Carbon Paper Gas Diffusion Layer Research Articles

Mechanical response of carbon paper gas diffusion layer under patterned compression

The Proton Exchange Membrane Fuel Cell (PEMFC) performances are strongly impacted by the compression of the Gas Diffusion Layer (GDL). Despite its fibrous microstructure, this material is usually considered as a continuous medium and characterized with uniform loading. However, the GDL is subjected to a heterogeneous compression onto rib/channel patterns in the fuel cell assembly. In the present study, a complex behavior of the GDL response is experimentally revealed when the material is loaded with a rib/channel pattern, compared to uniform compression. The tests are simulated by finite element modeling using a classical strain-dependent elastic law, using parameters fitted from uniform compression experiments. It is shown that the numerical results do not reproduce the effect of pattern observed experimentally. Hypotheses to interpret these results involve mechanisms at the fiber microscale including fiber fracture, cross-link breakage and fiber rearrangement, which are exacerbated by larger material deformation caused by the heterogeneous loading.

Effect of membrane electrode assembly characteristics on the performance of a proton exchange membrane fuel cell stack designed for unmanned aerial vehicle applications

ABSTRACT A proton exchange membrane fuel cell stack designed for unmanned aerial vehicle applications was used to evaluate the influence of several membrane electrode assembly (MEA) characteristics. MEAs prepared with catalytic layers (CLs) deposited directly on the membrane presented slightly better performance than those with CLs deposited in the gas diffusion layers (GDLs). Thinner membranes were beneficial to the stack performance due to lower activation and ohmic losses. Increasing the Pt loading on the cathode side from 0.5 to 2.0 mg Pt cm−2 did not improve the performance. Small differences were observed between carbon cloth and carbon paper GDLs, possibly due to the presence of a microporous layer (MPL). The hydrophobic treatment of the GDLs improved the performance due to a better humidification of the MEAs. The MPL was found fundamental for a high performance due to an improvement in the electric contact, decreasing ohmic losses. The stack operation under realistic conditions, consisting of a transient energy profile typical of an UAV flight, was also investigated. The stack presented higher dynamic performance than its steady-state one at low currents but slightly lower at high currents. Differences in temperature were observed, which probably contributed to this disparity.

Experimental study on a unitized regenerative fuel cell operated in constant electrode mode: Effect of cell design and operating conditions

Constant electrode (CE) operation of a unitized regenerative fuel cell (URFC) facilitates the optimization of each electrode. In this work, bipolar plates (BPPs) with different flow fields (FFs) are assessed, and a graphite BPP is compared with a titanium BPP in the cathode. Platinum coated titanium felt and carbon cloth and paper gas diffusion layers (GDLs) are tested in the cathode. The carbon BPP and GDLs operated with higher ohmic resistance than the titanium alternatives. However, the titanium GDL caused flooding in FC mode. Carbon GDLs can be applied in the cathode, where carbon paper operated with higher performance than carbon cloth. Titanium GDL is still required in the anode to resist the corrosive environment. A double-serpentine FF in the cathode and a parallel design in the anode were applied. Optimization of the URFC operating conditions under FC mode provided the best results at 80 °C, 2 barabs, with saturated air and dry hydrogen and stoichiometry ratios of 2 and 8. This resulted in a round-trip efficiency of 56% at 0.1 A cm−2, 48% at 0.3 A cm−2 and 22% at 1 A cm−2. In comparison, 56%, 52% and 37% were obtained with a discrete regenerative fuel cell (DRFC).

