Half-Wave Potential or Mass Activity? Characterizing Platinum Group

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Half-Wave Potential or Mass Activity? Characterizing Platinum Group Metal-Free Fuel Cell Catalysts by Rotating Disk Electrodes Downloaded via 31.40.210.32 on April 27, 2019 at 13:57:17 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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the scope of this work. Cell parameters that were observed include the type of counter electrode, electrolyte, ionomer-tocarbon mass ratio, and catalyst loading. Parameters that were extracted from data in the referenced literature include the half-wave potential, current density at 0.85 V, and limiting current density. The mass activity is generally not reported and is calculated from the current density at an IR-free voltage of 0.85 V and the active material loading on the RDE tip (see Figure 1). The limiting current density was an average of the

roton-exchange membrane fuel cells (PEMFCs) are becoming more promising as an alternative form of energy conversion for transportation and stationary applications. One challenge to the adoption of PEMFCs for fuel cell electric vehicles (FCEVs) is their cost, because they require a platinum or platinum alloy catalyst to achieve adequate power density and durability. Many groups are actively addressing this problem by making new PGM-free catalysts with the goal of performing to the level of platinum. Generally, synthesis researchers use a rotating-disk electrode (RDE) technique to screen and characterize the performance of their catalysts. However, there are challenges in comparing catalyst performance. The most common method is to compare half-wave potentials (E1/2). This metric has been useful, because it has allowed the PGM-free catalyst community to show that the half-wave potential of PGM-free catalysts can compete with Pt catalysts. However, the issue with this metric can be observed from a basic kinetic analysis in that E1/2 increases as RDE catalyst loading is increased. Thus, comparisons between catalysts by half-wave potential are biased toward studies that used higher loading, and loading dependence complicates comparison between studies of different loadings.Hence, we propose a shift to focusing on mass activity for PGM-free catalysts. Generally, mass activity is defined as the current density measured at a specified voltage where activation losses are dominant and normalized by the active material mass loading (e.g., μg of Pt).1,2 For PGM catalysts, an IR-free voltage of 0.9 V is typically used for the mass activity. In this analysis, we defined the mass activity using current density at an IR-free voltage of 0.85 V because of the commonly lower onset potentials. Also, from a practical standpoint, we chose a current density at 0.85 V so that we could more accurately extract current density values from publications. The mass used in our analysis is the total catalyst loading, including any additional carbon materials such as carbon black. We calculate the mass activity of catalysts reported in the literature and find several reports of high E1/2 with a low mass activity catalyst. In practice, mass activity is a key figure of merit in advancing fuel cells with PGM-free catalysts given the impact on electrode thickness and the transport losses that hinder power density. We further recommend lower RDE loadings to limit internal transport losses in relatively thick PGM-free catalyst films, which has the added benefit of providing more representative hydrogen peroxide yield measurements. In our survey, we restricted our analysis to publicly available RDE/RRDE tests for PGM-free catalysts with an acidic electrolyte. Although the oxygen reduction reaction (ORR) is considerably more facile in alkaline media,3−8 the fuel cell system-level constraints are significant enough to be beyond © XXXX American Chemical Society

Figure 1. PGM-free ORR catalysts are commonly evaluated by half-wave potential in RDE measurements. As depicted, the mass activity is a better metric of the intrinsic activity due to the lower influence of catalyst loading.

current density at 0.0 and 0.5 V. Table 1 lists the source and data extracted from the literature along with the calculated mass activity. Figure 2 presents the half-wave potentials versus the reported catalyst loading. As expected, there is a general trend of higher catalysts loadings exhibiting higher half-wave potentials. In contrast, Figure 3 plots the mass activity results and shows that there is no clear trend between mass activity and catalyst loading. It is interesting that the highest mass activity catalysts (UM and UB catalysts) have been evaluated at the highest loadings of ∼800 μg/cm2, partially contributing to their high half-wave potentials beyond just activity. The two catalysts with the highest mass activity at 0.85 V from Figure 3 is UM’s Fe 0.5−95028 and UB’s C-1.5 Fe-ZIF,21 with mass activities of 4.2 and 3.7 A/g, respectively. Common characteristics of these catalysts include a high density of active sites emanating from the MOF precursor and the lack of additional carbon supports. This lack of excess carbon mass is a significant advantage when constructing fuel cells with these catalysts, because it allows for the reduction of electrode Received: April 11, 2019 Accepted: April 15, 2019

