Optimization of cathode catalyst layer for direct methanol fuel cells: Part II: Computational modeling and design

The cathode catalyst layer in direct methanol fuel cells (DMFCs) features a large thickness and mass transport loss due to higher Pt loading, and therefore must be carefully designed to increase the performance. In this work, the effects of Nafion loading, porosity distribution, and macro-pores on electrochemical characteristics of a DMFC cathode CL have been studied with a macro-homogeneous model, to theoretically interpret the related experimental results. Transport properties in the cathode catalyst layers are correlated to both the composition and microstructure. The optimized ionomer weight fraction (22%) is found to be much smaller than that in H₂ polymer electrolyte fuel cells, as a result of an optimum balance of proton transport and oxygen diffusion. Different porosity distributions in the cathode CLs are investigated and a stepwise distribution is found to give the best performance and oxygen concentration profile. Influence of pore defects in the CLs is discussed and the location of macro-pores is found to play a dual role in affecting both oxygen transport and proton conduction, hence the performance. The reaction zone is extended toward the membrane side and the proton conduction is facilitated when the macro-pores are near the gas diffusion layer.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

The effect of two-layer cathode on the performance of the direct methanol fuel cell

To reduce the effect of methanol permeated from the anode, the structure of the cathode was modified from a single layer with Pt black catalyst to two-layer with PtRh black and Pt black catalysts, respectively. The current density of the direct methanol fuel cell (DMFC) using the two-layer cathode was improved to 228 mA/cm^-2 compared to that (180 mA/cm^-2) of the DMFC using the single layer cathode at 0.3 V and 303 K. From the cyclic voltammograms (CVs), it is indicated that the amount of adsorbates on the metal catalyst in the two-layer cathode is less than that of adsorbates in the single layer cathode after methanol test. In addition, the adsorbates were removed very rapidly by electrochemical oxidation from the two-layer cathode. It is suggested fromex situ X-ray absorption near edge structure analysis that the d-electron vacancy of Pt atom in the two-layer cathode is not changed by the methanol test. Thus, Pt is not covered with the adsorbates, which agrees well with the results of CV.     

The Effect of Methanol Crossover on the Cathode Overpotential of DMFCs

The effects of methanol crossover on cathode overpotential of direct methanol fuel cells (DMFCs) were investigated by focusing on a mixed potential effect and surface poisoning of the catalyst. Experiments using different membranes and catalyst loadings were performed and compared with a semi‐quantitative model to discuss the main cause of the cathode overpotential. When the measured methanol crossover increased, cathode overpotential increased at particular threshold values, which were 150 mA cm^–2 at 0.3 mg cm^–2 of cathode platinum (Pt) loading and above 200 mA cm^–2 at 1.1 mg cm^–2. The modelling results also supported this tendency, and showed that Pt surface was poisoned to a great extent above the threshold methanol crossover where the cathode overpotential increased sharply, while the cathode overpotential remained low and was explained solely by the mixed potential below the threshold value. The threshold methanol crossover can be regarded as the acceptable value, below which the cathode overpotential from methanol crossover remains low, and was related with the Pt loading in the cathode. The reduction of methanol crossover through membranes below the acceptable values will contribute greatly to a decrease in the cathode overpotential and to the reduction of catalyst loadings.     

Ruthenium cluster-like chalcogenide as a methanol tolerant cathode catalyst in air-breathing laminar flow fuel cells

This paper reports the incorporation of a cluster-like Ru_xSe_y as a methanol tolerant cathode catalyst in a laminar flow fuel cell. The effect on cell performance of several concentrations of methanol in the cathode stream was investigated for the Ru_xSe_y catalyst and compared to a conventional platinum catalyst. While the Pt catalyst exhibited up to ∼80% drop in power density, the Ru_xSe_y catalyst showed no decrease in performance when the cathode was exposed to methanol. At several methanol concentrations the Ru_xSe_y catalyst performed better than the Pt catalyst. This demonstration of a methanol tolerant catalyst in a laminar flow fuel cell opens up the way for further miniaturization of the cell design and simplification of its operation as the need for an electrolyte stream to prevent fuel crossover has been eliminated.     

