Materials research and development focus areas for low cost automotive proton-exchange membrane fuel cells

Journal Pre-proof Materials Research and Development Priorities for Low Cost Automotive PEM Fuel Cells Craig Gittleman, Anusorn Kongkanand, David Masten, Wenbin Gu PII:

S2451-9103(19)30156-5

DOI:

https://doi.org/10.1016/j.coelec.2019.10.009

Reference:

COELEC 469

To appear in:

Current Opinion in Electrochemistry

Received Date: 7 August 2019 Revised Date:

8 October 2019

Accepted Date: 10 October 2019

Please cite this article as: Gittleman C, Kongkanand A, Masten D, Gu W, Materials Research and Development Priorities for Low Cost Automotive PEM Fuel Cells, Current Opinion in Electrochemistry, https://doi.org/10.1016/j.coelec.2019.10.009. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

Materials Research and Development Priorities for Low Cost Automotive PEM Fuel Cells Craig Gittleman, Anusorn Kongkanand, David Masten, Wenbin Gu Fuel Cell Business, General Motors, Pontiac, Michigan 48340, USA Abstract Research and development of fuel cell materials often focuses on designing and discovering materials which will reduce the cost or improve the durability of an individual subcomponent. Examples of recent focus areas include non-Pt group metal (PGM) catalysts, non-carbon catalyst supports, and non-fluorinated membranes. These studies rarely look at the entire system to comprehend the impact of these materials on the cost-of-ownership to the customer, including vehicle and fuel costs. This perspective takes a holistic look at the impact of functional materials on automotive fuel cell systems and provides direction on which material properties will provide the greatest benefit. It also provides guidance on which material classes are the most likely to enable the achievement of systems which will result in the successful commercialization of lightduty fuel cell vehicles. Introduction When designing a fuel cell system (FCS), all high-level requirements including cost, performance, durability and robustness, must be considered.1 Performance metrics include power density, efficiency, and heat rejection. Power density and heat rejection are driven by highcurrent-density (HCD) performance whereas efficiency, for typical light-duty-vehicle (LDV) applications, is driven more by low-current-density performance. Durability issues include voltage degradation as well as failures that prevent a system from operating safely such as severe gas crossover across the membrane. Most of these metrics can be directly translated into cost. For example, power density and voltage degradation can be combined into an end-of-life power density requirement, which can often be “bought” by increasing stack size or adding more Pt. Recent materials research and development (R&D) for proton exchange membrane fuel cells for automotive applications has been focused on purported lower cost materials. These include non-PGM (non-Pt Group Metals, but also excludes other precious metals) catalysts and nonfluorinated membranes. Other areas of focus have been on materials that can withstand specific failure modes, such as non-carbon supports that can withstand elevated potentials experienced during non-mitigated start-stop conditions. These materials, in nearly all cases, do not perform as well as Pt or Pt-alloy catalysts supported on high surface area carbons with perfluorinated membranes. This could lead to the incorporation of larger stacks, and therefore, more expensive systems. This paper will discuss the materials trade-offs to simultaneously meet all automotive FCS requirements AND minimize cost, and how these trade-offs should be used to guide materials R&D. The FCS comprises of the stack where the electrochemical reactions take place and the Balance of Plant (BOP) which ensures the stack runs properly. As cost reduction of BOP components will mostly be achieved by engineering and business innovations, this article focuses on how materials research can impact fuel cell stack cost. We will use U.S. Department of Energy’s (DOE) 2018 80-kWnet LDV as our baseline system and identify high-impact areas for materials research which can lead to achieving DOE’s ultimate cost target. Otherwise stated, all analyses were done based on estimated component costs by Strategic Analysis, Inc. at 0.5 M system/year.2,3 Figure 1a shows the cost breakdown of the DOE’s 2018 80-kWnet stack 4 . As annual production volume increases, and economies of scale are realized, the cost of PGM becomes a

