Electrochemical Surface Area Loss Research Articles

Revealing the Nanostructure of Mesoporous Fuel Cell Catalyst Supports for Durable, High-Power Performance

Achieving high power performance and durability with low Pt loadings are critical challenges for proton exchange membrane fuel cells. PtCo catalysts developed on new carbon black supports show promise by simultaneously providing good oxygen reduction kinetics and local oxygen transport. We investigate the role of nanoscale morphology in the performance of these catalysts supported on accessible (HSC-e and HSC-f) and conventional (Ketjen Black) porous carbons using 3D electron tomography, nitrogen sorption, and electrochemical performance measurements. We find that the accessible porous carbons have hollow interiors with mesopores that are larger and more numerous than conventional porous carbons. However, mesopore-sized openings (>2 nm width) are too rare to account for significant oxygen transport. Instead we propose the primary oxygen transport pathway into the interior is through 1–2 nm microporous channels permeating the carbon. The increased mesoporosity in the accessible porous carbons results in a shorter diffusion pathlength through constrictive, tortuous micropores in the support shell leading to lower local oxygen transport resistance. In durability testing, the accessible porous carbons show faster rates of electrochemical surface area loss, likely from fewer constrictive pores that would mitigate coarsening, but maintain superior high current density performance at end of test from the improved local oxygen transport.

Electrochemical Stability and Degradation Mechanisms of Commercial Carbon-Supported Gold Nanoparticles in Acidic Media.

Electrochemical stability of a commercial Au/C catalyst in an acidic electrolyte has been investigated by an accelerated stress test (AST), which consisted of 10,000 voltammetric scans (1 V/s) in the potential range between 0.58 and 1.41 VRHE. Loss of Au electrochemical surface area (ESA) during the AST pointed out to the degradation of Au/C. Coupling of an electrochemical flow cell with ICP-MS showed that only a minor amount of gold is dissolved despite the substantial loss of gold ESA during the AST (∼35% of initial value remains at the end of the AST). According to the electrochemical mass spectrometry experiments, carbon corrosion occurs during the AST but to a minor extent. By using identical location scanning electron microscopy and identical location transmission electron microscopy, it was possible to discern that the dissolution of small Au particles (<5 nm) within the polydisperse Au/C sample is the main degradation mechanism. The mass of such particles gives only a minor contribution to the overall Au mass of the polydisperse sample while giving a major contribution to the overall ESA, which explains a significant loss of ESA and minor loss of mass during the AST. The addition of low amounts of chloride anions (10–4 M) substantially promoted the degradation of gold nanoparticles. At an even higher concentration of chlorides (10–2 M), the dissolution of gold was rather effective, which is useful from the recycling point of view when rapid leaching of gold is desirable.

Degradation Mapping of Polymer Electrolyte Fuel Cells During Start-Up/Shutdown Cycling

Significant performance variation exists across the lateral extent of a fuel cell electrode as a consequence of interconnected factors including reactant pressure drop, membrane humidification and catalyst utilisation. This can result in heterogeneous degradation rates, diminishing its lifetime. Here, current and temperature mapping are coupled with localised cyclic voltammetry (electrochemical surface area (ECSA)) and linear sweep voltammetry (hydrogen crossover) to study performance degradation in a 100 cm2 fuel cell, subjected to voltage cycling simulating repetitive start-up/shutdown procedures. With increased cycling, the current distribution below mid-currents become more heterogeneous, whilst their temperature maps remain relatively uniform. At higher currents, rising temperatures homogenises the current profiles. ECSA loss declines non-linearly, with no discernible changes in hydrogen crossover. Degradation rates vary across the active area; with differing areas of highest ECSA and performance loss. Results suggests that deteriorating water management, over ECSA loss from the carbon support corrosion, is the main contribution to the performance deterioration.

Fuel Cell Automotive Drive-Cycle Cathode Catalyst (Pt/C) Layer Durability: Dependence on Low Vs. High Carbon Supports and Ionomer-to-Carbon-Ratio

