Cycles Of Accelerated Durability Test Research Articles

Tailoring Zirconia Supported Intermetallic Platinum Alloy via Reactive Metal‐Support Interactions for High‐Performing Fuel Cells

AbstractDeveloping efficient and anti‐corrosive oxygen reduction reaction (ORR) catalysts is of great importance for the applications of proton exchange membrane fuel cells (PEMFCs). Herein, we report a novel approach to prepare metal oxides supported intermetallic Pt alloy nanoparticles (NPs) via the reactive metal‐support interaction (RMSI) as ORR catalysts, using Ni‐doped cubic ZrO2 (Ni/ZrO2) supported L10−PtNi NPs as a proof of concept. Benefiting from the Ni migration during RMSI, the oxygen vacancy concentrations in the support are increased, leading to an electron enrichment of Pt. The optimal L10−PtNi−Ni/ZrO2−RMSI catalyst achieves remarkably low mass activity (MA) loss (17.8 %) after 400,000 accelerated durability test cycles in a half‐cell and exceptional PEMFC performance (MA=0.76 A mgPt−1 at 0.9 V, peak power density=1.52/0.92 W cm−2 in H2−O2/−air, and 18.4 % MA decay after 30,000 cycles), representing the best reported Pt‐based ORR catalysts without carbon supports. Density functional theory (DFT) calculations reveal that L10−PtNi−Ni/ZrO2−RMSI requires a lower energetic barrier for ORR than L10−PtNi−Ni/ZrO2 (direct loading), which is ascribed to a decreased Bader charge transfer between Pt and *OH, and the improved stability of L10−PtNi−Ni/ZrO2−RMSI compared to L10−PtNi−C can be contributed to the increased adhesion energy and Ni vacancy formation energy within the PtNi alloy.

Tailoring Zirconia Supported Intermetallic Platinum Alloy via Reactive Metal-Support Interactions for High-Performing Fuel Cells.

Developing efficient and anti-corrosive oxygen reduction reaction (ORR) catalysts is of great importance for the applications of proton exchange membrane fuel cells (PEMFCs). Herein, we report a novel approach to prepare metal oxides supported intermetallic Pt alloy nanoparticles (NPs) via the reactive metal-support interaction (RMSI) as ORR catalysts, using Ni-doped cubic ZrO2 (Ni/ZrO2) supported L10-PtNi NPs as a proof of concept. Benefiting from the Ni migration during RMSI, the oxygen vacancy concentrations in the support are increased, leading to an electron enrichment of Pt. The optimal L10-PtNi-Ni/ZrO2-RMSI catalyst achieves remarkably low mass activity (MA) loss (17.8 %) after 400,000 accelerated durability test cycles in a half-cell and exceptional PEMFC performance (MA=0.76 A mgPt -1 at 0.9 V, peak power density=1.52/0.92 W cm-2 in H2-O2/-air, and 18.4 % MA decay after 30,000 cycles), representing the best reported Pt-based ORR catalysts without carbon supports. Density functional theory (DFT) calculations reveal that L10-PtNi-Ni/ZrO2-RMSI requires a lower energetic barrier for ORR than L10-PtNi-Ni/ZrO2 (direct loading), which is ascribed to a decreased Bader charge transfer between Pt and *OH, and the improved stability of L10-PtNi-Ni/ZrO2-RMSI compared to L10-PtNi-C can be contributed to the increased adhesion energy and Ni vacancy formation energy within the PtNi alloy.

Highly durable graphene-encapsulated platinum-based electrocatalyst for oxygen reduction reactions synthesized by solution plasma process

Though expensive platinum (Pt) is used as catalyst for oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs), insufficient durability remains as a bottleneck for commercialization of PEFCs. Improving both catalytic performance and durability by graphene encapsulation is an attractive strategy to solve this problem. In this study, graphene-encapsulated PtFe core-shell catalyst is synthesized with dimethyl formamide (DMF) and a pair of Pt and Fe electrodes without using any metal salts by utilizing the solution plasma (SP) process. TEM and EELS results show synthesized PtFe nanoparticles are encapsulated with close to single-layered highly crystallized graphene. Although commercial Pt/C showed significant performance degradation (ECSA −33%, MA −68%) after 50,000 cycles of accelerated durability test (ADT), PtFe core-shell catalyst shows remarkably improved durability (ECSA −13%, MA −19%) while graphene shell clearly remains. The improved durability is more prominent in the single cell test, the decrease in maximum power density after 6000 cycles of ADT was significantly lower as −1.2%, compared to that of Pt/C (−52.1%). This study introduces a novel and attractive catalyst synthesis process by the SP method followed by heat treatments and suggests graphene encapsulation can improve long-term durability of catalyst while maintaining ORR activity.

