Electrochemical Surface Area Of Pt Research Articles

Hollow core mesoporous carbon spheres as catalyst support for improved platinum utilization in phosphoric acid fuel cells

Using a well-known sol-gel technique, monodispersed silica spheres measuring 380 nm in size are generated in situ. These silica spheres serve as a template for the synthesis of hollow core mesoporous shell (HCMS) carbon spheres. Inside the pores of the template, a polymer is synthesized using azoisobutyronitrile and divinylbenzene polymerization route. Polymer carbonization followed by template remotion yielded HCMS carbon spheres. This HCMS carbon is mesoporous and offers uniform Pt crystallite distribution for acid fuel cell applications. As-prepared HCMS carbon is examined by field emission gun SEM, TEM, nitrogen adsorption/desorption isotherm (BET surface area and BJH pore size distribution analysis), and apparent density using Helium pycnometry. The HCMS carbon obtained demonstrates a BET surface area of 623 m2g-1, showcasing a uniform pore size distribution centered at 3.8 nm. This specific characteristic renders it an ideal support material for fuel cell catalysts. The electrochemical studies reveal the key parameters like corrosion resistance, bulk electrical conductivity, the electrochemical surface area of Pt chemically deposited on HCMS carbon, and unit fuel cell performance under phosphoric acid environment. These parameters are compared with the standard carbon powder, Vulcan XC72R and a commercial catalyst to evaluate the HCMS carbons’ suitability for fuel cell applications.

The importance of carefully choosing vertex potentials in hydrogen underpotential deposition

Accurate determination of electrochemical surface area (ECSA) is not always possible even for simple monometallic catalysts with well-known techniques. An example is Pt and the determination of its ECSA by hydrogen underpotential deposition (HUPD) used to determine the specific activity. Here, we show an important problem in determining the ECSA of Pt (both wire and foil electrodes) using HUPD which is the changing ECSA with the changing anodic and cathodic vertex potentials (Evertex) set in HUPD analysis. The results confirm that just by varying the Evertex, one could get a wide range of values for ECSA and the ECSA normalized specific activity for the same Pt electrode. This calls for instant remedial action as Pt is not just any catalyst but state-of-the-art in many energy conversion reactions and our study suggests that the Evertex should be chosen based on whether or not to have the effect of Pt oxidation on ECSA.

