Porous Electrocatalyst Research Articles

3D Hierarchical Porous Mo2 C for Efficient Hydrogen Evolution.

Porous electrocatalyst for hydrogen production. 3D hierarchical porous molybdenum carbide provides a low operating potential (97 mV at 10 mA cm(-2) ). These beneficial textures of large specific surface area (302 m(2) g(-1) ) and hierarchical porous architecture containing dominant pore size distribution peak at 11 Å in width can provide large surface active sites and facilitate proton mass transport.

Onion-like graphitic nanoshell structured Fe–N/C nanofibers derived from electrospinning for oxygen reduction reaction in acid media

Hollow onion-like graphitic nanoshell structured Fe–N/C nanofiber (Fe–N/CNF) catalyst with porous morphology was prepared by heat treating as-spun polyacrylonitrile/ferrous oxalate composite nanofibers in ammonia atmosphere for the first time. These porous electrocatalyst showed both excellent catalytic activity for oxygen reduction reaction (ORR) and much better stability than commercial Pt/C catalyst in acid solution. The Fe–N/CNF catalysts developed here could be easily fabricated on a large scale and show high potential in proton exchange membrane fuel cells (PEMFCs).

Pt/Carbon Catalyst Layer Microstructural Effects on Measured and Predicted Tafel Slopes for the Oxygen Reduction Reaction

The important effect of the physical and morphological properties of porous cathodes on the oxygen reduction reaction (ORR) Tafel slope, which is normally viewed as a mechanistic parameter, was investigated using a variety of Pt/carbon (Nafion) catalyst layers in room temperature O2-saturated sulfuric acid solutions. Consistent with previous theoretical predictions, it was found experimentally that increasing the pore length, the catalyst layer resistance, and the exchange current density, and decreasing the pore diameter, all serve to cause the Tafel slope to be larger than its mechanistically predicted value, thus leading to performance loss. Theoretical Tafel slopes, calculated using a model for the migration-induced distribution of potentials in an electroactive porous layer and employing the catalyst layer properties examined experimentally, showed very good agreement with the measured Tafel slopes. These results reveal the importance of minimizing the ohmic resistance of porous electrocatalyst layer...

The use of CO stripping for in situ fuel cell catalyst characterization

An important parameter in normalizing activities of porous electrocatalysts is real surface area of the catalyst. In this study CO stripping voltammetry was applied for the determination of the surface area of PtRu catalysts in a membrane electrode assembly. The difficulties in applying this method are summarized and a practical solution for the “accurate” voltammetric CO charge determination is suggested. The influence of adsorption time, potential, sweeping range and sweep rate on voltammetric CO charge determination is studied. It is shown that the CO charge determined in this way is independent on the sweep rate applied.

Performance evaluation of porous electrocatalysts via normalization of the active surface

When comparing the rate of electrochemical processes at different porous electrocatalysts a surface normalization should be used. It is shown for the case of methanol oxidation at PtRu layers electrodeposited on gold substrates that substantially different data are obtained for current, mass spectrometry signals and integrated IR band intensities of the products, with and without normalization of the catalyst surface used. Using stripping of saturated CO coverage as a normalization tool, cyclic voltammograms, on line MS and in situ FTIR spectroscopy give reasonable agreement of catalytic activity towards methanol oxidation.

The real-area scaling factor in electrocatalysis and in charge storage by supercapacitors

In comparative evaluation of high-area, porous electrocatalyst materials, an important “scaling factor” arises connected with the real-to-apparent area ratio, R , of the material. At a given operating current in a fuel-cell or electrolyser system, the polarisation and hence power consumption of the system is determined by the real current-density, taking into account the scaling factor R . In the case of comparative evaluation of various porous electrode materials, e.g. for H 2 evolution electrocatalysis, it is important to know the extent to which improvements to performance are due to intrinsic changes of electrocatalysis on account of fine-particle structure or are simply due to large R values. Procedures for real area evaluation are reviewed and experimental applications are made to evaluation of effectiveness of several electrodeposited composite materials for improvement of cathodic H 2 evolution performance in the water electrolysis process. Conditions under which real area determinations at porous electrodes can be made by the “double-layer” capacitance method are critically examined and a resulting satisfactory procedure is described.

