Studies of the performance of PEM fuel cell cathodes with the catalyst layer directly applied on Nafion membranes

The performance of a proton exchange membrane fuel cell (PEMFC) with gas diffusion cathodes having the catalyst layer applied directly onto Nafion membranes is investigated with the aim at characterizing the effects of the Nafion content, the catalyst loading in the electrode and also of the membrane thickness and gases pressures. At high current densities the best fuel cell performance was found for the electrode with 0.35 mg Nafion cm⁻² (15 wt.%), while at low current densities the cell performance is better for higher Nafion contents. It is also observed that a decrease of the usual Pt loading in the catalyst layer from 0.4 to ca. 0.1 mg Pt cm⁻² is possible, without introducing serious problems to the fuel cell performance. A decrease of the membrane thickness favors the fuel cell performance at all ranges of current densities. When pure oxygen is supplied to the cathode and for the thinner membranes there is a positive effect of the increase of the O₂ pressure, which raises the fuel cell current densities to very high values (>4.0A cm⁻², for Nafion 112—50 μm). This trend is not apparent for thicker membranes, for which there is a negligible effect of pressure at high current densities. For H₂/air PEMFCs, the positive effect of pressure is seen even for thick membranes.     

Effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell

The effects of hydrophobicity of the cathode catalyst layer on the performance of a PEM fuel cell are studied. The surface contact angle is measured to understand the changes of the hydrophobicity of the cathode catalyst layer upon the addition of hydrophobic dimethyl silicone oil (DSO). The results show that the contact angle increases with the DSO loadings in the cathode catalyst layer ranging from 0 to 0.65 mg/cm². The subsequent electrochemical measurements of the fuel cells with various cathodes reveal that the addition of DSO in the cathode catalyst layer can effectively prevent the cathode flooding at high current density, thus leading to a much higher limiting current density and the maximum power density when compared to the fuel cell with a normal cathode. An optimal DSO loading in the cathode catalyst layer is found to be around 0.5 mg/cm² under the testing conditions in this work. The fuel cell with cathode loaded with 0.5 mg/cm² can reach the maximum power density of 356 mW/cm² in H₂/air (or 709 mW/cm² in H₂/O₂) at room temperature, which is around 2.5 times in H₂/air (or 1.8 times in H₂/O₂) of that with normal cathode. All of the results indicate that the hydrophobicity of the cathode catalyst layer plays a crucial role in the water management of the fuel cell. The possible function of the DSO on improved oxygen solubility for the oxygen starved cathode during flooding warrants some further investigation.     

Electrogeneration of Hydrogen Peroxide in Acid Medium using Pyrolyzed Cobalt-based Catalysts: Influence of the Cobalt Content on the Electrode Performance

Cobalt tetramethoxyphenyl porphyrin (CoTMPP) adsorbed on a high area carbon support (Vulcan XC72-R) and heat-treated at 900 °C under inert atmosphere was studied as electrocatalyst for the reduction of O₂ to H₂O₂ in acid medium. Experiments performed on rotating ring-disc electrode (RRDE) and gas diffusion electrode (GDE) show that the catalyst performance depends on the cobalt loading, going through a maximum at 0.2 wt. % Co. For higher cobalt loadings, a growing part of oxygen is reduced into water, decreasing therefore the selectivity of the catalyst. These results are interpreted in terms of a further reduction of H₂O₂ on Co-based catalytic sites before leaving the catalytic layer. For a GDE polarized at −150 mV vs. saturated calomel electrode (SCE) and loaded with 0.9 μg cm⁻² of 0.2 wt. % Co-based catalyst, a H₂O₂ production rate of 300 μmol h⁻¹ cm⁻² was obtained which is five times higher than the H₂O₂ production rate measured with Vulcan. In these conditions, the selectivity of the Co-based catalyst for H₂O₂ production is 92%. The good agreement observed between RRDE and GDE results confirms the relevance of using RRDE experiment for screening these non-precious metal catalysts for further GDE applications.     

Influence of ionomer content on the structure and performance of PEFC membrane electrode assemblies

Nafion^® ionomer content of the cathode catalyst-layer of a polymer electrolyte fuel cell (PEFC), made by the “decal” hot pressing method, has been investigated for its effect on performance and structure of the membrane electrode assembly (MEA). Varying Nafion^® content was shown to have an effect on performance within the entire range of polarization curves (i.e. kinetic, ohmic, and mass-transport regions) as well as on the structure. AFM analysis shows the effect of Nafion on the dispersion of carbon aggregates. Further analysis using TEM demonstrates the effect of Nafion on both the dispersion of carbon aggregates and the distribution and thickness of the Nafion ionomer films surrounding the catalyst/carbon aggregates. The MEA structure change correlates well with the MEA performance on both kinetics and mass-transport region. The determining factors on the performance of MEA are the interfacial zone (between the ionomer and catalyst particle), the dispersion of catalyst/carbon aggregates and the distribution/thickness of Nafion films. An optimized Nafion^® content in the range of 27 ± 6 wt.% for the cathode was determined for an E-TEK 20% Pt₃Cr/C catalyst at a loading of 0.20 mg Pt/cm².     