Investigation of Pt/Fe-N-C Hybrids Towards ORR in Acidic Environment

Polymer electrolyte membrane fuel cells (PEMFCs) are recognized as one of the renewable energy sources in portable, automobile, and stationary applications. Currently, both low temperature (LT) and high temperature (HT) PEMFCs use platinum group metal (PGM) based catalysts for oxygen reduction reaction (ORR) containing usually Pt nanoparticles on carbon black. To reduce the total cost of PEMFC stack, worldwide researchers have considerable attention to find an inexpensive alternative catalyst. Recently, metal-nitrogen-carbon (M-N-C) compounds such as Fe-N-Cs are the most promising PGM-free catalysts for ORR. However, they suffer from insufficient volumetric activity and electrochemical stability in PEMFCs. [1,2] Xiao et al. have proven an improved electrochemical stability of Pt-Fe-N-C electrocatalysts consisting of atomically dispersed Pt and Fe atoms or Pt-Fe alloy nanoparticles in comparison with Fe-N-C only. [3] Mechler et al. have reported that the addition of 1-2 wt.% Pt in hybrid Pt/Fe-N-C catalyst performs well with an increased stability in LT-PEMFCs. [4] Liao et al. have recently shown better ORR activity of Pt/Fe-N-C than Pt/C. [5] With the overriding goal of reducing LT- and HT-PEMFC production costs, Pt/Fe-N-C activity, selectivity and stability with systematically reducing the Pt-content has not been investigated yet. In this study, Pt/Fe-N-C hybrids are synthesized using PMF-011904 from Pajarito Powder (USA) as catalyst support and wet-chemically precipitated Pt nanoparticles with targeted Pt-contents of 40, 5, 1 and 0 wt.%. First, ICP mass spectrometry is used for Pt quantification along all catalysts, and transmission electron microscopy is carried out to investigate morphology and Pt nanoparticle diameter and distribution on the Fe-N-C catalyst. Second, ORR activity and selectivity is investigated by rotating ring disc electrode (RRDE) experiments in 0.1 mol L-1 HClO4, and electrochemical surface area of Pt is calculated by hydrogen underpotential deposition and CO stripping voltammetry. Last, accelerated stress testing (AST) is performed with 5,000 triangle potential cycles between 0.6–1.5 VRHE to evaluate the catalyst stability. Figure 1 shows the initial ORR curves during the RRDE experiments. The mass activities of 40, 5 and 1 wt.% Pt/Fe-N-C determined at 0.9 VRHE are 222.5 ± 41.2 A gPt -1, 170.8 ± 80.4 A gPt -1 and 49.0 ± 8.6 A gPt -1, respectively. Thus, the mass activity depends on Pt-content in the hybrid catalyst strongly. In addition, the other electrochemical characterization results are also going to be discussed in terms of catalyst selectivity, stability and the correlation with morphological aspects during the presentation. Figure 1 ORR curves in O2-saturated 0.1 mol L-1 HClO4 electrolyte with rotation speed of 1,600 rpm and scan rate of 5 mV s-1 (averaged results of three separate electrodes per each catalyst).[1] Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev. 2020, 11, 3484–3524.[2] T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem. Commun. 2020, 118, 106795.[3] F. Xiao, G.-L. Xu, C.-J. Sun, I. Hwang, M. Xu, H.-W. Wu, Z. Wei, X. Pan, K. Amine, M. Shao, Nano Energy 2020, 77, 10592.[4] A. K. Mechler, N. R. Sahraie, V. Armel, A. Zitolo, M. T. Sougrati, J. N. Schwämmlein, D. J. Jones, F. Jaouen, J. Electrochem. Soc. 2018, 165, F1084.[5] W. Liao, S. Zhou, Z. Wang, F. Liu, H. Pan, T. Xie, Q. Wang, ChemCatChem 2021, 13, 23, 4925-4930. Figure 1

Gas flow Stimulated Hydrodynamics for Preparation and Application of Platinized Titanium Hollow Fibre Electrodes

AbstractIn this study, Pt was electrodeposited on the surface of Ti hollow fibres from PtCl62− solution at fixed potential and at variable gas flow rate. Partial Pt coverage of Ti was obtained at flow rates≤5 mL⋅min−1, while a more‐even geometrical distribution and linearly increasing Electrochemical Surface Area (ECSA) of Pt was obtained for flow rates of >5 and <20 mL⋅min−1. Above 20 mL⋅min−1, the morphology did not significantly change, and only a limited increase in ECSA of Pt (PtECSA) was observed. Second, the performance of the Pt functionalized Ti‐HFEs was evaluated at fixed potential in i) liquid phase oxidation of FeII to FeIII, and ii) the hydrogen (gas) evolution reaction (HER). Generally, for the oxidation of Fe(CN)64−, a linear increase in current from 3 to ≈9 mA was found as a function of increasing gas flow rate. For the HER, a gas evolving reaction, initially (in the range of 0–10 mL⋅min−1) a much larger enhancement in current (from ≈‐10 to −35 mA) was observed than consecutively at higher flow rates. The effect of gas flow rate on Pt deposition, and utilization of platinized Ti HFEs, is explained on the basis of increasing fractional participation of relatively small pores at high flow rates, and generally an increasing mass transfer coefficient (kM) associated with localized mixing of the electrolyte near the electrode/electrolyte interface. The kM is roughly in the same order of magnitude as obtained in experiments using rotating disc electrodes.

Cell Performance and Durability of Pt/C Cathode Catalyst Covered by Dopamine Derived Carbon Thin Layer for Polymer Electrolyte Fuel Cells

Pt nanoparticles supported on carbons (Pt/C) as the cathode catalyst for polymer electrolyte fuel cells (PEFCs) were modified with dopamine (DA) and heat-treated, and their performance and durability were investigated to test the concept of forming carbon thin layers on Pt surfaces. The coverage of adsorbed sulfonates from ionomer on the Pt surfaces was decreased with increasing the addition amount of DA, and as a consequence, the mass activity was improved. In addition, the retention of electrochemical surface area of Pt and the cell performance after durability test was improved with increasing the DA amount. The cell performance in a high current density region under wet conditions was, however, deteriorated by the modification. Thus, the modification of Pt surfaces by the overlayers is a promising method for improving both the activity and durability of the catalyst, but oxygen transport at the interface between Pt and ionomer remains an issue.

