Platinum Supported on Titanium–Ruthenium Oxide Is a Remarkably Stable Electrocatayst for Hydrogen Fuel Cell Vehicles

Platinum supported on titanium–ruthenium oxide is a remarkably stable electrocatayst for hydrogen fuel cell vehicles

Javier Parrondoa, Taehee Hanb, Ellazar Niangarb, Chunmei Wangb, Nilesh Daleb, Kev Adjemianb, and Vijay Ramania,1

aCenter for Electrochemical Science and Engineering, Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, IL 60616; and bFuel Cell and Battery Laboratory, Zero Emission-Research, Nissan Technical Center North America, Farmington Hills, MI 48331

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved November 27, 2013 (received for review October 24, 2013) We report a unique and highly stable electrocatalyst—platinum from the anode flow channels, leading to a mixed fuel–oxidant (Pt) supported on titanium–ruthenium oxide (TRO)—for hydrogen front that lasts for timescales corresponding to the residence fuel cell vehicles. The Pt/TRO electrocatalyst was exposed to strin- time in the flow channel. These mixed-reactant fronts result in gent accelerated test protocols designed to induce degradation significant electrode polarization; under these conditions, the and failure mechanisms identical to those seen during extended PEFC cathode can experience potentials as high as 1.5–2.0 V (6– normal operation of a fuel cell automobile—namely, support cor- 8), which corresponds to a significantly higher overpotential for rosion during vehicle startup and shutdown, and platinum dissolu- the carbon oxidation reaction (1.3–1.8 V as opposed to 0.4–0.6 V tion during vehicle acceleration and deceleration. These experiments during traditional operation). The electrochemical reaction rate were performed both ex situ (on supports and catalysts deposited constant, which increases exponentially with overpotential, is onto a glassy carbon rotating disk electrode) and in situ (in a mem- significantly enhanced during this period. Under these con- brane electrode assembly). The Pt/TRO was compared against a ditions, carbon corrosion is exacerbated. In a second mechanism, — state-of-the-art benchmark catalyst Pt supported on high surface- fuel starvation at the anode catalyst sites as a consequence of area carbon (Pt/HSAC). In ex situ tests, Pt/TRO lost only 18% of its fuel overutilization or flooding (lack of fuel access to catalyst initial oxygen reduction reaction mass activity and 3% of its oxygen site) also exacerbates carbon corrosion. In this case, carbon is reduction reaction-specific activity, whereas the corresponding oxidized to provide protons and electrons in place of the (absent) losses for Pt/HSAC were 52% and 22%. In in situ-accelerated deg- fuel (9, 10). From our previous work and that of others, it has radation tests performed on membrane electrode assemblies, the − been suggested that the presence of platinum accelerates the loss in cell voltage at 1 A · cm 2 at 100% RH was a negligible 15 mV oxidation of the carbon support; the CO generation rate was for Pt/TRO, whereas the loss was too high to permit operation at 2 1A· cm−2 for Pt/HSAC. We clearly show that electrocatalyst support observed to be higher for catalysts with higher platinum loading, as measured with online infrared-based CO2 measurement (11) corrosion induced during fuel cell startup and shutdown is a far – more potent failure mode than platinum dissolution during fuel and online mass spectroscopy (12 14). CHEMISTRY cell operation. Hence, we posit that the need for a highly stable + → + + + e−; Eo = : : [1] support (such as TRO) is paramount. Finally, we demonstrate that C 2H2O CO2 4H 4 0 0 207 V vs SHE the corrosion of carbon present in the gas diffusion layer of the i fuel cell is only of minor concern. The adverse consequences of carbon corrosion include ( ) platinum nanoparticle agglomeration/detachment; (ii) macroscopic electrode thinning/loss of porosity in the electrode; and (iii) noncarbon catalyst supports | PEFC | start–stop protocol | TiO2–RuO2 | carbon corrosion enhanced hydrophilicity of the remaining support surface. The

