Non precious metal catalysts for the PEM fuel cell cathode

Low temperature fuel cells, such as the proton exchange membrane (PEM) fuel cell, have required the use of highly active catalysts to promote both the fuel oxidation at the anode and oxygen reduction at the cathode. Attention has been particularly given to the oxygen reduction reaction (ORR) since this appears to be responsible for major voltage losses within the cell. To provide the requisite activity and minimse losses, precious metal catalysts (containing Pt) continue to be used for the cathode catalyst. At the same time, much research is in progress to reduce the costs associated with Pt cathode catalysts, by identifying and developing non-precious metal alternatives. This review outlines classes of non-precious metal systems that have been investigated over the past 10 years. Whilst none of these so far have provided the performance and durability of Pt systems some, such as transition metals supported on porous carbons, have demonstrated reasonable electrocatalytic activity. Of the newer catalysts, iron-based nanostructures on nitrogen-functionalised mesoporous carbons are beginning to emerge as possible contenders for future commercial PEMFC systems.     

Recent progress in carbon nanotubes support materials for Pt-based cathode catalysts in PEM fuel cells

Support materials have a significant impact on catalytic activity, stability, and performance of catalysts toward the oxygen reduction reaction (ORR). The properties of carbon-based materials have made them an excellent alternative for use as support for nanosized catalysts. Recently, carbon nanotubes (CNTs) have been explored as catalyst support materials, and their properties make them a promissory alternative. Furthermore, catalysts supported on CNTs exhibit higher resistance to electrochemical oxidation, better catalytic performance, and higher durability than catalysts supported on carbon black. In recent years, CNTs have acquired great relevance as catalysts support materials for ORR in acid media. This review addresses the most relevant studies on CNTs modification using methods such as functionalization, doping, and hybrid supports (CNTs-metal oxide) used as supports for Pt-based cathode catalysts in proton exchange membrane fuel cells.     

Recent advances in cathode electrocatalysts for PEM fuel cells

Great progress has been made in the past two decades in the development of the electrocatalysts for proton exchange membrane fuel cells (PEMFCs). This review article is focused on recent advances made in the kinetic-activity improvement on platinum- (Pt-) based cathode electrocatalysts for the oxygen reduction reaction (ORR). The origin of the limited ORR activity of Pt catalysts is discussed, followed by a review on the development of Pt alloy catalysts, Pt monolayer catalysts, and shape- and facet-controlled Pt-alloy nanocrystal catalysts. Mechanistic understanding is reviewed as well on the factors contributing to the enhanced ORR activity of these catalysts. Finally, future directions for PEMFC catalyst research are proposed.     

Pulse electrodeposited cathode catalyst layers for PEM fuel cells

Nanostructured Pt and Pt₃Co cathodes for proton exchange membrane fuel cells (PEMFCs) have been prepared by pulse electrodeposition. For high utilization the catalyst nanoparticles are directly deposited on the microporous layer (MPL) of a commercial available gas diffusion layer (GDL). In order to increase the hydrophilic nature of the substrate surface and thus improve drastically the electrodeposition process and the fuel cell performance, prior to electrodeposition, the carbon substrate is submitted to O₂/Ar plasma activation. Cathodes with different amounts and distributions of Aquivion ionomer within the cathode catalyst layer (CCL) thickness (“homogeneous”, “gradient” and “anti-gradient”), different catalysts (Pt and Pt₃Co) at varied plasma duration and catalyst loading have been prepared. The cathodes are analysed via attenuated total reflection (ATR-IR), goniometer, SEM, 0.5 M H₂SO₄ half-cell and 25 cm² H₂/Air single PEMFC. The highest single fuel cell performance is obtained for 2 min plasma activated Pt₃Co cathode.     

Visualization and quantification of cathode channel flooding in PEM fuel cells

An understanding of two-phase flow mechanisms in micro-channels is critical to water management in fuel cell applications. In this work, an in situ visualization study of cathode flooding in an operating fuel cell is presented. Gas relative humidities of 26%, 42% and 66%, current densities of 0.2, 0.5 and 0.8 A cm⁻²and flow stoichiometries ranging from 2 to 4 are used in this study which represent typical operating conditions for automotive applications. Results are presented in the form of a flow map depicting various two-phase flow patterns. The impact of flooding is also presented in terms of measurable parameters like two-phase pressure drop coefficient and voltage loss. A new parameter called wetted area ratio is introduced to characterize channel flooding and liquid water coverage on a gas diffusion layer, and its repeatability with multiple tests is demonstrated.     

