Cathode Catalyst Loading Research Articles

ECS H.H. Dow Student Achievement Award of the IEEE Division High Performance Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells

Introduction The hydrogen/air proton-exchange membrane fuel cell is a promising candidate for emission-free automotive power plants due to its high power output, energy conversion efficiency, and quick start-up. Of critical importance for fuel cell commercialization is membrane-electrode-assembly (MEA) cost and durability. The supported Pt/C catalyst used in the anode and cathode is expensive and corrosion of the carbon support during fuel cell start-up is a continuing issue that has drawn considerable research attention. Recently, R&D efforts have been directed at reducing Pt loading and fabricating catalysts with supports that can withstand the harsh automotive operating environment. Work in this area includes the investigation of metal alloys, core-shell nanostructures, and metal oxide supports.1,2 Researchers have also focused on creating oxygen reduction reaction catalysts that do not use platinum group metals.3 Another strategy to lower cathode catalyst loading while maintaining high fuel cell power output is to improve the electrode morphology in order to maximize catalyst contact with reactant gases while having a sufficient number of pathways for effective proton and electron conduction. Electrospinning offers the possibility of creating nanofiber structures from a rich variety of different catalytic materials. Previously, we have shown that a cathode electrospun from Johnson Matthey HiSpec 4000 catalyst with a Pt loading of 0.065 mgPt/cm2 produced 29% more power than a painted decal cathode at a much higher Pt loading of 0.104 mgPt/cm2 (360 mW/cm2 vs 280 mW/cm2 at 0.65 V).4 We have also shown that electrospinning is a robust electrode fabrication technique for creating both cathodes and anodes; fuel cell performance was largely insensitive to changes in binder content (28-45 wt%) and nanofiber diameter (250-475 nm).5 Additionally, the nanofiber cathodes exhibited less performance decay after accelerated stress tests that simulate automotive start-stop behavior. Experiments at Nissan Technical Center North America showed that the end-of-life performance of MEAs with nanofiber anodes and cathodes was superior to MEAs with sprayed electrodes with the same catalyst and loading.6 The outstanding performance of the electrospun cathodes is attributed to the unique inter-fiber and intra-fiber porosity of the nanofiber mat, where the catalyst and ionomer are well dispersed, resulting in high electrochemical surface area and high catalyst mass activity.4 Experimental Electrospinning inks were prepared by mixing catalyst with a polymeric binder in an appropriate solvent. For example4, a typical ink consists of: (a) Johnson Matthey HiSpec 4000 (40% Pt on Vulcan carbon), (b) a solution of 20% Nafion ionomer in a solvent of lower aliphatic alcohols + water (Aldrich) and (c) poly(acrylic acid) (PAA) (Mw=450,000, Aldrich). The total polymer and powder content of the spinning solution was 14.0 wt%, where the Pt/C:Nafion®:PAA weight ratio was 72:13:15. The ink was drawn into a 3 mL syringe and electrospun using a 22 gauge stainless steel needle spinneret, where the needle tip was polarized to a potential of +10.0 kV relative to a grounded stainless steel rotating drum collector. The spinneret-to-collector distance was fixed at 10 cm and the flow rate of the ink was 1.0 mL/h. The collector drum rotated at 100 rpm and oscillated horizontally to improve the uniformity of deposited nanofibers. Figure 1 shows a nanofiber mat created with Johnson Matthey HiSpec 4000 catalyst, Nafion, and poly(acrylic acid). MEAs (5 or 25 cm2) were made using a Nafion 211 proton-exchange membrane with electrospun anodes and cathodes. Electrodes were hot-pressed to the membrane at 140oC and 4 MPa for 1 minute after a 10 minute heating period at 140oC and 0 MPa. Carbon paper gas diffusion electrodes (Sigracet GDL 25 BC from Ion Power, Inc) were physically pressed onto the MEA’s anode and cathode in the fuel cell test fixture prior to MEA evaluation. In this paper, recent experimental results will be presented on new cathode catalysts (e.g., non platinum group metal powders and Pt catalysts that do not employ a carbon support), ultra-low Pt–loaded anodes, and binders other than Nafion. The goals of this work are to increase MEA power density, lower anode and cathode Pt loading, and further improve the durability of electrospun electrodes. Acknowledgments This work was funded by EMD Millipore and Nissan Technical Center North America. References Y. Ma, H. Zhang, H. Zhong, T. Xu, H. Jin, and X. Geng, Catalysis Communications, 11, 434-437 (2010).A. Kumar et al. ECS Meeting s MA 2012-02(13):1686.R. Othman, A. L. Dicks, and Z. Zhu, International Journal of Hydrogen Energy, 37, 357-372 (2012).M. Brodt, R. Wycisk, and P. N. Pintauro, Journal of the Electrochemical Society, 160, F744-F749 (2013).M. Brodt, R. Wycisk, P. Pintauro, T. Han, N. Dale, and K. Adjemian, ECS Transactions 58 (1) (2013) 381.T. Han et al. ECS Meeting s MA 2013-02 1306.

