Electrochemical Surface Area Loss Research Articles

Degradation Mechanisms of NaTi2(PO4)3 Aqueous Anodes and Mitigation Techniques

Aqueous rechargeable alkali-ion batteries offer an attractive alternative to Li-ion batteries for large scale installations because of their lower cost and higher degree of safety. However, with the exception of activated carbon anodes, demonstrated chemistries have generally showed poor cycle stability. Understanding the failure mechanisms of aqueous anode materials is of vital importance for commercialization of these batteries. In this study we have investigated three primary mechanism for capacity loss in NaTi2(PO4)3 (NTP) based anodes (1) dissolution of active material at high pH, (2) loss of active surface area due to gas entrapment from hydrogen evolution, and (3) alkaline oxidation of the carbon conductive additive. Inductively coupled mass spectrometry (ICP-MS) was used to quantify the solubility of NTP as a function of pH and helps explain the formation of a secondary reduction peak during extended low C-rate cycling. This degradation mechanism has been mitigated using a conformal polydopamine based carbon coating. Gas chromatography was used to quantify the production of H2, CO, and CO2 due to hydrogen evolution and alkaline oxidation of carbon. Electrochemical cycling followed by gas removal and re-infiltration of the porous electrodes was used to quantify the loss of electrochemical surface area due to gas entrapment and is found to be the primary cause of apparent capacity loss. A composite of the NTP electrode with a PTFE bound activated carbon was found to be effective in preventing gas entrapment with in the porous electrode.

Dissolution of NaTi2(PO4)3 in Alkaline Solutions and Its Implications for Use As an Aqueous Anode

There are two primary qualities batteries used for grid-scale storage should have (1) a low levelized cost of energy storage (LCOE) or a low $/kWh delivered over the lifetime of the battery and (2) a high degree of safety which includes low toxicity, low flammability, and low corrosiveness. Aqueous rechargeable alkali-ion batteries offer a promising alternative to Li-ion batteries for grid-scale electrochemical energy storage installations because of their lower cost of and higher safety. However, with the exception of systems that utilize activated carbon based anodes, demonstrated chemistries show significant capacity fade. Although a decent amount of research has been done on failure mechanism of intercalation cathodes in aqueous solution, few studies have been carried out to elucidate the role of the anode material in battery failure. Sodium super ionic conductor (NASICON) type NaTi2(PO4)3 is one of the most promising materials for aqueous anodes. It has good chemical stability, a high theoretical specific capacity of 133 mAh/g, low cost of manufacture, and a favorable redox potential. However, it is often reported to be cycled in neutral aqueous solution, which when given its low redox potential implies there will be some amount of water decomposition occurring. Water decomposition occurs due to both direct electrochemical hydrogen evolution, as well as direct chemical oxidation of the fully sodiated NaTi2(PO4)3. In this work we have set out to quantify the inherent chemical stability of NaTi2(PO4)3 in alkaline solutions using inductively coupled mass spectrometry (ICP-MS), x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), cyclic voltammeter (CV), and in-situ pH measurements. It was found that at pH > 11 dissolution begins to occur but does not become significant until pH > 12.7. At the highest tested pH an unidentified phase with an elongated morphology becomes thermodynamically favorable and begins to precipitate from solution. This secondary phase acts as a sink for dissolved titanium allowing for the continual dissolution of NaTi2(PO4)3 rather then eventual saturation of solvated titanium species. The in-situ pH measurement indicates that upon cycling in neutral solution the electrode undergoes a pH swing from lower pH to higher pH due to the generation of OH- ions, followed by their diffusion out of the electrode. This increase followed by decrease in pH exacerbates the dissolution of NaTi2(PO4)3 and leads to a continual dissolution precipitation reaction. The combination of FTIR and XRD indicate that these precipitated phases were a mixture of layered titantes of the form NaxH2-xTinO2n+1-yH2O. Although NaTi2(PO4)3 appears to be susceptible to dissolution at higher pH, the measured dissolution does not account for the overall capacity fade seen upon cycling. The authors are currently investigating the effect of gas generation and entrapment on the loss of electrochemical surface area and its effect on the apparent capacity fade.

