Degradations in the surface wettability and gas permeability characteristics of proton exchange membrane fuel cell electrodes under freeze-thaw cycles: Effects of ionomer type

Current state-of-the-art proton exchange membrane (PEM) fuel cell electrodes are typically comprised of either short-side-chain (SSC) or long-side-chain (LSC) ionomers, owing to their proven success in the electrode performance and durability under regular cell operation. However, the electrodes based on these two prominent ionomers have not been sufficiently investigated under sub-freezing conditions. In this study, the impact of ionomer type on the degradations of the surface wettability and gas permeability characteristics has been investigated for PEM fuel cell electrodes under freeze-thaw (F-T) cycles between 30 °C and −40 °C. The electrodes comprised of either SSC or LSC ionomers are manufactured with different catalyst loadings. It is found that the F-T cycles induce severe degradations in the electrodes, and the resulting surface morphologies differ greatly, depending on the ionomer type and catalyst loading. For a given catalyst loading, the SSC electrodes degrade more heavily than the LSC ones. Further, independent of the ionomer type, the high catalyst loading electrodes tend to degrade slower than their low catalyst loading counterparts. The SSC catalyst layers peel off from the electrodes virtually completely with the microporous layer largely degraded, inducing a highly corroded and heterogeneous surface morphology. The LSC electrodes experience relatively less degradations, thus the resulting surface morphologies are less corroded and more homogeneous. For all the electrodes, the morphological degradations cause a substantial increase in the gas permeability coefficients, but a decrease in the static contact angles. These increments and decrements correlate well with the severity of the surface degradations, and they are rapid and more substantial for the SSC electrodes.     

Characterization of electrode structures and the related performance of direct methanol fuel cells

The performance of membrane electrode assemblies (MEAs) in fuel cells is substantially affected by the structures of the electrodes. An increase of about 25% in power performance was achieved merely by controlling the pressure of hot press in the MEA fabrication process for a given Pt loading, instead of by employing pore formers and heat treatment-a widely accepted method-to modify the structures of the electrode. The microstructures of the different hot-pressed electrodes were examined by transmission electron microscopy, scanning electron microscopy, and small angle X-ray scattering to assess the effect of the pressure on the structures of the electrodes. Based on experimental observations, the improved performance of the MEA is attributed to the porosity of the cathode electrode, in which a network of macrofissures and sub-microfissures allows air to penetrate the electrode. Emphasis is also placed on the relationship between the total porosity of the electrodes and the MEA performance. Results of this study demonstrate that the specific power density nearly doubles when the total porosity increased from 57% to 76%. Also, the MEAs mounted in an air-breathing DMFC small pack were fabricated in-house to supply power for a mobile phone.     

Electric double-layer capacitors with sheet-type polarizable electrodes and application of the capacitors

Sheet-type polarizable electrodes with low sheet resistances for electric double-layer capacitors were outlined. The sheet-type electrodes consisted of activated carbon layers on aluminium foils. The sheet resistance of the sheet-type electrode mainly correlated with a filling density of activated carbons in the carbon layer. The species of activated carbons and the particle size of activated carbons affected the filling density of activated carbons in the layer. High filling ratio of activated carbons with small particle size resulted in the sheet-type electrodes with very low sheet resistance and high capacitance. Capacitors employing sheet-type electrodes showed very low internal resistance. Some application examples of the capacitor are described.     

Influence of structure and hydrophobic properties on the characteristics of carbon—air electrodes

The electrochemical parameters of carbon—oxygen gas-diffusion electrodes can be controlled over a wide range by varying the structure of the active carbon catalyst and the ratio of lyophilic and lyophobic pores in the catalyst particles. Two typical representatives of active carbon catalysts with significantly different hydrophobic properties have been investigated by mercury—alkali intrusion porosimetry and tested both in model floating electrodes and as the hydrophilic component of the active layer of two-layer, gas-diffusion working electrodes. The optimal electrolyte content in the active layer ensuring the maximum electrical characteristics of working electrodes has been found to depend on the structure and hydrophobic properties of the carbon catalyst. The gas pores in the carbon catalyst have been shown to play an essential role in the oxygen mass transfer process in the active layer.     

Effects of different methods of cobalt addition on the performance of nickel electrodes

Cobalt is an effective additive which is widely used in nickel electrodes. Three kinds of nickel electrodes with various cobalt levels are made with different methods of cobalt addition. The electrochemical properties, including charge–discharge characteristics of the nickel electrodes are investigated. It is found that the method used to add cobalt exerts a marked effect on electrode performance. Nickel electrodes with cobalt incorporated directly display excellent charge-discharge behaviour. By contrast, nickel electrodes co-precipitated with cobalt exhibit an adverse effect except at high-rate discharge.     

