Methanol oxidation reaction on core-shell structured Ruthenium-Palladium nanoparticles: Relationship between structure and electrochemical behavior

In this work the relationship between structural composition and electrochemical characteristics of Palladium(Pd)-Ruthenium(Ru) nanoparticles during alkaline methanol oxidation reaction is investigated. The comparative study of a standard alloyed and a precisely Ru-core-Pd-shell structured catalyst allows for a distinct investigation of the electronic effect and the bifunctional mechanism. Core-shell catalysts benefit from a strong electronic effect and an efficient Pd utilization. It is found that core-shell nanoparticles are highly active towards methanol oxidation reaction for potentials â¥0.6 V, whereas alloyed catalysts show higher current outputs in the lower potential range. However, differential electrochemical mass spectrometry (DEMS) experiments reveal that the methanol oxidation reaction on core-shell structured catalysts proceeds via the incomplete oxidation pathway yielding formaldehyde, formic acid or methyl formate. Contrary, the alloyed catalyst benefits from the Ru atoms at its surface. Those are found to be responsible for high methanol oxidation activity at lower potentials as well as for complete oxidation of CH3OH to CO2 via the bifunctional mechanism. Based on these findings a new Ru-core-Pd-shell-Ru-terrace catalyst was synthesized, which combines the advantages of the core-shell structure and the alloy. This novel catalyst shows high methanol electrooxidation activity as well as excellent selectivity for the complete oxidation pathway

A comb-like ionomer based on poly(2,6-dimethyl-1,4-phenylene oxide) for the use as anodic binder in anion-exchange membrane direct methanol fuel cells

Ionomeric binders are crucial to a proper use of fuel cells. In anion-exchange membrane direct methanol fuel cells (AEM-DMFCs) the requirements for the ionomeric binders are anionic conductivity, chemical and thermal stability and facilitation of the reactant and product transport in the three-phase boundary. In this study, we present the synthesis of a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) based comb-like ionomer and the physical and electrochemical characterization of this ionomer. The ionomer consists of a PPO backbone and a partially fluorinated cationic sidechain. In comparison to an imidazolium functionalized PPO ionomer, the comb-like ionomer showed higher stability regarding ionic conductivity and ion exchange capacity in alkaline media. Besides this, a low swelling ratio for 4 M methanol anodic fuel was found for the ionomer making it a suitable candidate for an application in the anodic catalyst layer in AEM-DMFCs. Thermogravimetric analysis coupled with a mass spectrometer proved the ionomer to be thermally stable in AEM-DMFC working conditions. Therefore, the comb-like ionomer was implemented in single cells as a catalyst binder. A single cell with an ionomer content of 25 wt% showed a 2.75 times higher power output compared to a single cell with PIPE as anode catalyst binder when run with KOH-free anodic fuel

Influence of Shell Thickness on Durability of Ru@Pt Core-Shell Catalysts for Reformate PEM Fuel Cells