Significance arbon has traditionally been the most common material of Cchoice for polymer electrolyte fuel cell (PEFC) electrocatalyst supports (1) due to its low cost, high abundance, high In this study, we showcase titanium–ruthenium oxide (TRO)— − electronic conductivity (30 S · cm 1), and high Brunauer, Emmett, a remarkably stable support material, and Pt/TRO, a derivative − and Teller (BET) surface area (200–300 m2 · g 1), which per- electrocatalyst that is also extremely stable. These materials mits good dispersion of the platinum (Pt) catalyst (2–4). The (in) have been tested to establish their catalytic activity and sta- stability of the carbon-supported platinum electrocatalyst is a key bility using accelerated tests that mimic conditions and degra- issue that currently precludes widespread commercialization of dation modes encountered during long-term fuel cell operation. PEFCs for automotive applications. Carbon is known to undergo We have evaluated Pt/TRO using these protocols, and provide electrochemical oxidation to carbon dioxide, as shown in Eq. 1. concrete evidence that this material is far more stable than Pt/C The standard electrode potential for this reaction is 0.207 V vs. and, would meet the requirements for use in an automotive fuel standard hydrogen potential (SHE; all potentials henceforth cell stack. Our cost analysis indicates the use of ruthenium is > reported vs. SHE, unless otherwise stated). Under normal PEFC not a factor given that 90% of catalyst cost resides in the operating conditions, the electrode potential is <0.1 V at the platinum metal; moreover, the exceptional stability of Pt/TRO anode and between 0.6 V and 0.8 V at the cathode. Despite the removes the needs for overdesign or replacement. fact that the cathode potential is usually significantly higher than Author contributions: T.H., E.N., C.W., N.D., K.A., and V.R. designed research; T.H., E.N., the standard potential for carbon oxidation (with an effective and C.W. performed research; N.D., K.A., and V.R. contributed new reagents/analytic overpotential of 0.4–0.6 V), the actual rate of carbon oxidation is tools; J.P., T.H., E.N., C.W., N.D., K.A., and V.R. analyzed data; and J.P. and V.R. wrote very slow due to intrinsic kinetic limitations; in other words, a the paper. very low standard heterogeneous rate constant (5). The authors declare no conflict of interest. During operation of automotive PEFC stacks, fuel/air mixed This article is a PNAS Direct Submission. fronts are known to occur during stack startup and shutdown. Air 1To whom correspondence should be addressed. E-mail: [email protected]. usually fills the flow channels when the stack is nonoperational. This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. During startup, the hydrogen fed into the stack displaces the air 1073/pnas.1319663111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1319663111 PNAS | January 7, 2014 | vol. 111 | no. 1 | 45–50 Downloaded by guest on September 24, 2021 former results in loss of catalyst active surface area and lower at 450 °C for 3 h in air to yield anhydrous, electron-conducting − mass activity resulting from reduced platinum utilization (9), TRO. This material had a BET surface area of 33 ± 2m2 · g 1 and − whereas the latter two result in a lower capacity to hold water an electron conductivity of 21 S · cm 1; both values are lower − and enhanced flooding, leading to severe condensed-phase mass than typically reported for Vulcan carbons (∼200 m2 · g 1 and − transport limitations. Clearly, both consequences directly impact 30 S · cm 1) but are reasonable for a catalyst support material PEFC cost and performance, especially in the context of auto- (2–4). Platinum nanoparticles were deposited on the catalyst motive stacks. It is of great value to automotive original equip- support by the reduction of hexachloroplatinic acid precursor ment manufacturers (OEMs) to eliminate or mitigate the issue with formic acid (36). The resultant Pt nanoparticles had diam- of support corrosion. eters ranging between 4 and 6 nm (Fig. S1). The relatively high Any viable alternative noncarbon support must