Porous Catalyst Films Research Articles

Boosting electrocatalytic oxygen evolution using ultrathin carbon protected iron–cobalt carbonate hydroxide nanoneedle arrays

Nonprecious metal hydroxides are promising candidates for oxygen evolution reaction (OER) catalysts. Enhancing charge transfer efficiency and optimizing their surface states are crucial to further improve their OER activity for practical applications. Herein, using a facile hydrothermal reaction and low-temperature calcination process, we synthesize ultrathin carbon coated, nitrogen and iron modified cobalt carbonate hydroxide needle arrays on carbon cloth (C@NFeCoCH/CC). The synergistic effect of Fe and N modification induces high OER activity of C@NFeCoCH/CC and a conversion of rate determining step for OER, which mainly benefits from highly active Co/Fe–N–C species in situ formed on the interface of carbon layer and needles. Thin carbon layer, N and Fe modification, along with continuous conductive carbon cloth guarantee fast and stable charge transfer from bulk of free-standing porous catalyst film to its interface. It is found that the optimized C@NFe1Co1CH/CC exhibits a low overpotential of 235 mV at 10 mA cm−2, a mass activity of 681.4 A g−1 and a superior stability over 30 h at 10 mA cm−2. Moreover, experiments demonstrate that the Co/Fe sites possess higher OER activity than the N–C sites, while the Co/Fe–N (-C) sites can obviously outperform the Co/Fe–O sites.

Nafion‐Free Carbon‐Supported Electrocatalysts with Superior Hydrogen Evolution Reaction Performance by Soft Templating

AbstractEfficient water electrolysis requires electrode coatings with high catalytic activity. Platinum efficiently catalyzes the hydrogen evolution reaction in acidic environments, but is a rare and expensive metal. The activity achieved per metal atom can be increased if small Pt particles are dispersed onto electrically conductive, highly accessible and stable support materials. However, the addition of Nafion, a typical binder material used in the manufacture of electrode coatings, can decrease catalytic activity by the blocking of pores and active surface sites. A new approach is reported for the direct synthesis of highly active Nafion‐free Pt/C catalyst films consisting of small Pt nanoparticles supported in size‐controlled mesopores of a conductive carbon film. The synthesis relies on the co‐deposition of suitable Pt and C precursors in the presence of polymer micelles, which act as pore templates. Subsequent carbonization in an inert atmosphere produces porous catalyst films with controlled film thickness, pore size and particle size. The catalysts clearly outperform all Nafion‐based Pt/C catalysts reported in the literature, particularly at high current densities.

Comparative study of the electrochemical promotion of CO2 hydrogenation on Ru using Na+, K+, H+ and O2 − conducting solid electrolytes

The kinetics and the electrochemical promotion of the hydrogenation of CO2 to CH4 and CO are compared for Ru porous catalyst films deposited on Na+, K+, H+ and O2− conducting solid electrolyte supports. It is found that in all four cases increasing catalyst potential and work function enhances the methanation rate and selectivity. Also in all four cases the rate is positive order in H2 and exhibits a maximum with respect to CO2. At the same time the reverse water gas shift reaction (RWGS) which occurs in parallel exhibits a maximum with increasing pH2 and is positive order in CO2. Also in all cases the selectivity to CH4 increases with increasing pH2 and decreases with increasing pCO2. These results provide a lucid demonstration of the rules of chemical and electrochemical promotion which imply that (∂r/∂Φ)(∂r/∂pD)>0 and (∂r/∂Φ)(∂r/∂pA)<0, where r denotes a catalytic rate, Φ is the catalyst work function and pDandpA denote the electron donor and electron acceptor reactant partial pressures respectively.

Evaluation of kinetic constants on porous, non-noble catalyst layers for oxygen reduction—A comparative study between SECM and hydrodynamic methods

An advanced approach based on scanning electrochemical microscopy (SECM) was used to investigate the kinetics of the oxygen reduction reaction (ORR) on multiwalled carbon nanotubes (MWCNTs) and a composite of MWCNTs and cobalt (IX) protoporphyrin (MWCNTs/CoP). The amount of hydrogen peroxide produced during ORR was studied as a function of catalyst loading in an electrolyte of pH 7. Additionally, a Pt ultra microelectrode (UME) was used to determine changes in interfacial oxygen concentration from which intrinsic rate constants of heterogeneous electron transfer during the ORR were calculated. The amount of hydrogen peroxide produced and the number of electrons exchanged during oxygen reduction, and the heterogeneous electron transfer rate constants determined using SECM were compared with the corresponding values obtained using methods based on forced convection, namely RRDE and RDE. It was found that SECM offers some advantages compared to RDE or RRDE with regard to accuracy in determining the number of electrons transferred during the ORR, particularly in the case of thick and porous catalyst films. However, the heterogeneous electron transfer rate constants were similar for both methods, indicating that the determination of the surface concentration of reactants using RC-SECM suffers from some drawbacks.