Anchoring Highly Dispersed Pt Electrocatalysts on TiOx with Strong Metal-Support Interactions via an Oxygen Vacancy-Assisted Strategy as Durable Catalysts for the Oxygen Reduction Reaction.

Pt electrocatalysts with high activity and durability have still crucial issues for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). In this study, a novel catalyst consisting of Pt nanoparticles (NPs) on TiOx/C composites (TiOx-Vo-H/C) with abundant oxygen vacancies (Vo) is proposed, which is abbreviated as PTO-Vo-H/C. The introduction of Vo helps anchor highly dispersed Pt NPs with low loading and strengthen the strong metal-support interaction (SMSI), which benefits to the enhanced ORR catalytic activity. Moreover, the accelerated durability test (ADT) demonstrates the higher retention of ORR activity for PTO-Vo-H/C. Experimental and theoretical analyses reveal that electronic interactions between Pt NPs and TiOx/C composite support give rise to an electron-rich Pt NPs and strong SMSI effect, which is favorable for the electron transfer and stabilization of Pt NPs. More importantly, the assembled PEMFC with PTO-Vo-H/C shows only 6.9% of decay on maximum power density after 3000 ADT cycles while the performance of Pt/C sharply decreased. This work provides a new insight into the unique vacancy regulation of dispersive Pt on metal oxides for superior ORR performance.

Degradation of Ni-Co Coated Ni Oxygen Evolution Electrodes in Alkaline Water Electrolysis Using Accelerated Durability Test Based on Reverse Current Phenomenon

INTRODUCTION Power to Gas (P2G) technology is expected to allow installation of a lot of renewable electricity without imbalance issue of electric power grid. Here, water electrolysis is a key device in the P2G. One of the conventional methods in water electrolysis is alkaline water electrolysis (AWE), which has the advantages of a low-cost material electrolyzer. On the other hand, there is an issue of electrode degradation caused by the reverse current generated by frequent start-stop of renewable energy power source. Therefore, further understanding of degradation phenomenon and development of durable electrodes are needed.In this study, degradation of NiCo spinel coated Ni anodes, which is a conventional anode for oxygen evolution electrode in alkaline medium has been investigated using accelerated durability test based on reverse current phenomenon [1]. The effect of stop condition of E min and short open circuit potential (OCP) holding before reverse current mode on degradation have been discussed with cyclic voltammetry (CV) as electrochemical measurement. EXPERIMENTAL The accelerated durability test (ADT) were performed in a three-electrode cell with 7 M KOH aqueous as electrolyte at 25℃. RHE, Ni coil and 1 cm2 electrode of NiCo spinel catalyst supported on Ni mesh substrate at 500 °C are used as reference, counter and working electrodes, respectively.Constant current electrolysis at 1 A cm-2 for 2 hours was conducted as pretreatment at 80℃ before ADT. The ADT protocol consisted of three steps: (1) constant current electrolysis at 600 mA cm-2 for 1 min, (2) potential scanning from open-circuit potential (OCP) to anodic potential E min (=0.3, 0.5, 0.7 V vs. RHE) at scanning rate of -500 mV s-1, and (3) holding at constant potential of E min for 1 min as shown in Figure 1.To evaluate the anode activity, CV (potential range: 0.5-2.0 V, scanning speed: 5, 50 mV s-1, 3 cycles) and AC impedance measurements (bias potential: 1.6, 1.7 V, frequency range: 106-10-1 Hz, amplitude: 10 mV) were performed at every 200 ADT cycles in 2,000 cycles of ADT. Furthermore, the same degradation evaluation was performed by the protocol inserting 10 s holding at OCP between (1) and (2) at Emin = 0.5 V vs. RHE to evaluate relaxation effect. RESULTS AND DISCUSSION Figure 2 shows the iR-free anode potential of CV at i =100 mA cm-2 as a function of the ADT cycles for various E min. The anode potential increased from ca. 1.6 to 1.8 V vs. RHE during 2,000 cycles of the ADT. There was no significant difference for degradation behavior and potential among various E mins after ADT. This was probably due to the significant consumption of the catalyst layer with detachment in earlier ADT cycles, because detached electrocatalyst powder precipitated at the bottom of the cell.Figure 3 also shows the iR-free anode potential as a function of the ADT cycles with and without 10 s of OCP at E min = 0.5 V. Anode potentials with and without OCP after ADT were almost same. However, potential with OCP was significantly smaller than that without OCP in the early cycles of the ADT. Figure 4 shows the anodic peak potential of the CV as a function of the ADT cycles. As inset of Fig. 4, the CV had two anodic peaks of (1)β-Ni(OH)2 /β-NiOOH at E 0 =1.33 V vs. RHE, and (2) NiOOH / NiO2 at E 0 =1.43 V vs. RHE. The peaks (1) and (2) correspond to active and inert site, respectively [2]. Therefore, the increase of the (2) and the decrease of the (1) with the ADT cycles would correspond to the degradation. In the early cycles, the peak currents with OCV were more active side than that without OCV. The insertion of OCP to suppress the potential increase in Fig. 3 would be affected by not only coherency of catalyst layer to suppress detachment but also the activation of catalyst itself. Acknowledgements This study was based on results obtained from the Development of Fundamental Technology for Advancement of Water Electrolysis Hydrogen Production in Advancement of Hydrogen Technologies and Utilization Project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Electrodes were provided by De Nora Permelec Ltd. We are grateful to all people concerned.