larger fraction of the stack cost (31% at 0.5M system/year). Stack cost reduction can be achieved by reduction of individual subcomponent costs, simplification and elimination of parts, and improvement in stack power density. While all these approaches have contributed to overall stack cost reduction, increased power density has been the largest recent enabler. This is because when stack power density increases, it allows for a smaller stack, hence reducing the cost of all stack components proportionally. This is illustrated in Figure 1b, which shows the progress in stack cost reduction and increase in stack power density over the past thirteen years. DOE has an ultimate FCS cost target of $30/kWnet, driven by economic parity with internal combustion engine (ICE) vehicles. Assuming ultimate FCS target will be achieved via equally proportional cost reduction of both the stack and BOP from the current status, we arrive at a target of $12.6/kWnet for the stack alone. Additionally, the USDRIVE Fuel Cell Tech Team has set a 2025 power density target of 1.8 W/cm2.5 This requires a current density of 2.7 A/cm2 at 0.67 V, which is the cell voltage target at rated power to enable a reasonably sized radiator for waste heat rejection at 94°C.6 Path to Cost Parity So, what does it take to reach the ultimate cost and performance targets? Figure 1c shows the voltage loss terms for the 2018 status membrane-electrode assembly (MEA) at 2 A/cm2 and 3 A/cm2.6 The two current densities approximately represent today’s state-of-the-art (SOA) MEA current density, and what is required to achieve 2025 power density targets. Oxygen reduction reaction (ORR) kinetics remains the largest source of voltage loss, followed by ohmic resistance losses and local-O2 transport losses. About half of the ohmic resistance is from membrane areaspecific (proton transport) resistance (ASR), while the other half is from other electronic resistances (mostly from the bipolar plate and gas diffusion layer interfaces). The ORR kinetic and local-O2 transport losses are associated with the cathode catalyst and cathode catalyst layer (CCL). The following sections will address how the losses attributed to the membrane and catalyst layers can be reduced. The targets cannot realistically be met by improving one of the above-mentioned loss terms alone. Figure 1d shows stack high volume cost and power density for cases with improved loss terms compared to today’s SOA MEA (with 24 s/cm local-O2 transport resistance7) at various cathode Pt loadings. Note that a 4X reduction in local-O2 transport alone is not sufficient. The analysis indicates that two or more of these losses must be reduced. As examples, we provide three additional cases: a) a case where there is a 4x improvement in local-O2 transport and a 10x increase in ORR kinetics, b) a case where there is a 4x reduction in local-O2 transport and 50% reduction in ohmic resistance, and c) a case where there is a 4x reduction in local-O2 transport, 3x increase in ORR kinetics, and 50% reduction in ohmic resistance. While these are challenging goals, these examples illustrate paths to 40% stack size reduction and Pt amounts as low as 35gPt/system. It is interesting to note that at these levels, the cost and performance sensitivity to total Pt content becomes much smaller. This brings FC vehicles to PGM-level parity with today’s ICE vehicles and also reduces sensitivity to Pt market supply-demand issues.6

Figure 1. a) Effect of production rate on stack cost for DOE 2018 LDV technology system (80 kWnet)1. b) Progress in stack cost reduction and power density improvement over the past 13 years assuming $48/gPt. c) Voltage loss contributions at 2 A/cm2 and 3 A/cm2 for DOE 2018 MEA technology. d) Hypothetical pathways to meet stack cost and power density targets. Rated power density is set constant at 0.67 V when operated in H2/air, 94°C, 65%RH, 250 kPaabs,outlet, A/C stoichiometries of 1.5/2.

