Current Status and Future Development of Catalyst Materials and

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Perspective

Current Status and Future Development of Catalyst Materials and Catalyst Layers for PEMFCs: An Industrial Perspective Dustin William H. Banham, and Siyu Ye ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00644 • Publication Date (Web): 03 Feb 2017 Downloaded from http://pubs.acs.org on February 7, 2017

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ACS Energy Letters

Current Status and Future Development of Catalyst Materials and Catalyst Layers for PEMFCs: An Industrial Perspective Dustin Banham, Siyu Ye* Ballard Power Systems, 9000 Glenlyon Parkway, Burnaby, BC, V5J 5J8, Canada

ABSTRACT: Proton exchange membrane fuel cells (PEMFCs) have already penetrated many commercial markets (e.g. portable power, backup power, materials handling, and bus), and are poised to greatly expand in the automotive market with both Toyota and Hyundai recently commercializing small fleets. As this occurs, catalysts for PEMFCs will experience ever greater demands on cost, activity, and durability. This perspective outlines the technology timeline and characteristics of the most promising catalysts currently being developed, and discusses the remaining challenges for both platinum group metal and non-precious metal catalysts. Finally, the importance of combined catalyst and catalyst layer design strategies is highlighted, and a brief discussion on the future outlook of this field is provided.

* Corresponding author. Tel: +1 604-412-4778; Fax: +1 604-412-4700 Email address: [email protected] (S. Ye)

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Table of Content Graphic


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In the face of ever growing environmental concerns, the need for advanced clean energy technology has never been more apparent. The global commitment to reduce our dependency on combustion of fossil fuels was recently highlighted on Dec. 12 2015, in the Paris Agreement which is the world’s first comprehensive climate agreement. 195 countries agreed to sign this broad-reaching agreement, as it is recognized that climate change is a global problem that will require a global solution. To meet the coming energy challenges, a broad suite of alternative energy/renewable resources will be required, including bio-mass, solar, hydro, wind, and nuclear to name a few. As many of these alternative energy sources are intermittent, it is widely recognized that energy storage will become increasingly important to help stabilize demand on the grid. Hydrogen has been identified as one of the most promising means for storing energy, and will certainly play a large role in helping to meet the coming energy challenges1. Due to their ability to efficiently convert the stored chemical energy in hydrogen into useable electrical energy, proton exchange membrane fuel cells (PEMFCs) will be a central technology in any envisioned version of the anticipated ‘hydrogen economy’. In fact, PEMFCs are already being commercialized for a broad range of applications, including portable power, backup power, materials handling, and bus. Additionally, some automotive OEMs have commenced sales of fuel cell electric vehicles (FCEVs)2. The penetration of PEMFCs into these commercial markets is expected to have exponential growth, as increased sales allow for decreased cost through economies of scale. This is true for almost every component of a PEMFC, including the bi-polar plates, gas diffusion layers, ionomer, and membrane. Unfortunately, there is still one component (platinum group metal catalysts) that could get more expensive as sales/volumes increase. In fact, it is anticipated that the contribution to total stack cost from PGMs (plus application) will increase from 21% at 1,000 fuel cell systems/year to 45% at 500,000 systems/year3-4. At both the cathode and anode of a PEMFC, platinum group metals (PGMs) are currently required to catalyze the desired redox reactions (hydrogen oxidation at the anode, and oxygen reduction at the cathode). As these metals are commodities, and are all quite scarce, increased demand for PEMFCs will only serve to increase the price of these catalysts if the loading is not reduced significantly from current levels. Due to the sluggish kinetics of the oxygen reduction reaction (ORR) (~ 5 orders of magnitude slower than hydrogen oxidation kinetics5), the majority of the PGMs are required at the cathode. This challenge is widely recognized in the PEMFC community, and has led to a strong focus on improving the catalysts used for the ORR at the cathode. This includes both improving the activity/utilization, as well as the durability/stability of these catalysts. A major commercial market for PEMFCs will be automotive, and thus it is this market that drives much of the long term cost analysis/catalyst requirements. An excellent overview on catalyst requirements for the automotive industry was recently published by General Motors (GM)6. However, in the short to mid-term, other markets which are currently more developed such as portable power, backup power, materials handling and bus will represent a larger total market share of PEMFC products. Each of these markets has different product (and catalyst) requirements. Thus, while it is prudent