The typical urban automotive drive-cycle for proton exchange membrane fuel cells (PEMFC) consists of frequent idle-to-peak power cathode voltage transients, with extremes ranging from 1.2 V to 0.6 V, respectively. Understanding the platinum electrochemical surface area (ECSA) loss mechanisms dependence on material and operational conditions have been extensively investigated with the objective of minimizing performance losses over the lifetime of PEMFC powered automobiles. Commonly, automotive accelerated stress tests (AST) are performed ex-situ on thin catalyst films. The standard catalyst consists of carbon supported platinum nanoparticles (Pt/C). This study compares triangle waveform (TW) ASTs monitoring ECSA loss for ex-situ (thin film in 0.1 HClO4) and in-situ (catalyst layers bond with ionomeric electrolyte in a complete membrane electrode assembly (MEA)). The ionomer-to-carbon ratios (I/C) of the MEAs is varied from 0.6 to 1.5 on either low surface area carbon (LSAC) or high surface area carbon (HSAC) supports. The AST profiles included 25k cycles at 10 sec intervals, between the upper potential limit of either 0.9 V or 1.2 V and the lower potential limit of 0.6V, with intermediate ECSA evaluations from the H-desorption current (0.02 V → 0.4 V).The electrolyte environment alters, liquid vs. ionomer, the dominant AST decay mechanism, Figure 1A. The decay in the liquid electrolyte is most sever upon initiation of the AST and the rate of decay decrease after the first 2000 cycles. While the MEA decay is less initial, but maintains the same rate of initial decay over 5000 cycles resulting in a higher overall loss of ECSA. ECSA decay is exacerbated at temperatures of 70°C. Increasing the I/C increases the interfacial acidity at the surface of the Pt/C and results in exacerbated ECSA loss, Figure 1B. In accordance with the Pourbaix diagram for Pt, it is less stable in a more acidic environment. The relative rate of ECSA loss is reduced on HSAC supports. Figure 1

Influence of Flow Rate on Catalyst Layer Degradation in Polymer Electrolyte Fuel Cells

With the U.S. Department of Energy (DOE) 2020 durability target for transportation applications of 5000 hours in mind [1], several studies have been performed to understand the factors influencing load cycle durability of the cathode catalyst layer subjected to various operating conditions. In general, high upper potential limit (> 1 V vs RHE), high temperature (> 90 °C) and high relative humidity (RH) during potential cycling have been shown to exacerbate degradation [2]. It is known that Electrochemical surface area (ECSA) loss occurs by Pt dissolution attributed to particle size growth via modified Ostwald ripening, crystal migration, detachment from carbon support due to carbon corrosion, and precipitation in the membrane by chemical reduction due to hydrogen crossover [3].Accelerated stress tests (ASTs) are often used to mimic material component degradation similar to real automotive driving conditions. Different cycle profiles have been shown to cause varying rates of degradation depending on the operating conditions. The influence of flow rate of reactant gases on degradation, however, has received little attention. Stariha et al. [4], showed that a relatively high flow rate AST had the highest degradation acceleration factor and resulted in non-uniform decay rates when compared to low flow rate conditions of nitrogen gas (N2) at the cathode. However, this phenomenon has not been further studied in detail.The current study is aimed at elucidating the effect of flow rate on cathode catalyst layer degradation subjected to a standard DOE square-wave AST protocol. Single cell studies with 5-cm2 active area were performed using Nafion® XL membrane (Ion Power Inc.) and SGL-22 BB gas diffusion electrodes (GDEs) as MEA materials and 1-serpentine flow field. All the MEA samples used in the present study had nominally identical Pt loading and were subjected to a consistent conditioning procedure prior to AST. A DOE square-wave cycling protocol [4] was executed: H2/N2 flows at anode/cathode, voltage cycling between 0.6 V and 0.95 V, and 3 seconds hold at each voltage. 30,000 cycles (total test time of 50 hours) were performed at 100% RH, 80°C, and atmospheric pressure. Three different cathode N2 flow rates of 20 sccm, 40 sccm and 80 sccm were used during the AST with fixed H2 flow of 40 sccm at the anode. ECSA loss was tracked using cyclic voltammograms at several intervals during the AST. Complete electrochemical characterization at beginning and the end of life (BOL and EOL), along with ex-situ diagnostic techniques such as TEM and GIXRD, were used to obtain particle size distribution and spatial degradation profiles. The results indicate higher performance loss (at EOL) for 80 sccm flow of N2 at the cathode compared to 20 sccm and 40 sccm flow rates, most likely due to inhomogeneous Pt loss from inlet to the outlet due to variations in local flow conditions by fully saturated N2 flow at different flow rates.