Catalyst Development ORR kinetics loss can be reduced by improving the Pt surface area, the area-specific activity, and the Pt utilization. Notable developments in this area include dispersion of Pt nanoparticles on high-surface-area carbon black, formation of Pt monolayers on core-shell nanoparticles 8 , and segregation of Pt-skins on alloy catalysts9. The current SOA ORR catalysts are dealloyed PtCo or PtNi nanoparticles, which gives a Pt-mass-specific activity of about 0.6 A/mgPGM.10 As discussed in Figure 1d, a further 3x to 10x improvement is required to meet targets. Is this achievable? Inspired by the exceptionally high ORR activity for thermally annealed PtNi(111) single crystal orientation reported by Stamenkovic et al11, many groups have prepared nanoscale PtNi with exclusively exposed (111) facets.12 Some have demonstrated activity as high as 7 A/mgPGM (11x activity) in ex-situ rotating disk electrode tests.13,14 Unfortunately, when tested in MEAs, the unique shape of the particles is quickly lost due to dissolution of Ni close to the surface.7,15,16 Still, this remains the most promising approach to date. Further research to enhance stability of these particles in an MEA and to elucidate the degradation mechanisms is warranted. Non-PGM catalysts have been a recent area of active research on the premise that removal of PGM from the cathode will significantly reduce the stack cost and U.S. reliance on foreign PGM supply.17,18 Although directionally correct, the goals of these projects do not account for the anticipated improvement in stack power density of Pt-based stacks. Smaller stacks also lead to

added benefits of lower BOP costs and total cost of ownership. For example, the lower pressure drop can lead to smaller compressors and less parasitic power losses. Figure 2a compares the stack cost of the two catalyst families as a function of power density. The performance and stability of non-PGM catalyst is currently so poor that it cannot be plotted on the same scale, we can only compare on the assumption that there is improvement in volumetric ORR activity over the current best non-PGM19 (as shown in black numbers, multiplier of iVol,0.9V = 0.9 A/cm3CCL). We also applied highly optimistic assumptions including a Tafel slope of 70 mV/decade, a breakthrough in ionomer with high proton conduction to allow the electrode to be thicker without excessive proton transport loss, and that the only difference in cost between a PGM and non-PGM MEA is the cost of the Pt. To investigate this benefit, we simulated two scenarios: (a) a cathode with equivalent transport properties as the current SOA PGM electrode (16µm thick, 0.8 mgcatalyst/cm2MEA) and (b) a cathode with a further 2x improvement in electrode proton conductivity and O2 transport (40µm thick, 2 mgcatalyst/cm2MEA). As shown in Figure 2a, indeed, there is cost benefit of PGM removal when compared to a Pt-based stack at the same power density. However, that benefit decreases quickly as power density increases, the stack becomes smaller, and the Pt amount decreases. For a given ORR activity, the 40µm non-PGM cathode shows higher power density and lower stack cost, thanks to the lower proton and O2 transport resistance. However, the benefit of electrode design is limited. In order for a non-PGM cathode-based stack to match the cost of a PGM-based stack that meets DOE targets, it must have 80-90% of the PGM stack power density. This advantage completely disappears at today’s Pt price of $27/gPt instead of the $48.23/gPt used in this (and DOE) analysis, or if Pt is recycled at a conservative rate of 60%, as currently achieved in the ICE market20. Furthermore, an FCS with a non-PGM cathode will still need a PGM anode. Therefore, precious metal recycling and resource management will still need to be established even for a non-PGM cathode fuel cell. Figure 2b shows that the low-power efficiency of a non-PGM FCS is similar to that of an optimized low-PGM fuel cell (Figure 2b). While exceptional advancement has been made in the non-PGM catalyst’s initial ORR activity in the last decade, the stability is still lagging considerably.18 Today, the volumetric ORR activity of 0.9 A/cm3CCL is maintained for less a day in an MEA,19,21 while the vehicle must last >10 years. SOA non-PGM catalysts are made of transition metal/nitrogen/carbon (M/N/C) active sites, with M=Fe or Co. The most accepted degradation mechanism is that the active site is attacked by its own ORR by-product, H2O2.18,21 Improvement in stability and activity must be made simultaneously by about 3 and 2 orders of magnitudes, respectively, from today’s SOA non-PGM catalyst. This translates to an initial activity at 0.9 V of 90-270 A/cm3CCL (or >0.4 A/cm2MEA). This is far more challenging than DOE’s 2025 target of >0.044 A/cm2MEA. 22 An entirely different material design approach from the current M/N/C catalysts is likely needed to meet these targets. Our analysis indicates that, with the R&D current focus areas, non-PGM catalysts are not on a trajectory to enable FCSs that are cost competitive with projected Pt-based systems. On the other hand, as discussed above for the crystalline-orientation-controlled nanoparticles, PGM catalysts already show promising results, albeit in ex-situ rotating disk electrode tests. Thus, more R&D in this area appears to be a much better investment.

