Graphitic Carbon as Durable Cathode‐Catalyst Support for PEFCs

Long‐term deterioration in the performance of PEFCs is attributed largely to reduction in active area of the platinum catalyst at cathode, usually caused by carbon‐support corrosion. It is found that the use of graphitic carbon as cathode‐catalyst support enhances its long‐term stability in relation to non‐graphitic carbon. This is because graphitic‐carbon‐supported‐Pt (Pt/GrC) cathodes exhibit higher resistance to carbon corrosion in‐relation to non‐graphitic‐carbon‐supported‐Pt (Pt/Non‐GrC) cathodes in PEFCs during accelerated stress test (AST) as evidenced by chronoamperometry and carbon dioxide studies. The corresponding change in electrochemical surface area (ESA), cell performance and charge‐transfer resistance are monitored through cyclic voltammetry (CV), cell polarisation and impedance measurements, respectively. The degradation in performance of PEFC with Pt/GrC cathode is found to be around 10% after 70 h of AST as against 77% for Pt/Non‐GrC cathode. It is noteworthy that Pt/GrC cathodes can withstand even up to 100 h of AST with nominal effect on their performance. X‐ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy and cross‐sectional field‐emission scanning electron microscopy (FE‐SEM) studies before and after AST suggest lesser deformation in catalyst layer and catalyst particles for Pt/GrC cathodes in relation to Pt/Non‐GrC cathodes, reflecting that graphitic carbon‐support resists carbon corrosion and helps mitigating aggregation of Pt‐particles.     

Tubular Cathode Prepared by a Dip‐Coating Method for Low Temperature DMFC

A novel tubular cathode for the direct methanol fuel cell (DMFC) is proposed, based on a tubular titanium mesh. A dip‐coating method has been developed for its fabrication. The tubular cathode is composed of titanium mesh, a cathode diffusion layer, a catalyst layer, and a recast Nafion® film. The titanium mesh is present at the inner circumference of the diffusion layer, while the recast Nafion® film is at the outer circumference of the catalyst layer. A DMFC single cell with a 3.5 mgPt cm^–2 tubular cathode was able to perform as well, in terms of power density, as a conventional planar DMFC. A peak power density of 9 mW cm^–2 was reached under atmospheric air at 25 °C.     

Investigation of a Novel Catalyst Coated Membrane Method to Prepare Low‐Platinum‐Loading Membrane Electrode Assemblies for PEMFCs

In this work, a novel catalyst coated membrane (CCM) approach–a catalyst‐sprayed membrane under irradiation (CSMUI)–was developed to prepare MEAs for proton exchange membrane fuel cell (PEMFC) application. Catalyst ink was sprayed directly onto the membrane and an infrared light was used simultaneously to evaporate the solvents. The resultant MEAs prepared by this method yielded very high performance. Based on this approach, the preparation of low‐platinum‐content MEAs was investigated. It was found that for the anode, even if the platinum loading was decreased from 0.2 to 0.03 mg cm^–2, only a very small performance decrease was observed; for the cathode, when the platinum loading was decreased from 0.3 to 0.15 mg cm^–2, just a 5% decrease was detected at 0.7 V, but a 35% decrease was observed when the loading was decreased from 0.15 to 0.06 mg cm^–2. These results indicate that this approach is much better than the catalyst coated gas diffusion layer (GDL) method, especially for the preparation of low‐platinum‐content MEAs. SEM and EIS measurements indicated ample interfacial contact between the catalyst layer and the membrane.     

Statistische Analyse des lokalen Wassertransportes einer Polymer‐Elektrolyt‐Brennstoffzelle

The water transport in the gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) was simulated using the Lattice Boltzmann method. By means of a stochastic geometry model an ensemble of microstructures was generated, all of them stochastically equivalent to the real structure of a GDL. The rough surface of the GDL defines the interface between the GDL and air channel of a PEFC. The water droplets emerging from the GDL were analyzed statistically regarding their contact angles. A short insight to the dynamics is given.     

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.     

Durability of Pt/C and Pt/MC‐PEDOT Catalysts under Simulated Start‐Stop Cycles in Polymer Electrolyte Fuel Cells

A composite of mesoporous carbon (MC) with poly(3,4‐ethylenedioxythiophene) (PEDOT) is studied as catalyst support for platinum nanoparticles. The durability of commercial Pt/carbon and Pt/MC‐PEDOT as cathode catalyst is investigated by invoking air‐fuel boundary at the anode side so as to foster carbon corrosion at the cathode side of a polymer electrolyte fuel cell (PEFC). Pt/MC‐PEDOT shows higher resistance to carbon corrosion in relation to Pt/C. Electrochemical techniques such as cyclic voltammetry (CV) and impedance measurements are used to evaluate the extent of degradation in the catalyst layer. It is surmised that the resistance of MC‐PEDOT as catalyst support toward electrochemical oxidation makes Pt/MC‐PEDOT a suitable and stable cathode catalyst for PEFCs.     