Drinking Water Purification by Electrosynthesis of Hydrogen Peroxide in a Power‐Producing PEM Fuel Cell

The industrial anthraquinone auto-oxidation process produces most of the world's supply of hydrogen peroxide. For applications that require small amounts of H2 O2 or have economically difficult transportation means, an alternate, on-site H2 O2 production method is needed. Advanced drinking water purification technologies use neutral-pH H2 O2 in combination with UV treatment to reach the desired water purity targets. To produce neutral H2 O2 on-site and on-demand for drinking water purification, the electroreduction of oxygen at the cathode of a proton exchange membrane (PEM) fuel cell operated in either electrolysis (power consuming) or fuel cell (power generating) mode could be a possible solution. The work presented here focuses on the H2 /O2 fuel cell mode to produce H2 O2 . The fuel cell reactor is operated with a continuous flow of carrier water through the cathode to remove the product H2 O2 . The impact of the cobalt-carbon composite cathode catalyst loading, Teflon content in the cathode gas diffusion layer, and cathode carrier water flowrate on the production of H2 O2 are examined. H2 O2 production rates of up to 200 μmol h(-1) cmgeometric (-2) are achieved using a continuous flow of carrier water operating at 30 % current efficiency. Operation times of more than 24 h have shown consistent H2 O2 and power production, with no degradation of the cobalt catalyst.

Nitrogen-doped ordered porous carbon catalyst for oxygen reduction reaction in proton exchange membrane fuel cells

Three different N-doped ordered porous carbons (CNx) were produced by a nanocasting process using polyaniline as the carbon and nitrogen precursor. A pyrolysis treatment of iron chloride-impregnated CNx under nitrogen is used in the preparation of the carbon composite catalysts, and this is followed by posttreatments and optimization of the iron loading and the pore size. Exploration of the catalytic activity of the CNx products for catalyzing the oxygen reduction reaction (ORR) using rotating disk electrode measurements and single-cell tests shows that the onset potential for ORR of the most effective catalyst in 0.5 M H2SO4 is as high as 0.9 V vs. the normal hydrogen electrode. A proton exchange membrane fuel cell constructed with the catalyst exhibits a current density as high as 0.52 A cm−2 at 0.6 V with 2 atm back pressure using a cathode catalyst loading of 6 mg cm−2. The average pore diameters of synthesized CNx-12, CNx-15, and CNx-16 are 0.7, 4.3, and 14 nm, respectively. It is observed that the pore size and specific surface area are an important factor for increased catalyst activity. The pore size of the most effective catalysts is found to be 4.3 nm.