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,  indicate non-PGM projections for increased ORR volumetric activity compared to iVol,0.9V =0.9 A/cm3.
16µm thick electrode scenario assuming sufficiently stability and comparable transport properties to the SOA PGM electrode.  40µm thick electrode scenario with 2x further improved transport properties. b) Projected FCS efficiency for the two catalyst systems. Pt cost is $48/gPt. Catalyst Support Development Local-O2 transport resistance is a major HCD voltage loss contributor. It is hypothesized to originate from the interaction between ionomer and the Pt surface.7 In general, approaches to decrease this interaction include segregation of ionomer from the Pt surface and use of an ionomer with a weaker adsorption energy to Pt or a restricted polymer structure conformation. Recently, it was shown that carbon supports with proper mesopore morphology could mitigate HCD voltage loss from local-O2 transport resistance. 23 The local resistance achieved by this approach decreases to about 6 s/cm (Fig 1d). Catalyst supports play an essential role in dispersing metal catalyst particles. Carbon supports with high surface area and high internal porosity generally provide better stabilization to the metal catalyst particles due to their morphology to confine the particles and their surface functional groups that bind to the particles.24,25 Carbon is relatively stable under normal fuel cell operation but can be oxidized quickly at unexpectedly high potentials (>1 V). Graphitization can improve high-V resistance somewhat, but is still insufficient, and also weakens the carbon’s anchoring strength to Pt particles. This creates a situation where a materials vs. system tradeoff must be made, to evaluate the frequency and impact of each high-stress event and how much it costs to prevent those events using system mitigation. DOE has a requirement for catalyst support stability under unmitigated start-stop operation of <40% activity and 30 mV loss after 5,000 cycles between 1.0 V and 1.5 V, 80ºC26. No catalysts in on-road fuel cell vehicles today, all of which use carbon supports, meet this requirement. OEMs currently use system controls27 to mitigate high-V exposure to the cathode during startstop operation which are projected to cost <$100/vehicle at high volumes. On the other hand, corrosion-resistant anode supports may improve fuel cell durability due to unexpected anode H2 starvation28. However, H2 starvation needs to be addressed for reasons besides anode corrosion, such as cathode carbon corrosion and membrane shorting29. For these reasons, the FCS cost savings of more corrosion and high V-resistant supports is expected to be minimal. Requirements for non-carbon (i.e. ceramic30) supports is essentially the same as carbon supports – they must match the electrical conductivity, provide good metal catalyst nanoparticle dispersion, and ensure ample porosity for reactant gas and product water transport. That being said, non-carbon materials hold significant promise in improving the Pt