Catalyst Degradation in Polymer Electrolyte Fuel Cells with Multi-Modal Techniques: Understanding Phenomena Under Varied Gas and Relative Humidity

For widespread commercialization of polymer electrolyte fuel cells (PEFCs), cost and durability need to be addressed. Reduction in cost should not negatively affect PEFC durability and performance. At high volume production, platinum (Pt) electro catalyst contributes to 41% of the total fuel cell stack cost for light duty vehicles. This cost contribution increases to 53% for middle and heavy-duty vehicles. When membrane electrode assemblies (MEAs) are designed for durability, this cost will increase further. A complex balance exists between cost, performance and durability. With smaller nanoparticle size better Pt dispersion is achieved which gives improved performance and reduces Pt loading and cost. But this negatively affects durability, as smaller nanoparticles have higher surface energy which causes faster electrocatalyst degradation1. During an actual drive cycle, PEFCs need to operate over a wide range of operating conditions such as temperature (-40C to 120C), potential (0V to 1.5V) and relative humidity which can further accelerate Pt dissolution Under load cycling during typical drive cycle, repeated oxidation and reduction of Pt surface leads to Pt dissolution2. Dissolved Pt ions redeposit on nearby nanoparticles effectively increasing their particle size2. This electrochemical surface area (ECSA) loss via increase in particle size is well known as Ostwald ripening. Some Pt ions are reduced in the electrolyte membrane by crossover hydrogen which leads to formation of a Pt band, usually at the membrane and cathode catalyst layer interface3. At > 1V, carbon corrosion can lead to Pt detachment2. This can happen during start-up/shut down cycles when anode is exposed to hydrogen and air. Such degradation mechanisms need to be studied systematically with effect of operating conditions like gas flow rates, temperature and relative humidity under dynamic load cycling.In this study, we investigate the effect of cathode gas environment and gas flow rates on Pt particle size during aging experiment. A DOE adopted accelerated stress test (AST) protocol simulating the drive cycle operating conditions is used. It consists of a 0.6 V – 0.95 V square wave with 3s hold time at each potential. In situ electrochemical characterization at regular stages is used to identify ECSA loss trends and polarization performance. Figure 1 shows normalized ECSA loss for the four cells cycled in different environments during catalyst AST. Measurements were taken beginning of life (BOL), after 1,000, 5,000, 15,000 and 30,000 cycles. We observed that the cell that was cycled in N2 and 100 % RH degraded the most , having 25 % of ECSA left after 30,000 cycles compared to BOL. Electrochemical measurements are supplemented by detailed post-mortem ex-situ x-ray characterization by using micro x-ray fluorescence, x- ray computed tomography and micro x-ray diffraction, SEM-EDS. 1cm2 active areas of the catalyst layer at the inlet, middle and outlet of the flow field were mapped using micro x-ray diffraction to understand Pt particle size distribution under land/channel. Catalyst layer loading before and after the AST were confirmed using micro x-ray fluorescence.References(1) Cherevko, S.; Kulyk, N.; Mayrhofer, K. J. J. Durability of Platinum-Based Fuel Cell Electrocatalysts: Dissolution of Bulk and Nanoscale Platinum. Nano Energy 2016, 29, 275–298. https://doi.org/10.1016/j.nanoen.2016.03.005.(2) Meier, J. C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H. J.; Topalov, A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K. J. J. Design Criteria for Stable Pt/C Fuel Cell Catalysts. Beilstein J. Nanotechnol. 2014, 5 (1), 44–67. https://doi.org/10.3762/bjnano.5.5.(3) Zhang, J.; Litteer, B. A.; Gu, W.; Liu, H.; Gasteiger, H. A. Effect of Hydrogen and Oxygen Partial Pressure on Pt Precipitation within the Membrane of PEMFCs. J. Electrochem. Soc. 2007, 154 (10), B1006. https://doi.org/10.1149/1.2764240. 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

Automotive Cold Start of a PEMFC System by Using a Methanol Solution as Antifreeze