Preparation and characterization of bimetallic Pt^Ni-P/CNT catalysts via galvanic displacement reaction on electrolessly-plated Ni-P/CNT

Platinum-based bimetallic catalysts have broad applications in polymer electrolyte membrane fuel cells and water splitting. In this work, galvanic displacement reaction was employed to prepare Pt^Ni-P/CNT catalysts using electrolessly-plated Ni-P/CNT. These catalysts were extensively characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Catalytic activities towards methanol oxidation and hydrogen evolution reactions were evaluated and benchmarked with a commercial Pt/C catalyst. Uniform dispersion of Pt on Ni-P particles led to high Pt utilization, and the electrochemical surface area of Pt^Ni-P/CNT with 12.1% Pt loading was found to be 126 m2 g−1, higher than that of a commercial Pt/C (77.9 m2 g−1). The Tafel slopes for the Pt^Ni-P/CNT catalysts were also found to be smaller than that of Pt/C indicating faster kinetics for hydrogen evolution reaction.

Sub-Zero Cell Reversal Tolerance in Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have gained considerable attention in recent years for application in electric vehicles, portable devices, and cogeneration systems. Large scale commercialization of PEMFCs mainly requires addressing the high cost issues associated with the fuel cell stack in addition to the challenges associated in building H2 infrastructure. Though, significant amount of research has been carried out in developing low Pt loaded Membrane Electrode Assemblies to meet the 2020 DOE performance targets for automotive applications, there are still challenges associated in meeting durability requirements due to catalyst layer and membrane degradation. It is well documented voltage cycling during start-up/shutdown process leads to Pt dissolution and loss of electrochemical surface area of Pt and thereby affecting the durability. Fuel cells stacks need to be robust against both specific and non-specific failure modes. Specific failure modes of MEA can be predicted by operating conditions/parameters, however non-specific failure mode such as cell reversal is difficult to predict. Though, system level mitigation can be employed to minimize the impact of cell reversal, there is a potential cost trade-off associated with this approach. Ballard1 was first to develop and report the impact of cell reversal tolerance above sub-zero conditions with and without cell reversal tolerance catalyst in the MEA. Cell reversals2 can occur mainly during reactant starvation and especially during H2 starvation under freeze start-up operation. This presentation will focus on evaluating the impact of operating parameters on cell reversal under sub-zero conditions and will highlight the mechanism and potential ways to improve cell reversal tolerance under sub-zero conditions. Figure 1 shows the impact of current density on cell reversal tolerance under sub-zero conditions3. As it can be seen, significant reduction in sub-zero reversal tolerance noticed with current density. To the best of our knowledge, we evaluated for the first time, the cell reversal tolerance under sub-zero conditions. References S.D.Knights, D.P.Wilkinson, S.A. Cambell, J.L.Taylor, J.M. Gascoyne and T.R. Ralph, Solid polymer fuel cell with improved voltage reversal tolerance. US Patent 6936370.C. Qin, J. Wang, D. Yang, B. Li and C. Zhang, Proton Exchange Membrane Fuel Cell Reversal:A Review, Catalysts, 6, 2016, 197.R.Bashyam, J.Bellerive, P.He, Fuel cell system and method of operating fuel cell system, German Patent application DE102016207786A1 Acknowledgement We thank Volkswagen AG and Audi for funding this work. Figure 1

Nanographene synthesized in triple-phase plasmas as a highly durable support of catalysts for polymer electrolyte fuel cells

Nanographene was synthesized in triple-phase plasmas comprising a gaseous phase, a gas–liquid boundary layer, and an in-liquid phase using a setup in which one electrode was placed in the gaseous phase while the other was immersed in the liquid phase. The triple-phase plasmas were generated using a pure alcohol, such as ethanol, 1-propanol, or 1-butanol, by applying a high voltage to a pair of electrodes made of copper or graphite. The nanographene synthesized using ethanol had high durability and thus could serve as a catalyst support in polymer electrolyte fuel cells (PEFCs). The PEFCs exhibited low degradation rates in the high-potential cycle test of a half-cell, as a result of which, a loss of only 10% was observed in the effective electrochemical surface area of Pt, even after 10,000 cycles.