Modification and durability of carbon paper gas diffusion layer in proton exchange membrane fuel cell

To solve the durability problem caused by carbon corrosion in the porous carbon paper gas diffusion layer (GDL) of proton exchange membrane fuel cell (PEMFC), a Cr7C3 ceramic coating on porous carbon paper by molten salt method was prepared. The obtained Cr7C3 coating is dense and shows good adherent with the carbon paper. Based on the corrosion resistance testes, it is concluded that the Cr7C3 coated carbon paper can keep high chemical stability in the long-time acid immersion accelerated corrosion tests and simulated cathodic proton exchange membrane fuel cells environment. The carbon paper with Cr7C3 coating shows a small current density with a value of about 1.1 × 10−5 A cm−2 in the potentiostatic polarization test under 1 M H2SO4 + 2 ppm HF at 70 °C and1.4 VSCE for 72 h. Meanwhile, the resistivity of carbon paper with Cr7C3 coating prepared is 30% lower than that of pure carbon paper, and the conductivity is 1.5 times that of pure carbon paper.

(Digital Presentation) Evaluating the Performance of Multi-Walled Carbon Nanotube Composite Microporous Layers Deposited on Carbon Felt Gas Diffusion Layers

Electrochemical energy storage devices (EESDs) such as metal-air batteries (MABs) and alkaline electrolyte membrane fuel cells (AEMFCs) have begun to gain vast amounts of attention due to their high theoretical energy densities and open-circuit voltage values. Although these devices present promising aspects for the environment and sustainability, they suffer from the slow reaction kinetics and degradation effects that occur at the catalyst layer interface of the air electrode. Typically, these effects are due to the accumulation of molecular species such as water or oxygen at catalyst active sites because of poor electrode hydrophobicity and fluid management. Herein, we perform experimentation to mitigate these effects by implementing mixed carbon composite microporous layers onto porous carbon felt based air electrodes. This adaptation of the air cathode was predominantly based on the aforementioned downfalls and the lack of availability of commercial carbon felts that have adequate wet proofing and conductivity like some of the related carbon paper gas diffusion layers. Microporous layers (MPLs) enhance electrical conductivity, surface area, and adhesion at the catalyst layer and gas diffusion layer interface. These layers typically consist of a hydrophobic polymer such as polytetrafluoroethylene (PTFE) and a conductive carbon black substrate such as Ketjen Black 600JD (KJB). The incorporation of multi-walled carbon nanotube (MWCNT) and carbon black composites into the structure of the MPLs drastically decreases ohmic polarization with minimal increases in specific resistance. The electrical resistance of the synthesized carbon substrate decreases at high rates of MWCNT dispersion, which corresponds directly to a low MWCNT weight percentage within the composite. The MPL increases the mass transport through the electrode by mitigating flooding effects that typically occur within the pores near the surface of the catalyst layer, ultimately aiding in charge transfer between the gas diffusion layer and catalyst active sites. Preliminary data from AEMFC symmetric cell tests suggests that a microporous layer consisting of a 10 wt.% MWCNT/KJB (m/m) substrate is the optimal loading of carbon nanotubes and leads to better cell performance. There is an increase in current density of 60 mA/cm2 compared to an air electrode with no MPL within the ohmic region of the polarization curve corresponding to approximately 0.7 V, while only an increase of 18 mA/cm2 is seen for an MPL consisting of a 30 wt.% MWCNT/KJB (m/m) carbon substrate. The increase of charge transfer by the 10 wt.% synthesized carbon substrate MPL was attributed to the ability of the nanotubes at high rates of dispersion to act as molecular wires within the electrode. Additionally, zinc-air battery tests using the optimally configured air electrode, determined from previous experiments, show that the current density at an operating voltage of 1.1 V was increased by 2.3 times that of an electrode without an MPL using the same catalyst material and loading. Significantly, this research will lead to a better understanding of optimization of the air electrode used within electrochemical devices and how to eliminate some of the adverse effects associated with it. Figure 1

Effect of Thickness and Outlet Area Fraction of Macroporous Gas Diffusion Layers on Oxygen Transport Resistance in Water Injection Simulations