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DOI: 10.1021/acsenergylett.9b00790 ACS Energy Lett. 2019, 4, 1158−1161

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Cite This: ACS Energy Lett. 2019, 4, 1158−1161

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ACS Energy Letters Table 1. Source of Catalyst, Catalyst Type, and Electrochemical Parametersa ref

institution acronym

catalyst sample

catalyst loading (μg/cm2)

half-wave potential (V)

mass activity at 0.85 V (A/g)

b

HZB UNM UNM (1) UNM (2) UNM (3) UNM LANL UB LANL (1) LANL (2) LANL (3) LANL UNM/CSM MPIE/UM MPIE/UM MPIE/UM MPIE/UM UB USC UB INRS UB ANL ANL NEU PEK UM UM

UK63 Fe-3PEI first cycle Fe-8CBDZ (1 cycle) Fe-8CBDZ-800C Fe-8CBDZ Fe-8AAPyr (CM-PANI)-Fe-C C-Fe-Z8-Ar PANI Fe C HT1 PANI Fe C HT2 PANI Fe C AL (CM + PANI-Fe-C(Zn)) FeNCB sample 4 FeNC-wet1 FeNC (20 °C) FeNC (50 °C) FeNC (70 °C) Fe ZIF 50 nm ethylene diamine C-1.5Fe-ZIF NC Ar NH3 20 Co NC 1100 Co/Zn(mlm)2-P FeIM/ZIF-8 FePhen @ MOF-ArNH3 300 nm ZIF-67 Fe 0.5−900 Fe 0.5−950

460 200 200 600 600 200 600 560 600 600 600 600 600 800 800 800 800 800 N/A 800 817 800 400 400 600 400 818 818

0.80 0.65 0.53 0.76 0.77 0.61 0.80 0.82 0.67 0.78 0.73 0.83 0.75 0.78 0.72 0.74 0.77 0.85 0.72 0.85 0.84 0.81 0.76 0.75 0.77 0.71 0.82 0.87

2.2 1.1 1.6 0.32 0.40 0 0.58 3.0 0.65 1.3 0.98 0.63 0 0.65 0.33 0.45 0.63 2.4 0.085 3.7 0.70 0.60 0.80 0.53 0.83 0.085 2.9 4.2

2 9b 10b

11b 12 13 14

15 16 17 18

19 20b 21 22 23 24c 25c 26d 27b 28

a Graphite rods were used as counter electrodes unless otherwise labeled. bPt counter electrode. cAu counter electrode. dUnknown counter electrode.

Figure 2. Half-wave potential E1/2 (V) vs catalyst loading (μg/cm2) for PGM-free catalysts. Different colored shapes refer to different counter electrodes used. Acronyms indicate the institutions and research groups that synthesized the catalyst in the respective reference.

Figure 3. Mass activity at 0.85 V (A/g) vs catalyst loading (μg/ cm2). Different colored shapes depict different counter electrode materials. Acronyms indicate the institutions and research groups that synthesized the catalyst in the respective reference.

considering the high density of Pt, the volumetric activity of leading PGM-free catalysts is roughly 1/10 that of its pure Pt counterpart and still further less than that of advanced Pt-alloy catalysts. Figure 4 presents the measured half-wave potential versus our calculated mass activity. The dashed line shows the approximate trend between mass activity and the half-wave potential, with a slope of 30 mV/(A/mg) or, if considered on a log10 scale, approximately 100 mV/dec. The roughly 100 mV/

thicknesses for lower transport losses. Through-plane transport losses are a significant challenge to achieving high power density with PGM-free catalysts. For comparison, the mass activity measured for a Pt loading of 400 μg/cm2 was 87.5 A/ gPt29 or, if we consider a Pt/C ratio of 40%, a mass activity based on the Pt/C catalyst loading of 25 A/g. Thus, the mass activity of current PGM-free catalysts is roughly 1/5 that of carbon-supported Pt. However, on a volumetric basis, 1159