Nanostructured membrane electrode assemblies from layer‐by‐layer composite/catalyst containing membranes and their fuel cell performances

In this study, two approaches are compared to develop nanostructured membrane electrode assemblies (MEA) using layer‐by‐layer (lbl) technique. The first is based on the direct deposition of polyallylamine hydrochloride (PAH) and sulfonated polyaniline (sPAni) on Nafion support to prepare lbl composite membrane. In the second approach, sPAni is coated on the support in the presence of platinum (Pt) salt, Nafion solution and Vulcan for obtaining catalyst containing membranes (CCMs). SEM and UV–vis analysis show that the multilayers are deposited on both sides of Nafion successfully. Although H₂/O₂ single cell performances of acid doped lbl composite membrane based MEA are found to be at the range of 126 and 160 mW cm^?2 depending on the number of deposited layers, the cell performance of MEA obtained from catalyst containing lbl self‐assembled thin membrane (PAH/sPAni‐H⁺)_10‐Pt is found to be 360 mW cm^?2 with a Pt utilization of 720 mW mgPt^?1. This performance is 82% higher as compared to original Nafion^®117 based MEA (198 mW cm^?2). From the cell performance evaluations for different structured MEAs, it is mainly found out that the use of lbl CCMs instead of composite membranes and fabrication of thinner electrolytes result in a higher H₂/O₂ cell activity due to significant reduction in ohmic resistivity. Also, it is observed that the use of sPAni slightly improves the cell performance due to an increased probability of the triple phase contact and it can lead to superior physicochemical properties such as conductivity and thermal stability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40314.     

Effect of silver catalyst on the activity and mechanism of a gas diffusion type oxygen cathode for chlor-alkali electrolysis

Application of a gas-diffusion type oxygen cathode will contribute to energy saving in chlor-alkali electrolysis. For this purpose the development of gas-diffusion electrodes with high performance and durability is essential. We have investigated the performance for oxygen reduction and the mechanism of its on gas-diffusion electrodes with and without Ag catalyst in order to develop such oxygen cathodes with high performance and durability. It has been found that an electrode with no catalyst, that is, carbon support only in the reaction layer, shows electrochemical activity for oxygen reduction in 32 wt % NaOH at 80 °C and 1 atm O₂, but loading of 2 mg cm^–2 Ag of particle size 300 nm, not only improves the activity by about 100 mV but promotes the four-electron reduction to produce OH^–, while H₂O₂ is the predominant reaction intermediate in the absence of the Ag catalyst. The production of H₂O₂ has been demonstrated by conducting CV measurements to detect H₂O₂ in the anodic scan after a cathodic sweep up to 0.3 V vs RHE. It has been shown that the gas-diffusion type oxygen cathode with Ag catalyst has the high performance and durability necessary for chlor-alkali electrolysis.     

Enhancement of oxygen reduction by incorporation of heteropolytungstate into the electrocatalytic ink of carbon supported platinum nanoparticles

Nafion stabilized inks of Vulcan XC-72 supported platinum (20 wt.%) nanoparticles (Pt/XC-72) were utilized to produce electrocatalytic films on glassy carbon. The catalysts were modified (activated) with phosphododecatungstic acid H₃PW₁₂O₄₀ (PW₁₂). Comparison was made to bare (PW₁₂-free) electrocatalytic films. Electroreduction of dioxygen was studied at 25 °C in 0.5 mol dm⁻³ H₂SO₄ electrolyte using rotating disk voltammetry. For the same loading of platinum (≈95 μg cm⁻²) and for the approximately identical distribution of the catalyst, the reduction of oxygen at a glassy carbon electrode modified with the ink containing PW₁₂ proceeded at ca. 30-60 mV more positive potential (depending on the PW₁₂ content), and the system was characterized by a higher kinetic parameter (rate of heterogeneous electron transfer), when compared to the PW₁₂-free electrocatalyst. Gas diffusion electrodes with Pt/XC-72 supported on carbon paper (Pt loading 1 mg cm⁻²) were also tested. Under the same experimental conditions, while the exchange current density and the total resistance contribution to polarization components, computed from the galvanostatic polarization curves were found to be clearly higher and lower, respectively, for the ink modified with PW₁₂ relative to the unmodified system. The results demonstrate that addition of heteropolytungstatic acid (together with Nafion) enhances the electrocatalytic activity of platinum towards reduction of oxygen.     