Activity and active sites of nitrogen-doped carbon nanotubes for oxygen reduction reaction

Nitrogen-doped carbon (CNx) nanotubes were synthesized by thermal decomposition of ferrocene/ethylenediamine mixture at 600–900 °C. The effect of the temperature on the growth and structure of CNx nanotubes was studied by transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. With increasing growth temperature, the total nitrogen content of CNx nanotubes was decreased from 8.93 to 6.01 at.%. The N configurations were changed from pyrrolic-N to quaternary-N when increasing the temperature. Examination of the catalytic activities of the nanotubes for oxygen reduction reaction by rotating disk electrode measurements and single-cell tests shows that the onset potential for oxygen reduction in 0.5 M H2SO4 of the most effective catalyst (CNx nanotubes synthesized at 900 °C) was 0.83 V versus the normal hydrogen electrode. A current density of 0.07 A cm−2 at 0.6 V was obtained in an H2/O2 proton-exchange membrane fuel cell at a cathode catalyst loading of 2 mg cm−2.

Electrocatalytic hydrogenation of acetophenone using a Polymer Electrolyte Membrane Electrochemical Reactor

The use of a solid polymeric electrolyte, spe, is not commonly found in organic electrosynthesis despite its inherent advantages such as the possible elimination of the electrolyte entailing simpler purification processes, a smaller sized reactor and lower energetic costs. In order to test if it were possible to use a spe in industrial organic electrosynthesis, we studied the synthesis of 1-phenylethanol through the electrochemical hydrogenation of acetophenone using Pd/C 30wt% with different loadings as cathode and a hydrogen gas diffusion anode. A Polymer Electrolyte Membrane Electrochemical Reactor, PEMER, with a fuel cell structure was chosen to carry out electrochemical reduction with a view to simplifying an industrial scale-up of the electrochemical process. We studied the influence of current density and cathode catalyst loading on this electroorganic synthesis. Selectivity for 1-phenylethanol was around 90% with only ethylbenzene and hydrogen detected as by-products.

Development of non-precious metal oxygen-reduction catalysts for PEM fuel cells based on N-doped ordered porous carbon

N-doped ordered porous carbon (CNx) was synthesized via a nano-casting process using polyacrylonitrile (PAN) as the carbon and nitrogen precursor and mesoporous silica as a hard template. Nitrogen adsorption/desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the synthesized CNx and the derived non-precious metal oxygen-reduction catalysts. The CNx exhibited a highly ordered porosity and high graphitization with a surface area of 1132m2g−1 and a nitrogen content of 6.88at.%. The non-precious metal oxygen-reduction catalysts were prepared by pyrolyzing iron acetate-impregnated CNx in argon, followed by post-treatments. Optimizations of the iron loading and the pyrolyzing temperature were also explored. The catalytic activities of the CNx products for the oxygen reduction reaction (ORR) were examined by rotating disc electrode (RDE) measurements and single-cell tests. The onset potential for oxygen reduction in 0.5M H2SO4 of the best catalyst was as high as 0.88V vs. normal hydrogen electrode (NHE). The current density obtained in an H2/O2 proton exchange membrane fuel cell (PEMFC) was as high as 0.6Acm−2 at 0.5V with a cathode catalyst loading of 2mgcm−2.

A model-based parametric analysis of a direct ethanol polymer electrolyte membrane fuel cell performance