IL-FE-SEM and ICP-MS Study for Degradation of Pt/C Catalyst

The carbon black-supported Pt nanoparticle catalyst (Pt/C) is a representative cathode catalyst of polymer electrolyte fuel cell (PEFC). Unfortunately, Pt/C deteriorates with loss of the electrochemical surface area (ECSA) under potential cycling operations and start-stop cycles, and is one of the major reasons for the deterioration of cell performance. Several degradation mechanisms have been considered to be responsible for the loss of ECSA[1] and then, we have been investigating the degradation mechanisms of individual Pt particles in Pt/C by “identical location field emission scanning electron microscopy (IL-FE-SEM)”, which is our original technique to visualize as well as monitor the change of Pt nanoparticles at nanometer scale, in order to quantitative analysis the degradation process as well as design durable catalyst[2]. In this presentation, the potential pulse acceleration degradation test (ADT) of Pt/C was carried out in HClO4 solution, and simultaneously the degradation of individual Pt particles was monitored by using IL-FE-SEM. Furthermore, Pt dissolution amount in the solution was also measured by using inductively coupled plasma-mass spectrometry (ICP-MS) to discuss the material balance owing to the deterioration of Pt/C.Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo, Pt weight = 46.7 wt%) was dispersed on a glassy carbon disk, and then Nafion thin film was formed on the catalyst (calculated thickness = 80 nm), which was employed as the test electrode. FE-SEM observation was carried out and the observation point was defined for IL-FE-SEM. After that, the test electrode was transferred to a glass half-electrochemical cell filled with 0.1 mol dm-3 HClO4 aqueous solution. Potential pulse ADT (0.6 V - 1.0 V, pulse time = 3 s, recommended by Fuel Cell Commercialization Conference of Japan; FCCJ) was carried out under N2 atmosphere at 60oC. IL-FE-SEM observation and cyclic voltammetry measurements were carried out after an arbitrary test period. We randomly selected 750 pieces of Pt nanoparticles to investigate the degradation mechanism. ICP-MS analysis was also carried out by sampling a tiny amount of the electrolyte solution during the ADT.Figure 1 shows typical IL-FE-SEM images of Pt/C catalyst taken during ADT. Individual (specific) Pt particles in Pt/C were clearly monitored. The typical morphological changes were observed for Pt particles such as shrinkage and growth of the particles, disappearance, migration, coalescence, precipitation on carbon support. The typical degradation processes were observed for Pt particles such as shrinkage and growth of the particles, disappearance, migration, coalescence, precipitation on carbon support. The ECSA decreased to 65% after 10,000th pulse. The total number of Pt particles was degreased from 750 to 697, showing the number of Pt particles is surely decreased during the ADT. Among the degradation processes, disappearance and shrinkage were recognized as main process, suggesting that dissolution of Pt should be severely taken place under the ADT. It was found from ICP-MS results that 2% of Pt was dissolved out from Pt/C catalyst after 10,000th pulse. In this presentation, the material balance of Pt during the ADT will be also discussed based on IL-FE-SEM and ICP-MS. Y. Shao-Horn, et al., Top. Catal., 46, 285 (2007). T. Kinumoto, et al., Electrochemistry, 83, 12 (2015). Figure 1

Innovative carbon-free low content Pt catalyst supported on Mo-doped titanium suboxide (Ti3O5-Mo) for stable and durable oxygen reduction reaction

Recently the use of titanium oxide and titanium suboxides (Magnéli phases) has been extensively investigated as an alternative catalyst support to carbon-based materials for the oxygen reduction reaction in low-temperature fuel cells. In this study, a 15wt.% Pt-based catalyst was developed on a unique, stable mix of titanium suboxides, with a prevailing of the Ti3O5 phase, doped with Mo, as a Ti3O5-Mo carbon-free support and compared to a commercial 20wt.% Pt/C (E-TEK). The Pt/Ti3O5-Mo catalyst exhibits excellent electroactivity and stability toward the ORR, reaching a performance of 73.3mAmg−1, slightly more than the double of the commercial Pt/C, with a current density of 1.1mAcm−2 at 0.9V vs RHE, and an half-wave potential of 0.86V vs RHE. A deep accelerated potential cycling between 0 and 1.2V vs RHE up to 5000 cycles demonstrated the remarkable stability of the Pt/Ti3O5-Mo catalyst, whose electrochemical surface area loss was accounted for only 11%, compared to the more than 81% loss of the commercial Pt/C reference

Analyzing the influence of high electrode potentials on intrinsic properties of catalyst coated membranes using impedance spectroscopy

Abstract Catalyst layers (CLs) with varying ionomer contents are produced using a stencil coating and screen printing technique. The optimum ionomer content of 31−34 wt% confirms the findings of other groups and performance is found to be independent of production technique. A new CL impedance transition line model is developed and fitted to in-situ data. The results indicate that the protonic contact resistance between CL and membrane is an important factor for the used transfer-decal process, especially for CLs with low ionomer loading. When subjected to potentials higher than 1.2 V, an increased performance is observed for low ionomer loading CLs. It is found that by applying the high potential to the electrode a significantly increased proton conductivity is counteracting and superimposing the loss of electrochemical surface area (ECSA) due to carbon corrosion. After aging, the performance of the 15 wt% CL is at the same level as the 31−34 wt% ionomer content CLs at the beginning of life, even though the ECSA is reduced due to carbon corrosion or platinum dissolution. The findings indicate that for the optimization of the ionomer loading, either the changing wetting properties or the redistribution of ionomer during lifetime have to be taken into account.