possess high particle size resulted in lower values for the electrochemically surface area and electron conductivity, in addition to being active surface area (ECA). Detailed characterization of the sup- highly corrosion resistant across the anticipated potential/pH port and catalyst are provided in our previous work (34). window (15). Tin oxide (16) and tin-doped indium oxide (ITO) The electrochemical stability of the support (TRO) and cat- have been proposed as an alternate electrocatalyst support; alyst (Pt/TRO) were evaluated using accelerated stress test however, the stability of tin oxide in acidic environments is protocols similar to those developed by the Fuel Cell Technical uncertain and it has hitherto been difficult to synthesize ITO Team of the US Drive Partnership in collaboration with the US nanoparticles with adequate surface area. TiO2 and WO3 are Department of Energy, with some minor differences as described particularly attractive as catalyst supports in PEFCs because they below (37). The idea behind establishing such protocols was to have very good chemical stability in acidic and oxidative envi- have set of standardized testing methods to evaluate fuel cell ronments (17). However, undoped titania is a semiconductor performance and durability across laboratories, thereby allowing and its electron conductivity is very low, as is that of WO3 (18). a proper comparison of the results obtained at different facilities Substoichiometric titanium oxides (Ti2O3,Ti4O7, Magnéli pha- and permitting proper evaluation of technologies/materials re- ses) obtained by heat treatment of TiO2 in a reducing environ- sulting from various funded projects. In this study, we used two ment (i.e., hydrogen, carbon) have electron conductivity similar different protocols that measure (i) the stability of the support − to graphite (∼1,000 S · cm 1) as a consequence of the presence of due to start/shutdown voltage spikes, either stand-alone support oxygen vacancies in the crystalline lattice (19–21). However, the or catalyzed support, the latter to investigate the impact of heat treatment process reduces the surface area of these mate- platinum catalyst on the support corrosion rate (start–stop pro- rials, precluding the preparation of supported electrocatalysts tocol) (38), and (ii) Pt catalyst degradation due to dissolution/ with good Pt dispersion (22). The literature contains several Ostwald ripening as a consequence of load cycling—excursions studies suggesting that Magnéli phase materials are suitable to near the open-circuit potential—during normal fuel cell oper- support materials for use in PEFCs; however, long-term experi- ation (load-cycling protocol) (38, 39). These protocols effectively ments are needed to ascertain whether these materials are stable imitate and induce, in an accelerated fashion, the degradation for extended periods without loss of electronic conductivity (oxi- mechanisms that occur during extended normal fuel cell vehicle dation of the substoichiometric oxide), especially when placed in operation. an oxidizing environment in the PEFC cathode (23, 24). It is generally accepted by automotive OEMs that the fuel cell Doping of titania with transition metals (e.g., Nb and Ta) has stack in an automobile should operate for at least 5,000 h and been used to enhance its electrical conductivity. Some of these 60,000 startup/shutdown cycles without any significant voltage doped titania supports have been catalyzed and evaluated for loss (38). To evaluate the stability of the support using the start– electrochemical stability and activity in a PEFC. Most of the stop protocol, the working electrode potential was cycled in a doped titania materials have electronic conductivities in the in- triangular waveform between 1.0 and 1.5 V at a scan rate of 500 − terval 0.1–1S· cm 1 (substantially lower than carbon catalyst sup- mV/s (triangular wave form) for 5,000 cycles (Fig. 1A). Cyclic ports), but showed activities for the oxygen reduction reaction that voltammograms (CV) were recorded initially (baseline) and after were comparable to commercial Pt/C catalysts