Enhanced water removal through the gas diffusion layer (GDL) is important for the design of high-performance proton exchange fuel cells. In this work, the effects of GDL thickness and open area fraction at the GDL/flow field interface are examined under water invasion for a carbon-paper GDL (similar to Toray TGP-H series). Both uncompressed and inhomogeneously compressed samples are considered. Transport in heterogeneous, macroporous GDLs is modeled by means of a hybrid 3D discrete/continuum formulation based on a subdivision of the porous medium into control volumes due to the lack of a well-defined separation between pore and layer scales. Capillary-dominated transport of liquid water is simulated with an invasion percolation algorithm, while oxygen diffusion is simulated with a continuum formulation. Model predictions are validated with previous numerical and experimental data. It is shown that the combination of thin GDLs (mathrm {thickness} sim 100; mu mathrm {m}) and high GDL/flow field open area fractions can facilitate water removal/oxygen supply from/to the catalyst layer and can provide a more uniform oxygen distribution over large cell active areas. In agreement with previous work, porous flow fields with pore sizes comparable to the GDL thickness are good candidates to meet the above requirements, while improving water removal from the flow field (higher gas-phase velocity than conventional millimeter-sized channels) and ensuring a more uniform assembly compression.

Investigation of Activation Protocols and Carbon Components for Core-Shell Mn@Mn3O4/Carbon Gas Diffusion Electrodes for Oxygen Reduction and Evolution Reactions

For the wide application of reversible fuel cells and metal-air batteries, highly efficient and cost-effective oxygen electrodes for both the oxygen reduction and evolution reactions (ORR/OER) are highly desired. Manganese oxides are considered to be one of the most promising bifunctional electrocatalyst candidates to replace the precious Pt/C (for ORR) and IrO2 (for OER) catalysts. Although a tremendous effort has been made to develop advanced materials, the impact of experimental protocols on electrode performance has not been well studied. The objective of this work is to optimize the ORR/OER performance of core-shell Mn@Mn3O4/C gas diffusion electrodes (GDEs) with a focus on designing an effective electrode activation protocol and exploring suitable carbon components, including both carbon additives and wet proofed carbon paper gas diffusion layers (GDLs).To adjust the electrode wetting, two approaches were adopted: i) controlling the weight ratio of Teflon on carbon paper, and ii) pre-treatment with warm HNO3 acid. The HNO3 pre-treatment on Teflon-coated GDLs can slightly increase hydrophilicity (introduces C-O bonds) resulting in enhancement of the ORR/OER activities of Mn@Mn3O4/Vulcan carbon GDEs and the polarization curves are identical regardless of Teflon loading.In order to improve the catalytic activity and durability of Mn@Mn3O4/C GDEs, we explored a set of different carbon additive combinations. Among the different combinations, graphene/Vulcan (1:1) reached the longest lifespan and the ORR/OER overpotentials of Mn@Mn3O4/Vulcan/graphene (before degradation) were as low as that of the Pt/C-IrO2-Vulcan-graphene benchmark.Since MnOx can be irreversibly activated or passivated, it is necessary to exercise caution when polarizing a pristine Mn@Mn3O4/Vulcan/graphene GDE in the first few cycles. For the design of activation protocols, different polarization methods were used, i.e., fast/slow cyclic voltammetry (CV) in different potential ranges or cycling under constant currents (CC) or constant potentials (CP). The highest catalytic activity was achieved by a CV-activated GDE that was obtained by cycling between the ORR and OER-related potentials at a slow scan rate of 1 mV s-1. This better performance might be due to the potential-driven MnOx phase transitions that develop R/α/γ type structures. Figure 1 shows the effect of different electrochemical protocols on the GDE. In comparison to the CC and CP-activated GDEs, it has a lower Mn site average oxidation state (AOS) of 3.1 and a larger contact angle, which suggests a higher population of Mn(III) active sites and OER-improved electrode wetting properties.By systematically tuning the carbon components and activation protocols, we found the ORR/OER performance of Mn@Mn3O4/C GDEs can be much improved when using Vulcan/graphene (1:1) additives, HNO3-treated wetproofed GDL, and applying cyclic voltammetry between ORR and OER-related potentials at a slow scan rate. Although this work focuses on improving MnOx/C GDEs, the developed approaches can also help establish protocols for reaching the full scope of other transition-metal-based electrodes. Figure 1