Abstract In the present work, a model-based parametric analysis of the performance of a direct ethanol polymer electrolyte membrane fuel cell (DE-PEMFC) is conducted with the purpose to investigate the effect of several parameters on the cell's operation. The analysis is based on a previously validated one-dimensional mathematical model that describes the operation of a DE-PEMFC in steady state. More precisely, the effect of several operational and structural parameters on (i) the ethanol crossover rate from the anode to the cathode side of the cell, (ii) the parasitic current generation (mixed potential formation) and (iii) the total cell performance is investigated. According to the model predictions it was found that the increase of the ethanol feed concentration leads to higher ethanol crossover rates, higher parasitic currents and higher mixed potential values resulting in the decrease of the cell's power density. However there is an optimum ethanol feed concentration (approximately 1.0 mol L −1 ) for which the cell power density reaches its highest value. The platinum (Pt) loading of the anode and the cathode catalytic layers affects strongly the cell performance. Higher values of Pt loading of the catalytic layers increase the specific reaction surface area resulting in higher cell power densities. An increase of the anode catalyst loading compared to an equal one of the cathode catalyst loading has greater impact on the cell's power density. Another interesting finding is that increasing the diffusion layers’ porosity up to a certain extent, improves the cell power density despite the fact that the parasitic current increases. This is explained by the fact that the reactants’ concentrations over the catalysts are increased, leading to lower activation overpotential values, which are the main source of the total cell overpotentials. Moreover, the use of a thicker membrane leads to lower ethanol crossover rate, lower parasitic current and lower mixed potential values in comparison to the use of a thinner one. Finally, according to the model predictions when the cell operates at low current densities the use of a thick membrane is necessary to reduce the negative effect of the ethanol crossover. However, in the case where the cell operates at higher current densities (lower ethanol crossover rates) a thinner membrane reduces the ohmic overpotential leading to higher power density values.

Platinum Black Polymer Electrolyte Membrane Based Electrodes Revisited

This study focuses on the fabrication and performance testing of unsupported platinum black electrodes for proton exchange membrane fuel cells. Experiments with platinum black coated diffusion media of varying anode and cathode catalyst loadings with demonstrate successful performance and stability characteristics for anode catalyst loadings down to while operating on pure and cathode catalyst loadings, without significant voltage losses. The voltage losses as a result of reducing the platinum black cathode catalyst loadings from 2.6 to are consistent with kinetic losses associated with the oxygen reduction reaction and lower electrocatalyst utilization. The study also highlights the durability and stability characteristics of these unsupported electrodes under extreme operating conditions. Optimization of the three-phase interface, namely electrode, electrolyte, and reactant gas, is shown to be dependent on the efficacy of the membrane–catalyst layer interface.

Experimental validation of a methanol crossover model in DMFC applications

A design of experiments (DOEs) coupled with a mathematical model was used to quantify the factors affecting methanol crossover in a direct methanol fuel cell (DMFC). The design of experiments examined the effects of temperature, cathode stoichiometry, anode methanol flow rate, clamping force, anode catalyst loading, cathode catalyst loading (CCL), and membrane thickness as a function of current and it also considered the interaction between any two of these factors. The analysis showed that significant factors affecting methanol crossover were temperature, anode catalyst layer thickness, and methanol concentration. The analysis also showed how these variables influence the total methanol crossover in different ways due to the effects on diffusion of methanol through the membrane, electroosmotic drag, and reaction rate of methanol at the anode and cathode. For example, as expected analysis showed that diffusion was significantly affected by the anode and cathode interfacial concentration, by the thickness of the anode catalyst layer and membrane, and by the diffusion coefficient in the membrane. Less obvious was the decrease in methanol crossover at low cathode flow rates were due to the formation of a methanol film at the membrane/cathode catalyst layer interface. The relative proportions of diffusion and electroosmotic drag in the membrane changed significantly with the cell current of the cell.

Preparation of Pt-Pd catalysts for direct formic acid fuel cell and their characteristics

Pt-Pd catalysts were prepared by using the spontaneous deposition method and their characteristics were analyzed in a direct formic acid fuel cell (DFAFC). Effects of calcination temperature and atmosphere on the cell performance were investigated. The calcination temperatures were 300, 400 and 500 °C and the calcination atmospheres were air and nitrogen. The fuel cell with the catalyst calcined at 400 °C showed the best cell performance of 58.8 mW/cm2. The effect of calcination atmosphere on the overall performance of fuel cell was negligible. The fuel cell with catalyst calcined at air atmosphere showed high open circuit potential (OCP) of 0.812 V. Also the effects of anode and cathode catalyst loadings on the DFAFC performance using Pt-Pd (1: 1) catalyst were investigated to optimize the catalyst loading. The catalyst loading had a significant effect on the fuel cell performance. Especially, the fuel cell with anode catalyst loading of 4 mg/cm2 and cathode catalyst loading of 5 mg/cm2 showed the best power density of 64.7 mW/cm2 at current density of 200 mA/cm2.