Synthesis of Chemically Ordered Pt3Fe/C Intermetallic Electrocatalysts for Oxygen Reduction Reaction with Enhanced Activity and Durability via a Removable Carbon Coating.

Recently, Pt3M (M = Fe, Ni, Co, Cu, etc.) intermetallic compounds have been highlighted as promising candidates for oxygen reduction reaction (ORR) catalysts. In general, to form those intermetallic compounds, alloy phase nanoparticles are synthesized and then heat-treated at a high temperature. However, nanoparticles easily agglomerate during the heat treatment, resulting in a decrease in electrochemical surface area (ECSA). In this study, we synthesized Pt-Fe alloy nanoparticles and employed carbon coating to protect the nanoparticles from agglomeration during heat treatment. As a result, Pt3Fe L12 structure was obtained without agglomeration of the nanoparticles; the ECSA of Pt-Fe alloy and intermetallic Pt3Fe/C was 37.6 and 33.3 m2 gPt-1, respectively. Pt3Fe/C exhibited excellent mass activity (0.454 A mgPt-1) and stability with superior resistances to nanoparticle agglomeration and iron leaching. Density functional theory (DFT) calculation revealed that, owing to the higher dissolution potential of Fe atoms on the Pt3Fe surface than those on the Pt-Fe alloy, Pt3Fe/C had better stability than Pt-Fe/C. A single cell fabricated with Pt3Fe/C showed higher initial performance and superior durability, compared to that with commercial Pt/C. We suggest that Pt3M chemically ordered electrocatalysts are excellent candidates that may become the most active and durable ORR catalysts available.

Evaluation of the Cell Performance and Load Cycle Durability for Membrane Electrode Assemblies Using Pt/Nb-SnO2 Cathode Catalyst Layers with Various Ionomer Contents