Gas Diffusion Layer: The Critical Player in Gases Distribution in the Proton Exchange Membrane Fuel Cell

Within the last decades, fuel cells have become the center of attention of research community, industry, and the general public. This is due to the emphasis on the sustainability of the society and the emerging concept of a hydrogen economy. Proton exchange membrane fuel cells (PEM FCs) represent a mature and established fuel cell technology. The flexibility of operation, the high degree of fuel utilization, and the output power density make them suitable for mobile applications such as hydrogen-fueled vehicles or on-site energy generation. Membrane electrode assembly (MEA) is the mainstay of the PEM FCs. The MEA consists of the proton exchange membrane, catalytic layers, and gas diffusion layers (GDLs). Single MEAs are interconnected by bipolar plates with flow field channels homogenously distributing reactants and realizing easy removal of the generated water. Individual cells are compressed together, forming the fuel cell stack. The GDL plays a three-fold role in the performance of PEM fuel cells. It has to offer good electrical conductivity, ensure good reactant distribution along the reaction zone, and facilitate the removal of the water produced by the fuel cell operation. The optimal combination of these three aspects of GDL allows the FC to achieve high performance. Furthermore, the homogeneous distribution of the reactants is crucial for the durability and lifespan of the FC. The inhomogeneous reactants distribution leads to a local starvation of the electrode, leading to high local overvoltage and thus shortening the lifespan of overloaded parts. Although GDLs play a crucial role in the operation of PEM fuel cells, so far, they have received little attention. Hence, the objective of our study is to compare commercially available GDLs with respect to their properties, namely permeability under conditions close to the operating fuel cell.The approach of combining advanced mathematical modeling with extensive experimental characterization was used to achieve this task. Testing cells allowing precise determination of the in-plane permeability of various GDLs were designed based on the computational fluid dynamics modeling. According to the contemporary literature, two different carbon based GDLs were examined, namely carbon-cloth and carbon-paper, which principally differ in the carbon-fiber orientation in their internal structure. The investigated properties of GDLs thus include thickness, porosity, internal structure, and the presence or absence of microporous layer.The achieved experimental results show that at given compression, thicker carbon paper GDLs are superior to carbon cloth samples in terms of permeability and deformation resistance. The obtained permeability values of GLDs at different degrees of compression were compared with predictions based on the Carman-Kozeny equation to assess the accuracy of this semi-empirical prediction. Finally, the importance of the optimal flow field and GDL combination in terms of reactant distribution in the fuel cell stack of industrial conditions is documented via mathematical modelling based on the obtained experimental data.This work was supported by the European Regional Development Fund-Project “Fuel Cells with Low Platinum Content” (No. CZ.02.1.01/0.0/0.0/16_025/0007414).

Effect of Clamping Compression on the Mechanical Performance of a Carbon Paper Gas Diffusion Layer in Polymer Electrolyte Membrane Fuel Cells.

During all the assembly stages of a polymer electrolyte membrane fuel cell (PEMFC) stack, gas diffusion layers (GDLs) endure clamping loads in the through-plane direction several times. Under such complicated assembly conditions, GDLs have to deform with the changes in structure, surface roughness, pore size, etc. A comprehensive understanding of the compressive performance of GDLs at different clamping phases is crucial to the assembly process improvement of PEMFCs. Two typical clamping compression was designed and performed to get close to the actual assembly conditions of PEMFCs. The results indicate that the initial clamping compression and the magnitude of the maximum clamping load have great impacts on the segmented compressive properties of GDLs. The nonlinear compressive performance of the GDL is mainly attributed to the unique microstructural information. The rough surface morphology contributes to the initial compressive characteristics where the big strain along with the small stress occurs, and the irreversible failures such as carbon fiber breakages and adhesive failures between fibers and binders account for the hysteresis between different compression stages. Importantly, it is found that the clamping compression hardly influences the small pore distribution below 175 μm but affects the large pore distribution over 200 μm.