Nanoporous oxide coatings on stainless steel to enable water splitting and reduce the hydrogen evolution overpotential

Nanoporous oxides (SiO₂, TiO₂, ZrO₂, and AlOOH) synthesized from sol–gel chemistry techniques were used as coatings for stainless steel electrodes in water electrolysis systems. These oxide coatings have been shown to provide corrosion protection of the stainless steel electrodes at potentials positive enough to evolve oxygen on the positive electrode. In addition, all four oxide coated electrodes showed a 100–200 mV lower overpotential for hydrogen evolution than an uncoated stainless steel electrode. This was attributed to the ability of the oxide coatings to adsorb hydrogen on the surface of the electrode. To verify gas production from these electrodes, a custom alkaline electrolyzer was built and tested with a constant applied current. The flow rate of gas was measured for five different electrode connection configurations, utilizing both monopolar and bipolar electrodes. The efficiency of the system was calculated to be between 66 and 75% as defined as the ratio of the higher heating value of hydrogen to the energy applied to the system. The oxide coated stainless steel electrodes were used without any additional catalysts, including the precious metals.     

The influence of fast charging on the performance of VRLA batteries

A valve regulated battery (VRLA) with thin tubular electrodes combines the endurance of robust positive tubular electrodes and the enhanced power properties of very thin electrodes. Fast charging is essential for many applications and it can compensate the lower energy density of lead-acid batteries. Thin tubular electrodes VRLA batteries were cycle life tested and influences of two charging regimes were investigated. A comparison is made between a newly developed fast charging regime with current step-down and a conventional charging regime as regards battery life and performance. The fast charging procedure was beneficial for the cycle life; however, associated beneficial effects were reduced by certain drawbacks. A measuring method was developed for analysing the current distribution over the electrodes at high and low currents and results are presented in this paper. Post-mortem battery analyses were carried out to determine the dominating degradation effects. The differences in the degradation effects caused by the charging regimes are discussed. The paper concentrates on the newly developed fast charging regime and compares the influence of the charging regime on ageing mechanisms of the VRLA batteries with tubular electrodes as opposed to a conventional charging regime.     

电极形式和布置方式对静电旋风除尘器性能的影响

静电旋风除尘器是在旋风除尘器内加入电极以引入高压静电场,实现离心除尘机理和静电除尘机理相结合的多机理除尘设备。静电除尘机理如何与离心除尘机理有机地结合从而发挥各自的优势,是静电旋风除尘器值得研究的主要问题,也是国内外学者研究的重点之一。静电旋风除尘器内的电极布置和单纯的管式静电除尘器和板式静电除尘器的电极布置差别很大。电极的形式和布置方式不仅关系到静电旋风除尘器能否很好地发挥电场力和离心力的综合作用,而且对电极的安装、电源的要求也是非常重要的。通过大量的实验,研究了静电旋风除尘器的电极形式和布置方式对除尘器性能的影响。     

A critical review on progress of the electrode materials of vanadium redox flow battery

Nowadays, renewable energy sources are taken great attention by the researchers and the investors around the world due to increasing energy demand of today's knowledge societies. Since these sources are non-continuous, the effective storage and re-use of the energy produced from renewable energy sources have great importance. Although classical energy storage systems such as lead acid batteries and Li-ion batteries can be used for this goal, the new generation energy storage system is needed for large-scale energy storage applications. In this point, vanadium redox flow batteries (VRFBs) are shinning like a star for this area. VRFBs consist of electrode, electrolyte, and membrane component. The battery electrodes as positive and negative electrodes play a key role on the performance and cyclic life of the system. In this work, electrode materials used as positive electrode, negative electrode, and both of electrodes in the latest literature were complained and presented. From graphene-coated and heteroatom-doped carbon-based electrodes to metal oxides decorated carbon-based electrodes, a large scale on the modification of carbon-based electrodes is available on the electrode materials of the VRFBs. By the discovering of novel electrode components for the battery system, the using of the VRFBs probably increase in a short time for many industrial and residential applications.     