Chemically synthesized reduced graphene oxide-carbon black based hybrid catalysts for PEM fuel cells

In this study, hybrid (synthesized rGO and commercial carbon black (CB) in various weight ratios) supported Pt catalysts were synthesized by using the supercritical carbon dioxide deposition (scCO₂) technique for PEM fuel cells. In hybrid materials, rGO to CB weight ratios were changed in between 90:10 to 50:50 which were compared to their plain materials. The physicochemical and the electrochemical characteristics of the materials were examined by using BET, XRD, TGA, TEM, contact angle and roughness measurements, CV and PEM fuel cell performance tests. All these characterizations showed that the hybrid supported Pt catalysts were successfully synthesized. TEM images of the catalysts confirmed the highly dispersed and small nanoparticle formation (1.9–2.9 nm) via scCO₂ deposition technique. Among the hybrid supported catalysts, catalyst having rGO:CB ratio of 70:30 showed the best PEM fuel cell performance. Electrochemical characterization either fuel cell performance test or CV results indicated significantly enhancement in activity with an increase in CB amount in the support.     

Non-platinum cathode catalysts for alkaline membrane fuel cells

Hydrogen–oxygen fuel cells using an alkaline anion exchange membrane were prepared and evaluated. Various non-platinum catalyst materials were investigated by fabricating membrane-electrode assemblies (MEAs) using Tokuyama membrane (# A201) and compared with commercial noble metal catalysts. Co and Fe phthalocyanine catalyst materials were synthesized using multi-walled carbon nanotubes (MWCNTs) as support materials. X-ray photoelectron spectroscopic study was conducted in order to examine the surface composition. The electroreduction of oxygen has been investigated on Fe phthalocyanine/MWCNT, Co phthalocyanine/MWCNT and commercial Pt/C catalysts. The oxygen reduction reaction kinetics on these catalyst materials were evaluated using rotating disk electrodes in 0.1 M KOH solution and the current density values were consistently higher for Co phthalocyanine based electrodes compared to Fe phthalocyanine. The fuel cell performance of the MEAs with Co and Fe phthalocyanines and Tanaka Kikinzoku Kogyo Pt/C cathode catalysts were 100, 60 and 120 mW cm⁻² using H₂ and O₂ gases.     

Development of cylindrical PEM fuel cells with semi-cylindrical cathode current collectors

Proton exchange membrane (PEM) fuel cells are attractive because of advantages such as low-temperature operation, no emission of harmful gases and high efficiency. However, the bipolar plates used in the state-of-the-art planar architecture are costly and increase the dead weight of the cell. In addition, the flow channels in the planar fuel cell increase the difficulty in removing the water produced in the cathode during cell operation. Cylindrical PEM fuel cells, on the other hand, do not require bipolar plates and there is no need for precisely machined flow channels. Thus, cylindrical PEM fuel cells are cheap, efficient in water management, and possess higher volumetric and gravimetric power density compared to planar PEM fuel cells. The design of a cylindrical fuel cell is very simple, but the fabrication of the same is fairly complex. In this work, a novel cathode current collector design for cylindrical PEM fuel cell has been developed. The cell performance was limited by low open circuit voltage and high ohmic resistance. The open circuit voltage of the cell is increased from 0.85 V to 0.95 V using an acrylic based adhesive to seal the membrane edges. The contact resistance of the cell is reduced from 75 mOhm to 50 mOhm by increasing the contact pressure on the membrane electrode assembly and it is further reduced to 30 mOhm by gold coating the current collectors. Furthermore, a cumulative 40% increase in peak power has been achieved from the optimization of cathode rib width and hydrogen flow rate. The optimized cell delivered a current density of 400 mA/cm² at 0.6 V and peak power of 2 W, which is appreciable considering the fact that the cell is air-breathing and operated with very minimal subsystems.     

A review on carbon and non-precious metal based cathode catalysts in microbial fuel cells

Microbial fuel cells, an emerging technology has been paid a great attention in recent years, due to its unique advantages in treating wastewater to portable water, together with the generation of useful electricity, with the help of bio-active anodes and electrochemical cathodes, simultaneously. When applying this technology in a practical scale, the indigenous bacteria present in the wastewater catalyze the breakdown of organic matter in the anode compartment, with generation of electrons and in the cathode compartment an oxidant, usually the oxygen present in the air, take the electron and reduce to water (oxygen reduction reaction, ORR). An ideal ORR catalyst should be highly active, durable, scalable, and most importantly it should be cost effective. Generally, platinum-based catalyst is utilized, however, due to the high cost of Pt based catalysts, many cheap, cost effective catalyst have been identified as efficient ORR catalyst. Carbon based catalysts known to possess good electronic conductivity, desirable surface area, high stability, together when doped with heteroatoms and cheap metals is found to remarkably enhance the ORR activity. Although a lot of research has been done in view of developing carbon based cheap, cost-effective catalysts, still their collective information has not been reviewed. In this article we anticipate reviewing various non-precious metal and metal-free catalysts that are synthesized and investigated for MFCs, factors that affect the ORR activity, catalyst designing strategies, membranes utilized for MFCs, together with the cost comparison of non-precious and metal-free catalysts with respect to Pt based catalysts have been summarized. We anticipate that this review could offer researchers an overview of the catalyst developed so far in the literatures and provides a direction to the young researchers.     