stability through their stronger metal-support interaction.31 Rather than focusing exclusively on high-V-resistance, catalyst support R&D should focus on materials and structures that promote HCD performance and PGM particle stability. Membrane Development Recent membrane development has focused on several areas, each with a different approach to reduce FCS cost. These approaches include (a) less expensive membrane materials, (b) more conductive membrane materials, and (c) membranes that enable drier and/or higher temperature operation. The less expensive materials include non-perfluorinated ionomer membranes which have been purported to be less expensive than the incumbent perfluorinated membranes32,33. In general, these hydrocarbon-based membranes tend to be less conductive and less durable that SOA PFSA membranes34, and while the raw materials used to make the membranes may be less expensive, there is no report confirming the lower cost of actual membrane products. Higher performing membrane materials development has focused on low equivalent weight (EW) ionomers to increase proton conductivity35. An example of this work is the development of ionomers with more than one acid group per side chain 36 . These materials present durability challenges as higher IEC membranes swell more presenting mechanical durability challenges37, and multi-acid sidechain ionomers have known chemical stability concerns38. Membranes which are conductive at drier conditions or anhydrous proton conductors could enable lower system costs by elimination of the cathode humidifier or operation at higher temperature, which would enable a smaller radiator. Approaches include additives designed for water retention at low RH or which contain highly conductive moieties such as acidfunctionalized inorganics39 ,40 or heteropolyacids41 . However, there are several reasons besides increased membrane conductivity to operate at wet conditions, such as reduced sensitivity to catalyst contaminants42,43 and less recoverable voltage losses44, which are especially important at low catalyst loadings45. Furthermore, while higher temperature can actually reduce the sensitivity to performance degradation caused by contamination 46 , higher temperature accelerates other degradation mechanisms such as Pt dissolution47 and membrane degradation29, and requires the use of higher-cost non-active materials such as seals and gaskets. One of the trade-offs which can be done for a given material set is to optimize the membrane thickness for a minimum cost of ownership. On one hand, there is a desire for the membrane to be as thin as possible because (a) less ionomer is required, lowering materials cost and (b) ASR decreases proportionally to thickness, thus decreasing ohmic losses and enabling increased stack power density. On the other hand, there is desire for a thicker membrane to reduce H2 gas crossover from anode to cathode, which reduces system efficiency. This enables a smaller storage system to maintain vehicle range requirements and reduces fuel costs over the life of the vehicle. We have conducted a study to determine the cost of ownership of a vehicle with the SOA MEA developed within our DOE funded project, “Durable High Power Membrane Electrode Assembly with Low Pt Loading”48. This FC-PAD membrane uses a low EW PFSA ionomer with an expanded poly [tetrafluoroethylene] (ePTFE) mechanical reinforcement and 0.125 mg/cm2 total Pt loading. In this study we consider the impact of hydrogen cost, duration of ownership, and fuel cell operating conditions. The cost of ownership relative to a system made with a 12µm membrane is shown in Figure 3a. The analysis assumes that the combined cost of H2 production and delivery meets the 2020 target of $4/kg49. Results indicate that customers with shorter terms of ownership (5yr) will have a minimum cost of ownership with membranes between 10-12µm thick. Thicker membranes would lead to a more expensive system, while thinner membranes would lead to greater fuel costs over the lifetime due to higher gas crossover. Those who keep their vehicles longer (15yr) are better off with thicker membranes as they’d save more on fuel. The relative cost of ownership is somewhat sensitive to operating conditions as

both permeability and conductivity increase with increasing RH. In all cases dryer conditions favor thinner membranes in terms of relative cost of ownership. Figure 3b explores the thickness at which the minimum cost of ownership occurs as functions of H2 cost, operating RH, and duration of ownership. When H2 is relatively inexpensive (≤$4/kg), the minimum stack cost occurs with between 10-12µm membranes. Higher fuel costs and wetter conditions favor thicker membranes. Drier conditions favor thinner membranes across all fuel costs. We also looked at cases when H2 costs are similar to today’s values, ($12-16/kg)50. These results are not shown in Fig 3a because at these fuel costs the minimum cost of ownership is at membrane thicknesses greater than 20µm. However, in order for fuel cells to be competitive with ICE vehicles, H2 costs need to be $4/kg or less, suggesting that the optimal membrane thickness is between 10-12µm.