For the use of polymer electrolyte membrane fuel cells (PEMFCs) in heavy duty applications, on-site generation of hydrogen rich gas by reforming hydrocarbons or methanol are a suitable alternative to pure H2 due to higher availability and higher energy density of hydrocarbon fuels [1]. The so-called reformate contains in addition of H2 also CO, which is a catalyst poison. PtRu alloys are the most promising catalysts for PEM fuel cells operated with reformate due to their high CO-tolerance [2]. It is known that the less noble Ru is unstable in acidic PEMFC conditions and is lowering the durability of the membrane electrode assembly (MEA) by dissolution and crossing over through the membrane to the cathode [3]. Ru dissolution and crossover result in a decreased CO-tolerance on the anode and a lower activity of the cathode towards oxygen reduction due to Ru blocking the Pt surface of the cathode catalyst [4]. To prolong the lifetime and CO-tolerance of reformate PEMFCs, the stability of the anode catalysts must be improved. One approach is nano-structuring the catalysts by encapsulating the less noble Ru-core with a Pt-shell. In doing so, the corrosion resistant Pt-shell supposes to protect the Ru-core from dissolution and thus mitigates the Ru crossover phenomenon.In this work, the influence of the Pt-shell thickness on the stability and performance of the Ru@Pt catalysts is investigated. Two catalysts with varying Pt-shell thicknesses were synthesized on Vulcan XC72R carbon via a two-step polyol. The catalyst with the thinner Pt-shell is called Ru@1Pt and with the thicker shell Ru@2Pt. The as-synthesized catalysts were physically characterized by XRD, ICP-OES, TEM and EDS. A representative EDS map of Ru@1Pt is shown in fig. 1a. The physical characterization of the obtained catalysts unveils a Ru@Pt core-shell structure with crystalline hcp Ru and fcc Pt. To further investigate the catalysts electrochemically, MEAs were manufactured using Ru@Pt catalysts as anodes and commercial membranes and cathode catalysts. An accelerated stress test (AST) was developed to target Ru dissolution by potential cycling and was applied to investigate and compare the stability of the catalysts. The MEAs were electrochemically characterized by cyclic voltammetry, CO-stripping and U-I curves before, during and after the AST. Post-mortem cross-section STEM analysis of the stressed cells were performed to evaluate the elemental composition and morphology of the anode and cathode after the stress test.During the AST, CO-stripping on the Ru@1Pt anode showed a peak shift to higher potentials, indicating a decrease in CO-tolerance due to Ru loss. By contrast, on the Ru@2Pt anode, the peak shifted to lower potentials implying a stronger promoting influence of Ru. On the cathode side, CV measurements before and after the AST revealed a double layer increase and a decrease in the HUPD. This change implies the poisoning of Pt with Ru and were more pronounced for Ru@1Pt compared to Ru@2Pt. As seen by the CO-stripping and CV results, the Ru dissolution and crossover was more severe for Ru@1Pt than for Ru@2Pt. Hence, a higher fuel cell performance loss was observed for the cell with Ru@1Pt in H2, as can be seen in fig. 1b. In addition, further U-I curves were measured under varying reformate conditions to identify the influence of CO on the cell performance. Cross-section EDS maps of the stressed MEAs were used to evaluate the Ru dissolution and its distribution in the different layers of the MEA. A higher Ru:Pt ratio was found on the cathode for the cell with Ru@1Pt compared to a cell with Ru@2Pt, which further verifies a higher degree of Ru crossover for the catalyst with the thinner Pt-shell. The above findings could evidence that the shell thickness plays a significant role in the corrosion resistance of core-shell reformate anode fuel cell catalysts.References[1] B. Du, R. Pollard, J.F. Elter, M. Ramani, in: F.N. Büchi, M. Inaba, T.J. Schmidt (Eds.), Polymer Electrolyte Fuel Cell Durability, Springer New York, New York, NY, 2009, pp. 341–366.[2] O.A. Petrii, J Solid State Electrochem 12 (2008) 609–642. https://doi.org/10.1007/s10008-007-0500-4.[3] E. Antolini, J Solid State Electrochem 15 (2011) 455–472. https://doi.org/10.1007/s10008-010-1124-7.[4] L. Gancs, B.N. Hult, N. Hakim, S. Mukerjee, Electrochem. Solid-State Lett. 10 (2007) B150. https://doi.org/10.1149/1.2754382.Figure

Methanol oxidation on Ru- or Ni-modified Pd-electrocatalysts in alkaline media: A comparative differential electrochemical mass spectrometry study

In this work, the methanol oxidation mechanism was investigated in detail on Ru- or Ni-modified, Pd-based electrocatalysts. The electrochemical activity towards methanol oxidation reaction, the surface inhibition behavior and the CO2 current efficiency was evaluated for the catalysts by cyclic voltammetry, chronoamperometry, CO adsorbate stripping and CH3OH adsorbate stripping experiments in a differential electrochemical mass spectrometry flow cell setup. Pd5Ni/C and Pd3Ru/C catalysts showed higher current densities, lower oxidation onset potentials and lower sensitivity to surface inhibition than Pd/C. Due to detailed studies on the CO2 current efficiencies, side products and adsorbate reactions via differential electrochemical mass spectrometry, novel insights into the electrooxidation of methanol on Pd5Ni/C and Pd3Ru/C were provided. While modification with Ru resulted in CO2 current efficiencies well over 80%, Ni-modified catalyst was not able to enhance the CO2 current efficiency