The physical–chemical properties and electrocatalytic performance of iridium oxide in oxygen evolution

An IrO₂ anode catalyst was prepared by using the Adams method for the application of a solid polymer electrolyte (SPE) water electrolyzer. The effect of calcination temperature on the physical–chemical properties and the electrochemical performance of IrO₂ were examined to obtain a low loading and a high catalytic activity of oxygen evolution at the electrode. The physical–chemical properties were studied via thermogravimetry–differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The electrochemical activity was investigated by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry in 0.1 mol L⁻¹ H₂SO₄ at room temperature. The optimum condition was found to be at the calcination temperature of 500 °C, where the total polarization reached a minimum at high current densities (>200 mA cm⁻²). The optimized catalyst was also applied to a membrane electrode assembly (MEA) and stationary current–potential relationships were investigated. With an optimized catalytic IrO₂ loading of 1.5 mg cm⁻² and a 40% Pt/C loading of 0.5 mg cm⁻², the terminal applied potential difference was 1.72 V at 2 A cm⁻² and 80 °C in a SPE water electrolysis cell.     

Novel KOH-free anion-exchange membrane fuel cell: Performance comparison of alternative anion-exchange ionomers in catalyst ink

Alkaline membrane electrode assemblies (MEAs) were fabricated and tested in 5 cm² single cell configuration. The fuel cell tests were preformed in the absence of any liquid electrolyte, such as KOH. This study shows fuel cell polarization curves for alkaline membrane fuel cell (AMFC) systems that were fabricated with novel anion-exchange ionomers. A comparison of two novel anion-exchange ionomers incorporated into the catalyst ink was achieved by comparing the performance under H₂/O₂ and H₂/air operating conditions. The results presented here indicate that the chemical and physical properties of the recast anion-exchange ionomer that is utilized in AMFC catalyst layers directly influence the obtainable fuel cell performance. It is shown that ionomer materials that are less prone to swelling from hydration and tend to pack closely together in the solid state will result in stronger catalyst-ionomer interfacial interactions. The O₂ transport properties in alkaline MEA cathodes are influenced by the resulting void volume of the electrode as defined by the structure and packing arrangement of the recast ionomer molecules.     

Pore‐forming technology and performance of MEA for direct methanol fuel cells

BACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent. RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm^?2 to 167.42 mW cm^?2 at 55 °C and 0.2 MPa O₂. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes. CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry     

Development and electrochemical studies of gas diffusion electrodes for polymer electrolyte fuel cells

Electrochemical studies on low catalyst loading gas diffusion electrodes for polymer electrolyte fuel cells are reported. The best performance is obtained with an electrode formed from 20 wt% Pt/C, 0.4 mg Pt cm^–2 and 1.1 mg Nafion^® cm^–2 in the catalyst layer and 15% PTFE in a diffusion layer of 50 µm thickness, for both the cathode and the anode. However, it is also observed that the platinum requirement can be diminished to values close to 0.2 mg Pt cm^–2 in the cathode and 0.1 mg pt cm^–2 in the anode, without appreciably affecting the good characteristics of the fuel cell response. The experimental fuel cell data were analysed using theoretical models of the electrode structure and of the fuel cell system. It is seen that most of the electrode systems present limiting currents and some also show linear diffusion components arising from diffusion limitations in the gas channels and/or in the thin film of electrolyte covering the catalyst particles.     

Kinetic analysis of the hydrogen oxidation reaction at Nafion film covered Pt-black rotating disk electrodes

The kinetics of the H₂ oxidation reaction at Nafion film covered Pt-black rotating disk electrodes (RDEs) in 0.5 M H₂SO₄ at 298 K was investigated by varying the Pt loading, Nafion film thickness, and rotating rate. The equation describing the H₂ oxidation kinetics at an RDE with a Nafion film covered porous Pt layer was derived, assuming a Tafel-Volmer mechanism and taking into account the mass transfer resistances in the aqueous electrolyte, Nafion film, and Pt layer. The H₂ oxidation reaction at the Pt layer was proved to be reversible and the measurable current density was determined entirely by the mass transfer of H₂ in the aqueous electrolyte and the Nafion film; the apparent kinetic current density measured was due to the experimental error. More accurate results of kinetic analysis were obtained in this work than our results reported previously.     