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     

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.     

Highly methanol-tolerant non-precious metal cathode catalysts for direct methanol fuel cell

This work reports on the oxygen reduction activity of several non-precious metal (non-PGM) catalysts for oxygen reduction reaction (ORR) at the fuel cell cathode, including pyrolyzed CoTPP, FeTPP, H₂TMPP, and CoTMPP. Of the studied catalysts, pyrolyzed CoTMPP (Co-tetramethoxyphenylporphyrin) was found to perform significantly better than other materials. The catalyst underwent a thorough testing in both hydrogen-air polymer electrolyte fuel cell (PEFC) and direct methanol fuel cell (DMFC). It was found that CoTMPP cathode can sustain currents that are only 2-3 times lower than those obtained with a conventional Pt-black cathode in an H₂-air PEFC. DMFC experiments, including methanol crossover and methanol tolerance measurements, indicate high ORR selectivity of the CoTMPP catalyst. Based on results obtained to date, the CoTMPP-based catalyst offers promise for the use in conventional and mixed-reactant DMFCs operating with concentrated methanol feeds. However, hydrogen-air fuel cell life data, consisting of over 800 h of continuous cell operation, indicate that improvement to long-term stability of the CoTMPP catalyst will be required to make it practical.     

Investigation on cathode degradation of direct methanol fuel cell

The cathode degradation of a direct methanol fuel cell (DMFC) was investigated after a 240 h discontinuous galvostatic operation at 80 °C. The catalyst coated membrane (CCM) and the cathode diffusion layer were not combined so as to isolate electrochemical and mass transport processes. It was indicated by the EDS and SEM tests that the loss of the cathode electrochemical surface area (ESA) was associated with the decays of the Pt/C catalyst and the interfacial contact. Furthermore, Ru crossover and higher methanol crossover resulting from the anode failure aggravated the degradation of the cathode. On the other hand, the change of the pore structure led to a higher wettability of the cathode microporous layer. Therefore, the oxygen transport was suppressed due to the decrease of hydrophobic passages.     

Performance of the Direct Methanol Carbonate Fuel Cell Using Anion Exchange Materials and Non‐Noble Metal Cathode Catalyst

A direct methanol fuel cell using a mixture of O₂ and CO₂ at the cathode was evaluated using anion exchange materials and cathode catalysts of Pt and a non‐Pt catalyst. The MEA based on non‐noble metal catalyst Acta 4020 showed superior performance than Pt/C based MEA in terms of open circuit potential and power density in carbonate environment. The fuel cell performance was improved by applying anion exchange ionomer in the catalyst layer. A maximum power density of 4.5 mW cm^–2 was achieved at 50 °C using 6.0 M methanol and 2.0 M K₂CO₃.     

Evaluation of the Performance of Carbon Supported Pt–Ru–Ni–P as Anode Catalyst for Methanol Electrooxidation

This research is aimed to improve the activity and stability of ternary alloy Pt–Ru–Ni/C catalyst. A novel anodic catalyst for direct methanol fuel cell (DMFC), carbon supported Pt–Ru–Ni–P nanoparticles, has been prepared by chemical reduction method by using NaH₂PO₂ as a reducing agent. One glassy carbon disc working electrode is used to test the catalytic performances of the homemade catalysts by cyclic voltammetric (CV), chronoamperometric (CA) and amperometric i–t measurements in a solution of 0.5 mol L^–1 H₂SO₄ and 0.5 mol L^–1 CH₃OH. The compositions, particle sizes and morphology of home‐made catalysts are evaluated by means of energy dispersive analysis of X‐ray (EDAX), X‐ray diffraction (XRD) and transmission electron micrographs (TEM), respectively. TEM images show that Pt–Ru–Ni–P nanoparticles have an even size distribution with an average diameter of less than 2 nm. The results of CV, CA and i–t curves indicate that the Pt–Ru–Ni–P/C catalyst shows significantly higher activity and stability for methanol electrooxidation due to the presence of non‐metal phosphorus in comparison to Pt–Ru–Ni/C one. All experimental results indicate that the addition of non‐metallic phosphorus into the Pt–Ru–Ni/C catalyst has definite value of research and practical application for enhancing the performance of DMFC.     