when evaluated in 100, 200, 500, 1,000, 2,000, and 5,000 cycles to characterize the a rotating disk electrode (RDE) (25–33). support by estimating the electrode pseudocapacitance (or, in an Mixed-metal oxides such as TiO2–RuO2 and SiO2–RuO2 have equivalent method, the current at 0.4 V in the capacitive region been shown to be an excellent alternative to carbon-based elec- of the CV). trocatalyst supports due to their high electrochemical stability Changes in fuel cell load occur as a consequence of the varying across a wide potential window (34). Ex situ stability tests (in an power demands that are incurred during a typical drive cycle. RDE) performed on these materials indicated no changes in the Although somewhat buffered by hybridization strategies, some double-layer pseudocapacitance for 10,000 cycles (under an ag- level of load cycling is inevitable. To evaluate the stability of the gressive start/stop stability protocol). The initial fuel cell perfor- platinum catalyst under load cycling, the cathode potential was mance obtained with Pt-catalyzed TiO2–RuO2 (Pt/TRO) was close cycled in a rectangular waveform from 0.95 V (near the open to that obtained with a commercial catalyst benchmark. circuit voltage; approaching no-load conditions) to 0.6 V (close In this work we further assess the efficacy of titanium–ruthe- to the maximum power; approaching full load conditions) for nium mixed oxides as a stable catalyst support, with an emphasis 10,000 cycles (Fig. 1A; note: the US Drive load-cycling protocol on membrane electrode assembly (MEA) testing. We evaluate is slightly different, and involves potential cycling from 0.65 to support and catalyst stability using accelerated degradation tests 1 V) CVs were recorded initially, and after 100, 200, 500, 1,000, (both ex situ and in situ). We assess the impact of stringent 2,000, 5,000, and 10,000 cycles. The stability of the catalyst was accelerated degradation tests on MEA performance. We also evaluated from the measured change in ECA and in electrode evaluate the oxygen reduction reaction (ORR) activity of the Pt/ polarization. TRO catalyst. We compare both the durability and the activity of The support and catalyst (Pt/HSAC, Pt/TRO) were examined this support and catalyst against an established commercial cat- with both protocols described above. The experiments were alyst benchmark, Pt/high surface-area carbon (HSAC; Tanaka performed both ex situ on supports/catalysts deposited onto a TEC10E50E; 50% Pt on HSAC). glassy carbon RDE, and in situ in a fully assembled fuel cell. The TRO (Ti:Ru mol ratio 1:1) support material was prepared by experiments were always performed with the working electrode precipitation of ruthenium hydroxide on commercial TiO2 nano- placed in a nitrogen environment to minimize side reactions. powder dispersed in deionized water (Aeroxide P25, BET surface The durability ex situ experiments were performed in an RDE − area 50 m2 · g 1; Acros Organics) (35). The powder was calcined setup at 60 °C using 0.1 M perchloric acid as the electrolyte, a

46 | www.pnas.org/cgi/doi/10.1073/pnas.1319663111 Parrondo et al. Downloaded by guest on September 24, 2021 Start/Stop protocol catalyst was studied using CV. The results are shown in Fig. S2. Pt/TRO did not show any sign of surface modification or in- Scan rate: 0.5 V/s stability, as observed for Pt/HSAC, and the H2 adsorption peak potential did not shift unlike in Pt/HSAC. Both observations 1s 1s indicated the superior stability of the TRO support upon po- 1.5 V tential cycling. The ratio of ionomer to support (well studied for Pt/C) was then optimized for the Pt/TRO catalyst via an ex situ 30s RDE study (Fig. S3). An optimal ionomer-to-catalyst ratio (I/C) − value of 0.58 g · g 1 was obtained (contrast with 0.43 for Pt/HSAC). The ECA; ORR mass and specific activities; number of electrons 1.0 V 2 s/cycle transferred during the ORR; and the Tafel slopes for the ORR were measured for both catalysts at their optimal I/C ratios. Initial hold potential Figs. S4–S7 and accompanying discussion in SI Text describe the results in more detail. Briefly, the