(Invited) The Influence of the Carbon Support and Platinum Particles Size on Degradation of Cathode for Low Platinum-Loading Catalyst Layer

We demonstrated the ability to control the platinum particle size in the synthesis of carbon supported Pt (Pt/C) electrocatalyst with reactive spray deposition technology (RSDT). The degradation of Pt cathode with loading of 0.1 mg/cm2 in membrane electrode assembly (MEA) under simulated drive cycles (30000 triangular wave cycles between 0.6 and 1.0 VRHE) showed similar electrochemical surface area (ECSA) at the end of life (EOL), regardless of the initial Pt particle size or carbon support type. However, it is possible to reduce the loss of ECSA using a gradient catalyst layer consisting of a 6 mm layer with Pt particles of 5 nm toward the membrane and another 6 mm layer with Pt particles of 2 nm toward the GDL. Microscopy studies suggested that the gradient catalyst layer have less Pt loss% into the electrolyte, thereby maintained higher ECSA at EOL.

N-Doped Reduced Graphene Oxide Composites for Hydrogen Generation in Presence of Chloride Ion Impurities in Alkaline Medium

The depletion in fossil fuel reserve leads to the search for a clean, alternate fuel which can replenish the energy demand without causing greater harm to the environment. Hydrogen is one such kind of promising sustainable energy fuel which can be produced from very cheap resources like water. The major hurdle behind the efficient and economical production of hydrogen through electrochemical conversion of water is the large thermodynamic overpotential related to it. Enormous volume of researches has been carried out towards the development of catalysts, improving support materials efficiency and electrochemical environment related to hydrogen evolution reaction (HER). In this work we are going to present a catalyst which shows appreciable enhancement in the electrochemical activity towards HER in alkanine medium and can withstand very high chloride ion impurities in operating conditions. Often, the performance and durability of a fuel cell are greatly influenced by the presence of impurities present in the feed stock, air stream or other components of fuel cell. Among the different contaminants chloride (Cl-) ion is one of the serious impurities which can diffuse through the gas diffusion system and can poison the electrocatalysts and membrane and ultimately leads to the decrease performance of the fuel cell system. Hence it is important to develop a robust catalyst which will have dual functionalities. Along with enhanced performance, it also should be able to withstand the extreme poisoning effect during hydrogen evolution reaction. In comparison to un-doped reduced graphene oxide (RGO)–Pt composites and commercial Pt /C catalysts, the nitrogen-doped graphene oxide (NGO)-Pt composites show low overall potential towards HER both in acidic and alkaline environment. Electro-catalytic performance of the prepared cathodes has been investigated by hydrogen evolution reaction in 0.1M perchloric acid, 0.5 M sulfuric acid and 0.1molar KOH solution. The controlled exposure of chloride ion impurities shows under similar condition NGO-Pt shows better stability and less reduction of electrochemical surface area (ESA) and hence less susceptible towards chloride poisoning under identical conditions. The loss of electrochemical surface area (ESA), before and after chloride poisoning, is much less in case of NGO-Pt . Figure 1 shows the comparison between the RGO-Pt,commercial-Pt and NGO-Pt for loss of ESA. Figure 1

Study of Stability of Ultrathin Pt Films on Titanium Nitride, Highly Oriented Pyrolytic Graphite and Sigradur G Glassy Carbon Substrates: The Role of Substrate Type and Catalyst Loading on the Degradation Mechanism

We studied degradation mechanism of platinum catalysts films of ultralow loading on titanium nitride, highly oriented pyrolytic graphite and Sigradur G glassy carbon substrates. The films were prepared by magnetron sputtering with platinum amount nominally corresponding to 1, 5 and 10 layers. The catalyst stability was tested by potential cycling in perchloric acid. We find that dependence of catalyst stability on the substrate type is very much function of catalyst loading. This is a result of different catalyst degradation mechanisms. For samples with 1 platinum layer we observe that three dimensional crystallites are formed from two dimensional initial nanoislands and degradation mechanism follows power laws derived for three dimensional nanoparticle growth; for samples with 5 platinum layers we observe formation of interconnected network of two dimensional islands. The electrochemical surface area loss does not depend much on the substrate type and the degradation mechanism follows power law developed for two dimensional particle growth. Catalyst films made by deposition of 10 platinum layers are continuous films on all three studied substrates and after potential cycling, the films stay continuous with small difference of the film stability for studied substrates. For studied platinum loadings carbon substrates performed better than titanium nitride substrate.