Rapid Microwave‐Assisted Solvothermal Synthesis of Methanol Tolerant Pt–Pd–Co Nanoalloy Electrocatalysts

We report here a microwave‐assisted solvothermal (MW‐ST) method to synthesise carbon‐supported multimetallic nanostructured alloys of Pt, Pd and Co with high crystallinity and homogeneity for electrocatalytic application in fuel cells. Multimetallic nanoalloy electrocatalysts have been synthesised by a one‐pot, rapid MW‐ST method within 15 min at <300 °C without any post‐annealing in reducing gas atmospheres. For a comparison, same multimetallic alloys were also synthesised by heat treatment of co‐precipitated metals. Significant differences were observed in the phase structure and surface composition of the alloys synthesised by the two methods, which were rationalised based on the synthesis procedures adopted. Further, the multimetallic alloys were also explored for their electrocatalytic applications as cathode catalysts for oxygen reduction reaction (ORR). The multimetallic alloys, synthesised by the MW‐ST method, show much higher ORR activity compared to their counterparts synthesised by the conventional borohydride reduction method. While the ORR activity of Pt₇₀Pd₂₀Co₁₀ is comparable to that of commercial Pt, the ORR activity of Pt₅₀Pd₃₀Co₂₀ in direct methanol fuel cells (DMFC) is superior to that of commercial Pt at high methanol concentrations due to its high tolerance to methanol that may crossover from the anode to the cathode.     

Electrochemical Evaluation of Porous Non‐Platinum Oxygen Reduction Catalysts for Polymer Electrolyte Fuel Cells

A porous non‐platinum electrocatalyst for the oxygen reduction reaction (ORR), obtained by pyrolysing a cobalt porphyrin precursor, was evaluated by electrochemical means. The reactivity of the non‐platinum ORR catalyst was investigated with a rotating disc electrode (RDE) experimental set up. RDE data were collected in an acidic electrolyte containing N₂, O₂, CO and under mixed reactant O₂/methanol conditions. The electrochemical performance of such‐obtained non‐platinum catalyst is discussed and compared to platinum‐based ORR catalysts. Based on the results collected here, we are able to propose and test possible proton exchange fuel cell (PEFC) operating conditions where non‐platinum ORR catalysts can be utilised. Direct methanol fuel cell (DMFC) data demonstrating a superior performance of the non‐platinum catalyst relative to platinum black, often perceived as the state‐of‐the‐art oxygen–reduction catalyst for the DMFC cathode is presented.     

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.     

Influence of copper oxide modification of a platinum cathode on the activity of direct methanol fuel cell

To determine whether a copper oxide modified Pt cathode (PtCuO_m) improves a performance of direct methanol fuel cells (DMFC), we performed structural and morphological analysis of the cathode and measured current-potential profile and impedance spectroscopy. Comparing with an unmodified Pt cathode, we found that PtCuO_m prepared by rf sputtering techniques induced higher oxygen reduction reaction rate and suppressed electrocatalytic oxidation of methanol, which is the main reason of the mixed potential occurred at a cathode. Therefore, PtCuO_m increased the power performance of DMFC applying both oxygen and air and electrochemical impedance spectra clearly supported the difference of the performance between unmodified and modified Pt electrodes. These results may play a role in better long-term stability of DMFC systems.     