An effective and durable interface structure design for oxygen reduction and methanol oxidation electrocatalyst

In order to inhibit the corrosion of carbon support and agglomeration of Pt nanoparticles in conventional Pt/XC-72 catalyst, the conventional Pt/C catalyst was modified by in-situ deposition of amorphous tungsten carbide (WC) at the catalyst/support interface. The results show that proper amount of WC can improve the performance and durability of catalyst, and excessive addition of W element will combine with oxygen and transform into WO3, thus covering Pt nanoparticles and reducing the active site of catalyst. When W content is controlled at 20 wt%, the half-wave potential (E1/2) of oxygen reduction reaction (ORR) polarization curve of Pt-WC (20 wt% W, PWC20) catalyst is positively shifted by 31 mV compared with that of conventional Pt/XC-72 catalyst, and the mass activity and specific activity of catalyst are increased by about 2.0 times at 0.85 V. After 10,000 cycles of accelerated durability test (ADT), the electrochemical active area (ECSA) of PWC20 catalyst decreased by 21.7% and the E1/2 was negatively shifted by 22 mV, while the ECSA of conventional Pt/XC-72 catalyst decreased by 49.49% and E1/2 shifted by 42 mV negatively. The peak current density of methanol oxidation reaction (MOR) is 1.41 times higher than that of conventional Pt/XC-72, and the forward scanning potential of methanol oxidation shifts 36 mV negatively. After 1000 cycles of ADT, the forward peak current density decreased by 15.23%, while the conventional Pt/XC-72 decreased by 26.62%. The results of CO stripping voltammetry and X-ray photoelectron spectroscopy (XPS) analysis show that the modification of catalyst/support interface by WC significantly improves the anti-CO toxicity ability of conventional Pt/C. The synergistic effect between WC and Pt enhances the metal-support interaction. The durability of catalyst toward ORR and MOR was improved by the modification of catalyst/support interface, and the performance and durability of proton exchange membrane fuel cell (PEMFC) cathode catalyst layer were also improved.