Optimization of the bipolar plate rib structure in proton exchange membrane fuel cells with an analytical method

In order to make proton exchange membrane fuel cell vehicles more marketable, not only should costs be reduced, but service life should also be further increased. Important factors determining the expected service life are the deformation and the stress distribution within the carbon paper gas diffusion layer (GDL), on which the rib structure of the bipolar plates (BPP) has a significant impact. Against this background, a new analytical method is firstly developed to predict the deformation and stress distribution within the GDL, due to compression by the ribs, with high accuracy and low computing resources. Based on the analytical method, a new parabolic rib geometry is then proposed, which can significantly reduce the maximum normal and shear stresses occurring within the GDL, thus reducing the possibility of its mechanical damage. The optimized rib design provides guidance for designing and processing commercial fuel cell BPPs.

A numerical study of the polarization effect of liquid water in the gas diffusion layer of a proton exchange membrane fuel cell

To improve the efficiency and lifetime of proton exchange membrane fuel cells, a deeper understanding of the physical and chemical factors affecting the water transport within the fuel cell is required. A factor that is often neglected in corresponding investigations is the polarization of the liquid water molecules in the unevenly distributed electric field. Therefore, in this paper, the effect of polarization on the transport of liquid water in the gas diffusion layers (GDL) is studied. Thereto, numerical simulations are conducted using the structural and physical properties of TORAY TGP-H-060 carbon paper GDL. The simulation findings indicate that the polarization effect has no substantial impact on the shape of liquid water droplets in the GDL. Nonetheless, polarization significantly lowers the internal pressure of the water droplets needed to penetrate the GDL.

Catalyst Electrodes with PtCu Nanowire Arrays In Situ Grown on Gas Diffusion Layers for Direct Formic Acid Fuel Cells.

The excellent performance and safety of direct formic acid fuel cells (DFAFCs) promote them as potential power sources for portable electronic devices. However, their real application is still highly challenging due to the poor power performance and high complexity in the fabrication of catalyst electrodes. In this work, we demonstrate a new gas diffusion electrode (GDE) with ultrathin PtCu alloy nanowire (NW) arrays in situ grown on the carbon paper gas diffusion layer surface. The growing process is achieved by a facile template- and surfactant-free self-growth assisted reduction method at room temperature. A finely controlled ion reduction process tunes the nucleation and crystal growth of Pt and Cu leading to the formation of alloy nanowires with an average diameter of about 4 nm. The GDE is directly used as the anode for DFAFCs. The results in the half-cell GDE measurement indicate that the introduction of Cu in PtCu NWs boosts the direct oxidation pathway for formic acid. The Pt3Cu1 NW GDE shows a 2.4-fold higher power density compared to the Pt NW GDE in the membrane electrode assembly test in single cells.

Investigation on Mechanical Properties of a Carbon Paper Gas Diffusion Layer through a 3-D Nonlinear and Orthotropic Constitutive Model

The mechanical loads that gas diffusion layers (GDLs) withstand in polymer electrolyte membrane fuel cell (PEMFC) stacks are sensitive to the assembly and working conditions. The mechanical properties of GDLs mostly depend on their composition materials, microstructural characteristics, operation conditions, etc. An accurate and comprehensive understanding of the mechanical performance of GDLs is significant for predicting the stress distribution and improving the assembly technology of PEMFC stacks. This study presented a novel 3-D nonlinear and orthotropic constitutive model of a carbon paper GDL to represent the material stiffness matrix with its compressive, tensile, and shear properties. Numerical simulations were performed based on the 3-D constitutive model, and the proposed 3-D model was validated against the experimental data reported previously. It is found that the simulation results of the 3-D constitutive model show a good agreement with the experimental results. Besides, the novel 3-D nonlinear and orthotropic model was applied in the overall stress simulation of a simplified PEMFC unit cell, compared to a conventional 3-D linear and isotropic model, and the simulation results of the two models show a significant difference.