Pt/TRO had lower ECA and Potential (V vs. RHE) Potential (V Open Open mass activities, but a higher specific activity than Pt/HSAC due circuit circuit to the larger platinum particle size (4–6 nm) in Pt/TRO. The number of electrons transferred during the ORR was estimated Time from a Levich plot to be 3.9 for Pt/HSAC and 3.2 for Pt/TRO; the Tafel slopes estimated from Koutecky–Levich analysis were Load cycling protocol 80 and 94, respectively. The ECA, mass activity (im), and specific activity (is) of Pt/TRO and Pt/HSAC were then estimated for Catalyst durability –Pt dissolution both catalysts upon exposure to the start–stop protocol ex situ Open Open (Fig. 2). The TRO support showed much better stability than circuit circuit carbon. The loss in ECA, mass activity, and specific activity after 3s 3s 5,000 start–stop cycles were, respectively, 16%, 18%, and 3% for 0.95 V Pt/TRO. In comparison, Pt/HSAC was much more severely de- graded; its ECA dropped by 39%, mass activity dropped by 52%, 30s and specific activity dropped by 22%. These ex situ studies suggested that TRO was a very stable support and that Pt/TRO was indeed a much more stable electrocatalyst than Pt/HSAC, 0.60 V albeit perhaps less active due to the larger platinum particle size. 6 s/cycle

Potential (V vs. RHE) Potential (V In situ accelerated degradation tests were then performed on MEAs. Fig. 3 shows the polarization curves obtained (at 100% Initial hold potential RH) on MEAs prepared with Pt/TRO and Pt/HSAC before and

after exposure to the start–stop protocol, in situ. There are two CHEMISTRY Time significant observations to note. First, despite the larger Pt par- Fig. 1. Start–stop and load-cycling protocols used to evaluate the stability ticle size, and concomitantly lower ECA and mass activity as of the support and catalyst, respectively. ascertained by ex situ RDE tests, the Pt/TRO electrocatalyst yielded an initial MEA performance that was slightly lower (especially at lower current densities) to that obtained with an glassy carbon rod counter electrode, and a hydrogen reference established benchmark (please note that this benchmark per- electrode. Both CV (at a scan rate of 50 mV/s) and linear po- formance is very much in line with industry standards and is − larization (scan rate of 10 mV · s 1, various rotation rates) were among the best available today). This result indicated that the performed at room temperature for ORR evaluation. The in situ experiments were performed in a 25 cm2 single fuel cell. MEAs were prepared using a Nafion 211 membrane, with − anode catalyst loading of 0.4 mg · cm 2 Pt/HSAC and cathode catalyst loading of 0.35 mg/cm2. The experiments were per- − formed at 80 °C, passing hydrogen (0.5 L · min 1) through the anode (counter and pseudoreference electrode) side and nitrogen − through the cathode/working electrode (0.5 L · min 1). The gases were humidified at either 100% relative humidity (RH) or 40% RH before entry into the cell. The 100% RH operating point was chosen to maximize carbon corrosion during the accelerated test (at high voltage, carbon corrosion requires water). The 40% RH condition was chosen as a possible operating point for the fuel cell stack in an automobile. CV and V–I polarization curves were obtained at the beginning and end of the potential cycling tests for each of the MEAs tested. The V–I polarization curves were obtained at 100% and 40% relative humidities, using hydrogen as fuel and air as oxidant. Initially, several preliminary experiments were performed to ascertain whether the in situ and ex situ approaches yielded similar results. Without a doubt, both methods yielded near Fig. 2. Comparison of ECA and ORR specific (is) and mass (im) activity for Pt/ identical results in terms of induced loss in ECA upon exposure HSAC and Pt/TRO catalysts before (BoL) and after (EoL) the start–stop pro-

to said protocols. Subsequently, both catalysts were exposed to tocol (ex situ experiments, performed on a RDE, in 0.1 M HClO4 electrolyte the start–stop protocol ex situ, and the impact of this test on the saturated with N2 at 60 °C).