Proton exchange membrane fuel cell vehicles are highly competitive due to their zero emissions, high efficiency and power density, quick response, low refuelling time and great autonomy. However, the lack of a sufficient hydrogen infrastructure and their required startup ability and durability at extreme winter conditions are limiting factors in their commercialisation. At subzero temperatures, the produced water during operation as well as the remained water in the cells after shutdown will freeze, hindering the cold start and causing several damages (e.g. cracks, pinholes, catalyst delamination or loss of electrochemical surface area). Commonly, the fuel cell system is purged with dried gases during shutdown to avoid that it freezes during the cold storage. Even though the dry gas purge methods have been extensively studied, thus reducing the degradation due to F/T cycles, there are still remaining unsolved issues, especially at very low temperatures. An alternative method that can mitigate this degradation and hence lower the temperature is the use of an antifreeze solution during the cold storage. Yet, it has been only tested in a single cell1. These results were quite promising, as the use of the antifreeze strongly diminished the degradation because of subzero temperatures and it was possible starting at -10°C. But they showed fairly poor cold start ability, since the residual methanol decreased the power density during the cold start. Therefore, the use of an antifreeze solution for a cold start should be further investigated in a stack.The aim of this study is to prove the cold start capability of a PEMFC system by using a methanol solution as antifreeze. For that, several cold starts at -10°C were carried out with a 4 kW commercial stack, a 25 vol% methanol solution to ensure no ice formation at -10°C and following next test protocol. A conditioning test and a polarization curve were conducted at room temperature. Then, the shutdown procedure, which involved soaking the stack with the methanol solution, was carried out before cooling down the system to -10°C. Following the cold storage, the antifreeze was removed from the stack and the cold start test was run. Finally, another polarization curve at room temperature was carried out. The cold start system ability with the antifreeze was compared with previous cold starts, which only used a dry gas purge method.All cold starts with antifreeze were successfully, thus proving the cold start capability of the PEMFC system by using the methanol solution. Indeed, they did not take longer than with the dry gas purge method. During the cold start the current generation did not decrease, which shows that the produced water did not freeze. Moreover, polarisation curves corroborated no performance losses after every cold start. These results are in concordance with those obtained at single cell level, as in both cases the antifreeze avoided ice formation during the cold storage. This significantly improvement with regard to the above results obtained with a single cell, is due to the ability of the stack to generate much more heat power and to minimize thermal losses, warming up the stack faster than a single cell.The obtained results allow confirming that a self-start of an automotive PEMFC system at subzero temperatures by using antifreeze solution is possible. This alternative method avoids ice formation during the cold storage, which reduces the degradation due to freezing and enables starting at lower temperatures. The decline in degradation increases the stack lifetime, thus can be achieved the strict automotive sector demands. The advantages of the antifreeze for automotive cold start can be used for others mobile applications too, such as trucks, trains, maritime or aircraft. In the future, cold starts with the antifreeze should be further investigated, and at temperatures down to -40°C.1. F. Knorr, D. Garcia, J. Schirmer, P. Gazdizcki, and KA. Friedrich. Appl. Energy, 238, 1 (2019).

Platinum-Nickel Nanotubes Array As Cathode for PEM Fuel Cell

Keywords: PEMFC, Anodic Alumina Oxide, Ni Nanowires, PtNi Alloy, Oxygen Reduction Reaction, Carbon-free electrode.Fuel cells technology have attracted great attention for future energy conversion and storage applications (1). The commercialization of polymer electrolyte membrane fuel cells (PEMFC) technologies is still a challenge due to durability and cost limitations of the catalyst (2). In order to enhance their achievement on the market, it is mandatory to optimize the high costly catalyst and improve its durability and stability or to innovate a new structure of electrodes.Commercial Pt/C catalysts (nanoparticles 3–5 nm) supported carbon black are considered as the current standard catalyst offering high surface areas and high specific activity. Unfortunately, they are limited by corrosion of the C and Pt dissolution/agglomeration through Electrochemical Ostwald ripening mechanism, which is reflected by a fast and significant loss of electrochemical surface area (ECSA) over time during fuel cell operation(3). This work focuses on reducing the amount of platinum and improving the performance and durability of membrane electrode assemblies (MEAs), aiming to reduce cost and thus encouraging the development of PEMFCs. Pt-based catalysts such as Pt-Co, Pt-Cu, and Pt-Ni etc. have shown to exhibit higher oxygen reduction reaction (ORR) activity than platinum for PEMFC. Accordingly, we aspire to develop and control a new carbon-free architecture of the cathode, made only with vertically aligned PtNi nanotubes supported onto a Nafion® membrane.Previous studies have demonstrated the benefits of platinum carbon-free nanostructures such as nanostructured thin films (NSTF) by 3M Company, USA (4), platinum opal nanostructures (5), or vertically aligned platinum nanotubes (6). They have shown very promising performance and stability for low Pt loading.A new carbon-free structure of electrodes made of vertically aligned PtNi nanotubes has been developed to improve performance and durability of PEMFC at low noble metal catalyst content. The process is based on an electrochemistry route of Ni electrodeposition inside anodic alumina oxide (AAO) mold, followed by a step of controlled spontaneous galvanic displacement (Figure 1), thermal treatment and acid treatment leading to the formation of nanotubes (~50nm in diameter and ~400nm in length). Ultraviolet spectrophotometric determination of platinum content showed a loading of ca. 0,1mg/cm². The latter are transferred onto a Nafion® membrane using an optimized hot pressing process. Finally, electrodes were integrated in a complete Membrane Electrode Assembly (MEA) in order to characterize their electrochemical and transport limitations in real operating conditions.The catalyst specific area estimated form the integration of Hupd desorption peaks at 30°C 100%RH in complete MEA, is approximately 6,7 cm²Pt/cm²geo. For comparison, we have elaborated a Pt/C standard cathode (Tanaka 50 wt.% catalyst onto Vulcan support) with low Pt loading (~20µg/cm²) which exhibits a close catalyst surface area of ~6,5 cm²Pt/cm²géo.The performance of the MEA was quantified by recording polarization curve under H2/O2 80°C (Figure 2 below). Promising results are obtained at a relative humidity of 80% at the anode side and 100% at the anode side, under a pressure of 1,38 bar / 1,5 bar respectively. Indeed, fully humified conditions at both anode and cathode lead to severe decay in performance as current density increases, ascribed to flooding. In addition, as shown in Figure 2, higher activities, estimated from the value of current density at 0.8V extracted from the polarization curve corrected from ohmic drop, are achieved with PtNiNTs comparing to Pt/C 20µg/cm². Figure 1