On the use of 3-D electrodes and pulsed voltage for the process intensification of alkaline water electrolysis

In this work, the potential of process intensification of alkaline water electrolysis has been studied when using 3-D nickel electrodes. First of all, it was shown that such macro-porous, 3-D electrodes, when used in combination with forced electrolyte flow, can have a strong impact on the hydrogen generation performance of the process. For optimal conditions of the imposed Reynolds numbers of both the catholyte and anolyte flow, a 45% increase in current was measured as compared to state-of-the-art 2-D electrodes for a cell potential of 2 V, the latter being a typical value for industrial electrolysers. For the 2-D electrodes, forced flow did not induce any significant improvement. Secondly, the use of pulsed electrical power has been studied as well, with pulse widths in the range 2–200 ms. A significant synergistic effect was observed when using 3-D electrodes in combination with both forced flow and pulsed voltage, allowing in the case of 2 ms pulses an almost fivefold increase in current at 2 V.     

Thermal and water transfer in PEMFCs: Investigating the role of the microporous layer

The objective of this work was to investigate experimentally the effects of the microporous layer (MPL) within a PEMFC. The experiments consisted in measuring, at the anode and at the cathode, the average temperature of the electrodes using small platinum wires, heat fluxes using heat flux sensors and water fluxes by means of water balance for two builds of cell; one with porous layers and MPL and another without MPL. Three thermal configurations related to the imposed temperature of the plates were studied. The measurements put forward a new role of the microporous layer on heat transfer. Indeed, the MPL implies an increase of the electrodes temperature by adding a thermal resistance. This higher temperature enables to avoid the saturation at the electrodes and improve the water removal towards the flow field plates. In addition, the effective thermal conductivity of microporous layer, a key parameter for the analysis of heat transfer in the fuel cell, was estimated in situ.     

Effect of electrode configuration on the thermal behavior of a lithium-polymer battery

A thermal modeling was performed to study the effect of the electrode configuration on the thermal behavior of a lithium-polymer battery. It was examined the effect of the configuration of the electrodes such as the aspect ratio of the electrodes and the placing of current collecting tabs as well as the discharge rates on the thermal behavior of the battery. The potential and current density distribution on the electrodes of a lithium-polymer battery were predicted as a function of discharge time by using the finite element method. Then, based on the results of the modeling of potential and current density distributions, the temperature distributions of the lithium-polymer battery were calculated. The temperature distributions from the modeling were in good agreement with those from the experimental measurement for the batteries with three different types of electrodes at the discharge rates of 1C, 3C, and 5C.     

Role of the binder on the failure mechanism of Si nano-composite electrodes for Li-ion batteries

In this work a novel method is used to fabricate uniform thin layer electrodes based on nano-silicon composite material. The manufacture process, based on a combination of Laser assisted Chemical Vapor Pyrolysis and Electro-Spray Deposition, allows the forming of highly uniform and highly porous surface electrodes. By controlling the particle synthesis and deposition parameters it is possible to obtain samples that are highly reproducible in terms of their morphology. Electrosprayed nano-composite electrodes show high specific capacity. The capacity retention of these types of electrodes is strongly dependant on the binder's nature. The capacity fading mechanism has been further investigated and we suggest a simple mechanical model, which is supported by one of the two CMC binding mechanism already proposed in literature.     

Electrocatalytic properties of Co decorated graphene and graphene oxide for small organic molecules oxidation

In this work, research on the electrocatalytic oxidation of organic compounds on composite electrodes containing cobalt and nanometric carbon fillers is presented. The selected organic compounds for electrolytic oxidation were methanol and urea. The obtained composite electrodes were characterized by a greater degree of surface development and larger surface coverage of redox species than those of cobalt electrodes. The composite electrodes containing nanometric carbon forms showed favorable properties in the oxidation of organic compounds. The results obtained in both the methanol and urea solutions indicated the repeatability and stability of the processes occurring on the surface of the electrodes. In addition, composite electrodes containing graphene were characterized by higher electrocatalytic activity in the oxidation of organic compounds.     

Effects of brush lengths and fiber loadings on the performance of microbial fuel cells using graphite fiber brush anodes

An alternative method for fabricating graphite fiber brush (GFB) electrodes was proposed. Two series of GFB electrodes with different lengths (L) and loaded fiber masses (m) were fabricated. The effects of m/L on the biomass distribution, active biomass content, electrochemical behavior and MFC performance were investigated. For the electrodes with a similar m but different L, substrate supply within the interior of GFB electrodes improved with L, leading to higher biomass content and consequently the improved performance. However, a complex trend was found for the electrodes with different m and similar L, due to the opposing trends of substrate supply and actual functional area for electrochemically active bacteria with m. Furthermore, m-normalized biomass content and power density of the GFB electrodes increased with decreasing of m/L ratio due to the improved graphite fiber utilization until 0.014 g mm⁻¹, below which they remained constant since the utilization of graphite fibers plateaued.     

