR). It is very ingenious that DMF is selectively coordinated with Pt ions, thus greatly bridging the deposition potential gap between Pt and Co, and promoting the formation of alloy catalysts. The optimal Pt4Co alloy sample exhibits a remarkable specific activity as high as 1.52 mA·cm−2Pt at 0.9 V (vs. RHE) towards the ORR, 7 times of that for the commercial Pt/C catalyst. It is believed that the remarkable ORR electrocatalytic activity is mainly ascribed to the synergetic effect from two aspects dealing with a DMF selective coordination with Pt ions as well as the generation of highly porous Pt-rich surfaces.

Structures of Atomically Designed PtCo Alloy Catalysts and Durability/Activity Towards Oxygen Reduction Reaction

For polymer electrolyte fuel cells, Pt-based alloy catalysts have been widely studied. We have developed a novel nanocapsule method1,2 for the synthesis of controlled PtCo alloy nanoparticles (PtCo/GCB, graphitized carbon black support to mitigate carbon corrosion), and modified the method3for covering hydrogen-treated PtCo/GCB (H-PtCo/GCB) with Pt-skin layers. Figure 1 shows images obtained with a transmission electron microscope (TEM) of PtCo/GCB, H-PtCo/GCB, and Pt-skin coated PtCo catalyst (PtxAL-PtCo/GCB) with the histograms of the particle-size distributions. Particle sizes were very uniform with the diameters of 2.5±0.2 nm for PtCo/GCB and H-PtCo/GCB, and of 3.0±0.3 nm for PtxAL-PtCo/GCB. By the electron diffraction (Table 1), the lattice of PtCo/GCB was determined as fcc. After the hydrogen treatment at 400 oC,3 the lattice became fct for H-PtCo/GCB. After the addition of a Pt-skin on H-PtCo/GCB, the lattice was mostly (ca. 97%) fct and partially (ca. 3%) fcc, presumably due to the addition of a Pt-skin. By X-ray absorption fine structure (XAFS) measurements, Pt and Co atoms were found to be very uniformly distributed in PtCo/GCB. For H-PtCo/GCB, neither the atomic distances nor the coordination numbers showed a large change, whereas for PtxAL-PtCo/GCB, the Pt-Pt coordination number increased showing the formation of a Pt-skin layer. The effect of the Pt-skin layers was examined in a 0.1-M HClO4 solution at 65 oC with a multi-channel flow double-electrode cell2 on the activity towards the oxygen reduction reaction and the durability examined by the accelerated durability test according to the protocol for load-change cycles4 recommended by the Fuel Cell Commercialization Conference of Japan (FCCJ) (Fig. 2). Both the durability and the mass activity evaluated at 0.85 V vs. the reversible hydrogen electrode were the highest at PtxAL-PtCo/GCB followed by H-PtCo/GCB. By in situ XAFS measurements, the layers of Pt skins were indicated to exist at the surface of the alloy particles in 0.1 M HClO4. After 30,000 cycles, the initial high activity at PtCo/GCB was lost and became almost identical to that at a commercial Pt catalyst supported on carbon black (c-Pt/CB) because of Co leaching out.5,6 The efficiency/usefulness of the atomically-controlled Pt-skin layers was clearly demonstrated. Detailed structural analysis during the durability test is now in progress. Acknowledgements This work was carried out under the “HiPer-FC” and “SPer-FC” projects, NEDO, Japan. The synchrotron radiation experiments were performed at BL14B2 and BL16B2 of SPring-8 (Proposal No. 2015B1851, 2015B1852, 2015B3388, 2015B5390, 2016A5390). References 1) H.Yano, M. Kataoka, H. Yamashita, H. Uchida, M. Watanabe, Langmuir, 23, 6438 (2007). 2) H. Yano, J. M. Song, H. Uchida, M. Watanabe, J. Phys. Chem. C, 112, 8372 (2008). 3) M. Watanabe, H. Yano, D. A. Tryk, H. Uchida, J. Electrochem. Soc., 163, F455 (2016). 4) http://fccj.jp/pdf/23_01_kt.pdf in Japanese. 5) M. Watanabe, D. A. Tryk, M. Wakisaka, H. Yano, H. Uchida, Electrochim. Acta, 84, 187 (2012). 6) K. Okaya, H. Yano, K. Kakinuma, H. Uchida M. Watanabe, ACS Appl. Mater. Interfaces, 4, 6982 (2012). Figure 1