Degradation study of a proton exchange membrane water electrolyzer under dynamic operation conditions

Understanding degradation phenomena of polymer electrolyte water electrolyzers operating under dynamic conditions is imperative for developing and implementing efficient and reliable means of energy storage from fluctuating and intermittent renewable energy sources. Herein, a commercial membrane electrode assembly with an amorphous IrOx anode is subjected to potential sweeping (1.4–1.8 V) with short and long holds and few steady-state interims for overall 830 h at 60 °C and ambient pressure to simulate frequent alternating idle and nominal operation regimes. Systematic electrochemical diagnostics and physicochemical methods are applied to identify degradation sources. Mild kinetic deactivation (2.6 μV/h) is observed independent on the dynamic protocol, due to loss of electrochemical surface area via crystallization. The amount of Ir dissolved and redeposited in the ionomer anode phase and in the membrane is negligible in comparison to the current state-of-the-art Ir loadings. As compared to kinetic losses, the irreversible resistive losses are one order of magnitude higher and are thought to be caused by the degradation of the membrane close to the anode catalyst layer. These resistive losses are associated specifically with dynamic operation. The two orders of magnitude higher, but recoverable, degradation during steady-state interims is attributed to the growth of inhibiting or long-lived species.

Lifetime prediction for proton exchange membrane fuel cell under real driving cycles based on platinum particle dissolve model

Despite the great progress of proton exchange membrane fuel cell (PEMFC) vehicle, the durability and cost of PEMFC still remain challenges. In this paper, a lifetime prediction model of PEMFC is developed by considering the platinum (Pt) electrochemical surface area (ECSA) degradation caused by steady power and transient power. The direct and continuous relationship between lifetime and real driving cycles is built by the proposed model. Firstly, the steady ECSA degradation model is deduced, and both the chemical and electrochemical dissolution of Pt particles are considered in the catalyst layer. The ECSA loss rate for steady power condition can be calculated by this model. Secondly, transient ECSA loss formula is obtained by fitting experimental data of PEMFC. This transient ECSA loss formula is used to calculate the ECSA loss rate under power changes condition. Thirdly, by applying the power voltage relationship, for a given power, the voltage can be calculated and applied to the two ECSA degradation models. Finally, the lifetime prediction method of PEMFC is proposed, and it is verified by PEMFC under three different driving cycles. Experimental test results show that the proposed lifetime prediction model accurately predict the lifetime of PEMFC under different driving cycles. The proposed method in this paper is more accurate compared with the method which divides the degradation into 4 conditions.

Voltage Cycling-Induced Pt Degradation in Proton Exchange Membrane Fuel Cells: Effect of Cycle Profiles.

To understand the processes of voltage cycling-induced catalyst degradation, influence of cycle profiles on Pt degradation is investigated using a mathematical method. Results show that the electrochemical surface area (ECSA) loss rate increases significantly with longer dwell time at the upper potential limit (UPL), which is mainly attributed to the enhanced Pt mass loss. The scan rate is also found to have little impact on the ECSA loss in the range of 1-37.5 mV/s as a lower scan rate will increase the Pt mass loss but mitigate Ostwald ripening. However, too long dwell time at the UPL or too slow scan rate would promote the formation of a more steady-state Pt oxide coverage, which is speculated to mitigate Pt dissolution. Decreasing cycle number has also been demonstrated to be the main contributor to the lower ECSA loss due to Ostwald ripening. Additionally, Ostwald ripening and Pt mass loss have comparable contributions to catalyst degradation at the UPL ≤ 0.9 V, while Pt mass loss contributes more at a higher UPL, which suggests that different load cycling strategies should be proposed for mitigating catalyst degradation at different UPLs.