Pt or PtCo alloys supported on carbon (Pt/C or PtCo/C) are commonly used as cathode catalysts for polymer electrolyte fuel cells (PEFCs). The carbon support degrades at high potential and during startup/shutdown (SU/SD) cycles. The degradation leads to the aggregation and detachment of Pt and decreases the cell performance. We synthesized a non-carbon support, Nb-doped tin oxide (Nb-SnO2), which has a fused aggregate network structure, by use of a flame oxide synthesis method, and loaded the Pt catalyst on the Nb-SnO2 support (Pt/Nb-SnO2) by use of a colloidal method. The Pt/Nb-SnO2 catalyst has high activity for the oxygen reduction reaction and high durability versus the SU/SD process compared with Pt supported on graphitized carbon black (Pt/GCB).1-4 In this research, we evaluated the cell performance and load cycle durability for membrane electrode assemblies (MEAs) using Pt/Nb-SnO2 cathode catalyst layers (CL) with various ionomer contents. The Brunauer-Emmett-Teller (BET) surface area of the Nb-SnO2 support was 228 m2 cm-3. The Pt loading amount of the Pt/Nb-SnO2 cathode was 0.05 mg cm-2. The cell performances were evaluated under H2/air at 80% RH and 80 ℃. The cell durability was evaluated by use of simulated load cycling with potential steps (0.6~1.0 V vs. RHE; holding time, 3 s at each potential), which was developed by the Fuel Cell Commercialization Conference of Japan (FCCJ). The ionomer contents (ionomer/support volume ratio, I/S) of the each cathode CL were varied as follows: 0.12, 0.24, 0.45 and 0.67. We also evaluated the cathode CL using Pt/GCB (I/S = 0.7, optimized ratio; BET surface area of GCB, 325 m2 cm-3) as a reference. The Pt particle sizes and the cross-sectional images of the MEA before cycling and after 50,000 cycles of simulated load cycling were analyzed by use of scanning transmission electron microscopy (STEM, HD2700, Hitachi High Technologies Co.). The initial cell performances and those after 50,000 simulated load cycles are shown in Fig. 1. The initial mass activity at 0.80 V (Fig. 2) and current density at 0.60 V (Fig. 3) improved with decreasing I/S and reached a maximum at I/S = 0.12. The optimized I/S for Pt/Nb-SnO2 was quite different from that for Pt/GCB. The electrochemical surface area of Pt on Nb-SnO2 was much higher than that on GCB. The images of transmission electron microscopy with low acceleration voltage indicated that the Nafion ionomer was uniformly covered on the surface of Pt/Nb-SnO2 in comparison with that on Pt/GCB. The results suggested that the optimum value of I/S varies with both hydrophilicity and surface area of the support. After 50,000 load cycles, the current density for the cell using the Pt/Nb-SnO2 cathode CL (I/S = 0.12) at 0.60 V was 2.5 times higher than that for Pt/GCB (I/S = 0.7), as shown in Fig. 1. The Pt particles on the Nb-SnO2 cathode CL after the 50,000 cycles maintained their high dispersion, while those on GCB became aggregated. It was detected from the STEM images that the orientations of the Pt particles were arranged according to those of the Nb-SnO2. The strong chemical bonding would mitigate the migration of Pt particles and would prevent the degradation of the Pt/Nb-SnO2. Moreover, no Pt band was observed in the membrane after the load cycling in the MEA using the Pt/Nb-SnO2 cathode, while a band was observed in the MEA using Pt/GCB (I/S = 0.7). These results also suggest that the differences of surface properties between Nb-SnO2 and GCB would affect the Pt dissolution, prevent the formation of Pt bands in the membrane, and would also contribute to the improvement of the durability. This work was supported by funds for the “SPer-FC” project of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. (1) Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv. 4 (2014) 32180. (2) Y. Senoo, K. Taniguchi, K. Kakinuma, M. Uchida, et al. Electrochem. Commun. 51 (2015) 37. (3) Y. Chino, K. Taniguchi, Y.Senoo, K. Kakinuma, M.Hara, M.Watanabe, M.Uchida J. Electrochem. Soc. 162 (2015) F736. (4) Y. Chino, K. Kakinuma, D.A. Tryk, M. Watanabe, M. Uchida J.Electrochem.Soc. 163 (2016) F97. Figure 1

Electrochemical impedance spectroscopy investigation on the ionomer degradation modeling of the anode using a porous microelectrode

In this study, the modeled degradation behavior of ionomer at the anode was investigated by combining the electrochemical impedance spectroscopy (EIS) and porous microelectrode techniques. We prepared the PME packed with the powder mixtures of Pt/C and Nafion with different ratios of Pt/C:Nafion = 1:1, 1:0.6, and 1:0 for modeling non-degraded, partly degraded and completely degraded ionomer. Based on the EIS results, both the charge transfer and mass transfer resistances increased with the decrease of Nafion content. The increased charge transfer resistance suggests a suppression of the hydrogen oxidation reaction rate at the electrode with degraded ionomer due to the decrease in the electrochemical surface area of Pt. On the other hand, the increase of mass transfer resistance is attributed to the decrease of Nafion content, which suppressed the ionic conductivity and blocked the reactant pathway. Moreover, a significantly higher adsorption resistance of the electrode with completely degraded ionomer was observed, indicating the adsorption site of the intermediates, i.e., the electrochemical surface area of Pt on the electrode without Nafion is lower than that of the electrode packed with the powder mixture of Pt/C and Nafion.

Simple Preparation of Pd Core Nanoparticles for Pd Core/Pt Shell Catalyst and Evaluation of Activity and Durability for Oxygen Reduction Reaction