Performance of fluidized bed electrode in a molten carbonate fuel cell anode

A fluidized bed electrode could lower concentration polarization and activation polarization because of its high mass and heat transfer coefficient. The polarization characteristics of the fluidized bed electrode are systematically investigated in a molten carbonate fuel cell anode with an O₂/CO₂/gold reference electrode. The results show that polarization performance of the anode is improved by selecting proper flow rates of H₂, O₂ and CO₂, choosing suitable nickel particle content together with appropriate O₂/CO₂ ratio, and increasing reaction temperature as well as the area of the current collector. Limiting current density of 115.56 mA·cm⁻² is achieved under optimum performance as follows: a cylindrically curved nickel plate current collector, nickel particle content of 7.89%, the reaction temperature of 923 K, H₂ flow rate of 275 mL·min⁻¹, O₂/CO₂ flow rate of 10/20 mL·min⁻¹ and O₂/CO₂ ratio of 1 : 2.     

Oxygen reduction reaction on carbon-supported CoSe2 nanoparticles in an acidic medium

We investigated the effect of CoSe₂/C nanoparticle loading rate on oxygen reduction reaction (ORR) activity and H₂O₂ production using the rotating disk electrode and the rotating ring-disk electrode techniques. We prepared carbon-supported CoSe₂ nanoparticles with different nominal loading rates and evaluated these samples by means of powder X-ray diffraction. All the catalysts had an OCP value of 0.81 V vs. RHE. H₂O₂ production during the ORR process decreased with an increase in catalytic layer thickness. This decrease was related to the CoSe₂ loading on the disk electrode. H₂O₂ production also decreased with increasing catalytic site density, a phenomenon related to the CoSe₂ loading rate on the carbon substrate. The cathodic current density significantly increased with increasing catalytic layer thickness, but decreased with increasing catalytic site density. In the case of 20 wt% CoSe₂/C nanoparticles at 22 μg cm⁻², we determined that the transfer process involves about 3.5 electrons.     

The Influence of Pt Loading,Support and Nafion Content on the Performance of Direct Methanol Fuel Cells: Examined on the Example of the Cathode

Nano‐sized Pt colloids were prepared using the polyol method and supported on Ketjen black EC 600J (KB), Vulcan XC‐72 (VC) and high surface area graphite 300 (HG). The effects of the Nafion ionomer content, and the Pt loading of the cathode catalyst layer as well as the Pt loading on the support on the performance of direct methanol fuel cells (DMFCs), were studied. The membrane electrode assemblies (MEAs) were analysed using current–voltage curves, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and adsorbed CO stripping voltammetry. Optimum Nafion to carbon (N/C) ratios (N/C being defined as the weight ratio of the Nafion ionomer to the carbon) were determined. The optimum N/C ratios were found to depend on the support as follows, 1.4, 0.7 and 0.5 for Pt/KB, Pt/VC and Pt/HG, respectively and to be independent of the Pt/C loading range of 20–80 wt% tested in this work. The highest DMFC performances, as well as the highest electrochemical active surface areas, and improved gas diffusivities, were achieved using these ratios. For the catalysts prepared in this work, the average Pt crystallite size was found to decrease with increasing surface area of the support for a particular Pt loading. MEAs made using KB as support and the optimal N/C ratio of 1.4 showed the best performances, i.e. higher than the VC and HG supports for any N/C ratio. The highest DMFC performance was observed using 60 wt% Pt on KB cathode electrodes of 1 mg Pt cm^–2 loading and an N/C value of 1.4. For all three supports studied, the 60 wt% Pt on carbon loading resulted in the best DMFC performance. This may be linked to the Pt particle size and catalyst preparation method used in this work. In comparison to literature results, high DMFC performances were achieved using relatively ‘low' Pt and Ru loadings. For example, a maximum power density of >100 mW cm^–2 at 60 °C was observed using a 1 mg Pt cm^–2 cathode loading and a 2 mg PtRu cm^–2 anode loading.     