Operating Conditions Effect on PEM FC Durability Using Design of Experiment Approach

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]. While numerous efforts have focused on stabilizing the PtCo nanoparticle, the improvements so far have been marginal. Under severe environment of the fuel cell operation, the less noble alloying element tend to dissolve rapidly and contaminate the MEA. The actual operating conditions of the PEMFC can have a significant impact on the PtCo degradation. In our initial study, we had shown that the relative humidity of fuel cell operation can have a significant impact on Pt electrochemical surface area (ECSA) loss and Co dissolution [3]. 35% ECSA loss was observed for 30,000 voltage cycles in H2-N2 at 100% RH, vs. zero ECSA loss for similar voltage cycling test at 25% RH. 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.To study the impact of operating conditions in depth, a design of experiment approach was utilized. The design of experiments approach allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. A 20-run design was created to study the various factors, such as relative humidity, temperature, upper potential limit, upper potential hold time etc at three different levels. The factors and its corresponding levels studied are listed in Table 1. Voltage cycling tests was conducted in H2/N2 environment at different operating conditions up to 60,000 voltage cycles. 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 were 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. A fundamental model was developed to predict the ECSA and activity loss as a function of operation conditions.

Efficient method to obtain Platinum–Cobalt supported on graphene oxide and electrocatalyst development

We have prepared a highly efficient and stable platinum–cobalt catalyst supported on graphene oxide by using a one-step synthesis microwave-irradiation process. The structure and composition of two different compositions (Pt:Co(2.5:1)/rGO, Pt:Co(2:1)/rGO) have been investigated by Fourier infrared spectroscopy (FT-IR), X-ray Photoelectron spectroscopy (XPS), specific surface area (BET), Raman spectroscopy. Their electrocatalytic activity was investigated and the electrochemical response from cyclic voltammetry revealed the high efficiency and stability as well as the potential application as cathode electrode. The electrocatalysts exhibited a superior durability comparing with commercial Pt/C catalyst after accelerated stress test, indicating a lower loss of electrochemical surface area in the case of prepared samples. Moreover, this study extends the applicability of this synthesis method for the preparation of other noble or transitional metal nanoparticles decorated on reduced graphene oxide.

Design of gradient cathode catalyst layer (CCL) structure for mitigating Pt degradation in proton exchange membrane fuel cells (PEMFCs) using mathematical method

The durability of PEMFCs is greatly influenced by the non-uniform degradation of catalyst particles across the thickness of cathode catalyst layer (CCL). Gradient CCL is one of the effective structures to improve both cell performance and durability. Here we employ mathematical modeling to evaluate several gradient CCL structures with Pt/C catalysts in terms of the evolution of electrochemical surface area (ECSA) and Pt mass during cycling. Results based on the two-layer models considering particle size gradient only show that larger Pt particles near the CCL/membrane interface can remarkably mitigate the loss of ECSA and Pt mass, but accelerate the Pt dissolution near the gas diffusion layer (GDL). An increase in the number of layers, corresponding to a more smooth decrease of particle size from the CCL/membrane interface to GDL, can effectively retain high ECSA after cycling. The CCL with an optimum performance is achieved by manipulating the particle size gradient and Pt loading gradient simultaneously, which gains more uniform distribution of Pt active surface for improving the end-of-test performance and durability. This work provides a strategy to achieve highly durable PEMFCs by combining the gradient of particle size and Pt loading in the CCL structure.