Graphene Oxide Modified Pt/C and Pt2Ru3/C for Enhanced Carbon Monoxide Tolerable Fuel Cell Anode Catalyst

The commercial viability of residential fuel cell technology operating on reformed H2 strongly demands to find efficient, CO tolerable and long lasting anode catalyst.1 Currently, Pt-based electrocatalysts are widely used for this purpose. However, one of the major drawbacks of Pt-based anode is the surface poisoning by carbon monoxide (CO) leading to severe decrease in efficiency and long-term performance of catalyst.2 This problem has been mitigated by either (i) synthesis of catalysts with higher CO oxidation capability or (ii) suppressing the adsorption of CO on the catalyst surface. Within the first context, additives often partially cover the alloy surface and lead to a decrease in the electrochemically active Pt surface area and thus the HOR activity. Therefore, in the current work, the second strategy has been pursued. Here we use graphene oxide (GO) to improve the CO tolerance of Pt/C and Pt2Ru3/C catalysts by suppressing the adsorption of CO on the catalyst surface. Graphite oxide was prepared from graphite (Z-5F, Average particle size 4μm, ≥98.8 %, Ito Graphite Industry Co., Ltd.) by a modified Hummers' oxidation method.3 Graphite oxide was then dispersed in 2-propanol to exfoliate graphite oxide into GO. Commercial Pt/C (TKK TEC10E50E, 46.6 mass% Pt) and Pt2Ru3/C (TKK TEC61E54, 30 mass% Pt and 23.3 mass% Ru) were used as-received. GO–Pt/C and GO–Pt2Ru3/C were prepared by stirring, sonication and drying of a mixture of GO colloid with Pt/C and Pt2Ru3/C powder respectively. COad stripping measurement was performed by cyclic voltammetry (CV) at 10 mV s-1 in N2 saturated 0.1 M HClO4. Chronoamperometry (CA) was conducted in 0.1 M HClO4 saturated with either pure H2 or 250 ppm CO/H2 at 20 mV vs. RHE with 400 rpm rotation. Accelerated durability test (ADT) was conducted by square-wave potential stepping between 5 and 400 mV vs. RHE for 3000 cycles with a holding time of 3 seconds at each potential. TEM images confirm that GO wraps around Pt/C or Pt2Ru3/C and are distributed uniformly. XRD and XPS data support the co-existence of GO and Pt/C or Pt2Ru3/C. COad stripping of Pt/C and GO–Pt/C suggest that in contrast to Pt/C, in case of GO–Pt/C a pre-oxidation peak was observed at 0.5-0.7 V before the main CO oxidation peak (Fig. 1a). This pre-oxidation peak may be attributed to oxidation of weakly adsorbed CO. The apparent HOR activities in pure H2 for Pt/C (190 A (g-Pt)−1) and Pt2Ru3/C (148 A (g-PtRu)−1) do not change after the addition of GO, thus GO does not interfere with the HOR. Fig. 1b compares CA (HOR current) of Pt/C, GO–Pt/C, Pt2Ru3/C and GO–Pt2Ru3/C in 250 ppm CO/H2 saturated HClO4 after 3000 cycles of ADT. The HOR current for GO–Pt/C was 109 A (g-Pt)−1, which is 1.3 times higher than that of Pt/C (84 A (g-Pt)−1. The HOR current for GO–Pt2Ru3/C (89 A (g-PtRu)−1) is 1.3 times higher than that of commercial Pt2Ru3/C (68.4 A (g-PtRu)−1). The enhanced CO tolerance and durability of GO–Pt/C and GO–Pt2Ru3/C may be attributed to i) GO protects the coagulation of Pt particles and ensure the sufficient stability of Pt/C catalyst, ii) steric hindrances arise from oxygen-containing functional groups of GO surface decrease the adsorption of CO on Pt surface and iii) the electron transfer between the oxygen atoms and Pt nanoparticles could lead to the enhanced electron density of Pt via a ligand effect which in turns decrease the attraction towards CO. The current findings suggest that the prepared GO–Pt/C and GO–Pt2Ru3/C catalysts have great potential for use in fuel cell anode. Acknowledgments This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan. References E. Antolini, Appl. Catal. B Environ., 74, 324 (2007), E. Antolini, Appl. Catal. B Environ., 74, 337 (2007).M. Watanabe, T. Sato, K. Kunimatsu, and H. Uchida, Electrochem. Acta, 53, 6928 (2008).S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 80, 1339(1958). Figure 1

Kinetics and Mechanism of Redox Processes of Pt/C and Pt3Co/C Cathode Electrocatalysts in a Polymer Electrolyte Fuel Cell during an Accelerated Durability Test