Improved gas diffusion electrodes for hybrid polymer electrolyte fuel cells

In this study, the performance of the anionic electrodes for hybrid polymer electrolyte fuel cells was improved. The anion exchange membrane (AEM) electrodes were initially characterized as the cathode on a proton exchange membrane (PEM) anode/membrane half-assembly (i.e. hybrid polymer electrolyte fuel cell). The electrode performance was improved by tailoring the ionomer distribution within the electrode structure so as to better balance the electronic, ionic, and reactant transport within the catalyst layer. An ionomer impregnation method was used to achieve a non-uniform ionomer distribution and higher performance. Traditional electrode fabrication methods (i.e. thin-film method) lead to a uniform ionomer distribution. The peak power density at 70 °C for a H₂/O₂ hybrid fuel cell was 44 mW cm⁻² using the thin-film electrode, and 120 mW cm⁻² using the ionomer impregnated electrode. A hydrophobic additive used in the catalyst layer further improved the electrode performance, giving a peak power density of 315 mW cm⁻² for H₂/O₂ at 70 °C. Electrochemical impedance spectroscopy was used as an in situ diagnostic tool to help understand the origin of the electrode improvements. The increase in performance was attributed to improved catalyst utilization due to the creation of facile gas transport domains in the AEM electrode structure. Similarly, the AEM anode prepared by ionomer impregnation with polytetrafluoroethylene resulted in a three-fold increase in the peak power density compared to ones made by the thin-film method, which has no polytetrafluoroethylene.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

Membrane Electrode Assembly with Ultra Low Platinum Loading for Cathode Electrode of PEM Fuel Cell by Using Sputter Deposition

Cathode electrodes of proton exchange membrane fuel cells were fabricated by using Pt sputter deposition to increase the gravimetric power density (W mg_Pt⁻¹) with reduced Pt loading. Ultra low Pt‐based electrodes having Pt loading in between 0.0011 and 0.06 mg_Pt cm⁻² were prepared by a radio frequency (RF) sputter deposition method on the surface of a non‐catalyzed gas diffusion layer (GDL) substrate by changing the sputtering time (20, 90, 180, 1050 s). The effect of cathode Pt loading on the performance of membrane electrode assembly were investigated using polarization curve, impedance, H₂ crossover and cyclic voltammetry techniques. The effect of backpressure on PEMFC performance was also investigated. Sputter1050 (0.06 mg_Pt cm⁻²) exhibited the best power density at 80 °C cell temperature and without backpressure for H₂/O₂, 100 %RH (297 mW cm⁻² and 5 W mg_Pt⁻¹ at 0.6 V). On the other hand sputter90 (0.005 mg_Pt cm⁻²) showed the peak gravimetric power density (15 W mg_Pt⁻¹ and 75 mW cm⁻² at 0.6 V). The Pt utilization efficiency increased as the Pt loading decreased. Sputter20 and sputter90 electrodes yielded insufficient electrochemical surface area (ECSA), higher charge transfer and ohmic resistance, but sputter180 and sputter1050 yielded sufficient ECSA and lower charge transfer and ohmic resistance.     

Development of non-precious oxygen reduction reaction catalyst for polymer electrolyte membrane fuel cells based on substituted cobalt porphyrins

Active and stable cobalt-based non-precious metal catalysts for the oxygen reduction reaction (ORR) in PEM fuel cells were developed through high-temperature pyrolysis of metal-porphyrins supported on carbon. The roles of substituted porphyrins, carbon support, and catalyst loading on ORR activity were studied using rotating disc electrode (RDE) measurements. It was observed that the carbon support plays a major role in improving the catalytic activity. The results showed that among the supported catalysts, the homemade mesocarbon-supported cobalt-porphyrin catalyst with 20 wt% loading displayed higher ORR activity; the cell performance showed maximum current density of 1.1 A cm⁻² at 0.13 V in H₂/O₂ fuel cells.     

PEM fuel cell reaction kinetics in the temperature range of 23-120 °C

The performance of a Nafion 112 based proton exchange membrane (PEM) fuel cell was tested at a temperature range from 23 °C to 120 °C. The fuel cell polarization curves were divided into two different ranges based on current density, namely, <0.4 A/cm² and >0.4 A/cm², respectively. These two ranges were treated separately with respect to electrode kinetics and mass transfer. In the high current density range, a linear increase in membrane electrode assembly (MEA) power density with increasing temperature was observed, indicating the advantages of high temperature operation.Simulation based on electrode reaction kinetic theory, experimental polarization curves, and measured cathodic apparent exchange current densities all gave temperature dependent apparent exchange current densities. Both the calculated partial pressures of O₂ and H₂ gas in the feed streams and the measured electrochemical Pt surface areas (EPSAs) decrease with increasing temperature. They were also used to obtain the intrinsic exchange current densities. A monotonic increase of the intrinsic exchange current densities with increasing temperature in the range of 23-120 °C was observed, suggesting that increasing the temperature does promote intrinsic kinetics of fuel cell reactions.There are two sets of cathode apparent exchange current densities obtained, one set is for the low current density range, and the other is for the high current density range. The different values of cathode current densities in the two current density ranges can be attributed to the different states of the cathode Pt catalyst surface. In the low current density range, the cathode catalyst surface is a Pt/PtO, and in the high current density range, the catalyst surface becomes pure Pt.