Problem of in situ real-area determination in evaluation of performance of rough or porous, gas-evolving electrocatalysts. Part 2.—Unfolding of the electrochemically accessible surface of rough or porous electrodes: a case-study with an electrodeposited porous Pt electrode

The problem of evaluation of the electrochemically effective area of porous electrocatalyst materials is treated by examination of the kinetics of the hydrogen evolution reaction (HER) from aqueous KOH at a plated porous Pt electrode in relation to in situ real-area determination by means of scanning electron microscopy (SEM), cyclic voltammetry, potential relaxation and impedance measurements. With regard to its electrochemical accessibility, the rough or shallow-pore electrode surface can be ‘unfolded’ experimentally by increasing the electrolyte conductivity, i.e. simplifying the transmission-line model by diminishing the distributed electrolyte resistance. It was found in the impedance studies that, for the same porous plated Pt electrode, the constant phase element exponent, α, associated with double-layer capacitance varies from 0.56 to 1.0 for the HER in 0.5 and 2.0 mol dm–3 KOH solution, respectively. That is, α is not an intrinsic property of a given porous electrode, but depends strongly on the conductivity of electrolyte solution in the pores.

A hydrodynamic model for water decomposition in electrolyzers with a solid polymer electrolyte

A new hydrodynamic model for two-phase water-electrolysis filtration in porous electrocatalysts and current collectors using a solid polymer electrolyte has been developed. The model takes into account the effect due to the internal structure of the porous elements on gas-liquid distribution and transport and electrochemical characteristics. Owing to the finite hydrodynamic permeability of porous elements, a limiting mass-exchange current was shown to set in, with the voltage tending to infinity because of the electrolyte depletion within the electrochemical reaction zone. The efficiency of the porous current collectors of various types has been considered under specified process conditions using computational modelling.

Reaction kinetics in porous electrocatalysts

In a theoretical and experimental study of the effect of pore diffusion on the electrode kinetics of non-first-order reactions (e.g., hydrocarbon fuel cell reactions), a simple pore model was used successfully to describe the reaction in the pores of a partially wettable electrocatalyst. Experiments on the electrogenerative hydrogenation of gaseous olefins (ethylene through butylene) and of liquid benzene on polytetrafluoroethylene-bonded porous electrodes, activated with 9 mg/sq cm platinum or palladium black, were in good agreement with calculations by the model and showed that partially wettable porous catalytic electrodes behave kinetically similar to conventional catalysts when pore diffusion is slow. Kinetic studies on porous electrocatalysts can, therefore, be meaningful for catalyst characterization and for the design of electrochemical reactors.

Mass Transport Processes at Porous Electrocatalysts in the Electrogenerative Reactor

Mass transport limitations were investigated in the electrolyte, gas phase, and in the pores of platinum and palladium black porous electrocatalysts using the electrogenerative reactor. Limiting currents were identical for both catalysts, depending on a power of the bulk reactant concentration, , where ; can be the electrolyte concentration of hydrogen ions or of dissolved ethylene. Mathematical modeling and comparison with the experimental equation provided information regarding limiting processes. In a matrix cell diffusion limited ionic transport at the ethylene cathode. In free electrolytes, free convection can assist mass transport and improve limiting currents. Gas phase limiting current densities were examined in terms of a model involving limiting radial diffusion of dissolved ethylene with a concentration‐dependent surface area and pore utilization.

Electrocatalysis; selective electrogenerative reduction of organic halides

Reduction of vinyl and ethyl fluorides and chlorides has been investigated under the operating conditions of the electrogenerative reactor. Reaction selectivity and rate were examined over palladium, platinum, ruthenium and silver black porous electrocatalysts operating at positive potentials with special attention given to vinyl fluoride reduction. Substrate partial pressure and flow rate as well as electrolyte concentration were considered as potential parameters for selectivity control. The effect of the position of the halide has also been studied. Results are compared and contrasted with conventional heterogeneous catalytic and electrochemical reduction. Palladium is the most selective electrocatalyst for double bond hydrogenation, while platinum catalyzes cleavage preferentially. Differences in selectivity are discussed in terms of an insertion-type mechanism and an electrochemical hydrodehalogenation step leading to cleavage.