An investigation of the oxygen cathode with a silver catalyst polarized in the region of negative potentials

Different types of oxygen diffusion cathodes have been tested to determine whether cathodic polarization of the electrodes at negative potentials has an influence on their further catalytic activity for oxygen ionization, and whether under these conditions potassium from the KOH solution penetrates into the silver catalyst.The results show that even if these electrodes were kept in the hydrogen evolution potential range for a long time, there were no changes in the catalytic activity of the silver in oxygen gas electrodes. No evidence of potassium penetration into the silver catalyst was found.     

Oxygen evolution reaction on Pt sphere and Ir-modified Pt sphere electrodes with porous structures

Oxygen evolution reactions (OER) have received recent attention because of their importance in electrochemical water splitting for sustainable energy sources. Here, the electrochemical OER activity on electrodeposited Pt sphere electrodes with porous structures was investigated. The dependence of the OER activity on the porosity of the Pt sphere electrodes was systemically examined by changing the charge during the electrodeposition of the Pt spheres. Significantly enhanced OER activities were observed on the porous Pt sphere electrodes compared to that on a flat Pt electrode, and the activity increased as the porosity of the Pt sphere increased. Pt spheres with rough nanoscale surface features exhibited greater OER activities than the smooth Pt sphere electrodes. The surfaces of the Pt sphere electrodes were modified with Ir by spontaneous replacement reactions, which resulted in an OER activity enhancement relative to the Pt sphere electrodes. The OER activity of Pt electrodes depending on the porosity and surface roughness demonstrated in this work gives insight into the fabrication of efficient OER electrocatalysts.     

Electrodes based on nafion and epoxy-graphene composites for improving the performance and durability of open cathode fuel cells,prepared by electrospray deposition

Fabrication of electrodes for polymer electrolyte fuel cells is a intriguing process in which a balance between gas transport, electrical conductivity, proton transport and water managing must be optimized. In this work four different electrodes prepared by electrospray deposition have been studied using different catalytic inks, in which Nafion and epoxy doped with Graphene-Nanoplatelets were used as binders. After studying the behavior of those electrodes in a single open cathode fuel cell proton electrolyte membrane, it is clear that the addition of epoxy as binder doped with graphene, improves the performance of the fuel cell and increase the mechanical stability of the electrode avoiding the loose of catalyst during the electrode manipulation in the fuel cell assembly process and the durability of the fuel cell. To explain this behavior, an ex-situ study was carried out, in which properties such as its surface morphology, hydrophobicity and electrical and thermal conductivity of those electrodes were studied. From the results of this study, such improvement in the performance of the fuel cell was justified on the basis of the increase in the electrical conductivity, a diminution in its thermal conductivity and an enhancement of hydrophobicity (surface morphology) of the deposited catalyst layer, when an optimum quantity of epoxy is added to the catalytic ink that makes to improve the mechanical properties of those electrodes.     

Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies

The effect of freezing of a membrane-electrode assembly on its physical properties and performance was investigated. It was found that freeze-thaw cycles caused the electrode (i.e., catalyst layer) of a fully hydrated membrane-electrode assembly (MEA), either as a freestanding piece or as assembled in a cell, to crack. Accompanying the cracking was a reduction in the electrochemical active surface areas of the electrodes as measured by cyclic voltammetry, but the short-term performance of the fuel cell did not show much effect. When dry reactants were used to remove some water from a cell that had been previously tested at fully hydrated condition, freeze-thaw cycling did not cause apparent damage to the appearance of the electrodes. Also, for freestanding MEAs that were taken directly from the manufacturing line and only exposed to ambient temperature (e.g., 23 °C) and relative humidity (e.g., <50% RH), freezing did not cause apparent damage to the appearance of the electrodes.     

Preparation and performance of plastic-bonded-carbon bromine electrodes

A range of carbon blacks has been used in conjunction with a polypropylene binder to form electrodes for zinc/bromine cells.The resistivity of each of these carbon/plastic materials has been measured on hot-pressed pellets. The respective electrochemical activity has been determined from polarization tests using 2 cm² electrodes. These tests have shown that a two-layer electrode structure, with a surface layer containing a high fraction of carbon black, is necessary to obtain satisfactory activity. Best results have been achieved when this surface layer has been composed of blacks with a low bulk density and a high surface area.The best of the carbon/plastic compositions has been used to prepare 16 cm² bromine electrodes for testing in zinc/bromine cells. It has been possible to make electrodes with an initial polarization resistance of 0.3 ohm cm² and an initial ohmic resistance of 0.8 ohm cm². These values increase when the electrodes are left in contact with bromine-containing solutions, regardless of whether the electrodes have been charge/discharge cycled or not.