The Importance of Temperature and Potential Window in Stability Evaluation of Supported Pt-Based Oxygen Reduction Reaction Electrocatalysts in Thin Film Rotating Disc Electrode Setup

The present study showcases the importance of temperature and potential window for evaluation of Pt-based supported electrocatalyst stability. A platinum based commercial material with an average size of Pt nanoparticles between 2–3 nm (Pt/C) and its thermally annealed analogue with an average particle size of ∼5 nm (Pt/C-HT) are considered. X-ray diffraction (XRD), ex situ transmission electron microscopy (TEM) imaging and thin film rotating disc electrode (TF-RDE) along with proprietary high-temperature disc electrode (HT-DE) are used for electrocatalysts inspection. The study shows a clear dependence between the electrochemical surface area (ECSA) loss and the temperature increase during the potentiodynamic accelerated degradation test (ADT). Additionally it is demonstrated that selection of the lower and upper potential limits in ADT protocol plays an important role in ECSA loss. Comparing various results obtained on Pt/C and Pt/C-HT, we show that varying ADT conditions of temperature and different potential windows is crucial for adequate evaluation and stability interpretation of potentially promising novel electrocatalysts and that relatively mild ADT conditions (i.e. 0.4–1.0 VRHE, RT) can be potentially misleading.

Boosting electrochemical stability of ultralow-Pt nanoparticle with Matryoshka-like structure in polymer electrolyte membrane fuel cells

Electrochemical catalysts with a core-shell structure have received much attention because of their enhanced efficiency and activity. Among them, those with a CuPd alloy core exhibit better activity than the ones with a single metal Pd core, which is known to be an excellent core-material. However, the superior performance of previously reported Pt catalysts with CuPd core has only been observed in half-cells, and was not reflected or even expanded to single-cells. We report catalysts having a Matryoshka-like structure with a Pt outer-layer, Cu interlayer, and Pd core for oxygen reduction reaction. This catalyst has 3.4 times higher Pt mass activity than the commercial Pt/C in half-cells, and also performs better in single-cells at only 0.056 mgPt cm−2. Particularly, the stability of this catalyst satisfies the 2020 DOE target, and electrochemical surface area loss during the stability test is only 40 % for this catalyst, while that of Pt/C is 80 %.

Particle Size-Controlled Growth of Carbon-Supported Platinum Nanoparticles (Pt/C) through Water-Assisted Polyol Synthesis.

A water-assisted control of Pt nanoparticle size during a surfactant-free, microwave-assisted polyol synthesis of the carbon-supported platinum nanoparticles (Pt/C) in a mixture of ethylene glycol and water using (NH4)2PtCl6 as the Pt precursor is demonstrated. The particle size was tuned between ∼2 and ∼6 nm by varying either the H2O volume percent or the Pt precursor concentration during synthesis. The electrochemical surface area (ECSA) and the oxygen-reduction reaction activity obtained for the Pt/C electrocatalyst show a catalytic performance competitive to that of the state-of-the-art commercial Pt/C electrocatalysts used for polymer electrolyte membrane fuel cell electrodes (ECSA: ∼70 m2/g; half-wave potential for oxygen reduction reaction: 0.83 V vs reversible hydrogen electrode). The synthesized Pt/C electrocatalysts show durability equivalent to or better than that of the commercial Pt/C. The durability was found to improve with increasing particle size, with the ECSA loss values being ∼70 and ∼55% for the particle sizes of 2.1 and 4.3 nm, respectively. The study may be used as a route to synthesize Pt/C electrocatalysts from a convenient and economic Pt precursor (NH4)2PtCl6 and avoiding the use of alkaline media.

Degradation of core-shell Pt3Co catalysts in proton exchange membrane fuel cells (PEMFCs) studied by mathematical modeling

Understanding the degradation process of Pt-based catalysts is a crucial step to improve the durability of PEMFCs. A mathematical model is established to study the electrochemical surface area (ECA) loss and structural/compositional evolutions of core-shell Pt3Co catalysts along the cathode under a certain circumstance. Three major processes are included in this model: Pt dissolution and re-precipitation on the Pt shell, Co leaching from the bulk and deposition of Pt ions in the membrane due to crossover hydrogen. The results show that the significant ECA loss next to the membrane is mainly attributed to the severe particle corrosion rather than variation of the particle size distribution. The Pt shell thickness and Pt/Co atomic ratio decrease from gas diffusion layer (GDL)/CCL interface to the cathode catalysts layer (CCL)/membrane interface, while the Co mass loss shows an inverse trend. This work demonstrates that mathematical modeling is effective to predict the structural and compositional evolutions of the catalyst particles across the CCL, and therefore is useful to evaluate the complete performance degradation of Pt3M catalysts for PEMFCs in the future.