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to strive towards commercial automotive targets, it is also wise for catalyst developers to keep the requirements of these other markets in mind, as they may offer more immediate opportunities to commercialize some of the next generation catalysts currently being developed. Clarification of terminology When considering the activity of a PGM catalyst, there are two primary metrics: 1) specific activity (µA/cm2), and mass activity (A/mg). The specific activity of a catalyst provides information on the inherent turn-over frequency, and is thus an important parameter for researchers working to tune catalyst structure/electronic properties of their catalyst to achieve the maximum exchange current density towards the oxygen reduction reaction. As specific activity is only concerned with the processes occurring at the surface of the catalyst, it is an excellent metric for single crystal studies where the mass of the electrode is not a factor of interest. In fact, it is through analyzing and comparing specific activities that many of the most significant catalyst advances have been made, including the seminal work on Pt3Ni(111)7, which demonstrated a specific activity 90x higher than commercial Pt/C and had a profound impact on global catalysis research efforts. However, for industrial applications, cost is the primary consideration, and thus the mass activity is ultimately what matters as this metric is directly transferable to cost. Importantly, this metric considers both inherent activity (specific activity) and utilization (ratio of surface atoms to bulk). Thus, for this perspective, when discussing next generation ORR catalysts, the primary activity metric will be mass activity. Another important concept prior to delving into deeper analysis is ‘Pt utilization’ which can have very different meanings depending on whether a catalyst or catalyst layer perspective is taken. Definition 1: For catalyst researchers, Pt utilization is based on the concept of ‘dispersion’, and is a measure of the ratio of surface atoms to bulk. In this sense, Pt utilization has no dependence on how well the cathode catalyst layer (CCL) is designed. Definition 2: For catalyst layer researchers, Pt utilization can mean the ratio of electrochemically accessible Pt area in the MEA to the expected Pt area (based on TEM, XRD, RDE, or other ex-situ measurement). Definition 3: Also for catalyst layer researchers, Pt utilization can mean access of reactants to the Pt surface at a given current density. Clearly, all three interpretations are important, but it is necessary to clarify which one is being discussed as they have very different meanings. In this perspective, it will be explicitly stated which type of ‘Pt utilization’ is meant whenever this term is used. Finally, the terms ‘electrochemical surface area’ (ECSA) and electrode platinum surface area (EPSA)8 must be defined. ECSA is typically reported in units of m2/g, and refers to the catalyst surface area normalized to the mass of the catalyst particle. This is the measurement of interest to most synthetic chemists/catalyst researchers as it relates to Definition 1 for Pt utilization. EPSA is a measure of real platinum surface area to geometric surface area of an MEA, and has units of cm2Pt/cm2MEA. Due to the close analogy to traditional ‘roughness factor’ in electrochemistry/materials science9, EPSA is often also referred to as the MEA roughness factor6, 10-11. EPSA is a critical value for catalyst layer researchers, and is related to Definition 2 for ‘Pt utilization’. Classification and current status of ORR catalysts


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ACS Energy Letters

During the past decade, a wide variety of highly promising ORR catalysts have been developed. Broadly, these catalysts can be categorized as: 1) Pt/C, 2) Pt and Pt alloy/de-alloy, 3) core-shell, 4) nonprecious metal catalysts, 5) shape controlled nanocrystals, and 6) nanoframes. The approximate development timelines for each catalyst type is shown below in Fig. 1. The timeline in Fig. 1 is highly subjective, and is only meant to give a general ranking of technology readiness for each catalyst family. No specific dates are provided as this would be rather presumptuous, and will depend largely on where current/future research efforts are focused. It should be noted that these catalysts are usually supported on carbon and other nanomaterials. As this perspective is not focused on support material, readers are referred to other review papers published on this topic12-13. Additionally, the nanostructured thin film (NSTF) catalyst developed by 3M can be considered a hybrid of several of the catalysts listed above, and is also highly promising14-15. However, as this catalyst is a free-standing structure, and typically ‘ionomer-free’ when used as a catalyst layer, it is fundamentally different from those listed in Fig. 1, with many unique advantages/challenges. Thus, this particular catalyst will not be covered in this perspective, and for further understanding of the NSTF, readers are referred to an overview published by 3M15.