Pd core nanoparticles less than 5 nm in mean size were prepared on carbon black (CB) without any stabilizer by using palladium acetate as a precursor and CO as a reducing agent, and then used for preparing Pd core/Pt shell nanoparticles-loaded CB (Pt/Pd/CB). The mean size of Pd nanoparticles could be controlled by the concentration of palladium acetate and the CO bubbling time. The cyclic voltammograms of two Pd nanoparticles-loaded CB (Pd4.2/CB, Pd3.3/CB) electrodes whose mean size was 4.2 and 3.3 nm, respectively, had characteristics similar to a Pt electrode after the formation of a Pt monolayer shell, suggesting that the Pd core nanoparticles were almost covered with the Pt monolayer shell. The oxygen reduction reaction (ORR) on both Pt/Pd/CB proceeded in 4-electron reduction mechanism. Both Pt/Pd/CB electrodes was ca. 1.5 times higher in ORR activity per electrochemical surface area of Pt (specific activity, SA) than the commercial Pt nanoparticles-loaded CB (Tanaka Kikinzoku Kogyo, Pt/CB-TKK) electrode, and the Pt/Pd3.3/CB electrode had higher SA than the Pt/Pd4.2/CB electrode. The ORR activity per unit mass of Pt for both Pt/Pd/CB electrodes was 5.0 and 5.5 times as high as that for the Pt/CB-TKK electrode, respectively. The durability of both Pt/Pd/CB electrodes was comparable to that of Pt/CB-TKK.

Effect of oleylamine concentration on the structure and oxygen reduction activity of carbon-supported surface-Pt-enriched Pt 3 Au electrocatalysts

For carbon-supported Pt-enriched Pt3Au nanoparticles (Pt3Au/C), the effect of oleylamine (OA) concentration during nanoparticle synthesis is investigated. When the OA concentration is increased from 0.12 mM to 1.2 mM, the particle size of Pt3Au gradually decreased from 5.1 nm to 3.3 nm, whereas the crystal structure and surface Pt concentration are not significantly influenced. With higher OA concentration, the oxygen reduction reaction activity of the Pt3Au/C is largely enhanced, which can be explained by the combined effect of increased specific activity and electrochemical surface area of Pt. It is experimentally confirmed using a bulk CO-oxidation technique that smaller particle size (larger OA concentration) leads to decreased OH adsorption strength, which originates from the double layer overlapping effect of closely adjacent nanoparticles.

Preparation of Extremely Small Pt Nanoparticle Catalysts and Their Electrocatalytic Activity for Oxygen Reduction Reaction

Introduction Polymer electrolyte fuel cells (PEFCs) have attracted great attention as the next generation energy device for electric vehicles due to their high energy conversion efficiency and low emission of pollutants. Pt nanoparticles-loaded carbon black (Pt/CB) is widely used as an electrocatalyst in PEFCs. However, it is necessary to minimize the consumption of Pt because of its high cost and diminishing reserves.CO is known to be strongly adsorbed on Pt atoms. If Pt nanoparticles are prepared in a CO atmosphere, the growth of Pt may be suppressed. Based on the above idea, we have prepared a Pt-carbonyl cluster complex ([Pt3(CO)3 (μ-CO)3]3 2−) by bubbling CO in an acetonitrile-water (1:1 in volume ratio) mixed solution containing PtCl6 2−.1 The complex spontaneously adsorbed on CB, and oxidatively broke and aggregated to form Pt nanoparticles in air. The resultant Pt nanoparticles had the monodispersed particle size of 1.8±0.3 nm. If the Pt-carbonyl cluster complex were fixed on CB with a stabilizer, the aggregation of the resultant Pt nanoparticles could be suppressed. In this study, we designed to inhibit the growth of Pt nanoparticles by electrostatically binding several kinds of stabilizers with the anionic complex on CB before converting to Pt. We characterized the resultant extremely small Pt nanoparticles-loaded CB and evaluated its catalysis for oxygen reduction reaction. Experimental Several cationic molecules, polydiallyldimethylammonium-chloride (PDDA), methyl-viologen(MV), benzyl-viologen(BV), cetyltrimethylammonium-bromide(CTA), pyridine(Pyr), bipyridine(Bpy) and terpyridine (Tpy), were used as a stabilizer in this study. They were mixed after the Pt-carbonyl cluster complex dianion was adsorbed on CB, leading to the production of ionic couples due to electrostatic interaction. Results and Discussion Pt nanoparticles prepared with stabilizers except CTA were smaller in mean size than conventional Pt nanoparticles (ca. 1.8 nm) prepared without stabilizer, suggesting that each stabilizer inhibited the growth of Pt nanoparticles on CB.In ORR, specific activity (SA), activity per unit electrochemical surface area of Pt, and mass activity (MA), activity per mass of Pt, for the Pt/CB catalyst prepared with bipyridine (Pt/CB+Bpy) were ca. 1.4 and 1.6 times higher than those of the commercial catalyst (Pt/CB-TKK(Tanaka Kikinzoku Kogyo TEC10E50E)), respectively. X-ray photoelectron spectra exhibited that in Pt(4f) core level spectra of Pt/CB+Bpy, a peak assigned to metallic Pt shifted to higher binding energy because of the final state effect and a decrease in electron density of Pt surface atoms due to adsorption of electron-withdrawing stabilizers, which seems to weaken Pt-O bond and increase SA and MA. Reference 1. E. Higuchi, A. Taguchi, K. Hayashi, H. Inoue, J. Electroanal. Chem., 663, 83-89 (2011).