Parrondo et al. PNAS | January 7, 2014 | vol. 111 | no. 1 | 47 Downloaded by guest on September 24, 2021 Both in situ and ex situ experiments confirmed that TRO (TiO2–RuO2) is an exceptionally stable catalyst support, and that Pt/TRO is an exceptionally stable electrocatalyst that yields initial (and final) fuel cell performance slightly lower (especially at lower current densities) to the best benchmark commercial Pt/ HSAC catalyst. We report herein an alternate catalyst support to carbon that demonstrates both excellent (equivalent to benchmark) initial performance, and such exceptional stability upon exposure to an extremely stringent accelerated test that has been confirmed by automotive OEMs to accurately induce degradation and failure mechanisms seen during extended normal operation with regular start–stop cycles. A Note on Choice of Benchmark Used One might argue that HSAC is perhaps not the best support to use as a comparator, and that more stable carbon supports exist. We believe the latter is indeed true. However, the Pt/HSAC catalyst used here represents an internal industry benchmark in Fig. 3. Comparison of fuel cell performance at 100% RH obtained with Pt/ terms of performance. We used it to illustrate the exceptional HSAC and Pt/TRO before and after exposure to the start–stop protocol (1,000 initial (and as it turns out) final performance of the Pt/TRO cycles). Experiment performed in a 25-cm2 single fuel cell at 80 °C and 100% electrocatalyst. Furthermore, we have also subjected many other, − − RH. Catalyst loadings: anode, 0.40 mg Pt · cm 2; cathode, 0.35 mg Pt · cm 2; I/C more stable, carbon supports and derivative electrocatalysts to ratios, optimal for each electrode; polymer electrolyte membrane, Nafion similar test protocols—all of them show unacceptable losses in 211. Closed symbols: beginning of life (BoL); open symbols: end of life (EoL). performance upon exposure to the start–stop protocol. A Note on Costs and Viability Pt/TRO catalyst was very much viable in terms of catalytic activity and performance. Second, and even more significantly, An argument is frequently made that the presence of ruthenium whereas the Pt/HSAC MEA revealed a very significant (and in the catalyst support is undesirable because of its high cost most likely catastrophic) loss in performance, the Pt/TRO showed relative to carbon (which is very inexpensive). This project was minimal loss in performance upon exposure to 1,000 start–stop performed with active participation from Nissan Technical − cycles. The loss in cell voltage at 1 A · cm 2 at 100% RH was only Center, North America, an automotive OEM that is well aware ∼15 mV for Pt/TRO, whereas the corresponding loss was too of the cost factor. Our cost modeling has revealed that regardless − high to permit operation at 1 A · cm 2 for Pt/HSAC; this MEA of the support used, over 95% of the cost of the catalyst origi- − failed at a current density of ∼0.4 A · cm 2. The 40% RH data, nated from the use of platinum as the active electrocatalyst. shown in Fig. 4, revealed a similar trend in terms of stability— Moreover, when the much-enhanced stability of TRO is taken exceptional stability for Pt/TRO as opposed to very poor stability into account, the Pt/TRO electrocatalyst is more than viable for Pt/HSAC. These observations were attributed to the much from a cost perspective, compared with Pt/HSAC (or any other higher stability of the TRO support compared with HSAC. MEAs equivalent carbon support). We would be happy to share this prepared with each catalyst were then exposed to the load-cycling analysis with any interested reader. protocol. Though a significant loss in ECA was observed in each case, consistent with platinum dissolution and agglomeration, there was minimal impact on performance (see Fig. S8 and accompanying discussion in SI Text. This result suggested that the stability of the support was much more important, from the context of cell and stack failure, than the stability of the platinum particles that are loaded onto the support. Finally, we measured the carbon dioxide concentration in the cathode exit stream during the accelerated degradation test (start–stop protocol) and found extremely low levels of CO2 (between 3 and 10 ppm) in the case of Pt/TRO (Fig. 5). In contrast, the CO2 emission levels from a conventional Pt/HSAC catalyst were ∼200 ppm. Of course, the Pt/TRO is carbon-free, and no CO2 emission would emanate from this material. This observation was, however, a clear indicator that the main source of carbon being oxidized to carbon dioxide in an MEA was the carbon catalyst support, and not the gas diffusion layer (GDL) or the graphite flowfields. Both MEAs in this study used identical GDLs and flowfields. The small amount of CO2 observed in the MEA prepared with Pt/TRO arose almost certainly from the corrosion of carbon in the microporous layer of GDL. We Fig. 4. Comparison of fuel cell performance at 40% RH obtained with Pt/ believe this to be a unique method to quantify the corrosion rate – (in situ) of the carbon in the GDL microporous layer. We pro- HSAC and Pt/TRO before and after exposure to the start stop protocol (1,000 cycles). Experiment performed in a 25-cm2 single fuel cell at 80 °C and 40% − − pose that the Pt/TRO catalyst can be used in the future in con- RH. Catalyst loadings: Anode: 0.40 mg Pt · cm 2; cathode: 0.35 mg Pt · cm 2; I/C junction with carbon dioxide monitoring to measure the corrosion ratios: Optimal for each electrode; Polymer electrolyte membrane: Nafion rate of candidate GDLs. 211. Closed symbols: beginning of life (BoL); Open symbols: end of life (EoL).

