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.     

The improved methanol tolerance using Pt/C in cathode of direct methanol fuel cell

Membrane electrode assemblies (MEA) were prepared using PtRu black and 60 wt.% carbon-supported platinum (Pt/C) as their anode and cathode catalysts, respectively. The cathode catalyst layers were fabricated using various amounts of Pt (0.5 mg cm⁻², 1.0 mg cm⁻², 2.0 mg cm⁻², and 3.0 mg cm⁻²). To study the effect of carbon support on performance, a MEA in which Pt black was used as the cathode catalyst was fabricated. In addition, the effect of methanol crossover on the Pt/C on the cathode side of a direct methanol fuel cell (DMFC) was investigated. The performance of the single cell that used Pt/C as the cathode catalyst was higher than single cell that used Pt black and this result was pronounced when highly concentrated methanol (above 2.0 M) was used as the fuel.     

Enhanced performance and improved interfacial properties of polymer electrolyte membrane fuel cells fabricated using sputter-deposited Pt thin layers

For this study, catalyst layers for polymer electrolyte membrane fuel cells (PEMFC) were prepared by spraying and sputtering to deposit Pt amount of 0.1 and 0.01 mg cm⁻², respectively. These Pt layers were then assembled to fabricate membrane electrode assemblies (MEA) having either single- or double-layered catalysts. The PEM fuel cell with double layers showed a current density of 777 mA cm⁻² at a cell voltage of 0.6 V, which is a higher current density than state-of-the-art fuel cells at 643 mA cm⁻². These results indicate that Pt loading in state-of-the-art PEMFCs could be reduced by approximately 50% with no performance loss by using both spraying and sputtering method in the MEA fabrication process.     

Simulations and study of electrochemical hydrogen energy conversion in EasyTest Cell

An EasyTest Cell concept is applied to study the performance characteristics of the electrochemical processor for polymer electrolyte membrane electrochemical hydrogen energy converters (PEM EHEC), broadly known as a membrane electrode assembly (MEA). A series of MEAs consisting of Nafion 117 polymer electrolyte and magnetron sputtered Pt, IrO_x, and composite IrO_x/Pt/IrO_x catalysts with varying catalytic loadings were investigated. The partial electrode reactions proceeding in the real PEM EHEC, namely hydrogen oxidation (HOR), hydrogen evolution (HER), oxygen reduction (ORR), and oxygen evolution (OER), are simulated and studied in a recently developed test cell with a unitized gas compartment. The EasyTest Cell design gives possibilities for strict control of the experimental conditions by avoiding the usage of any auxilliary gas conditioning equipment. By varying the thickness of the sputtered Pt film, the catalyst loading is remarkably reduced (from 0.5 to 0.06 mg cm⁻² or about 8 times) for both HOR and HER without any sacrifice of the electrode performance. The electrode with 0.2 mg cm⁻² sputtered IrO_x shows the best OER performance. The composite IrO_x/Pt/IrO_x electrode demonstrated a bi-functional catalytic activity toward both OER and ORR, as well as improved gas diffusion properties toward ORR compared to the single Pt layer with the same catalytic loading.A phenomenological criterion for evaluating the gas diffusion properties of the electrodes is proposed. The applied testing approach is validated via comparison of the results obtained in the EasyTestCell and the common laboratory PEM electrolytic cell.     

Preparation of low-platinum-loading electrocatalysts using electroless deposition method for proton exchange membrane fuel cell systems