Figure 1: Development timelines for Pt, Pt alloy/de-alloy, core-shell16, nonprecious metal17, shape controlled18, and nanoframe19 ORR electrocatalysts.


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As mentioned, the ‘current status’ and development timeline shown in Fig. 1 is subjective, and among other factors, depends largely on what applications are being considered. For example, from a purely automotive perspective, the non-precious metal catalysts (NPMCs) would likely be considered the furthest from commercialization, as they currently do not meet performance, durability (performance loss during voltage cycling20), stability (performance loss during potentiostatic/galvanostatic experiments20), or power density requirements. However, NPMCs are now close to meeting the requirements for portable power applications. In addition to differences in the development timeline of each family of catalyst, each one offers unique advantages/disadvantages from an industrial/commercialization perspective. At a high level, these differences are summarized in Fig. 2, and will be discussed in more depth below. Pt/C: For the past decade, commercial PEMFC products have relied heavily on Pt/C catalysts. When first introduced, these catalysts offered significant advantages over un-supported Pt black due to the much smaller nanoparticles that are achievable with supported catalysts compared to other catalysts in Fig. 2. The simplicity of these catalysts is both a benefit and a drawback. From a synthetic perspective, there is little room to tailor activity/durability when limiting the design to a single element (Pt). In fact, further improvements in activity and durability with conventional Pt/C now rely on advances in catalyst supports resulting in ‘catalyst-support’ interactions12, 21 which have been reported to enhance both activity and durability of PGM-based ORR catalysts. While promising, these approaches are unlikely to meet long term mass activity requirements using conventional Pt nanoparticles. From a manufacturing perspective, simpler systems are advantageous. However, while having only one component in the synthesis (Pt) may be considered an advantage, the reality is that at large scales, catalyst manufacturing costs are minimal in comparison to PGM costs22. Pt-alloy: For these reasons, Pt-alloy (e.g. PtCo, PtNi) catalysts are becoming the new baseline catalyst at the commercial level as they are able to achieve high mass activities while also demonstrating similar/better durability. The technological maturity of these catalysts is highlighted by Toyota’s recent announcement that a PtCo-alloy is currently used in the Mirai2. The improved electrocatalytic activity of Pt-alloys (such as PtCo, PtNi, PtFe, PtCr, PtV, PtTi, PtW, PtAl, PtAg) has been attributed to (i) the smaller Pt–Pt bond distances resulting in more favorable sites that enhance the dissociative adsorption of oxygen; and (ii) the structure-sensitive inhibiting effect of OHads. Considerable work has been carried out over the past decades on carbon supported binary alloys or ternary alloys that demonstrate 2-3 times higher mass activity vs. Pt/C23-24. In order to improve the stability and durability of these catalysts, further work on pre-leaching of Pt-alloy catalysts has been performed to remove base-metal (deposited on the carbon surface or poorly alloyed to the Pt)25. Post-treatments by either acid leaching (starting with Co/Ni rich alloys) and/or heat treatment on Pt alloys have also been performed, resulting in increased stability and activity, due to the formation of a Pt-rich skin26-28. These approaches have proven quite successful in improving the durability of alloy catalysts, with similar/improved durability vs. Pt/C reported at the MEA/stack level25, 29-30.


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ACS Energy Letters

Figure 2: Benefits and remaining challenges for each of the primary categories of electrocatalysts. Core-shell: Significant progress has been made in recent years on the highly promising ‘core-shell’ family of ORR catalysts. As shown in Fig. 2, the core-shell concept relies on having the active ORR catalyst (Pt) located only on the surface of the nanoparticle, with another metal (most typically Pd) making up the bulk. This unique design/concept theoretically can allow for the highest possible Pt utilization (surface Pt to bulk), and thus from a cost perspective, is highly attractive (provided less expensive cores can be developed). Additionally, the inherent rate of the ORR can be tuned through changing the core, which has both structural and electronic impacts on the Pt shell31-34. However, perhaps the largest potential benefit of this class of catalyst is the extraordinarily high ECSA afforded by the high Pt dispersion. This benefit goes beyond the obvious cost advantage, and as will be discussed under the ‘Remaining Challenges’ section, could be critical to achieving PGM loadings of < 0.1 mg/cm2 at the cathode. In addition to having an improved activity/ECSA vs. conventional PGM catalysts, core-shell catalysts have demonstrated improved durability during voltage cycling16, 32, 35. One reason for this is the ‘self-healing’ mechanism that has been identified by Dr. Adzic at BNL32. It is known that these core-shell catalysts often have imperfections in the shell, and the less noble core is prone to dissolution during voltage cycling. However, as this