48 | www.pnas.org/cgi/doi/10.1073/pnas.1319663111 Parrondo et al. Downloaded by guest on September 24, 2021 protocol, both at 100% RH and at 40% RH. The voltage loss was − on the order of 10–15 mV at 1 A · cm 2, which is eminently ac- ceptable given the harshness of this accelerated test. On the contrary, the voltage losses were too high to permit operation at − − anywhere close to 1A · cm 2 (or even 0.5 A · cm 2) for Pt/HSAC at either 100% or 40% RH, a result that would equate to catastrophic failure if it were to occur in an automotive fuel cell stack. Despite the larger Pt particle size in Pt/TRO, and concomitantly lower ECA and mass activity as ascertained by RDE tests, the Pt/TRO electrocatalyst yielded initial MEA performance slightly lower (especially at lower current densities) to that obtained with the benchmark Pt/HSAC electrocatalyst. Because the beginning-of-life and end-of-life performances were nearly identical, we can state that we have identified in Pt/TRO an electrocatalyst that meet durability targets for automotive fuel cell stacks. We note that by using appropriate processing methods to lower Pt particle size, we can further enhance the activity of the Pt/TRO, and we are working toward this now. When the catalysts were subjected to the load-cycling protocol for 10,000 cycles, there was a significant loss in ECA observed in Fig. 5. Evolution of carbon dioxide in the cathode exit stream during the in each case, consistent with platinum dissolution and agglomera- situ support durability test (start–stop protocol) for Pt/HSAC and Pt/TRO. tion, and this was in line with expectations. Though TRO is a corrosion-resistant catalyst support, it was not designed to mitigate Pt dissolution (which would be rather difficult given the Conclusions facility of this process). However, most interestingly, there was The electrochemical stability of a unique electrocatalyst—platinum minimal detrimental impact on performance (for either catalyst) supported on a highly stable TRO support—was investigated using despite the large losses seen. This result suggests that the stability both ex situ (RDE) and in situ (MEA) tests. The accelerated test of the support is far more important than the stability of the protocols used to evaluate stability were carefully designed in platinum particles that are loaded onto the support in terms of conjunction with Nissan Technical Center, North America. The avoiding stack failure. The fuel cell stack is likely to be much start–stop protocol mimicked the potential transients that are more forgiving of platinum dissolution and agglomeration during observed during fuel cell stack startup and shutdown and that load cycling than of support corrosion and related effects arising contribute to severe electrocatalyst support corrosion, whereas the from startup and shutdown cycles. In conjunction with the fact load-cycling protocol mimicked potential transients seen during that hybridization methods will inevitably be used to minimize full-load to no-load transitions that are sometimes encountered load cycling, we posit that identifying a corrosion-resistant sup- CHEMISTRY during fuel cell operation, and that contribute to platinum port is a key priority, and we offer TRO as one such outstanding dissolution. corrosion-resistant support that also yields highly active elec- Ex situ studies were performed in a rotating ring electrode trocatalysts with excellent performance. using electrocatalyst inks with individually optimized ionomer to Finally, we measured the carbon dioxide concentration in the − − catalyst ratios of 0.58 g · g 1 for Pt/TRO and 0.43 g · g 1 for Pt/HSAC. MEA cathode