One promising preparative method that offers the potential for improved platinum (Pt) dispersion of electrocatalysts is electroless deposition (ED). In this study, the effects of multiwalled carbon nanotubes (MWCNTs) pretreatment and synthesis procedure on properties of the four catalysts, synthesized by ED method, have been considered. The results of energy-dispersive X-ray spectroscopy (EDS), X-ray dot-mapping, X-ray fluorescence (XRF) and cyclic voltammetry (CV) analyses showed that using palladium (Pd) precursor during two-step sensitization-activation coating procedure gives uniform Pt particles distribution on MWCNTs with low aggregation and high specific surface area (∼80 m² g⁻¹). In addition, to investigate the performance of the synthesized catalysts in experimental fuel cell system, thin-film method was used to fabricate the membrane electrode assemblies (MEAs). Obtaining the polarization curves for the fabricated MEAs (Pt loading ∼0.4 mg cm⁻²) and a commercial MEA (ElectroChem, Pt loading ∼1 mg cm⁻²) demonstrated that the catalyst prepared by two-step sensitization-activation coating procedure possesses a good performance despite of its lower Pt content.     

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.     

Cobalt based non-precious electrocatalysts for oxygen reduction reaction in proton exchange membrane fuel cells

Cobalt based non-precious metal catalysts were synthesized using chelation of cobalt (II) by imidazole followed by heat-treatment process and investigated as a promising alternative of platinum (Pt)-based electrocatalysts in proton exchange membrane fuel cells (PEMFCs). Transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were used to characterize the synthesized CoN_x/C catalysts. The activities of the catalysts towards oxygen reduction reaction (ORR) were investigated by electrochemical measurements and single cell tests, respectively. Optimization of the heat-treatment temperature was also explored. The results indicate that the as-prepared catalyst presents a promising electrochemical activity for the ORR with an approximate four-electron process. The maximum power density obtained in a H₂/O₂ PEMFC is as high as 200 mW cm⁻² with CoN_x/C loading of 2.0 mg cm⁻².     

Novel layer wise anode structure with improved CO-tolerance capability for PEM fuel cell

This study proposes a novel layer wise anode structure to improve the CO-tolerance ability and utilization efficiency of catalyst. The layer wise structure consists of an outer and an inner catalyst layer. The outer catalyst layer acting as a CO barrier is composed of two nano-Ru layers (0.06 mg cm⁻²) by magnetron sputtering deposition method and a Pt₅₀-Ru₅₀ layer (0.10 mg cm⁻²) by screen-printing method on the GDL. The inner catalyst layer providing the hydrogen oxidation reaction is a pure Pt layer (0.07 mg cm⁻²) prepared by direct-printing method on PEM. The roles of the outer and inner catalyst layer relating to the improvement of CO-tolerance ability and utilization efficiency of catalyst for the proposed catalyst layer structure are investigated in this paper. SEM, X-ray, EDS and EPMA analysis were used to characterize microstructures, phases, chemical composition and distributions for the obtained electrocatalyst layers. The hydrogen fuel containing 50 ppm CO/hydrogen fuel containing 50 ppm CO + 2% O₂ is continuously fed to the anode side to investigate the dependence of CO-tolerance ability over time for the MEAs, respectively. The results demonstrate that this proposed anode catalyst layer structure presents a superior CO-tolerance ability and performance to those of conventional and Huag's structures in both oxygen free and oxygen present CO containing hydrogen fuels as well as pure hydrogen fuel. The filtering effect of the outer catalyst layer causes the improved CO-tolerance capability.     

Gold supported catalytic layer: An intermediate step between fundamental and applied fuel cell studies

We report the preparation and electrochemical characterization of gold supported catalytic layer electrodes which have been prepared using a similar methodology to that employed in the preparation of conventional catalyst coated membranes. Consequently, the so-prepared catalytic layers have comparable properties (morphology and thickness) than those employed in direct liquid fuel cells. Using this working electrode configuration, and the so-called hydrogen adsorption-desorption region, fundamental electrochemical parameters such as electrochemically accessible Pt surface area, roughness factor and Pt-catalyst utilization of the catalytic layer have been evaluated. The electrochemically accessible Pt surface area, roughness factor and Pt-catalyst utilization have been found independent of the metal loading in the range of 0.1-0.5 mg Pt cm⁻². However for 1 mg Pt cm⁻², an important decrease on electrochemically accessible Pt surface area and Pt-catalyst utilization has been observed. On the other hand, when formic acid electrooxidation is used, a sudden decrease on the electrocatalytic activity has been observed as metal loading increases. These results clearly indicate that the formic acid electrooxidation process is strongly dependant of the accessibility of the reactant into the inner part of the electrodes, highlighting its mass-transport controlled reaction properties. These findings point out that, from an applied point of view, it is recommendable using catalytic layers as thinner as possible as well as high formic acid concentrations. These experimental conditions will maximize the Pt-catalyst utilization by minimizing the accessibility problems of the reactant into the inner part of the electrodes.     