Cathode Carbon Corrosion and Electrode Layer Compaction in PEM Fuel Cells

PEM fuel cells (PEMFCs) show great promise to increase the fuel efficiency for transportation applications. However, cost reduction and improvement in durability is required for mass commercialization. To achieve high platinum dispersions, platinum is typically supported on high surface area carbon. However, elemental carbon is not the thermodynamically favored state under PEMFC operating conditions of acidity and potential. Catalyst support, e.g., carbon, corrosion occurs during fuel cell operation, which can be greatly exacerbated by SD/SU cycles when hydrogen/air fronts are present on the fuel cell anode. Carbon corrosion results in thinning of the catalyst layer and changes in surface hydrophobicity, contributing to degradation of the catalyst and accelerating performance losses. The high cost of the noble metal(s) used as the catalyst makes this a crucial area requiring durability improvement. Porous electrodes lose substantial porosity during PEMFC operation, which does not appear to correlate directly with loss of carbon during corrosion reactions. Compaction of the electrode layer reduces the pore volume within the electrode and can negatively impact performance due to increased mass transport losses. To examine the stability of electrode structures and further understand the combined effects of carbon corrosion on performance, we have tested various types of carbon support materials, as well as platinum and platinum-alloy catalysts under simulated drive-cycles and accelerated stress tests. Post characterization has included porosity measurements by mercury intrusion porosimetry (MIP), transmission electron microscopy (TEM) characterization of electrode cross-sections and 2D image analysis, and focus ion beam-SEM (FIB-SEM). Advanced alloy catalysts maintain their electrode structure for some period of time during ASTs, but eventually display similar pore collapse and loss as that observed for Pt-only catalysts supported on high-surface-area-carbons. The loss of pore volume is shown by impedance measurements to result in large increases in mass transport losses. The initial pore size distribution within the catalyst layer shows a range of sizes with an average pore diameter of 120 nm. This average pore diameter holds for up to 500 AST cycles; however, after 1000 AST cycles the average pore diameter is reduced to 40 nm. The initial pore sizes exhibit a distribution centered around 50-99 nm, but range in size up to ~ 300 nm (See Figure 1a). After the AST, most of the remaining pores are < 50 nm, with a few pores measured above 50 nm (See Figure 1b). During the carbon corrosion AST, PtCo alloy catalyst particles show a loss of Co, changing from 18 at.% Co initially to 14 at.% Co after the AST, further degrading the performance of the electrode. Acknowledgments The authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead: Dimitrios Papageorgopoulus and Technology Development Manager: Nancy Garland. The authors also wish to acknowledge General Motors and Ion Power, Inc. for supplying materials used in this study. Figure 1

Durability of Pt-Co Alloy Cathode Catalysts for PEM Fuel Cells Under Cyclic Potentials

Alloying Pt with transition metals, such as Co and Ni, enhances the catalytic activity for the oxygen reduction reaction (ORR) at cathodes of polymer electrolyte fuel cells (PEFCs). The alloying process necessarily involves a high-temperature thermal annealing step that grows the size of catalyst nanoparticles above 4-5 nm. Particles of this size are known to be more stable under cyclic potentials as they are less prone to Pt dissolution and coalescence than finer scale 2-3 nm Pt particles. However, transition metals are unstable at operating fuel cell potentials and can leach out of the catalyst in the cathode and into the ionomer, resulting in loss of ORR activity. Three formulations of Pt-Co alloy catalysts supported on high surface area carbon were subjected to accelerated stress tests (ASTs) in H2-N2 environment. The AST protocol consisted of repeated square waves with 0.6 V lower potential limit (LPL), 0.95 V upper potential limit (UPL), and 3-s holds at LPL and UPL. Changes in cell performance were monitored by measuring polarization in H2-air, high frequency resistance (HFR), electrochemical surface area (ECSA), impedance in air and helox (79% He, 21% O2), and mass activity. These measurements were made at beginning of life (BOL) and after 1000 (1k), 20,000 (20k) and 30,000 (30k) cycles. The three catalysts had a different initial Co content (34%, 20%, and 15%) and electrode microstructure. At BOL, transmission electron microscopy (TEM) analysis of the high-Co content (H) catalyst electrode, 0.2 mg/cm2 Pt loading, showed a “spongy” porous (hollow) catalyst nanoparticle morphology with ~6.7 nm median Pt-Co particle size after conditioning. The BOL medium-Co content (M) catalyst electrode, 0.1 mg/cm2 Pt loading, showed a crystalline fully alloyed catalyst nanparticle morphology with ~4.4 nm particle size after conditioning. The BOL low-Co content (L) catalyst electrode, 0.1 mg/cm2 Pt loading, also showed catalysts with a fully alloyed crystalline morphology with ~4.5 nm median particle size.Post-mortem TEM analysis of aged electrode specimens showed different amounts of Co dissolution from the nanoparticles. There is an apparent direct relationship between the initial alloy Co content the amount of Co dissolved after potential cycles. WAXS analysis confirms significant Co loss from the nanoparticles and also indicates that the Pt-Pt d-spacing in Pt-Co alloy approaches the values typical of pure Pt particles. The effect of Co dissolution on the loss in specific activity for ORR is summarized in Fig. 1a. The data suggest that the specific activity depends not only on the initial Co content but also on the catalyst nanoparticle morphology and size. Even with 27-50% Co loss, the specific activity of Pt-Co alloy remains higher than 1000 mA/cm2-Pt, and exceeds the activity (650 mA/cm2-Pt) of pure Pt of similar particle size. The measured specific activities are consistent with a kinetic model and catalyst-specific choice of kinetic parameters.The modeled ORR kinetic losses depend on specific activity, ECSA, catalyst loading, and kinetic parameters (electrode morphology). As shown in Fig. 1b, the H-catalyst electrode shows the smallest kinetic overpotentials (ηc s) due to high Pt loading, but has the fastest increase in ηc s with ageing because of low Co retention in the electrode. The L-catalyst electrode shows higher ηc s due to lower Pt loading but has a smaller increase in ηc s with ageing because of high Co retention. For this catalyst, increase in is mostly due to loss of ECSA with ageing. The M-catalyst electrode shows similar increase in ηc s with ageing as the L-catalyst electrode, but this increase is due to the combined effects of Co dissolution and ECSA loss. The modeled increase in mass transfer overpotentials (ηm) with ageing correlates with Pt loading and ECSA loss and depends on the initial electrode morphology. There is no clear evidence of a relationship between Co dissolution (i.e., effect of Co2+ on O2 permeability in ionomer film) and the resulting increase in ηm. AcknowledgmentsThe authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead, Dr. Dimitrios Papageorgopoulus, and Technology Development Manager, Dr. Nancy Garland. The authors also wish to acknowledge General Motors, IRD and Umicore for supplying materials used in this study. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1