DOI: 10.1021/acsenergylett.9b00790 ACS Energy Lett. 2019, 4, 1158−1161

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ACS Energy Letters

specifically review hydrogen peroxide yields because they are less commonly reported, our suggestion to use lower loadings and use a mass activity for PGM-free catalysts will provide improved characterization of the four-electron ORR selectivity. We do wish to note that it will be common for many PGMfree catalysts to have a greater lower bound of loading or thickness compared to carbon-supported PGM catalysts that is due to the catalyst morphology. Many of the PGM-free catalysts are synthesized in the form of large aggregates (1−20 μm) that are difficult to break up into smaller particles. Given the importance of smooth RDE films of uniform thickness in minimizing flow disturbance at the film surface and generating accurate current densities, the largest particle size can set a threshold to the minimum thickness and loading.

Diana E. Beltrán Shawn Litster*

Figure 4. E1/2 (V) vs mass activity at 0.85 V (A/g) graph. Different shapes depict different catalyst loadings. Acronyms indicate the institutions and research groups that synthesized the catalyst in the respective reference.

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dec scaling is not surprising given the typical Tafel slopes for ORR. We note that these are only approximate scalings given the variation in loading in these data points. In any case, the results show that increases in half-wave potential on the order of 50 mV can be achieved by a 3-fold increase in loading (e.g., 200 to 600 μg/cm2). For instance, in Figure 4, there are several catalysts with high half-wave potentials that have a comparatively lower mass activity, where the high half-wave potential has likely been imparted by a high loading. The impact of the ionomer to catalyst ratio on mass activity was also investigated, but I/C did not seem to have a systematic effect on the mass activity of the PGM-free catalysts. Although, recent studies in our group and others have shown that for a given type of catalyst the I/C ratio can have a strong impact on the mass activity measured by RDE. Ideally, mass activity should not be dependent on catalyst loading, but RDE catalysts loadings are so high that the effects of the electrode thickness cannot be negligible. Generally, PGM-free electrodes that are made for membrane electrode assemblies (MEAs) have a loading and thickness of 4 mg/cm2 and 100 μm, respectively. Thus, the RDE loading range of 200−800 μg/cm2 should correspond to RDE thicknesses of 5− 20 μm with PGM-free catalysts. These thicknesses are similar to the full thickness of Pt-based cathodes in MEAs and cannot be treated as a thin film. Furthermore, given transport losses through the thicknesses of the electrode, there will be diminishing returns to increasing RDE loading to increase the half-wave potential. Most importantly, the transport losses with high loadings may be diminishing the mass activity metric. Thus, there is a significant benefit to improved evaluations of ORR performance if the mass activity is used as the figure of merit with measurements done using lower loadings. As Suntivich et al.30 discuss, the reporting of low hydrogen peroxide yields and four-electron ORR selectivity for carbonbased PGM-free electrodes in rotating ring-disk electrode (RRDE) studies may be misleading due to the thick electrodes. If the electrodes are thick, there is a higher probability that the hydrogen peroxide generated internally by the two-electron ORR will internally reduce to water so that less hydrogen peroxide is reduced at the ring electrode. On the other hand, if the electrode is thin, there is less chance for an internal reduction reaction and a more representative collection of hydrogen peroxide at the ring.30 Although we did not

Department of Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shawn Litster: 0000-0003-1973-1834 Notes

Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076. We would like to thank Dr. Chang Hyuck Choi, Dr. Gang Wu, and Hanguang Zhang for replying to correspondence concerning their papers.

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ACRONYMS UNM University of New Mexico HZB Helmholtz-Zentrum Berlin ANL Argonne National Lab LANL Los Alamos National Lab UB University of Buffalo PEK Peking University UM Université Montpelier INRS Insitiut National de la Recherche Scientifique CSM Colorado School of Mines NEU Northeastern University USC University of South Carolina MPIE Max Planck Institute für Eisenforschung

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DOI: 10.1021/acsenergylett.9b00790 ACS Energy Lett. 2019, 4, 1158−1161