Huge Instability of Pt/C Catalysts in Alkaline Medium

The stability of carbon-supported electrocatalysts has been largely investigated in acidic electrolytes, but the literature is much scarcer regarding similar stability studies in alkaline medium. Herein, the degradation of Vulcan XC-72-supported platinum nanoparticles (noted Pt/C), a state-of-the-art proton exchange membrane fuel cell electrocatalyst, is investigated in alkaline medium by combining electrochemical measurements and identical location transmission electron microscopy; electrochemical surface area (ECSA) losses were bridged to electrocatalyst morphological changes. The results demonstrate that the degradation in 0.1 M NaOH at 25 °C is severe (60% of ECSA loss after only 150 cycles between 0.1 and 1.23 V vs RHE), which is about 3 times worse than in acidic media for this soft accelerated stress test. Severe carbon corrosion has been ruled out according to Raman spectroscopy and X-ray photoelectron spectroscopy measurements, and it seems that the chemistry of the carbon support (in particular, the interface (chemical bounding)) between the Pt nanoparticles and their carbon substrate does play a significant role in the observed degradations.

Enhanced stability of multilayer graphene-supported catalysts for polymer electrolyte membrane fuel cell cathodes

One of the biggest challenges in the field of polymer electrolyte membrane fuel cells (PEMFC) is to enhance the lifetime and the long-term stability of PEMFC electrodes, especially of cathodes, furthermore, to reduce their platinum loading, which could lead to a cost reduction for efficient PEMFCs. These demands could be achieved with a new catalyst support architecture consisting of a composite of carbon structures with significant different morphologies. A highly porous cathode catalyst support layer is prepared by addition of various carbon types (carbon black particles, multi-walled carbon nanotubes (MWCNT)) to multilayer graphene (MLG). The reported optimized cathodes shows extremely high durability and similar performance to commercial standard cathodes but with 89% lower Pt loading. The accelerated aging protocol (AAP) on the membrane electrode assemblies (MEA) shows that the presence of MLG increases drastically the durability and the Pt-extended electrochemical surface area (ECSA). In fact, after the AAP slightly enhanced performance can be observed for the MLG-containing cathodes instead of a performance loss, which is typical for the commercial carbon-based cathodes. Furthermore, the presence of MLG drastically decreases the ECSA loss rate. The MLG-containing cathodes show up to 6.8 times higher mass-normalized Pt-extended ECSA compared to the commercial standard systems.

Pt Electrocatalyst Supported on a 3D-Nanoporous Carbon Shows a High Performance in a High-Temperature Polymer Electrolyte Fuel Cell

Considerable attention has been focused on the high-temperature PEFCs (> 100 oC), due to their benefiting PEFCs with higher carbon monoxide (CO) tolerance, faster electrochemical kinetics, and better water management than those of conventional PEFCs. However, the high-temperature PEFCs also suffer from their low durability in terms of carbon corrosion and platinum nanoparticles (Pt-NPs) aggregation resulting in the detachment of the Pt-NPs from the catalysts and loss in electrochemical surface area (ECSA) as well as the degradation of FC performance. Thus, enhancement in durability is highly demanded for the commercialization of the next-generation PEFCs. We have already reported that poly[2,2’-(2,6-pyridine)-5,5’-bibenzimidazole] (PyPBI) is an efficient dispersant for carbon supporting materials and provides the anchor sites for the Pt-nanoparticles (NPs) as well as creating the so-called three-phase boundary structure[1-7]. In this paper, we used a 3D nanoporous carbon (NanoPC) with a high specific surface area of 1,037 m2/g as a carbon support for high-temperature polymer electrolyte fuel cell, and fabricated an electrocatalyst (NanoPC/PyPBI/Pt) having platinum nanoparticles of ca. ~2.2 nm diameter deposited on the NanoPC that was wrapped by poly[2,2’-(2,6-pyridine)-5,5’-bibenzimidazole] (PyPBI). Even after 10,000 start-up/shutdown cycles in the range of 1.0 to 1.5 V vs. RHE, the NanoPC/PyPBI/Pt showed almost no loss in electrochemical surface area (ECSA), which was much higher durability than those of a CB/PyPBI/Pt (~32%-loss), in which conventional CB was used in place of the NanoPC, and conventional CB/Pt (~46%-loss). FC performance of the assembled MEAs was evaluated at 120 ºC without any external humidification using a computer-controlled fuel cell test system. The polarization and the power density curves were measured under the atmospheric pressure by flowing dry hydrogen (flow rate; 100 mL/min) and dry air (flow rate; 200 mL/min) at the anode and the cathode, respectively. The power density of the NanoPC/PyPBI/Pt was 342 mW/cm2, which was higher than those of the CB/PyPBI/Pt (183 mW/cm2) and CB/Pt (115 mW/cm2). The present study provides useful information for the preparation of an electrocatalyst with a high durability and performance in high temperature PEFCs.