Coupling catalysis and electrocatalysis for hydrogen production in a solid electrolyte membrane reactor

This study reports the production of H2 via catalytic methane decomposition on Pt supported on yttria-stabilized zirconia together with its electrochemical regeneration in a solid electrolyte membrane reactor. Hence, a Pt-YSZporous/YSZ/Pt double-chamber solid electrolyte cell was prepared and tested under two reaction regimes. In the first regime, under open-circuit conditions, hydrogen and carbon are produced on the catalytically active Pt-YSZ porous catalyst via methane decomposition reaction (CH4 (g) → C (s)+2H2 (g)). In the second regime, under polarization conditions, steam is electrolyzed at the Pt cathode of the cell (H2O+2e−→H2+O2−) and the produced O2− ions were simultaneously electrochemically pumped to the Pt/YSZ porous catalyst (anode), thereby allowing removal of the previously deposited carbon (C (s)+O2−→CO2 (g)) and finally regenerating the Pt/YSZ porous catalyst film. We demonstrated that the carbon generated in the methane decomposition step serves as a depolarizating agent in the steam electrolysis process, thus decreasing the electrical energy input required for electrochemically producing pure H2. In addition, during the regeneration step, C2 hydrocarbons (e.g., ethane and ethylene) were obtained as a result of the electrocatalytic methane oxidative coupling on the Pt/YSZ porous catalyst film. The performance and durability of the system was also verified for long operation times in view of the possible practical application of this novel reactor configuration, which combines gas-phase catalysis and electrocatalysis for hydrogen production.

Layer reduction method for fabricating Pd-coated Ni foams as high-performance ethanol electrode for anion-exchange membrane fuel cells

An ideal electrode architecture that boosts the performance of anion-exchange membrane direct liquid fuel cells needs the electrode design to meet all the requirements of electrochemical kinetics and mass and charge transport characteristics. In this regard, here we propose a facile, well-controlled, and binder-free layer reduction method for preparing a three-dimensional foam electrode, which enables the catalytic particles to be directly reduced on the surface of the metal foam. This innovative layer reduction method not only avoids the formation of large catalytic aggregation, but also promises a thin and porous catalyst film that presents a sponge-like morphology uniformly coated onto the skeleton, thus both improving the electrochemical kinetics and enhancing the mass and charge transport of species. The results demonstrate that the application of the layer-reduced Pd/Ni foam electrode in an anion-exchange membrane direct ethanol fuel cell enables a peak power density and the maximum current density as high as 164 mW cm−2 and 1.34 A cm−2 at 60 °C, respectively, which are 1.03 and 1.16 times higher than those of the conventional design.

Electrochemical promotion of propane oxidation on Pt deposited on a dense β″-Al2O3 ceramic Ag(+) conductor.

A new kind of electrochemical catalyst based on a Pt porous catalyst film deposited on a β″-Al2O3 ceramic Ag+ conductor was developed and evaluated during propane oxidation. It was observed that, upon anodic polarization, the rate of propane combustion was significantly electropromoted up to 400%. Moreover, for the first time, exponential increase of the catalytic rate was evidenced during galvanostatic transient experiment in excellent agreement with EPOC equation.

Electrochemical Promotion of NO Reduction by C2H4 in Excess O2 Using a Monolithic Electropromoted Reactor and Pt–Rh Sputtered Electrodes

The reduction of NO by ethylene in the presence of excess oxygen was investigated in a recently developed monolithic electropromoted reactor (MEPR). In this novel dismantlable monolithic-type electrochemically promoted catalytic reactor, thin (~40 nm) porous catalyst films are sputter-deposited on thin (0.25 mm) parallel solid electrolyte plates supported in the grooves of a ceramic monolithic holder and serve as electropromoted catalyst elements. Using Pt–Rh(1:1)/YSZ/Au-type catalyst elements, the 8-plate reactor operated with apparent Faradaic efficiency exceeding unity achieving significant and reversible enhancement in the rates of C2H4 and NO consumption in presence of up to 10% O2 in the feed at gas flow rates of 1,000 cc/min. The reactor, which is a hybrid between a monolithic catalytic reactor and a flat-plate solid oxide fuel cell, permits easy practical utilization of the electrochemical promotion of catalysis.

Monolithic electrochemically promoted reactors: A step for the practical utilization of electrochemical promotion

A novel dismantlable monolithic-type electrochemically promoted catalytic reactor and “smart” sensor-catalytic reactor unit has been constructed and tested for hydrocarbon oxidation and NO reduction by C 2H 4 in the presence of O 2. In this novel reactor, thin (∼ 40 nm) porous catalyst films made of two different materials are sputter-deposited on opposing surfaces of thin (0.25 mm) parallel solid electrolyte plates supported in the grooves of a ceramic monolithic holder and serve as sensor or electropromoted catalyst elements. The catalyst dispersion was higher than 10%. A 22 flat plate reactor operated with apparent Faradaic efficiency up to 100, at near complete reactants conversion, at gas flow rates up to 30 l/min. The novel design has only two external electrical connections and thus significantly facilitates the practical utilization of electrochemical promotion of catalysis.

Novel monolithic electrochemically promoted catalytic reactor for environmentally important reactions

A novel dismantlable monolithic-type electrochemically promoted catalytic reactor and “smart” sensor-catalytic reactor unit has been constructed and tested for hydrocarbon oxidation and NO reduction by C2H4 in presence of O2. In this novel reactor, thin (∼20–40nm) porous catalyst films made of two different materials are sputter-deposited on opposing surfaces of thin (0.25mm) parallel solid electrolyte plates supported in the grooves of a ceramic monolithic holder and serve as sensor or electropromoted catalyst elements. Using Rh/YSZ/Pt-type catalyst elements, the 22-plate reactor operated with apparent Faradaic efficiency exceeding 25 achieving near complete fuel and NO conversion at 300°C in presence of up to 1.1% O2 in the feed at gas flow rates exceeding 1.3l/min. The metal catalyst dispersion was of the order of at least 15%. The novel reactor design requires only two external electrical connections and permits easy practical utilization of the electrochemical promotion of catalysis.