The degradation of Pt electrocatalysts in membrane electrode assemblies (MEAs) of polymer electrolyte fuel cells under working conditions is a serious problem for their practical use. Here we report the kinetics and mechanism of redox reactions at the surfaces of Pt/C and Pt3Co/C cathode electrocatalysts during catalyst degradation processes by an accelerated durability test (ADT) studied by operando time-resolved X-ray absorption fine structure (XAFS) spectroscopy. Systematic analysis of a series of Pt LIII-edge time-resolved XAFS spectra measured every 100 ms at different degradation stages revealed changes in the kinetics of Pt redox reactions on Pt/C and Pt3Co/C cathode electrocatalysts. In the case of Pt/C, as the number of ADT cycles increased, structural changes for Pt redox reactions (charging, surface, and subsurface oxidation) became less sensitive because of the agglomeration of catalyst particles. It was found that their rate constants were almost constant independent of the agglomeration of t...

Titanium cobalt nitride supported platinum catalyst with high activity and stability for oxygen reduction reaction

We describe a facile route to the development of novel robust non-carbon titanium cobalt nitride (Ti0.9Co0.1N) used as a support for Pt, and the catalyst exhibits high activity and stability for the oxygen reduction reaction (ORR). XRD and TEM results show that the synthesized Ti0.9Co0.1N is formed as a single-phase solid solution with high purity. The XPS measurements verified the strong metal/support interaction between Pt nanoparticles (NPs) and the Ti0.9Co0.1N support. Most importantly, Ti0.9Co0.1N supported Pt catalyst (Pt/Ti0.9Co0.1N) exhibits a much higher mass activity and durability than that of the commercial JM Pt/C electrocatalyst for ORR. The accelerated durability test (ADT) reveals that the novel Ti0.9Co0.1N support can dramatically enhance the durability of the catalyst and maintain the electrochemical surface area (ECSA) of Pt. Pt/Ti0.9Co0.1N shows great improvement in ECSA preservation, with only 35% of the initial ECSA drop even after 10 000 ADT cycles. The experimental data indicate that the electronic structure of Pt can be modified by Co doping, and there exists a strong interaction between Pt and the Ti0.9Co0.1N support, both of them are playing an important role in improving the activity and durability of the Pt/Ti0.9Co0.1N catalyst.

Hollow and porous titanium nitride nanotubes as high-performance catalyst supports for oxygen reduction reaction

We describe a route to the development of robust non-carbon TiN nanotubes (TiN NTs) used as a support for Pt, and the catalyst exhibits high activity and stability for the oxygen reduction reaction (ORR). The accelerated durability test (ADT) reveals that the novel supporting material can dramatically enhance the durability of the catalyst and maintain the electrochemically active surface area (ECSA) of Pt, which shows great improvement in ECSA loss with 77% of the initial ECSA remaining even after 12 000 ADT cycles, which is much higher than the commercial Pt/C (E-TEK) catalyst. The experimental data verified the strong metal/support interaction between Pt nanoparticles (NPs) and the TiN NTs support. The surface of the TiN NTs is composed of dendrite nanocrystals, which may function as ‘hunters’ for re-capture and re-nucleation of the Pt species (atoms or clusters) that would dissolve into the electrolyte, which prevents leaching and migrating of dissolved Pt NPs. This new approach opens a reliable path for designing various TiN NT-supported catalysts for a wide range of applications in energy conversion processes.

Anomalously increased oxygen reduction reaction activity with accelerated durability test cycles for platinum on thiolated carbon nanotubes

We report an anomalous phenomenon in Pt supported on thiolated multi-walled carbon nanotubes (Pt-S-MWNT): oxygen reduction reaction (ORR) activity increases with accelerated durability test (ADT) cycles. Sub-nanometer-sized Pt clusters on S-MWNT were gradually agglomerated to an optimal size with ADT cycles, and finally showed increased ORR activity after the ADT.