Fabrication of microstructure controlled cathode catalyst layers and their effect on water management in polymer electrolyte fuel cells

Although cathode catalyst layers (CCLs) are at the center of water management in polymer electrolyte fuel cells (PEFCs), the understanding of water movement in CCLs and their roll on fuel cell performance is still limited. In this present study, several CCLs with controlled microstructure, including main pore size, pore volume and porosity ranging from 30 to 70 nm, 0.443 to 0.962 cm³/g_Pt/C, and 45.4 to 64.4%, respectively, were prepared by changing the hot-pressing pressure in a decal process, and their water management ability and cell performance were evaluated. The electrochemical analyses reveal that, as the pore size and pore volume of CCLs increase, the diffusion resistance mainly arising from water accumulation in the pores is evidently reduced by capillary water equilibrium, which leads to better cell performance. Water balancing between accumulation and discharging in the pores also depends on the CCL pore structure, and the CCLs with greater pore sizes and larger pore volumes reveal more stable cell performance by better water management in steady state operation, even under extremely humid conditions. Based on these MEA technologies such as fabrication of CCLs, further study will be performed to understand microscopic phenomena in nano pores of CCLs by combining the experimental approach with CCL numerical modeling.     

自呼吸式直接甲醇燃料电池性能及其传质特性

针对有效面积为1 cm2的自呼吸式直接甲醇燃料电池（direct methanol fuel cell,DMFC）单电池,阳极采用燃料罐供液,将阴极侧集流体和夹具设计为一体式结构,并用自制的七合一膜电极组件对其进行测试,讨论了催化剂类型、扩散层材料、集流体结构等因素对其性能的影响,分析了电池内部的传质特性,优化了电池特别是其在中高电流密度条件下的性能。实验结果表明：采用Pt黑、Pt-Ru黑催化剂制作的自呼吸式DMFC能强化反应物的传质;采用碳布制作的膜电极更倾向于获得更高的极限电流密度;低电流密度时,因甲醇渗透电池电压随着甲醇浓度的增加而降低,但在中高电流密度下,电池性能随甲醇浓度的增大先升高后降低;平行集流体有利于阴阳极生成物的排出和反应物的传质,因此易获得较高的电池性能。     

Catalytic Oxidation of Methanol by Aqueous POM on Al2O3 Supported Catalysts and Electrochemical Performance of POM

Phosphomolybdic acid (H₃PMo₁₂O₄₀, POM) was attempted to be used as the energy‐storage agent in this paper to avoid some problems of the direct methanol fuel cell (DMFC), such as catalyst poisoning and methanol permeation. Catalytic oxidation of methanol by aqueous POM on Al₂O₃ supported catalysts with Pt and Ru active metal was evaluated in the presence of liquid water. The process takes advantage of the high catalytic activities of platinum for methanol oxidation. The effects of temperature, reaction time, and methanol concentration on activity were observed. The catalytic activity of Pt/Al₂O₃ is better than that of Ru/Al₂O₃ for the oxidation of methanol by POM. The methanol conversion rate reached 93.55% on the Pt/Al₂O₃ at 80 °C after reaction for 1 h. The electrochemical experiments indicate that POM shows a larger current density in redox processes on an Au electrode than methanol. The redox process of reduced POM is a reversible multi‐electron transfer process.     

Methanol crossover effect on the cathode potential of a direct PEM fuel cell

The potential of the oxygen cathode in a direct methanol fuel cell is strongly influenced by the crossover of methanol through the poly-electrolyte membrane. In the presence of methanol, oxygen is reduced at the cathode already at open circuit and the equivalent amount of methanol is oxidized. This results in the formation of a mixed potential, up to 200 mV negative to the original oxygen potential. In this work, the anode and cathode potentials of a DMFC are monitored in situ, using a dynamic hydrogen electrode (DHE). For the first time, the effect of crossover on the cathode potential as function of time is presented. Methanol and ethanol as fuels are compared. Changing from methanol to hydrogen, the influence of methanol crossover on the cathode potential can also be followed as function of current density. It is already known that, in addition to the consumption of fuel and oxygen with the formation of a mixed potential, a purely chemical reaction takes place at the platinum surface. A quantitative determination of the respective CO₂ formation is presented here.