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occurs, the Pt shell experiences lattice contraction which leads not only to higher specific activity, but also to a higher dissolution potential thus reducing overall Pt dissolution16. Additionally, it was shown that using a Pd-Au alloy core can further enhance the stability of these catalysts as the Au preferentially diffuses to any defects in the Pt shell, thus preventing further dissolution of the core36. Before moving to the next family of catalyst, a brief comment on the ECSA of core-shell catalysts must be made. While extraordinarily high ECSAs are often reported for core-shell catalysts when considering Pt only, the true ECSA must consider all PGMs in the catalyst. At the core of the most promising core-shell catalysts is often Pd, which when included, decreases the ECSA by ~ 60%37. Thus, for practical use, it is imperative that less expensive cores (PGM alloy or non-PGM) are explored. Fortunately, this work is already underway, with highly promising results at the RDE level38-39. Shape Controlled Nanocrystals: While at an earlier stage of development than core-shell catalysts, shape controlled catalysts (Fig. 2) appear to be a highly promising class of ORR catalyst due to their extremely high mass activities 40. As described in the introduction, mass activity is dependent on specific activity and Pt utilization (Pt dispersion). Core-shell catalysts most strongly exemplify the ‘Pt utilization’ strategy to generate high mass activities. Conversely, shape controlled catalysts rely primarily on achieving extraordinarily high specific activities to generate high mass activities. In this way, these catalysts have the closest ties to the fundamental single crystal studies mentioned earlier in this perspective. In principle, these catalysts attempt to recreate the ideal crystal structure identified by single crystal studies7, but at the nanometer scale40. An excellent example of this is the 9 nm Pt2.5Ni Octahedra developed at the Georgia Institute of Technology. Despite the relatively poor Pt utilization (Pt dispersion) afforded by the large 9 nm particles in this study, a mass activity of 3.3 A/mg was achieved (more than 7 x higher than the 2020 DOE target, albeit at the RDE level)41. This was accomplished through maintaining the ideal Pt2.5Ni(111) crystal structure (which has >50x higher specific activity vs. commercial Pt/C) at the nm scale. Despite the great promise shown by these catalysts, it is still too early in the development timeline to make firm conclusions on their commercial viability (the majority of testing on this family of catalyst has been at the RDE level only). In particular, there are two issues which may prove highly challenging for these catalysts when used in an operating fuel cell: 1) ECSA, and 2) durability/stability. ECSA: As described previously, to achieve high performance at high current densities with a PGM loading of 0.1 mg/cm2, an ECSA of ~ 50 m2/g will be required assuming no substantial advances in ionomer (discussed later on in this manuscript). Unfortunately, it has proven difficult to maintain the desired crystal face as the particle size is decreased to 50 m2/g is less of a concern, and the much higher mass activity of these advanced catalysts vs. commercial Pt (or Pt alloy) catalysts could provide a valuable increase in efficiency for products used in these other important markets. Durability/Stability: It has been reported that shape controlled catalysts have lower stability than commercial Pt/C when