Concentration and ohmic losses in free-breathing PEMFC

Non-isothermal and three-dimensional simulations were carried out to study concentration and ohmic losses in free-breathing PEMFC under diverse conditions. Flow fields, species transport, transport of water in polymer membrane and movement of liquid water in cathode and anode porous layers were determined. Numerical results were obtained under different hydrating conditions, cell temperatures, cathode catalyst loadings and channel lengths. Current density and polymer electrolyte water content distributions as well as average power density were used as main output variables to study effects of operative conditions on performance. Results show that slow oxygen transport to active sites constitutes the most limiting factor to consider. Dehydrating conditions slightly affect free-breathing PEMFC performance. The numerical model showed to be suitable to study diverse phenomenon involved in free-breathing PEMFC performance.

Dependence of PEM fuel cell performance on catalyst loading

Abstract This study focuses on a determination of the cell voltage losses observed for Pt and PtRu loading reductions in H2/air and reformate/air polymer/electrolyte-membrane fuel cells (PEMFC). Experiments with catalyst-coated membranes (CCM) of varying anode and cathode catalyst loadings with H2/O2 and H2/air demonstrate that the anode catalyst loading in state-of-the-art membrane electrode assemblies (MEAs) operating on pure H2 can be reduced to 0.05 mgPt/cm2 without significant voltage losses, while the cell voltage losses upon a reduction of the cathode catalyst loading from 0.40 to 0.20 mgPt/cm2 for optimized MEAs amounts to 10–20 mV, consistent with purely kinetic losses due to the oxygen reduction reaction. It is shown that H2/air operation with state-of-the-art MEAs very closely approaches the Pt-specific power density (in units of gPt/kW) for large-scale automotive fuel cell applications with pure H2 feed. For reformate/air operation, PtRu anode loadings can be reduced to 0.20 mgPtRu/cm2 for reformate containing 100 ppm CO with a 2% air-bleed. Any further reduction will, however, require either a change in operating conditions (i.e. lower CO concentration or cell temperature ⪢80 °C) or novel, more CO-tolerant anode catalysts.

Current efficiency for soybean oil hydrogenation in a solid polymer electrolyte reactor

Soybean oil has been hydrogenated electrocatalytically in a solid polymer electrolyte (SPE) reactor, similar to that in H2/O2 fuel cells, with water as the anode feed and source of hydrogen. The key component of the reactor was a membrane electrode assembly (MEA), composed of a precious metal-black cathode, a RuO2 powder anode, and a Nafion® 117 cation-exchange membrane. The SPE reactor was operated in a batch recycle mode at 60°C and one atmosphere pressure using a commercial-grade soybean oil as the cathode feed. Various factors that might affect the oil hydrogenation current efficiency were investigated, including the type of cathode catalyst, catalyst loading, the cathode catalyst binder loading, current density, and reactant flow rate. The current efficiency ordering of different cathode catalysts was found to be Pd>Pt>Rh>Ru>Ir. Oil hydrogenation current efficiencies with a Pd-black cathode decreased with increasing current density and ranged from about 70% at 0.050Acm−2 to 25% at 0.490Acm−2. Current pulsing for frequencies in the range of 0.25–60Hz had no effect on current efficiencies. The optimum cathode catalyst loading for both Pd and Pt was 2.0mgcm−2. Soybean oil hydrogenation current efficiencies were unaffected by Nafion® and PTFE cathode catalyst binders, as long as the total binder content was ≤30wt% (based on the dry catalyst weight). When the oil feed flow rate was increased from 80to 300mlmin−1, the oil hydrogenation current efficiency at 0.100Acm−2 increased from 60% to 70%. A high (70%) current efficiency was achieved at 80mlmin−1 by inserting a nickel screen turbulence promoter into the oil stream.