Oxygen mass transport limitations at the cathode of polymer electrolyte membrane fuel cells

Oxygen transport across the cathode gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells was examined by varying the O₂/N₂ ratio and by varying the area of the GDL extending laterally from the gas flow channel under the bipolar plate (under the land). As the cathode is depleted of oxygen, the current density becomes limited by oxygen transport across the GDL. Oxygen depletion from O₂/N₂ mixtures limits catalyst utilization, especially under the land.The local current density with air fed PEM fuel cells falls to practically zero at lateral distances under the land more than 3 times the GDL thickness; on the other hand, catalyst utilization was not limited when the fuel cell cathode was fed with 100% oxygen. The ratio of GDL thickness to the extent of the land is thus critical to the effective utilization of the catalyst in an air fed PEM fuel cell. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Waterproof Breathable Membrane Used as Gas Diffusion Layer in Activated Carbon Air Cathode Microbial Fuel Cells

One of the main limiting factors for scaling up microbial fuel cells (MFCs) technology is to develop low‐cost and high‐efficiency cathode. A new and simplified approach was developed by using a commercial waterproof breathable membrane (WBM) as gas diffusion layer (GDL) material as substitution for conventional polytetrafluoroethylene (PTFE) GDL. Air‐cathode with the WBM pasted (AC‐P) onto the stainless steel mesh (SSM) achieved a maximum power density of 611 ± 10 mWm⁻², which was similar to that using a PTFE GDL by rolling method (645 ± 12 mWm⁻², AC‐R). Physical and electrochemical techniques were employed to investigate the morphology and electrochemical characteristics of the cathode. The result demonstrated that AC‐P had a higher current density and internal resistance than AC‐R. Besides, the WBM had a higher porosity and uniform texture. The study showed that the WBM was a kind of good GDL material for easy preparation, low cost and stable performance of cathode construction.     

A Simple and Accurate Method for High‐Temperature PEM Fuel Cell Characterisation

A set of basic parameters for any polymer electrolyte membrane fuel cell (PEMFC) includes the Tafel slope b and the exchange current density j_* of the cathode catalyst, the oxygen diffusion coefficient D_b in the cathode gas‐diffusion layer and the cell resistivity R_cell. Based on the analytical model of a PEMFC [A. A. Kulikovsky, Electrochim. Acta (2004) 617], we propose a two‐step procedure allowing to evaluate these parameters for a high‐temperature PEMFC. The procedure requires two polarisation curves measured at different oxygen (air) stoichiometries. The method is validated using the experimental data obtained with the in‐house designed cell. High quality of fitting confirms validity and accuracy of this approach. The physical background of the method is discussed.     

Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore‐Network Models

Water management remains a critical issue for polymer electrolyte fuel cell performance and durability, especially at lower temperatures and with ultrathin electrodes. To understand and explain experimental observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore‐network models. X‐ray computed tomography was used to extract GDL material parameters for use in the pore‐network model. The simulations were conducted to explain experimental observations associated with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting current density. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphology allowed more efficient water movement through the anode and higher temperatures at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temperature performance with the stacked anode GDL.     

Carbon‐supported perovskite oxides as oxygen reduction reaction catalyst in single chambered microbial fuel cells

BACKGROUND: Pt‐free cathodic catalyst is needed for microbial fuel cells (MFCs). Perovskite‐type oxide could be a substitute for Pt because it has been proved to be a highly active and low‐cost oxygen reduction catalyst in chemical fuel cells. RESULTS: A nano‐sized La_0.4Ca_0.6Co_0.9Fe_0.1O₃ perovskite‐type oxide on a carbon support (LCCF/C) was prepared and tested for its performance and stability (15 cycles) in MFCs. An exchange current density of 7.030 × 10^?5 (A cm^?2) was obtained with fresh LCCF/C cathode and is increased to 7.438 × 10^?5 (A cm^?2) after 15 cycles operating in MFCs. A power density of 405 mW m^?2 was achieved with the LCCF/C cathode at the 2nd cycle which was between those of Pt/C (560 mW m^?2) and C (339 mW m^?2) cathodes. At the end of the 15th cycle, the lowest decay (due to biofouling) rate on the open circuit voltage (2%) and the maximum power density (15%) were observed with LCCF/C cathode compared with those of Pt/C (4%, 17%) and C (22%, 69%) cathodes, respectively. CONCLUSIONS: This study demonstrated that perovskite‐type oxide on carbon support catalysts could be a potential substitute for Pt for cathodic oxygen reduction reaction (ORR) in air‐cathode MFCs. © 2012 Society of Chemical Industry     