Development of Accelerated Stress Tests for Polymer Electrolyte Membrane Fuel Cells

The durability of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is one of the key barriers to their commercialization in automotive applications.1 The development of accelerated stress tests (ASTs) is a critical step in the evaluation and improvement of the durability of PEMFCs. The availability of validated component ASTs can lead to the accelerated development of materials with improved durability. However the ASTs need to faithfully reproduce the degradation mechanisms of interest and not introduce new degradation modes while attempting to accelerate the degradation rates. Furthermore, the acceleration factors with respect to the intended application need to be determined accurately to provide manufacturers with a valuable tool that enables informed decision making regarding cost, performance and durability. For example the DOE – Fuel Cell Technologies program has set lifetime durability targets of 5500 hours for automotive fuel cells and the U.S. Drive Fuel Cell Technical Team (FCTT) has developed AST protocols for the various components.2,3 In this paper we will compare the degradation observed during the ASTs to the degradation observed during simulated drive cycle operation and report on the acceleration factors observed for various materials. We will also propose new ASTs that have greater acceleration factors than the reported FCTT ASTs while retaining the degradation mechanisms of interest. The FCTT has recommended potential cycling from 0.6 V to 1.0 V at 50 mV/sec as the electrocatalyst AST. This potential cycling is performed at 80 °C in 100% RH with H2 (anode) and N2 (cathode) and results in Pt particle growth and loss in electrochemical surface area (ECSA). This loss in ECSA and associated performance loss correlates well to the degradation observed in the field and during simulated drive cycle operation. Figure 1 compares the degradation observed during this AST (Old AST) to that observed during the wet portion of the FCTT: Drive cycle (current cycling from 0.02 to 1.2 A/cm2 at 80 °C and saturated conditions). It is seen that this AST has a 5X acceleration factor for 3 different MEAs that were evaluated. In order to further accelerate this AST, we modified the triangle wave to a square wave and lowered the upper potential to 0.95 V. This new AST consisted of a square wave with upper and lower potentials of 0.95 V and 0.6 V with 3 seconds duration, and was based on literature reports.4 This New AST (Fig. 1) demonstrated a 100X acceleration factor compared to the drive cycle which is a 20 times improvement over the old AST. Moreover, this new AST retained the Pt growth mechanism while minimizing carbon corrosion. The FCTT has 2 recommended ASTs for membranes with one focusing on chemical degradation (OCV hold at 90 °C and 30% RH) and another focusing on mechanical degradation (RH cycling in Air). Our previous results have indicated that both these ASTs are not representative of membrane failure in the field.5 In this paper we will present a new AST that combines the mechanical and chemical degradation and is representative of field failure. This AST consists of cycling the RH between saturated and dry conditions in a H2/Air atmosphere at 90 °C. The duration of the wet and dry cycles are 30s and 45s respectively. The fluoride release rate observed during this AST using DuPont XL® membranes, illustrated in Figure 2, is comparable to the chemical AST and higher than in the mechanical AST.