A Hierarchical Nanoporous PtCu Alloy as an Oxygen-Reduction Reaction Electrocatalyst with High Activity and Durability.

A hierarchical nanoporous (HNP) PtCu alloy was successfully fabricated by two-step dealloying of a well-designed PtCuAl precursor alloy combined with an annealing operation. The as-made PtCu alloy has an interconnected hierarchical network architecture with controllable bimodal ligament/pore distributions obtained by adjusting the annealing temperature and the redealloying time. HNP-PtCu exhibits superior specific and mass activities for the oxygen-reduction reaction (ORR) compared with single-modal nanoporous PtCu and commercial Pt/C catalysts. More importantly, the HNP-PtCu alloy shows much higher structure stability with less loss of the electrochemical surface area of Pt and ORR activity upon long-term potential scans in acid solution. The excellent electrocatalytic performance for ORR makes the HNP-PtCu alloy attractive as efficient cathode electrocatalysts in fuel-cell-related technologies.

Deterioration Behavior of Pt in a Pt/Au Nanoparticle-Loaded Carbon Black Catalyst Analyzed by an Accelerated Durability Test

Au nanoparticles with 2.5 ± 0.5 nm in size were prepared on carbon black by bubbling CO as a reducing agent in a KAuCl4 aqueous solution containing glutathione as a stabilizer (Au-G/CB). The Au nanoparticles after heat treatment (Au-G/CB-HT) had the mean particle size and size distribution of 2.8 ± 0.5 nm. Pt monolayer was deposited on the Au nanoparticles by underpotential deposition and the following galvanic replacement (Pt/Au-G/CB-HT). The activity per unit mass of Pt for oxygen reduction reaction for the Pt/Au-G/CB-HT at 0.9 V was ca. 6 times as high as that of commercial Pt/CB. In durability tests, the Pt/Au-G/CB-HT/GC electrode was more stable than commercial Pt/CB electrode. The decrease in electrochemical surface area of Pt for the Pt/Au-G/CB-HT during the initial 2000 cycles was caused by both anodic dissolution of Pt into the electrolyte solution and formation of solid solutions of Pt and Au, while after 2000 cycles it was significantly suppressed.

Durability of a PEMFC Pt–Co cathode catalyst layer during voltage cycling tests under supersaturated humidity conditions

We examined the durability of a carbon-supported Pt-based Co alloy catalyst (Pt–Co/C) during voltage cycling tests under supersaturated humidity conditions to simulate events such as flooding. The voltage cycling tests were conducted from 0.4V to 1V for up to 50,000 cycles using 100% relative humidity (RH) hydrogen as the anode gas and 189% RH nitrogen as the cathode gas. We performed cyclic voltammetry (CV) to evaluate the electrochemical surface area (ECSA) and investigated polarization properties to evaluate the activity of Pt–Co/C. The ECSA of Pt–Co/C decreased more slowly than that of a platinum catalyst (Pt/C) during tests (percentages of initial ECSA were 62% and 37% for Pt–Co/C and Pt/C, respectively), either because (1) initial Pt–Co/C was not dispersed unlike Pt/C, or (2) Pt/C exhibited stronger resistance to agglomeration than Pt–Co/C. We concluded that Pt–Co/C exhibited high oxygen reduction reaction (ORR) activity and durability during voltage cycling tests because the mass activity of Pt–Co/C was about twice that of Pt/C throughout the 50,000 cycles. Electrocatalytic specific activities of Pt–Co/C were constant and about twice those of Pt/C for 4000–50,000 cycles, indicating that alloying can increase intrinsic activity for up to 50,000 cycles.