On the design of a comb-shaped, poly(phenylene oxide)-based anodic binder for anion-exchange membrane direct methanol fuel cell (AEM-DMFC)

In this study, we present the synthesis of a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) based comb-like ionomer and the characterization of this ionomer. The ionomer consists of a PPO backbone and a partially fluorinated cationic sidechain. The comb-like ionomer showed high stability regarding ionic conductivity and ion exchange capacity in alkaline media. Besides this, a low swelling ratio for 4M methanol anodic fuel was found for the ionomer making it a suitable candidate for an application in the anodic catalyst layer in AEM-DMFCs. Evaluation of the methanol diffusion with a mass exchanger unit coupled to a mass spectrometer showed that the partially fluorinated sidechain lowers methanol diffusion compared to a non-fluorinated ionomer. Nevertheless, the comb-like ionomer was implemented in single cells as a catalyst binder and provided strong enhancement in single cell performance when the cell was run with KOH-free anodic fuel

Functionalization of multi walled carbon nanotubes with indazole

In this work, the functionalization of carbon nanotubes (CNTs) with indazole groups covalently bound to the CNTs’ surface is reported. The novel material was structurally characterized via near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) and successful functionalization was proven. As the novel material is a potential candidate for catalyst support application in high-temperature proton-exchange membrane fuel cells (HT-PEMFCs), thermal and electrochemical stability of the novel material was investigated. Measurements via thermogravimetric analysis coupled to a mass spectrometer (TGA-MS) showed that the indazole-functionalized CNTs are thermally stable until a temperature of approx. 300 °C is reached. The thermal degradation of the functional group was tracked by monitoring the evolution of NOX gases. Furthermore, electrochemical stability of the novel material was evaluated using high-temperature differential electrochemical mass spectrometry (HT-DEMS) under gas-phase conditions. Compared to unmodified CNTs, it was shown that the functionalization leads to a slightly increased electrochemical carbon corrosion. However, the indazole-functionalized CNTs show a higher electrochemical stability than carbon black (Vulcan XC72R) typically used as catalyst support in HT-PEMFCs. In comparison to unmodified CNTs, functionality tests of the indazole-functionalized CNTs showed a better dispersibility in water and a lower contact angle with concentrated H3PO4, which is the electrolyte in HT-PEMFCs. Ultimately, ion exchange capacity measurements showed that the indazole-functionalized CNTs are able to bind protons in the catalyst layer and, therefore, potentially improve the catalyst-electrolyte interface as well as the proton conductivity in the catalyst layer

Impact of Carbon Support Functionalization on the Electrochemical Stability of Pt Fuel Cell Catalysts

Nitrogen-enriched porous carbons have been discussed as supports for Pt nanoparticle catalysts deployed at cathode layers of polymer electrolyte membrane fuel cells (PEMFC). Here, we present an analysis of the chemical process of carbon surface modification using ammonolysis of preoxidized carbon blacks, and correlate their chemical structure with their catalytic activity and stability using in situ analytical techniques. Upon ammonolysis, the support materials were characterized with respect to their elemental composition, the physical surface area, and the surface zeta potential. The nature of the introduced N-functionalities was assessed by X-ray photoelectron spectroscopy. At lower ammonolysis temperatures, pyrrolic-N were invariably the most abundant surface species while at elevated treatment temperatures pyridinic-N prevailed. The corrosion stability under electrochemical conditions was assessed by in situ high-temperature differential electrochemical mass spectroscopy in a single gas diffusion layer electrode; this test revealed exceptional improvements in corrosion resistance for a specific type of nitrogen modification. Finally, Pt nanoparticles were deposited on the modified supports. In situ X-ray scattering techniques (X-ray diffraction and small-angle X-ray scattering) revealed the time evolution of the active Pt phase during accelerated electrochemical stress tests in electrode potential ranges where the catalytic oxygen reduction reaction proceeds. Data suggest that abundance of pyrrolic nitrogen moieties lower carbon corrosion and lead to superior catalyst stability compared to state-of-the-art Pt catalysts. Our study suggests with specific materials science strategies how chemically tailored carbon supports improve the performance of electrode layers in PEMFC devices