Enhanced Durability with Cu-Containing Chemically Ordered Pt-Alloy Catalyst for Oxygen Reduction Reaction in Polymer Electrolyte Fuel Cell

Large-scale commercialization of polymer electrolyte fuel cells (PEFCs) requires great improvement in activity and durability of the cathode catalyst [1]. We have reported enhanced activity and durability of the Pt-based intermetallics, Pt2FeCo/C, with L10 ordered face-centered tetragonal (fct) structure both in acidic solution [2] and as the cathode catalyst of a membrane-electrode assembly [3]. In this work, Cu with higher standard reduction potential was employed to form ordered fct-Pt2FeCu/C, and its activity of oxygen reduction reaction (ORR) and durability were compared with its counterpart bimetallic catalyst, fct-PtFe/C, and the commercial catalyst from Tanaka Kikinzoku Kogyo (TKK-Pt/C). Pt2FeCu/C and PtFe/C were fabricated by the heat treatment of the mixture of metallic precursors and carbon black at 800 °C under 20% H2 in N2 for 2 h. The metal content of the catalyst was around 40 wt%. ORR activity of the catalysts was evaluated in 0.1 M HClO4solution using the rotating disk electrode technique. The durability of the catalysts was examined in the same solution at 60 °C by applying square wave potential cycling, 0.6 V for 3 s and 1.0 V for 3 s, according to load cycle durability testing protocol recommended by the Fuel Cell Commercialization Conference of Japan (FCCJ). XRD patterns of Pt2FeCu/C and PtFe/C showed the formation of chemically ordered fct structures. Both the fct catalysts initially showed high mass activity above 0.5 A/mg-Pt. Figure 1 shows the change in electrochemical surface area (ECSA) and mass activity during load-cycle durability test at 60 °C. Cu-containing fct-Pt2FeCu/C retained more than 70% of mass activity and ECSA at 10K durability cycles, while fct-PtFe/C and TKK-Pt/C catalysts showed a significant loss of activity and ECSA: retention was about 50% for TKK-Pt/C and 40% for fct-PtFe/C. The mass activity of fct-Pt2FeCu/C catalyst was enhanced by more than twice with respect to TKK-Pt/C catalyst, both at the initial state and after 10K durability cycles. STEM-EDX line-scans of the catalysts before and after durability cycles showed that dissolution of Fe and Cu was much less in fct-Pt2FeCu/C than in fct-PtFe/C. The enhancement in the durability of the fct-Pt2FeCu/C catalyst may be ascribed to the synergistic effects of the presence of Cu with higher redox potential and the ordered structure of catalyst.

Instability Of Commercial Pt/C And Pd/C Electrocatalysts In Alkaline Media

Alkaline fuel cells are interesting alternatives to proton-exchange membrane fuel cells, owing to the possibility to feed them directly with complex fuels (such as ethanol, methanol, ammonia, hydrazine, sodium borohydride, etc.) with acceptable performances. In that frame, studying the stability of carbon-supported catalysts nanoparticles in alkaline media is mandatory, these properties having largely been investigated in acidic electrolytes in the literature so far. Whether some works have focused on general aspects of carbon stability in alkaline media, to date none of them have investigated the stability of metal nanoparticles on carbon supports. Here, we studied the degradation of commercial (E-TEK) Pt and Pd nanoparticle electrocatalysts supported on Vulcan XC-72 (noted Pt/C and Pd/C) in NaOH electrolyte solutions, and compared the fate of the electrocatalysts to that in H2SO4, HClO4electrolyte solutions. Electrochemical measurements, combined with Identical-Location Transmission Electron Microscopy (ILTEM) allowed to demonstrate that the catalysts degradation in 0.1 M NaOH at 25°C is extremely severe for a mild accelerated stress tests (AST – 150 cycles, at 100 mV sec-1 and 25°C, between 0.1 and 1.23 V vs.RHE). The AST performed directly on TEM grids (Gold + Lacey carbon) for Pt/C in Identical-Location Transmission Electron Microscopy (ILTEM) imaging (Fig.1.a-b) demonstrated extensive nanoparticles loss (either by dissolution or detachment from the carbon surface) and non-negligible extent of agglomeration. These observations were further bridged with the results obtained with the CO-stripping technique (Fig.1.c), which clearly highlight the appearance of a pre-peak associated to the Pt agglomerates, features that are generally observed for much harsher and/or longer degradation protocols and at higher temperature value in acidic media1,2,3. As a result, the electrochemical surface area (ECSA) losses are about 3 times worse in alkaline than in acidic media (60 % of ECSA loss vs. ca. 20% in H2SO4 and HClO4, Fig.1.d) for such a “mild” AST. The effect of the electrocatalyst material nature (Pd or Pt) and size effects on the degradation profile and rate have also been investigated. In these harsh degradation in alkaline medium, extensive carbon corrosion has been ruled out, according to Raman spectroscopy measurements that show nearly no sign of structural changes of the carbon, although such effect is often observed in acidic media4. Therefore, we attributed the huge changes of Pt/C and Pd/C morphology upon AST in alkaline medium to the modification of the carbon surface carbon chemistry, in agreement with XPS measurements, which simply destroys the anchoring sites of the Pt (resp. Pd) nanoparticles and favors their simple detachment from the carbon surface. We also cannot rule out cathodic corrosion of the Pt (resp. Pd) nanoparticles at the lower vertex potential of the AST, as experienced by the group of Koper 5. References (1) F. Nikkuni, E. Ticianelli, L. Dubau, M. Chatenet, Electrocatal., 4 (2013) 104-116. (2) L. Dubau, L. Castanheira, G. Berthomé, F. Maillard Electrochim. Acta, 110 (2013) 273– 281. (3) Meier, J. C., Galeano, C., Katsounaros, I., Witte, J., Bongard, H. J., Topalov, A. a, Mayrhofer, K. J. J. (2014). Beilstein Journal of Nanotechnology, 5, 44–67. (4) Dubau, L., Castanheira, L., Chatenet, M., Maillard, F., Dillet, J., Maranzana, G., (2014). International Journal of Hydrogen Energy, 39(36), 21902–21914. (5) Yanson, A. I.; Rodriguez, P.; Garcia-Araez, N.; Mom, R. V.; Tichelaar, F. D.; Koper, M. T. M. Angew. Chem. Int. Ed. 2011, 50, 6346. Figure 1