Performance and durability of Pt/C cathode catalysts with different kinds of carbons for polymer electrolyte fuel cells characterized by electrochemical and in situ XAFS techniques

The electrochemical activity and durability of Pt nanoparticles on different kinds of carbon supports in oxygen reduction reactions (ORR) were investigated using rotating disc electrodes (RDE) and the membrane electrode assemblies (MEA) of polymer electrolyte fuel cells (PEFC). The mass activity of Pt/C catalysts (ORR activity per 1 mg of Pt) at the RDE decreased, according to the type of carbon support, in the following order; Ketjenblack (KB) > acetylene black (AB) > graphene > multiwall carbon nanotube (MW-CNT) > carbon black (CB), whereas the average size of the Pt nanoparticles and the surface specific activity (ORR activity per electrochemical surface area) did not vary significantly between these carbon supports. These results indicate that the different mass activities of the Pt/C catalysts may originate from the differences in the fraction of Pt on the carbon supports which is available for utilization. The durability of the MEAs of the top two active catalysts Pt/KB and Pt/AB among the five catalysts was examined based on ORR performance, TEM and in situ XAFS. It was found that the performance of the Pt/KB cathode catalyst in PEFC MEA decreased significantly over 500 accelerated durability test (ADT) cycles, whereas the performance of the Pt/AB cathode catalyst in PEFC MEA did not decrease significantly during 500 ADT cycles, it was also found that the Pt/AB possesses 8 times higher durability compared with the Pt/KB. In situ Pt LIII-edge XAFS data in the ADT cycles and stepwise potential operations revealed the different oxidation-reduction behaviors of the Pt nanoparticles on the KB and AB supports. The Pt/KB was oxidized to form surface PtO layers more easily than the Pt/AB in the increasing potential operation from 0.4 VRHE to 1.4 VRHE, and the surface PtO layers of the Pt/AB were reduced to the metallic Pt state more readily than those of the Pt/KB in the decreasing potential operation from 1.4 VRHE to 0.4 VRHE. The XAFS analysis for the Pt valences and the coordination numbers of Pt-O and Pt-Pt demonstrated that the Pt/AB catalyst is more durable than the Pt/KB catalyst in PEFC MEAs.

Stabilization of High-Performance Oxygen Reduction Reaction Pt Electrocatalyst Supported on Reduced Graphene Oxide/Carbon Black Composite

Oxygen reduction reaction (ORR) catalyst supported by hybrid composite materials is prepared by well-mixing carbon black (CB) with Pt-loaded reduced graphene oxide (RGO). With the insertion of CB particles between RGO sheets, stacking of RGO can be effectively prevented, promoting diffusion of oxygen molecules through the RGO sheets and enhancing the ORR electrocatalytic activity. The accelerated durability test (ADT) demonstrates that the hybrid supporting material can dramatically enhance the durability of the catalyst and retain the electrochemical surface area (ECSA) of Pt: the final ECSA of the Pt nanocrystal on the hybrid support after 20 000 ADT cycles is retained at >95%, much higher than the commercially available catalyst. We suggest that the unique 2D profile of the RGO functions as a barrier, preventing leaching of Pt into the electrolyte, and the CB in the vicinity acts as active sites to recapture/renucleate the dissolved Pt species. We furthermore demonstrate that the working mechanism can be applied to the commercial Pt/C product to greatly enhance its durability.

Oxygen Reduction Reaction on Active and Stable Nanoscale TiSi2 Supported Electrocatalysts

Simultaneous electrochemical activity and kinetic stability of the electrode materials are of paramount importance to the commercialization of proton exchange membrane fuel cells. In this paper, we report a new material, titanium disilicide (TiSi2), to support nano Pt electrocatalysts that were synthesized via a colloidal approach. Accelerated durability test (ADT) shows that the oxygen reduction electrode made of Pt/TiSi2 exhibits substantial improvement over commercial Pt/C electrode in terms of the kinetic stability. After 2000 ADT cycles, the electrochemical surface area of Pt/TiSi2 decreases to ∼73% of the initial magnitude, compared to 36% of Pt/C electrodes. The ADT results are consistent with the analysis of Pt particle size by transmission electron microscopy, which shows aggregation of Pt nanoparticles in Pt/C and marginal growth of Pt with TiSi2 as the support. Oxygen reduction reactions on Pt/TiSi2 take place via a four-electron electrochemical process. A similar Tafel behavior is observed in Pt/TiSi2 and Pt/C electrodes, suggesting the presence of similar oxygen reduction mechanisms in both electrodes.