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ACS Energy Letters

subjected to voltage cycling40, 42. In fact, it has been suggested that this type of catalyst exists only in a ‘metastable’ state, and will inevitably change to the thermodynamically preferred ‘round’ shape following voltage cycling42. The use of core-shell type strategies may help to overcome this limitation, with one report showing almost no loss in activity following an aggressive voltage cycling protocol for a PtPd-Ni core-shell octahedral catalyst43. Ultimately, MEA testing will be required to properly understand the merits/limitations of this catalyst type. In this regard, some MEA testing performed at GM has already indicated that some of these shape controlled catalysts (e.g. octahedral PtNi) are quite unstable, thus emphasizing the need for further MEA evaluation6. Nanoframe: The most recent family of PGM ORR catalysts is the ‘nanoframe’19, 44-46 . These catalysts consistently show significantly higher mass activity than commercial Pt/C (up to 20x higher than commercial Pt/C19, 45 based on RDE studies). However, the largest advantage of this catalyst type is their superb stability/durability during voltage cycling (Fig. 2) 19, 45-46. The unique design of nanoframe catalysts allows them to benefit from the high activity and stability typically associated with extended platinum surfaces47-48, while still achieving excellent ECSAs (> 50 m2/g19, 46) due to their relatively thin frames ( 15x) by catalyst researchers at the RDE level (Table 1). While seemingly impressive, these results actually represent one of the challenges for PGM catalysts, that is, gauging ‘state-of-the-art’ based on RDE testing. Firstly, due to the significant differences between RDE and MEA testing, it is hard to draw any conclusions on expected MEA-level activities based on the values reported in Table 1. This is particularly important to recognize in light of the fact that the FCTO targets are referenced at the MEA level. In fact, it has been repeatedly demonstrated that mass activities measured at the RDE level rarely translate to mass activities at the MEA level57. To the best of our knowledge, the highest reported MEA mass activity is a far more modest 0.7 A/mg50. Secondly, even within the RDE literature, the great variability in testing methods and approaches makes comparison between labs nearly impossible, and comments such as ‘world’s highest’ or ‘record setting’ are somewhat meaningless as these activities are as much a function of how they are measured as they are the true catalytic activity of the catalyst being studied. If comparisons must be made, it is likely best to refer to ‘improvement factors’ vs. a commercially available Pt/C catalyst (as opposed to absolute activities), as this at least helps to somewhat normalize for lab-to-lab differences. Further normalization of RDE data may be achieved in the future through following the recommendations of an excellent study performed at the National Renewable Energy Laboratory which can serve as a guide for global RDE testing58-59. It should be mentioned that the above discussion is not at all meant to diminish the use of RDE which the authors do believe can provide useful information for screening


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potential catalysts early in the development cycle. However, it does highlight the fact that caution must be applied when reviewing catalyst literature which report impressively high mass activities, and clearly demonstrates that future efforts should be focused largely on MEA evaluation as little is left to be achieved at the RDE level. Table 1: Recent published reports on PGM catalyst activity and durability (by RDE). Reference

PGM Mass Activity (A/mg)

Durability

ECSA (m2/g)

Huang et al.60

6.98

5.5 % loss*

68

Chen et al.19

5.7

0 %*

67

Choi et al. 43

1.6

1.7%*

90

Li et al.61

13.6

12 %

118

* See original references for details on voltage cycling protocols Challenge 2: ECSA - While not currently listed in the FCTO list of electrocatalyst requirements, it has become clear that the electrochemical surface area (ECSA) is also a critical parameter if cathode catalyst loadings of 1.5 A/cm2 10, 62-63. Currently, it is believed this additional performance loss is due to oxygen transport resistance through the ionomer film covering the catalyst surface. As the roughness factor is decreased, the local oxygen flux at each individual catalyst particle is greatly increased, leading to a local increase in transport resistance10, 62. An excellent discussion of this phenomenon was recently published by GM6, with one of the key findings shown in Fig. 3.

Figure 3: Non-Fickian O2 transport vs. roughness factor (or EPSA) for different catalysts. Pt/C ( × ), PtCo/C (○), Pt-ML/Pd/C (+), NSTF (■), and NSTF with 2−4 nm ionomer coatings (▲). Reprinted with permission from ref 37. Copyright 2016 American Chemical Society.