Effects of Repetitive Freeze-Thaw Thermal Cycles on the In-Plane Effective Thermal Conductivity of a Gas Diffusion Layer in Polymer Electrolyte Membrane Fuel Cells

As the fundamental components of polymer electrolyte membrane fuel cells, the thermal properties of gas diffusion layers (GDLs) play a significant role in influencing their overall heat conduction and management. We have experimentally characterized the temperature dependence of the in-plane thermal conductivity of a commercial carbon paper GDL, including the progressively in-plane thermal degradation of GDLs through a series of repetitive freeze-thaw thermal aging cycles. Even though the thermal properties of carbon fibers are crucial for the thermal performance determination of GDLs, the temperature-dependent in-plane thermal conductivity of pristine GDLs depends mainly on the thermal characteristics of the binder that connects carbon fibers and the polytetrafluoroethylene (PTFE) coated on the microporous layer. The microstructural failure of aged GDLs (breakage of carbon fibers, delamination, adhesive failure of binder, and PTFE cracks) caused by the freeze-thaw thermal aging treatment dominantly affects the in-plane effective thermal degradation at different thermal aging phases. Overall, the breakage of carbon fibers positively contributes, and delamination, adhesive failure of binder, and PTFE cacks negatively contribute to the in-plane effective thermal conductivity of GDLs. In particular, the in-plane thermal conductivity of aged GDLs through 300 cycles reaches a peak.

Fabrication of Large Area Ag Gas Diffusion Electrode via Electrodeposition for Electrochemical CO2 Reduction

For the improvement for the commercialization of electrochemical carbon dioxide (CO2) conversion technology, it is important to develop a large area Ag gas diffusion electrode (GDE), that exhibits a high electrochemical CO2 conversion efficiency and high cell performance in a membrane electrode assembly (MEA)-type CO2 electrolyzer. In this study, the electrodeposition of Ag on a carbon-paper gas diffusion layer was performed to fabricate a large area (25.5 and 136 cm2) Ag GDE for application to an MEA-type CO2 electrolyzer. To achieve uniformity throughout this large area, an optimization of the electrodeposition variables, such as the electrodes system, electrodes arrangement, deposition current and deposition time was performed with respect to the total electrolysis current, CO production current, Faradaic efficiency (FE), and deposition morphology. The optimal conditions, that is, galvanostatic deposition at 0.83 mA/cm2 for 50 min in a horizontal, two-electrode system with a working-counter electrode distance of 4 cm, did ensure a uniform performance throughout the electrode. The position-averaged CO current densities of 2.72 and 2.76 mA/cm2 and FEs of 83.78% (with a variation of 3.25%) and 82.78% (with a variation of 8.68%) were obtained for 25.5 and 136 cm2 Ag GDEs, respectively. The fabricated 136 cm2 Ag GDE was further used in MEA-type CO2 electrolyzers having an active geometric area of 107.44 cm2, giving potential-dependent CO conversion efficiencies of 41.99%–57.75% at Vcell = 2.2–2.6 V.

Impact of cyclic mechanical compression on the electrical contact resistance between the gas diffusion layer and the bipolar plate of a polymer electrolyte membrane fuel cell

The electrical contact resistance between the Gas Diffusion Layer (GDL) and the BiPolar Plate (BPP) used in Polymer Electrolyte Membrane Fuel cells (PEMFCs) is responsible for a substantial amount of Ohmic losses in the electrical power generator. This contact resistance was measured for a variety of carbon paper GDLs under cyclic mechanical compression between 0 and 8 MPa according to the Transmission Line Method (TLM). Contact resistance and strain hysteresis were noticed as a result of cyclic compression. The effect of GDL structure and composition on the electrical contact resistance and its cyclic behaviour has been evaluated. The contact resistance was found to decrease non-linearly with compression; more than 75% of reduction was attained at 2.5 MPa. The electrical contact resistance’s difference between the different cycles of compression decreased with compression loads. Graphitised straight fibre Toray GDLs demonstrated the smallest contact resistance followed by the MicroPorous Layer (MPL) and the felt fibre substrate. The SGL straight fibre substrates exhibited the highest contact resistance. The felt fibre structure exhibited the smallest difference rates between the cycles of compression.