Catalytic activity vs. size correlation in platinum catalysts of PEM fuel cells prepared on carbon black by different methods

In this work nanoparticulated platinum catalysts have been prepared on carbon Vulcan XC-72 using three methods starting with chloroplatinic acid as a precursor: (i) formic acid as a reductor agent; (ii) impregnation method followed by reduction in hydrogen atmosphere at moderated temperature; and (iii) microwave-assisted reduction in ethylene glycol. The catalytic and size studies were also performed on a commercial Pt catalyst (E-Tek, De Nora).     

Analysis of liquid water transport in cathode catalyst layer of PEM fuel cells

The performance of a polymer electrolyte membrane (PEM) fuel cell is significantly affected by liquid water generated at the cathode catalyst layer (CCL) potentially causing water flooding of cathode; while the ionic conductivity of PEM is directly proportional to its water content. Therefore, it is essential to maintain a delicate water balance, which requires a good understanding of the liquid water transport in the PEM fuel cells. In this study, a one-dimensional analytical solution of liquid water transport across the CCL is derived from the fundamental transport equations to investigate the water transport in the CCL of a PEM fuel cell. The effect of CCL wettability on liquid water transport and the effect of excessive liquid water, which is also known as “flooding”, on reactant transport and cell performance have also been investigated. It has been observed that the wetting characteristic of a CCL plays significant role on the liquid water transport and cell performance. Further, the liquid water saturation in a hydrophilic CCL can be significantly reduced by increasing the surface wettability or lowering the contact angle. Based on a dimensionless time constant analysis, it has been shown that the liquid water production from the phase change process is negligible compared to the production from the electrochemical process.     

Transient analysis for the cathode gas diffusion layer of PEM fuel cells

A one-dimensional, non-isothermal, two-phase transient model has been developed to study the transient behaviour of water transport in the cathode gas diffusion layer of PEM fuel cells. The effects of four parameters, namely the liquid water saturation at the interface of the gas diffusion layer and flow channels, the proportion of liquid water to all of the water at the interface of the cathode catalyst layer and the gas diffusion layer, the current density, and the contact or wetting angle, on the transient distribution of liquid water saturation in the cathode gas diffusion layer are investigated. Especially, the time needed for liquid water saturation to reach steady state and the liquid water saturation at the interface of the cathode catalyst layer and gas diffusion layer are plotted as functions of the above four parameters. The ranges of water vapour condensation and liquid water evaporation are identified across the thickness of the gas diffusion layer. In addition, the effects of the above four parameters on the steady state distributions of gas phase pressure, water vapour concentration, oxygen concentration and temperature are also presented. It is found that increasing any one of the first three parameters will increase the water saturation at the interface of the catalyst layer and gas diffusion layer, but decrease the time needed for the liquid water saturation to reach steady state. When the liquid water saturation at the interface of the gas diffusion layer and flow channels is high enough (≥0.1), the liquid water saturation at steady state is almost uniformly distributed across the thickness of the gas diffusion layer. It is also found that, under the given initial and boundary conditions in this paper, evaporation takes place within the gas diffusion layer close to the channel side and is the major process for water phase change at low current density (<2000 A m⁻²); condensation occurs close to the catalyst layer side within the gas diffusion layer and dominates the phase change at high current density (>5000 A m⁻²). For hydrophilic gas diffusion layers, both the time needed for liquid water saturation to reach steady state and the water saturation at the interface of the catalyst layer and gas diffusion layer will increase when the contact angle increases; but for hydrophobic gas diffusion layers, both of them decrease when the contact angle increases.     

A study on cathode structure and water transport in air-breathing PEM fuel cells

For portable electronics, air-breathing PEMFCs are being developed without external air-feeding and humidification systems. To optimize the cathode structure and to investigate characteristics of such air-breathing PEMFCs, the effects of catalyst loading and gas diffusion layer (GDL) properties on the cell performance are examined. Water transport in air-breathing PEMFCs is interpreted in terms of the net water drag coefficient, which was obtained by changing the current density, relative humidity, temperature and the hydrogen stoichiometry to understand the water transport phenomena under various conditions.     