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The results in Fig. 3 demonstrate the critical importance of EPSA (roughness factor) on this oxygen transport loss phenomenon, and highlight the importance of developing catalysts with high ECSAs. Specifically, the EPSA (or roughness factor) is a product of the PGM loading (mg PGM/cm2 MEA) and ECSA (m2/g). The PGM loading is set by cost requirements, with the U.S. Department of Energy (DOE) targeting a total PGM loading of 0.125 mg/cm241 by 2020. Allowing only 0.025 mg/cm2 for the anode (likely the lowest loading achievable without losing performance from HOR kinetics64-66), this leaves only 0.1 mg/cm2 PGM for the cathode. At this target loading, the x axis in Fig. 3 can be directly converted to catalyst ECSA. Equation 1 gives an example of this assuming a catalyst with an ECSA of 50 m2/g at the target loading of 0.1 mg/cm2.  m 2 Pt  g 10000cm 2 Pt cm 2 Pt  ⋅ ECSA 50 500 ⋅ = g  1000mg mg 1m 2 Pt  (Equation 1)   cm 2 Pt  mg cm 2 Pt   ⋅ 0.1 2  ECSA 500 = EPSA 50 2 mg  cm MEA   cm MEA  Thus, any catalyst to be used at a target loading of 0.1 mg/cm2 must have an ECSA of ≥ 50 m2/g or it will suffer from high transport losses at high current densities. In fact, an ECSA of 50 m2/g should be considered an end of life target, as some loss of ECSA will almost certainly occur during the operating life of a PEMFC vehicle due to voltage degradation mechanisms and possible ECSA loss from contaminants (e.g. SOx, NOx). With this in mind, it is clear that simply achieving a high mass activity will not be sufficient to meet high current density targets. Fortunately, many of the next-generation catalysts being developed have achieved ECSAs > 50 m2/g (Table 1), but maintaining this ECSA at end of life may prove to be a challenge (further MEA testing is required). In particular, the core-shell catalyst concept will likely be a key design as automotive original equipment manufacturers (OEM)s progress towards ever lower PGM loading targets. However, recent work has demonstrated that while these catalysts can result in excellent beginning of life performance, significant durability improvements are still required for the scaled-up catalyst to meet real-world applications37. Challenge 3: Scale-up - Fortunately, scale up of some of these advanced catalysts is already underway. Core-shell catalysts have already been scaled up, and are commercially available from N.E. Chemcat 32, 37, 67 (although further optimization of the scale up is required to achieve high durability37). For shape controlled catalysts, the synthesis is typically performed in small reactors to produce mg quantities of catalyst. Scaling up this synthesis by simply using larger reactors is not feasible, as chemical and thermal gradients become increasingly severe as the volume of the reactor increases68. To overcome this issue, Xia et al. have been developing a droplet flow reactor design, which minimizes the reaction volume (< 1 mL/droplet69) and theoretically will allow quantities as high as kg/day68. This approach has already been successfully demonstrated for PtxNi octahedra at quantities of ~ 3.5 g/day, with no inherent limitation for scaling to higher quantities. The droplet flow reactor strategy is also being explored by ANL to scale up the PtxNi nanoframe catalyst70. This work is critical, as CCL design and innovation is not possible at the current scale (µg/mg) these catalysts are typically prepared at. Further discussion on the importance of CCL design strategies, and possible directions, is provided later in this perspective.