Die Rolle der Gasdiffusionselektroden in der Zink‐Luft‐ und Lithium‐Luft‐Batterie

Metal‐air batteries, especially the rechargeable type, are currently of great interest for many industrial sectors. Besides the potentially low‐cost zinc‐air system, the potentially high‐energy lithium‐air system, originally intended as a high‐energy storage device for electric mobility, has been investigated a lot within the last ten years. Within this article, rechargeable zinc‐air and lithium‐air systems will be discussed. Especially the role of the gas diffusion electrode will be described and general trends for further developments will be given.     

Electrochemical performance and stability of thin film electrodes with metal oxides in polymer electrolyte fuel cells

Thin film electrodes are prepared by thermal evaporation of nanometer thick layers of metal oxide and platinum on a gas diffusion layer (GDL), in order to evaluate different metal oxides’ impact on the activity and stability of the platinum cathode catalyst in the polymer electrolyte fuel cell. Platinum deposited on tin, tantalum, titanium, tungsten and zirconium oxide is investigated and the morphology and chemistry of the catalysts are examined with scanning electron microscopy and X-ray photoelectron spectroscopy. Cyclic sweeps in oxygen and nitrogen are performed prior and after potential cycling degradation tests. Platinum seems to disperse better on the metal oxides than on the GDL and increased electrochemically active surface area (ECSA) of platinum is observed on tin, titanium and tungsten oxide. A thicker layer metal oxide results in a higher ECSA. Platinum deposited on tungsten performs better than sole platinum in the polarisation curves and displays higher Tafel slopes at higher current densities than all other samples. The stability does also seem to be improved by the addition of tungsten oxide, electrodes with 3 nm platinum on 3, 10 and 20 nm tungsten oxide, performs better than all other electrodes after the accelerated degradation tests.     

Water Evolution in Direct Methanol Fuel Cell Cathodes Studied by Synchrotron X‐Ray Radiography

Water evolution, distribution, and removal in the cathodes of a running direct methanol fuel cell were investigated by means of synchrotron X‐ray radiography. Radiographs with a spatial resolution of around 5 μm were taken every 5 s. Special cell designs allowing for through‐plane and in‐plane viewing were developed, featuring two mirror‐symmetrical flow field structures consisting of one channel with the through‐plane design. Evolution and discharge of water droplets and the occurrence of water accumulations in selected regions of the channels were investigated. These measurements revealed a nonuniform distribution of water in the channels. Both irregular and periodic formation of water droplets were observed. In‐plane measurements revealed, that the droplets evolve between adjacent carbon fiber bundles of the gas diffusion layer. The water distribution within the channel cross‐section fits very well to the pressure difference between cathode channel inlet and outlet. The quick discharge of water droplets causes sudden decreases of the pressure difference up to 4.5 mbar.     

A one‐step way to novel carbon‐niobium nitride nanoparticles for efficient oxygen reduction

In recent years, researchers have been exploring various Pt‐free electrocatalysts to optimize the performance of regenerative fuel cell and rechargeable metal‐air battery. However, similar studies in ceramic fields are still stalled. In this work, as an efficient oxygen reduction reaction (ORR) catalyst, carbon‐niobium nitride (C‐NbN) nanostructure was successfully synthesized via a facile solid phase method. Microstructure characteristic analysis include SEM, TEM and element mapping spectra visually demonstrated C‐NbN nanoparticles, and its unique carbon coating layer. Moreover, the state of the carbon layer of C‐NbN was investigated by X‐ray photoelectron spectroscopy (XPS), Raman scattering spectrum, and energy‐dispersive X‐ray spectroscopy (EDX). After a series of electrochemical tests, it was found that such a novel carbon layer play two important roles in the ORR, leading to superior ORR performance (with 0.9 V onset potential, vs RHE) and outstanding durability (retained 97.02% of initial current for a duration of 10 000 second chronoamperometry test and excellent methanol tolerance) of C‐NbN catalyst. This method might could be implemented in the synthesis of other carbon‐transition nitride ceramics to enhance their ORR performance.     