CO tolerance and catalytic activity of Pt/Sn/SnO2 nanowires loaded on a carbon paper

Electrochemical activities and structural features of Pt/Sn catalysts supported by hydrogen-reduced SnO₂ nanowires (SnO₂NW) are studied, using cyclic voltammetry, CO stripping voltammetry, scanning electron microscopy, and X-ray diffraction analysis. The SnO₂NW supports have been grown on a carbon paper which is commercially available for gas diffusion purposes. Partial reduction of SnO₂NW raises the CO tolerance of the Pt/Sn catalyst considerably. The zero-valence tin plays a significant role in lowering the oxidation potential of CO_ads. For a carbon paper electrode loaded with 0.1 mg cm⁻² Pt and 0.4 mg cm⁻² SnO₂NW, a conversion of 54% SnO₂NW into Sn metal (0.17 mg cm⁻²) initiates the CO_ads oxidation reaction at 0.08 V (vs. Ag/AgCl), shifts the peak position by 0.21 V, and maximizes the CO tolerance. Further reduction damages the support structure, reduces the surface area, and deteriorates the catalytic activity. The presence of Sn metal enhances the activities of both methanol and ethanol oxidation, with a more pronounced effect on the oxidation current of ethanol whose optimal value is analogous to those of PtSn/C catalysts reported in literature. In comparison with a commercial PtRu/C catalyst, the optimal Pt/Sn/SnO₂NW/CP exhibits a somewhat inferior activity toward methanol, and a superior activity toward ethanol oxidation.     

Substrate effect on oxygen reduction electrocatalysis

The oxygen reduction reaction (ORR) was investigated on carbon (XC-72) supported platinum nanoparticles, generated via the carbonyl chemical route and on oxide composites supported platinum generated via the UV-photo-deposition technique in sulfuric acid medium. The behavior of Pt/C was examined using a careful dosing of the catalyst loading spanning the range from 4.3 to 131 μg cm⁻². The ORR electrochemical response of Pt/C (in line with recent literature data) is put into contrast with the Pt/oxide-composite systems. Our results point out that it is possible to use smaller amounts of catalyst for the ORR when platinum atoms interact with the oxide (anatase) surface of the substrate composite. Evidence of the incipient metal-substrate interaction is discussed in the light of the results of XRD experiments.     

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Influence of sputtering power on oxygen reduction reaction activity of zirconium oxides prepared by radio frequency reactive sputtering

Zirconium oxides (ZrO_2−x) have been investigated as new cathodes for direct methanol fuel cells without platinum. ZrO_2−x films were prepared using a radio frequency (RF) magnetron sputtering at RF powers from 75 to 175 W. The influence of the RF power on the catalytic activity for the oxygen reduction reaction (ORR) and properties of the ZrO_2−x films were examined. The ORR activity of the ZrO_2−x catalyst increased with the RF power in the range we studied. The onset potential for ORR over ZrO_2−x deposited at 175 W was 0.88 V vs RHE. In addition, the relationship between the ORR activity and the composition, crystallinity, electric conductivity, as well as the ionization potential has been investigated. The zirconium oxide with an oxygen defected state and the higher electric conductivity showed the higher ORR activity, and the electrocatalytic activity for ORR increased with the decreasing in the ionization potential of the ZrO_2−x catalyst.     