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Challenges for NPMCs Long term challenges (for automotive applications): Performance, durability, stability - The remaining challenges for NPMCs depend largely on the fuel cell application being targeted. For automotive applications, enormous challenges still remain to achieve the required performance, durability, and stability. Additionally, as these CCLs are typically much thicker (~ 100 µm) 52 than traditional PGM-based CCLs (~ 10 µm) 71, obtaining sufficient high current density performance will almost certainly be a challenge due to transport related losses. For these reasons, it is likely better to target (at least in the short term), applications with less demanding catalyst requirements, such as backup power and portable power applications. In addition to the lower performance, durability and stability requirements of these applications, the operating current densities are relatively low (< 1 A/cm2) which greatly alleviates the transport concerns through these relatively thick CCLs. Short-term challenge (portable power/backup power): Stability - It is an unfortunate reality that the most active NPMC are consistently found to be the least stable20. In fact, there is presently no NPMC that has demonstrated sufficient stability for typical portable power applications (~ 1000 h with < 10% performance loss). Thus, the largest remaining challenge for commercializing NPMCs is improving the stability of the most active NPMCs. In order to achieve this challenging goal, a concerted effort is needed to properly study and understand the degradation mechanisms occurring during potentiostatic and/or galvanostatic experiments. As described in a previous review by our group20, this understanding has been somewhat hampered by the incorrect use of accelerated stress tests (ASTs) designed for PGM catalysts, which are not always appropriate for NPMCs. In particular, ASTs performed under N2 can provide meaningful data on Pt dissolution, but do not properly target degradation mechanisms occurring for NPMCs. Fortunately, mechanistic studies are currently underway72-76, and will hopefully result in more stable NPMCs in the near future. Importance of catalyst layer design strategies Looking again at the FCTO electrocatalyst requirements for PGM catalysts, it is clear the largest remaining gaps are: (1) Loss in initial catalytic activity (require < 40%, currently can achieve 66%), (2) Platinum group metal total content (both electrodes) (require < 0.125 g/kW, currently can achieve 0.16 g/kW), (3) Performance loss at 1.5 A/cm2 (require < 30 mV, currently can achieve 65 mV)41. It should be noted that both 1) (loss in initial activity) and 2) (PGM total content) were achieved using 3M’s NSTF as opposed to a supported PGM catalyst. The loss in initial mass activity (Gap 1) is primarily a catalyst/materials level characteristic, possibly not requiring advanced CCL design concepts to be achieved. In fact, based on Table 1 it would appear that some strong catalyst candidates are already available. However, as discussed previously, MEA testing will still be required to properly verify the durability of these catalysts. Gaps (2) and (3) are a combination of both catalyst and catalyst layer requirements, and will almost certainly require advanced catalyst layer design concepts as these are performance requirements which must be achieved while operating at relatively high current densities. Fig. 4 shows one example of how a catalyst layer design strategy can be used to overcome challenges at high current densities.


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In Fig. 4a, a commercial Pt alloy catalyst is compared against a traditional Pt/C catalyst. In the low current density region, the Pt alloy catalyst demonstrates improved performance vs. Pt, as expected based on the higher mass activity of Pt alloy catalysts vs. Pt alone. However, at currents ≥ 1.5 A/cm2, the Pt alloy catalyst shows significantly higher transport limitations. This is a commonly observed phenomenon77, and has been attributed to dissolution of the base metal leading to cation contamination in the ionomer, causing either proton transport limitations77 and/or reduced O2 permeability78. It should be noted that this loss occurs despite the fact that extensive acid washing is performed on most commercial Pt alloy catalysts. Following voltage cycling, further base metal dissolution occurs which exacerbates this problem, as shown in Fig. 4b. By drawing on an understanding of the mechanisms behind this performance loss, Ballard data indicates this challenge can be overcome through redesign of the CCL (Fig. 4a.). Not only can this redesigned CCL maintain the expected performance benefit from the Pt alloy catalyst over all current densities, it showed improved durability vs. the Pt baseline following voltage cycling. This was achieved without changing the total PGM content in the CCL. Overall, this result highlights the significant opportunities at the CCL level for closing remaining gaps in high current density performance and durability.

Figure 4: (a) Air polarization curves for a commercial Pt/C catalyst, and a Pt-alloy catalyst with two different catalyst layer designs. In each design, the catalyst loading is 0.4 mg/cm2 (b) Performance loss for each design following 5000 voltage cycles between 0.6 – 1.0 V under air. Future Outlook GM has reported that the DOE PGM target loading of 0.125 mg/cm2 would equate to ~ 11 g PGM/vehicle6. It should be noted that this PGM/vehicle calculation is highly dependent on stack power, efficiency point, operating conditions, allowable degradation rate, etc. However, it does seem clear that to achieve the necessary reduction in PGM loadings for automotive applications will require catalysts to demonstrate a mass activity of ≥ 0.44 A/mg PGM. Even allowing for an order of magnitude decrease in mass activity when going from RDE to MEA tests, some of the most active catalysts would still be expected to exceed this target, while demonstrating excellent voltage cycling durability19, 60. Thus, with the strong pipeline of next-gen electrocatalysts that have been developed (Fig. 1), there appears to be no physical limitation (at the catalyst level) to reducing the PGM content of automotive PEMFCs to well below 10 g/vehicle. Most of the remaining challenges will be on catalyst scale-up and CCL design.