Mesoscopic modeling of two-phase behavior and flooding phenomena in polymer electrolyte fuel cells

A key performance limitation in polymer electrolyte fuel cells (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. Liquid water covers the electrochemically active sites in the catalyst layer (CL) rendering reduced catalytic activity and blocks the available pore space in the porous CL and fibrous gas diffusion layer (GDL) resulting in hindered oxygen transport to the active reaction sites. The cathode CL and the GDL play a major role in the mass transport loss and hence in the water management of a PEFC. In this work the development of a mesoscopic modeling formalism coupled with realistic microstructural delineation is presented to study the influence of the pore structure and surface wettability on liquid water transport and interfacial dynamics in the PEFC catalyst layer and gas diffusion layer. The two-phase regime transition phenomenon in the capillary dominated transport in the CL and the influence of the mixed wetting characteristics on the flooding dynamics in the GDL are highlighted.     

A 2<Superscript>7-3</Superscript> fractional factorial optimization of polybenzimidazole based membrane electrode assemblies for H<Subscript>2</Subscript>/O<Subscript>2</Subscript> fuel cells

We describe the usefulness of a statistical fractional factorial design to obtain consistent and reproducible behavior of a membrane-electrode-assembly (MEA) based on a phosphoric acid (PA) doped polybenzimidazole (PBI) membrane, which allows a H₂/O₂ fuel cell to operate above 150 °C. Different parameters involved during the MEA fabrication including the catalyst loading, amount of binder, processing conditions like temperature and compaction load and also the amount of carbon in the gas diffusion layers (GDL) have been systematically varied according to a 2^7-3 fractional factorial design and the data thus obtained have been analyzed using Yates’s algorithm. The mean effects estimated in this way suggest the crucial role played by carbon loading in the gas diffusion layer, hot compaction temperature and the binder to catalyst ratio in the catalyst layer for enabling continuous performance. These statistically designed electrodes provide a maximum current density and power density of 1,800 mA cm⁻² and 280 mW cm⁻², respectively, at 160 °C using hydrogen and oxygen under ambient pressure.     

PtCo/Polypyrrole‐Multiwalled Carbon Nanotube Complex Cathode Catalyst Containing Two Types of Oxygen Reduction Active Sites Used in Direct Methanol Fuel Cells

Low oxygen reduction reaction (ORR) activity and high cost of noble metal catalysts are two major challenges in direct methanol fuel cells (DMFCs). Pt‐based catalysts are considered as an ideal alternative to deal with these two problems. While the second component metals play only the role of synergy effect with Pt, they themselves are inert towards activity towards ORR. It is necessary to design a new route to ultilize the second component metal by forming CoN_x ORR active site on the base of PtM catalyst. In this paper, PtCo/polypyrrole‐multiwalled carbon nanotubes (PtCo/PPy‐MWCNTs) catalyst containing two types of ORR active site (Pt and CoN_x) was synthesized by one pot synthesis route. The effect and dynamic mechanism of the named CoN_x active site towards ORR was discussed by X‐ray photoelectron sprectroscopy and linear sweep voltammetry. PtCo/PPy‐MWCNTs cathode catalyst showed improved activity towards ORR and great potential in DMFCs.     

PEMFC阴极扩散层结构特性对水淹影响的数值分析

建立质子交换膜燃料电池一维两相传递模型,通过达西定律和菲克定律的联立求解得到扩散层中的液体饱和度和氧气浓度分布。考察扩散层特性参数孔隙率、厚度、接触角、渗透率对阴极水淹的影响,结果表明扩散层表面憎水将有助于液态水移出,但当达到憎水条件后,增大接触角对液态水传输和氧气传质的影响逐渐变小。憎水条件下孔隙率和厚度对液态水传输的影响不是很明显,但孔隙率增大和扩散层厚度减小均有利于氧气传质,实际应用中孔隙率增大的同时,厚度也要适当增大,极限电流密度相差不大。模型计算结果与文献中不同PTFE含量条件下实验的Tafel斜率和极限电流密度比较,吻合较好。     

High Active Hollow Nitrogen‐Doped Carbon Microspheres for Oxygen Reduction in Alkaline Media

Dopamine, a sustainable and cheap raw material, was selected as the carbon and nitrogen sources to synthesize hollow nitrogen‐doped carbon microspheres (HNCMS). The obtained HNCMS were used as a non‐noble‐metal electrocatalyst for oxygen reduction in alkaline solution and show high electrocatalytic activity, excellent long‐term stability, and tolerance to crossover effect of methanol.