Kinetics and PEMFC performance of RuxMoySez nanoparticles as a cathode catalyst

Kinetics of Ru_xMo_ySe_z nanoparticles dispersed on carbon powder was studied in 0.5 M H₂SO₄ electrolyte towards the oxygen reduction reaction (ORR) and as cathode catalysts for a proton exchange membrane fuel cell (PEMFC). Ru_xMo_ySe_z catalyst was synthesized by decarbonylation of transition-metal carbonyl compounds for 3 h in organic solvent. The powder was characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM) techniques. Catalyst is composed of uniform agglomerates of nanocrystalline particles with an estimated composition of Ru₆Mo₁Se₃, embedded in an amorphous phase. The electrochemical activity was studied by rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. Tafel slopes for the ORR remain invariant with temperature at −0.116 V dec⁻¹ with an increase of the charge transfer coefficient in dα/dT = 1.6 × 10⁻³, attributed to an entropy turnover contribution to the electrocatalytic reaction. The effect of temperature on the ORR kinetics was analyzed resulting in an apparent activation energy of 45.6 ± 0.5 kJ mol⁻¹. The catalyst generates less than 2.5% hydrogen peroxide during oxygen reduction. The Ru_xMo_ySe_z nanoparticles dispersed on a carbon powder were tested as cathode electrocatalyst in a single fuel cell. The membrane-electrode assembly (MEA), included Nafion^® 112 as polymer electrolyte membrane and commercial carbon supported Pt (10 wt%Pt/C-Etek) as anode catalyst. It was found that the maximum performance achieved for the electro-reduction of oxygen was with a loading of 1.0 mg cm⁻² Ru_xMo_ySe_z 20 wt%/C, arriving to a power density of 240 mW cm⁻² at 0.3 V and 80 °C.     

Novel carbon-supported Fe-N electrocatalysts synthesized through heat treatment of iron tripyridyl triazine complexes for the PEM fuel cell oxygen reduction reaction

2,4,6-Tris(2-pyridyl)-1,3,5-triazine (TPTZ) was used as a ligand to prepare iron-TPTZ (Fe-TPTZ) complexes for the development of a new oxygen reduction reaction (ORR) catalyst. The prepared Fe-TPTZ complexes were then heat-treated at temperatures ranging from 400 °C to 1100 °C to obtain carbon-supported Fe-N catalysts (Fe-N/C). These catalysts were characterized in terms of catalyst composition, structure, and morphology by several instrumental methods such as energy dispersive X-ray, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. With respect to the ORR activity, the Fe-N/C catalysts were also evaluated by cyclic voltammetry, as well as rotating disk and ring-disk electrodes. The results showed that among the heat-treated catalysts, that obtained at a heat-treatment temperature of 800 °C is the most active ORR catalyst. The overall electron transfer number for the catalyzed ORR was determined to be between 3.5 and 3.8, with 10-30% H₂O₂ production. The ORR catalytic activity of this catalyst was also tested in a hydrogen-air proton exchange membrane (PEM) fuel cell. At a cell voltage of 0.30 V, this fuel cell can give a current density of 0.23 A cm⁻² with a maximum MEA power density of 0.070 W cm⁻² indicating that this catalyst has potential to be used as a non-noble catalyst in PEM fuel cells.     

Electroreduction of dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalysts—effect of the presence of methanol

Ag/C catalysts with different loading were prepared using a colloidal route to obtain well dispersed catalysts on carbon, with a particle size close to 15 nm. An amount of 20 wt.% Ag on carbon was found to be the best loading in terms of current density and mass activity. The 20 wt.% Ag/C catalyst was then studied and the kinetics towards ORR was determined and compared with that of a 20 wt.% Pt/C catalyst. The number of exchanged electrons for the ORR was found to be close to four with the rotating disk electrode (RDE) as well as with the rotating ring disc electrode (RRDE) techniques. From the RDE results, the Tafel slopes b, the diffusion limiting current density inside the catalytic film (j_l^film) and the exchange current density (j₀) were evaluated. The Tafel slopes b and diffusion limiting current densities inside the catalytic film (j_l^film) were found to be in the same order for both catalysts, whereas the exchange current density (j₀), which is a suitable estimation of the activity of the catalyst, was at least 10 times higher at the Pt/C catalyst than at the Ag/C catalyst. The behavior of both catalysts in methanol containing electrolyte was investigated and it was found that at a low methanol concentration, the Pt/C catalyst was quasi-tolerant to methanol. But, at a high methanol concentration, the ORR at a Pt/C was affected. However, the Pt/C catalyst showed in each case better activity towards ORR than the Ag/C catalyst, even if the latter one was less affected by the presence of methanol than the former one.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