Voltage Cycling Degradation Dependence on O2 Pressure: A Comparative Voltage-Loss Analysis

Polymer-electrolyte-membrane fuel cells (PEMFCs) have proven their applicability as a clean energy conversion technology. At the same time, a clear understanding of their degradation under dynamic operation is critical to project and ultimately improve long-term durability. In order to mimic real-life operation stressors, accelerated stress tests (ASTs) are used to enhance a particular type of degradation mechanism. After the AST application, various physico- and electrochemical diagnostic tools aid to understand and catalogue the observed performance losses. Since the ultimate goal is to develop in situ mitigation and recovery strategies for automotive applications, a comprehensive voltage loss study describing the degradation effects on performance losses is desired. Particularly inside ASTs, voltage cycling tests reproduce conditions that mimic the dynamic operation of a fuel cell stack, and which are directly linked to the degradation of catalyst layers1 in a membrane electrode assembly (MEA). In a semi-empirical study, voltage cycling tests were applied by Kneer et al. 2 to promote electrochemical surface area (ECSA) losses of a carbon-supported platinum catalyst (Pt/C) for the oxygen reduction reaction (-ORR), establishing an ECSA loss semiempirical model for a wide range of voltage cycling conditions. Subsequently, this model was used to project performance losses in H2/air curves based on a theoretical relationship between ECSA evolution and the ORR observed kinetic and transport losses. However, the authors observed that their model prediction resulted in a substantial unaccounted voltage loss (compared to experimental data), suggesting that deeper analyses are required to understand voltage cycling induced performance losses, with the ultimate goal to develop in situ mitigation and recovery strategies. Therefore, our study will make use of voltage cycling ASTs, which will correlate voltage losses to degradation phenomena measured by various diagnostic tests3. These include the ORR mass activity (via H2/O2 measurements), proton transport resistance through the membrane and catalyst layer (via impedance analyses), and O2 transport resistance (via limiting current measurements). By performing a standardized protocol, the differences in MEA aging during voltage cycling tests conducted with H2/air vs. the more frequently used H2/N2 (anode and cathode, respectively) will be separately studied. Furthermore, as observed in Figure 1a, lowering the relative humidity (RH) of the reactants significantly reduces ECSA losses during voltage cycling tests, in comparison to the same protocol performed at 100% RH. More importantly, the H2/air performance losses are substantially lowered after low RH testing (in comparison to 100% RH) (Figure 1b). The unaccounted voltage losses vs. specific current density (ispec ) (the difference between the theoretical 70 mV/dec. line (black line in Figure 1c) and the corrected experimental cell voltages) are lowered, as well. Therefore, the effect of further operational stressors (e.g. cell temperature) on voltage cycling aging for H2/N2 vs. H2/air will be presented. Ultimately, we will seek to correlate the observed losses to those under automotive operating conditions. This insight leads to a better understanding of catalyst degradation and its subsequent suppression via appropriate mitigation strategies through the correct application of operating conditions as described by single cell experiments. Figure 1: a) ECSA losses vs. number of cycles. The insert depicts the used voltage profile (1 cycle equals to 8s at the lower voltage limit (LVL) followed by 8s at the higher voltage limit (HVL), voltage ramp >4V/s). Voltage cycling tests were performed as described by Harzer et. al. in a single-cell PEMFC with 5 cm2 active area (loading: 0.1 and 0.4 mgPt cm-2 in the anode and cathode, respectively; 100 kPaabs, 80°C and H2/N2 flows of 200 nccm/75 nccm on anode/cathode, respectively)3. b) H2/air performance curves (conditions: differential flow, 80°C, 170 kPaabs inlet-controlled pressure, anode: 2000 nccm of H2, cathode: 5000 nccm of air). c) Tafel representation of I-V curves after voltage cycling AST at 30% RH (20k cycles) and 100% RH (10k cycles). Ecell was corrected for HFR, H+ resistance through the catalyst layer, and O2 mass transport resistance; specific current density (ispec ) is corrected for cell shorting resistance and H2 crossover current. References Kneer, A.; Wagner, N.; Sadeler, C.; Scherzer, A. C.; Gerteisen, D., Effect of Dwell Time and Scan Rate during Voltage Cycling on Catalyst Degradation in PEM Fuel Cells. J Electrochem Soc 2018, 165 (10), F805-F812. Kneer, A.; Wagner, N., A Semi-Empirical Catalyst Degradation Model Based on Voltage Cycling under Automotive Operating Conditions in PEM Fuel Cells. J Electrochem Soc 2019, 166 (2), F120-F127. Harzer, G. S.; Schwämmlein, J. N.; Damjanovic, A. M.; Ghosh, S.; Gasteiger, H. A., Cathode Loading Impact on Voltage Cycling Induced PEMFC Degradation: A Voltage Loss Analysis. J Electrochem Soc 2018, 165 (6), F3118-F3131. Figure 1