Studies on flooding in PEM fuel cell cathode channels

The main subject of this study are the flooding phenomena in the cathode channels of low-temperature PEM fuel cell. Transparent acrylic materials are used to make various fuel cell models for the experiments. Parameters considered in the experiments include the rate of water injected into the models, the velocity and the temperature of the humidified gas in the cathode channels, the types of flow field, and the temperature of the models. It is found that the parallel and interdigitated flow channels are easily flooded under certain conditions. In order to decrease the chance of flooding, the design of the flow field path should fit the streamline pattern. Furthermore, fuel cells with two different types of flow channels and two different electrode sizes (25, 100 cm²) were made, and their performances were compared to some of the flooding results observed from the transparent physical models.     

A three-dimensional agglomerate model for the cathode catalyst layer of PEM fuel cells

In this work, a three-dimensional, steady-state, multi-agglomerate model of cathode catalyst layer in polymer electrolyte membrane (PEM) fuel cells has been developed to assess the activation polarization and the current densities in the cathode catalyst layer. A finite element technique is used for the numerical solution to the model developed. The cathode activation overpotentials, and the membrane and solid phase current densities are calculated for different operating conditions. Three different configurations of agglomerate arrangements are considered, an in-line and two staggered arrangements. All the three arrangements are simulated for typical operating conditions inside the PEM fuel cell in order to investigate the oxygen transport process through the cathode catalyst layer, and its impact on the activation polarization. A comprehensive validation with the well-established two-dimensional “axi-symmetric model” has been performed to validate the three-dimensional numerical model results. Present results show a lowest activation overpotential when the agglomerate arrangement is in-line. For more realistic scenarios, staggered arrangements, the activation overpotentials are higher due to the slower oxygen transport and lesser passage or void region available around the individual agglomerate. The present study elucidates that the cathode overpotential reduction is possible through the changing of agglomerate arrangements. Hence, the approaches to cathode overpotential reduction through the optimization of agglomerate arrangement will be helpful for the next generation fuel cell design.     

A two-phase flow and transport model for the cathode of PEM fuel cells

A unified two-phase flow mixture model has been developed to describe the flow and transport in the cathode for PEM fuel cells. The boundary condition at the gas diffuser/catalyst layer interface couples the flow, transport, electrical potential and current density in the anode, cathode catalyst layer and membrane. Fuel cell performance predicted by this model is compared with experimental results and reasonable agreements are achieved. Typical two-phase flow distributions in the cathode gas diffuser and gas channel are presented. The main parameters influencing water transport across the membrane are also discussed. By studying the influences of water and thermal management on two-phase flow, it is found that two-phase flow characteristics in the cathode depend on the current density, operating temperature, and cathode and anode humidification temperatures.     

Development of tellurium-modified carbon catalysts for oxygen reduction reaction in PEM fuel cells

Tellurium (Te)-modified carbon catalyst for oxygen reduction reaction was prepared through chemical reduction of telluric acid followed by the pyrolysis process at elevated temperatures. The catalyst was found to be active for oxygen reduction reaction. High-temperature pyrolysis plays a crucial role in the formation of the active sites of the catalysts. When the pyrolysis was conducted at 1000 °C, the catalyst exhibited the onset potential for oxygen reduction as high as 0.78 V vs. NHE and generated less than 1% H₂O₂ during oxygen reduction. The performance of the membrane–electrode assembly prepared with the Te-modified carbon catalyst was also evaluated.     

Computational modeling and experimental verification of cathode catalyst layer on PEM fuel cells

Fuel cell systems are environmentally friendly energy converters that directly transform the chemical energy of the fuel to electricity. The proton exchange membrane (PEM) fuel cells are one of the most common type of fuel cells since they deliver high power density and are lighter and smaller when compared to the other cells. However, commercialization of the PEM fuel cells is challenging due to the high cost of its components. In addition to high catalyst costs, the problem of poor water management is also a vital issue that needs to be overcome. While the gas diffusion layer of a fuel cell is essential for removing the by-product water, the Nafion solution contained in the catalyst layer has hydrophobic properties and is crucial for both preventing the water accumulation and increasing the effectiveness of the fuel cell. In this study, the effects of Carbon:Nafion ratio on the reduction potential was investigated. The cyclic voltammograms (CV) was produced for each ratio, and it was shown that the CVs exhibit characteristics of hydrogen adsorption/desorption peaks. All the linear sweep voltammogram (LSV) curves revealed well distinguished regions of kinetic, mixed and diffusion limited reaction rate. As a result, it was observed that the ratio of 1:5 resulted higher reduction potential compared to 1:3 and 1:7. Finally, a mathematical model was purposed, in which related the rotation rate and platinum coating with the current density, in order to gain insight about the responses of the fuel cell system. The constructed model is tested and validated experimentally for various parameters that are present in the system, and it may be utilized to determine oxygen reaction activities of the catalysts without performing any unnecessary electrochemical tests.