The influence of a new fabrication procedure on the catalytic activity of ruthenium-selenium catalysts

A new procedure has been introduced to enhance catalytic activity of ruthenium-selenium electro-catalysts for oxygen reduction, in which materials are treated under hydrogen atmosphere at elevated temperatures. The characterisation using scanning electron microscopy, energy dispersive spectroscopy or energy dispersive X-ray spectroscopy exhibited that the treatment at 400 °C made catalysts denser while their porous nature remained, led to a good degree of crystallinity and an optimum Se:Ru ratio. The half cell test confirms feasibility of the new procedure; the catalyst treated at 400 °C gave the highest reduction current (55.9 mA cm⁻² at −0.4 V) and a low methanol oxidation effect coefficient (3.8%). The direct methanol fuel cell with the RuSe 400 °C cathode catalyst (2 mg RuSe cm⁻²) generated a power density of 33.8 mW cm⁻² using 2 M methanol and 2 bar oxygen at 90 °C. The new procedure produced the catalysts with low decay rates. The best sample was compared to the Pt and to the reported ruthenium-selenium catalyst. Possible reasons for the observations are discussed.     

Platinum nanopaticles supported on stacked-cup carbon nanofibers as electrocatalysts for proton exchange membrane fuel cell

Inexpensive stacked-cup carbon nanofibers (SC-CNFs) supported Pt nanoparticles with a loading from 5 to 30 wt.% were prepared through a modified ethylene glycol method. XRD and TEM characterizations show that the average Pt particle sizes increase with increasing metal loading, and they can be controlled <5 nm with a uniform dispersion. A self-developed filtration process was employed to fabricate Pt/SC-CNFs film-based membrane electrode assembly (MEA), and the catalyst transfer efficiency can reach nearly 100% using a super-hydrophobic polycarbonate filter. The thickness of catalyst layer can be accurately controlled through altering Pt loadings of the catalyst and electrode, this is in good agreement with our theoretical calculation. For Pt/SC-CNFs-based-MEAs, Pt cathode loading was found more critical than Pt anode loading on proton exchange membrane fuel cell (PEMFC) performance. The Pt/SC-CNFs-based MEA with an optimized 50 wt.% Nafion content demonstrates higher PEMFC performance than the carbon black-based MEA with an optimized 30 wt.% Nafion content. SC-CNFs possess much larger length-to-diameter ratio than carbon black particles, it makes Pt/SC-CNFs more easily form continuously conductive networks in the Nafion matrix than carbon black. Therefore, the Pt/SC-CNFs-based MEA demonstrates higher Pt utilization than carbon black-based MEA, which implies possible reduction in Pt loading of MEA.     

CO tolerance effects of tungsten-based PEMFC anodes

The performance of proton exchange membrane fuel cells (PEMFC) fed with CO-contaminated hydrogen was investigated for anodes with PtWO_x/C and phosphotungstic acid (PTA) impregnated Pt/C electrocatalysts. A quite high performance was achieved for the PEMFC fed with H₂ + 100 ppm CO with anodes containing 0.4 mg PtWO_x cm⁻² and also for those with 0.4 mg Pt cm⁻² impregnated with ca. 1 mg PTA cm⁻². A decay of the single cell performance with time is observed, and this was attributed to an increase of the membrane resistance due to the polymer degradation promoted by the crossover of the tungsten species throughout the membrane.