Degradation heterogeneities induced by repetitive start/stop events in proton exchange membrane fuel cell: Inlet vs. outlet and channel vs. land

This paper investigates aging heterogeneities that set up during PEMFC operation in start-up and shut down conditions. The spatially-resolved analyses were based on in situ measurements of local current densities, electrochemical surface area of Pt at the cathode in a segmented cell and overall CO2 generation. In complement, ex situ physicochemical analyses were performed after the PEMFC testing, using scanning and transmission electron microscopy as well as focused ion beam scanning electron microscopy, to probe the micro and nano-scale of the cathode catalyst layer. In the present cell configuration (counter-flow mode, impact of the shut-down events negligible versus that of start-up), two kinds of aging heterogeneities are witnessed. Firstly, the performances loss at the air outlet/H2 inlet is less important than at the air inlet/H2 outlet; fuel starvation events are prevalent in this latter region; the resulting local loss of performances are linked to the distribution of the internal currents along the cell, and to larger physicochemical changes of the cathode catalyst layer in the air outlet/H2 inlet region. The faradic part of the internal currents (generated during the fuel starvation events) was identified to not only come from the electrooxidation of the carbon support of the cathode catalytic layer (CL): internal currents may also originate from Pt dissolution, carbon corrosion in the gas diffusion layer (GDL) and/or water oxidation. Secondly, the degradations at the air inlet/H2 outlet also depend on the position during the aging, either under a channel or a land. Fuel starvation events are more pronounced in land regions, due to slower removal of oxygen trapped under a land (the source of the fuel starvation) in the anode compartment during a start-up. Finally, it is wise pointing out that the particular degradation mechanism witnessed in this study would not have been observed if the MEA had been aged following a standardized stress test (potential cycling under N2-atmosphere); therefore, one may question whether the DOE standardized stress-test procedure is relevant to mimic real fuel cell operation.

Nanoporous PtCo and PtNi alloy ribbons for methanol electrooxidation

Nanoporous (NP) PtCo and PtNi alloy ribbons with predetermined bimetallic compositions are easily fabricated by one step of mild dealloying, which are characterized by uniform three-dimensional bicontinuous network architecture with the ligament size as small as 3 nm. Compared with E-TEK Pt/C catalyst, the as-made NP-PtCo(Ni) alloys exhibit superior specific activity with the lower peak potential and enhanced CO-tolerance toward methanol electrooxidation. More importantly, these nanomaterials also show much higher structure stability with little loss of the electrochemical surface area of Pt upon 5000 potential cycles in acid solution. X-ray photoelectron spectroscopy and DFT calculations revealed that alloying with Co or Ni modifies the electronic structure of Pt with the downshift of Pt d-band center, thus resulting in the improved methanol oxidation activity and decreased CO poisoning.

Microwave-assisted microemulsion synthesis of carbon supported Pt-WO3 nanoparticles as an electrocatalyst for methanol oxidation

A microwave-assisted microemulsion synthesis of carbon supported Pt-WO3 nanoparticles is presented. Amorphous WO3 nanoparticles of 1.0nm in size were first deposited onto carbon from an alkaline tungstate containing microemulsion via mixing with an acid containing microemulsion under controlled microwave exposure. Platinum was subsequently deposited onto the carbon supported WO3 nanoparticles by reducing H2PtCl6 in a microemulsion under controlled microwave exposure forming nanoparticles with an average size of 2.5nm. This method produced a homogeneous distribution of nanoparticles on the carbon support with a uniform Pt:W ratio. The Pt:W can be well controlled by varying the metal precursors’ Pt:W ratio. Hydrogen adsorption, CO-stripping and Cu-UPD stripping method were used to estimate the electrochemical surface area of Pt in the Pt-WO3 mixed catalyst system. Cyclic voltammetry and chronopotentiometry experiments demonstrate that a 1:1 ratio of Pt:W has the highest electrocatalytic activity based on Pt mass for methanol oxidation in sulfuric acid. The enhanced electrocatalytic activity is attributed to both physical and chemical influence of the WO3 component.