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When developing CCL design strategies, consideration must be given to all stages of the CCL design process. While the diagrams in Fig. 1 convey the advanced structures of the next-gen catalysts being developed, and are certainly impressive to visualize, they also highlight the challenges that may be expected even at the ink-mixing stage of CCL design. Many commercial catalyst ink-mixing technologies exist for traditional Pt and Ptalloy catalysts, but it is not implicit that these methods will be compatible with the more advanced catalyst shown in Fig. 1, as the same advanced structures that provide these catalysts with excellent activity and voltage cycling durability may cause them to be more susceptible to physical damage79. At the CCL level, catalyst gradients have been explored and have demonstrated the ability to greatly improve both performance and durability80. Currently, one of the most important areas of research is the tuning of ionomer properties to match the requirements in the electrode. Traditionally, the same (or very similar) polymer that is used in the membrane has also been used in the electrodes. This is not ideal when considering that a key requirement of the membrane is low gas permeability, whereas high H2 (anode) or O2 (cathode) permeabilities are required in the electrodes (Table 2). This strategy is currently being investigated, with early work demonstrating promising MEA performance for cathode ionomers having high oxygen permeability6, 81. Beyond an expected performance improvement by increasing the oxygen concentration in the electrode, this work may prove critical to achieving high performance at high current densities for ≤ 0.125 mg/cm2 PGM loaded MEAs as it should help to alleviate the widely acknowledged oxygen transport problems in these low PGM loaded designs 6, 10, 62. Table 2: High level requirements of the polymer in the anode, membrane and cathode Requirement Anode Membrane Cathode + H conductivity High High High O2 solubility Low Low High H2 solubility High Low Low Importantly, non-automotive applications may have an important role to play in the commercialization of some of these advanced catalysts. These alternative markets typically do not demand high current density performance, which currently appears to be one of the large potential challenges for several families of PGM catalysts (shape controlled and nanoframe) and NPMCs. The efficiency/cost advantages afforded by these advanced catalysts could provide immediate benefits to nonautomotive markets, and the data from these real-world applications would also provide catalyst researchers with invaluable information on how their materials perform outside of the lab. Overall, while it is clear that challenges still remain at the CCL/MEA level, there is also reason for great optimism when considering the advances made in both PGM and NPMCs over the past several decades. With a concerted effort on scaling up some of the most promising catalysts, and improving/innovating at the CCL level, there appear to be no fundamental catalyst limitations to achieving widespread commercialization of PEMFCs for both automotive and non-automotive applications.


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Biographies: Dustin Banham received his Ph.D. in electrochemistry from the University of Calgary in 2012, and is currently a Senior Research Scientist at Ballard Power Systems. He has 10 years of experience in PEMFC catalyst research. Currently, his work at Ballard is focused on advanced electrocatalyst and catalyst layer development.

Siyu Ye received his Ph.D. in electrochemistry from Xiamen University in 1988. He is an Adjunct professor at University of British Columbia, University of Waterloo, and South China University of Technology. Currently, Dr. Ye is a Principal Research Scientist at Ballard and is leading Ballard’s next generation catalyst technology development. Link to website: www.ballard.com


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Acknowledgements The authors would like to thank all of our Ballard colleagues for many engaging discussions, useful insights, and critical feedback on this work. In particular, we would like to acknowledge Shanna Knights, Julie Bellerive, Lijun Yang, Ping He and Mauricio Blanco.


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Quote 1:While it is prudent to strive towards commercial automotive targets, it is also wise for catalyst developers to keep the requirements of these other markets in mind, as they may offer more immediate opportunities to commercialize some of the next generation catalysts currently being developed. Quote 2: This discussion does highlight the fact that caution must be applied when reviewing catalyst literature which report impressively high mass activities, and clearly demonstrates that future efforts should be focused largely on MEA evaluation as little is left to be achieved at the RDE level. Quote 3: In order to achieve this challenging goal, a concerted effort is needed to properly study and understand the degradation mechanisms occurring during potentiostatic and/or galvanostatic experiments. Quote 4: Overall, this result highlights the significant opportunities at the CCL level for closing remaining gaps in high current density performance and durability.


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Current Status and Future Development of Catalyst Materials and

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