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Figure 3. a) Total cost of FC vehicle ownership at 500,000 vehicle/yr relative to a 12µm thick membrane at the same durations of ownership and operating RH values; b) Thickness at minimum cost of ownership as a function H2 cost, operating RH, and duration of ownership This study indicates that there are limits to how thin of a membrane one would choose depending on ownership and operating parameters. A question arises as to what materials development could enable the use of thinner membrane without a cost-of-ownership penalty. This could be addressed by developing materials that have lower H2 permeability and, therefore, less crossover. To date, there has been only limited development focus on this area. However, non-perfluorinated PEMs that have been developed for the purposes of cost reduction have shown up to 10 times lower H2 permeability than PFSA ionomers, depending on operating conditions 33, 51,52 . As noted above, such hydrocarbon-based PEMs typically suffer from a reduction in proton conductivity. Thus, we have conducted a theoretical investigation to understand the independent relative system cost impact of membrane proton conductivity and H2 permeability. In this study we held the overall net stack power at 80kW, and fixed the drive cycle efficiency. We made the assumption that membrane proton conductivity and H2 permeability could be improved without increasing mass specific ionomer cost. We independently varied proton conductivity or H2 permeability 50-fold, holding the other fixed, and then adjusted the membrane thickness and cell count to maintain the stack power and efficiency. As we have a fixed thickness ePTFE support layer, there is a minimum membrane thickness of 2µm. Figure 4 shows the relative costs of a stack made with higher conductivity or lower permeability membranes compared to one made with the 12µm FC-PAD membrane. The impact of conductivity is shown in Fig 4a by plotting relative stack cost as a function of resistivity (1/conductivity). In this case the membrane thickness is fixed at 12µm in order to maintain efficiency, and the stack cost decreases linearly with resistivity as the more conductive membrane leads to increased power density and reduced cell count. Here a 50% reduction in resistivity enables $0.24/kW savings at

100% RH and $0.35/kW savings at 50% RH, and a 98% resistivity reduction leads to $0.48/kW and $1.01/kW savings at 100% RH and 50% RH, respectively. The impact of H2 permeability is displayed in Fig 4b, showing that the membrane thickness is reduced linearly with permeability to maintain efficiency. The thinner membranes in turn lead to less proton transport resistance, which translates to increased power density and reduced cell count. Thus, with reduced permeability, cost reductions are achieved by using both less ionomer per cell and fewer total cells. Here a 50% reduction in permeability enables $0.88/kW savings at 100% RH and $1.18/kW savings at 50% RH, and a 98% permeability reduction leads to $1.48/kW and $1.92/kW savings at 100% RH and 50% RH, respectively. The results indicate that for the same relative amount of improvement, lower permeability is more valuable than higher conductivity. Conductivity effect at fixed Permeability

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Figure 4. Stack cost (relative to one made with a 12µm FC-PAD membrane) and membrane thickness as a function of (a) membrane resistivity and (b) membrane H2 permeability at 100% RH and 50% RH. There is a valid concern that thinner membranes may have a greater durability risk. However, this will not be an issue if degradation is driven by radicals formed due to gas crossover53, as the crossover rate here is fixed. Additionally, GM has run accelerated durability tests, using two different ionomers at each thickness, showing no difference in lifetimes between 12 and 18µm thick ePTFE-supported membranes. While this does not guarantee that there will be no durability issues with very thin membranes, it provides promise for cost reduction via the use of thin, low permeability membranes, and justification for R&D focus on low permeance, highly conductive ionomers. Summary Significant challenges remain to meet the long-term cost targets for fuel cell vehicles. Much of the cost reduction can be achieved though materials R&D, but it is essential that this R&D is focused on the right types of materials. The best approach to cost reduction is development of materials which enable higher power density stacks. This includes high-activity, high-ECSA PGM catalysts, where most of the PGM materials are accessible to reactants, and improving the stability of these catalysts. This includes catalyst support materials and structures that promote HCD operation and PGM stability. It includes low-H2-permeability membranes that enable the use of thinner membranes without compromising efficiency. While none of these are easy and will require substantial investments, improvement in these areas represent the most likely paths to achieving the ultimate cost target. Disproportionate spending on materials that are not projected to lead to significant FCS cost reduction, such as non-PGM catalysts and corrosion-resistant supports, is not justified.

Acknowledgements This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grants DE-EE0007271 and DE-EE0007651. The authors thank Ruichun Jiang for provided membrane resistivity and permeability models; Brian James and Jennie Huya-Kouadio of Strategic Analysis, Inc. for stack component cost estimates.

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