exit stream during the accelerated start–stop The initial ECSA and mass activities were lower for Pt/TRO than degradation tests for both catalysts. The CO2 emission levels for Pt/HSAC, which was attributed to the larger platinum particle from the Pt/HSAC catalyst was ∼200 ppm. In contrast, extremely size on TRO. The lower ECSA also resulted in a higher specific low levels of CO2 (between 3 and 10 ppm) were observed in the activity for Pt/TRO (we note although that for practical purposes, case of Pt/TRO. This CO2 was clearly a result of carbon corro- the mass activity is more relevant). The number of electrons sion in the gas diffusion layer and/or the bipolar plates (TRO has transferred for the ORR was estimated from Levich plots to be no carbon). This finding provides a quantitative measure of the 3.2 for Pt/TRO and 3.9 for Pt/HSAC. The Tafel slopes, derived extent of carbon corrosion arising from sources other than car- from a Koutecky–Levich analysis, were 94 mV per decade and bon support, and confirms that the carbon in the gas diffusion 80 mV per decade respectively, for Pt/TRO and Pt/HSAC. layer does not corrode anywhere near the same extent as the The Pt/TRO was much more stable that Pt/HSAC by all carbon electrocatalyst support. measures. After 5,000 start–stop cycles, the loss in ECSA was For further information on detailed materials and methods, 16% for Pt/TRO compared with 39% for Pt/HSAC. The loss in see SI Text, which contains the following: RDE electrode prep- ORR mass and specific activities were 18% and 3% for Pt/TRO, aration and protocols used in the experiments; transmission compared with 52% and 22% for Pt/HSAC. These ex situ electron microscope micrographs of the Pt/TRO catalyst; cyclic screening studies indicated that the Pt/TRO was an electro- voltammograms obtained on 40% Pt/TRO and 50% Pt/HSAC catalyst with exceptionally high stability, but lower activity as a catalysts tested ex situ (RDE) using the start–stop protocol; result of larger platinum particle size. optimization of ionomer (Nafion) to catalyst weight ratio for Though ex situ tests are a useful way to screen new candidate 40% Pt/TRO; cyclic voltammograms in nitrogen-degassed 0.1 M materials, the true viability of an electrocatalyst can only be perchloric acid and ORR polarization curves obtained in oxygen- ascertained from in situ tests in a MEA; these were therefore saturated 0.1 M perchloric acid for 40% Pt/TRO and 50% Pt/ performed. MEA tests demonstrated both the high performance HSAC catalysts; comparison of ECA and activity toward the and, more importantly, the exceptional stability of the Pt/TRO ORR of 40% Pt/TRO and 50% Pt/HSAC catalyst; Levich plot, electrocatalyst. Polarization curves (obtained at 100% and 40% Koutecky–Levich plot, and Tafel slope for 40% Pt/TRO; Levich RH) on MEA prepared with Pt/HSAC revealed a very significant plot, Koutecky–Levich plot, and Tafel slope for 50% Pt/HSAC; loss in performance upon exposure to 1,000 cycles of the start– and fuel cell performance of 40% Pt/TRO and Pt/HSAC catalyst stop protocol. The MEA with Pt/TRO showed very little change upon exposure to the load-cycling protocol (80 °C and 40% rel- in performance upon exposure to 1,000 cycles of the start–stop ative humidity) for 10,000 cycles.

Parrondo et al. PNAS | January 7, 2014 | vol. 111 | no. 1 | 49 Downloaded by guest on September 24, 2021 ACKNOWLEDGMENTS. We thank Dr. Chih-Ping Lo for preparing and Department of Energy Office of Energy Efficiency and Renewable Energy characterizing several samples used in this work. Funding was provided by Grant DE-EE-0000461.

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