Durable Pt Electrocatalyst Supported on a 3D Nanoporous Carbon Shows High Performance in a High-Temperature Polymer Electrolyte Fuel Cell.

In this paper, we used a 3D nanoporous carbon (NanoPC) with a high specific surface area of 1037 m(2)/g as a carbon support for high-temperature polymer electrolyte fuel cell, and fabricated an electrocatalyst (NanoPC/PyPBI/Pt) having platinum nanoparticles of ∼2.2 nm diameter deposited on the NanoPC that was wrapped by poly[2,2'-(2,6-pyridine)-5,5'-bibenzimidazole] (PyPBI). Even after 10,000 start-up/shutdown cycles in the range of 1.0 to 1.5 V vs. RHE, the NanoPC/PyPBI/Pt showed almost no loss in electrochemical surface area (ECSA), which indicated much higher durability than those of a CB/PyPBI/Pt (∼32% loss), in which conventional carbon black (CB) was used in place of the NanoPC, and conventional CB/Pt (∼46% loss). The power density of the NanoPC/PyPBI/Pt was 342 mW/cm(2), which was much higher than those of the CB/PyPBI/Pt (183 mW/cm(2)) and CB/Pt (115 mW/cm(2)).

Catalyst Degradation in Fuel Cell Electrodes: Accelerated Stress Tests and Model-based Analysis

Catalyst degradation is the major hurdle in the development of polymer electrolyte fuel cells for automotive applications. This article presents an analysis of degradation data obtained in accelerated stress tests under ex situ conditions. The influence of the range of potential variation, potential waveform and temperature on the loss of electrochemical surface area was analyzed. Higher upper potential limit, square waveform of potential cycling, and increased temperature were found to accelerate degradation. A Pt counter electrode affects stress test results because it reaches potentials, at which it dissolves. This effect markedly distorts results for dissolution and redepostion rates at the working electrode; it can be eliminated by using an alternative counter electrode material. Experimental data were analyzed with a dynamic model that couples degradation mechanisms by dissolution/redeposition, coagulation and detachment. Detachment plays a role only at higher potentials (1.2V vs. RHE), at which carbon corrosion of the substrate is strongly enhanced. In this potential range, coagulation and dissolution/redeposition seemingly have a similar impact, in addition to detachment. A square wave potential cycle enhances the contribution of dissolution and redeposition, as compared to a triangular potential cycle. At the lower upper potential limit of 0.9V, degradation was significantly lower. The current analysis does not resolve the ambiguity of coagulation and dissolution/redeposition mechanisms.

Magneli phase titanium sub-oxide conductive ceramic Ti n O2n−1 as support for electrocatalyst toward oxygen reduction reaction with high activity and stability

Magneli phase titanium sub-oxide conductive ceramic Ti n O2n−1 was used as the support for Pt due to its excellent resistance to electrochemical oxidation, and Pt/Ti n O2n−1 composites were prepared by the impregnation-reduction method. The electrochemical stability of Ti n O2n−1 was investigated and the results show almost no change in the redox region after oxidation for 20 h at 1.2 V (vs NHE) in 0.5 mol/L H2SO4 aqueous solution. The catalytic activity and stability of the Pt/Ti n O2n−1 toward the oxygen reduction reaction (ORR) in 0.5 mol/L H2SO4 solution were investigated through the accelerated aging tests (AAT), and the morphology of the catalysts before and after the AAT was observed by transmission electron microscopy. At the potential of 0.55 V (vs SCE), the specific kinetic current density of the ORR on the Pt/Ti n O2n−1 is about 1.5 times that of the Pt/C. The LSV curves for the Pt/C shift negatively obviously with the half-wave potential shifting about 0.02 V after 8000 cycles AAT, while no obvious change takes place for the LSV curves for the Pt/Ti n O2n−1. The Pt particles supported on the carbon aggregate obviously, while the morphology of the Pt supported on Ti n O2n−1 remains almost unchanged, which contributes to the electrochemical surface area loss of Pt/C being about 2 times that of the Pt/Ti n O2n−1. The superior catalytic stability of Pt/Ti n O2n−1 toward the ORR could be attributed to the excellent stability of the Ti n O2n−1 and the electronic interaction between the metals and the support.

A One-Dimensional Pt Degradation Model for Polymer Electrolyte Fuel Cells

A one-dimensional model is developed and validated to study platinum degradation and the subsequent electrochemical surface area (ECA) loss in the cathode catalyst layer (CL) of polymer electrolyte fuel cells (PEFCs). The model includes two mechanisms of Pt degradation: Ostwald ripening on carbon support and Pt dissolution-re-precipitation through the ionomer phase. Impact of H2 | N2 or H2 | Air operation, operating temperature, and relative humidity (RH) on Pt degradation during voltage cycling is explored. It is shown that ECA loss is non-uniform across the cathode CL with a zone of exacerbated Pt degradation and hence much lower ECA found near the membrane. This non-uniform Pt degradation is caused by consumption of Pt ions by crossover H2 in both H2 | N2 and H2 | Air systems. An important consequence is that thinning the cathode electrode in a fuel cell would lead to more ECA loss as a higher fraction of the thin CL would fall in this exacerbated degradation zone. We have quantified the effect of thin cathode CLs on Pt degradation for the first time.

Platinum Dissolution in Fuel Cell Electrodes: Enhanced Degradation from Surface Area Assessment in Automotive Accelerated Stress Tests

Automotive fuel cells are plagued by high platinum-based cathode catalyst costs exacerbated by high catalyst loadings to circumvent performance losses induced by typical urban drive cycles. Accelerated durability tests are commonly used to evaluate the impact of nano-structure and/or operating conditions on cathode catalyst loss rates. To map out loss rates the electrochemical surface area (ECSA) of the catalyst is periodically monitored. Herein the accelerated stress testing was performed 25,000 times by imposing perturbations from 0.9 V to 0.6 V in a triangular wave profile while the ECSA was measured by assessing the hydrogen adsorption/desorption region with a lower potential limit of 0.025 V at intermittent points during the triangular wave perturbations. The ECSA loss observed during the 25,000 triangle wave cycles is 7% when measured only 2 times, but 26.3% when measured 250 times. The exacerbated losses during frequent low potential ECSA measurements suggest a restructuring of the catalyst surface due to the formation of a protective β-oxide, as seen by alterations in the cyclic voltammtric profile and particle size distribution.

Study of the degradation mechanisms of carbon-supported platinum fuel cells catalyst via different accelerated stress test

A combination method based on three different accelerated stress test (AST) protocols along with the monitoring of electrochemical surface area (ECSA), oxygen reduction reaction (ORR) activities, X-Ray photoelectron spectrometer (XPS), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) response is introduced to investigate the degradation mechanisms of carbon-supported platinum (Pt/C) catalyst. By comparing the ECSA and ORR activity loss under different AST protocols, we revealed that the activity loss in AST can be divided into recoverable activity loss and unrecoverable activity loss. The recoverable activity loss is attributed to the reduction of Pt oxide or partially due to the removal of CO formed during carbon corrosion. The unrecoverable activity loss is ascribed to the Pt dissolution/re-deposition, agglomeration, detachment and carbon corrosion. XPS results show that the Pt dissolution/re-deposition in AST can be detected by using a more negative potential window. TEM images and analysis confirmed that the Pt growth mode in this study is mainly due to the Pt agglomeration rather than dissolution/re-deposition. EIS analysis reveals that the alternative decomposition/formation of oxygen containing groups over time is the main corrosion pathway of carbon support. These findings are very important for understanding Pt/C catalyst degradation and are also useful for developing fast test protocol for screening new durable catalyst materials.