Effect of PEM fuel cell porous media compression on in-plane transport phenomena

Abstract Liquid-gas two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane fuel cells is investigated using an ex-situ experimental setup. The mass transport phenomena is investigated in carbon paper and carbon cloth GDLs and at different compressions. Water percolation within the plane of the GDL is visualized with a CCD camera while its injection pressure is measured. Similarly, air percolation within the plane of GDL samples which were initially saturated with water is investigated. Experiments are conducted for the three flow regimes of stable displacement, capillary fingering, and viscous fingering. Images are analyzed to obtain the normalized wetted area. It is observed that while the GDL compression directly affects normalized wetted area for stable displacement and viscous fingering flow regimes, it has no impact on this parameter for capillary fingering flow regime. For stable displacement flow regime and for both carbon paper and carbon cloth samples, water percolation pressure increases with GDL compression. However, the water percolation pressure data obtained for capillary fingering flow regime does not suggest any discernible trend as a function of GDL compression. The findings in this study can be used to validate percolation models proposed by different schemes such as pore-network models.

Impact of carbon paper structural parameters on the performance of a polymer electrolyte fuel cell cathode via lattice Boltzmann method

Polymer electrolyte fuel cells (PEFCs) being employed in fuel cell electric vehicles (FCEVs) are promising power generators producing electric power from fuel stream via porous electrodes. Structure of carbon paper gas diffusion layers (GDLs) applying in the porous electrodes can greatly affect the PEFC performance, especially at the cathode side where electrochemical reaction is more sluggish. To discover the role of carbon paper GDL structure on the mass transfer properties, different cathode electrodes with dissimilar structural parameters are simulated via lattice Boltzmann method (LBM). 3D contours of oxygen and water vapor concentration through the GDL as well as the 2D contours of current density on the catalyst layer are illustrated and examined. The results indicate that the carbon fiber diameter has a negligible impact on the current density while the impact of carbon paper thickness and porosity is significant. In fact, increasing of carbon paper thickness or porosity leads to lack of cell performance.

The Effect of CO2 Bubble Distribution on Power Generation Performance of a Direct Formic Acid Fuel Cell

Introduction Recently, formic acid is expected as one of the energy career for renewable energy. Especially, direct formic acid fuel cells (DFAFCs) which use formic acid as fuel, have received considerable attention since they can generate higher power density than the other direct liquid fuel cells [1]. However, CO2 gas is generated in the anode reaction when DFAFCs are operating. If CO2 bubbles stay in the anode gas diffusion layer (GDL) the mass transport resistance in GDL increases and the power generation performance of DFAFCs decreases with the lapse of time. In this study, we have investigated the effect of CO2 bubble distribution on power generation performance by visualizing the CO2 bubble distribution in DFAFCs during power generation using X-ray CT. Experimental A single cell with 1 cm2 of active area was used. We used NR-212 as proton exchange membrane, PEM. Pd/C was used for anode catalyst and Pt/C was used for cathode catalyst. Moreover, two types of GDL with different structures of carbon paper and carbon cloth were used. The voltage sweep was performed at a sweep rate of -5 mV / sec until the cell voltage turned from the open circuit voltage to 0 V, and the cell voltage and current density during the sweep were recorded. A three-dimensional measurement X-ray CT apparatus (TDM-1000H-Ⅱ (2K), Yamato Scientific) was used for visualizing CO2 bubble distribution in DFAFCs. It took 15 minutes to visualize and the spatial resolution was 2.7 μm. Results and discussion From the measurement of the power generation characteristics, it was found that the maximum current density is higher with carbon cloth GDL than with carbon paper GDL, despite the lower open circuit voltage. In addition, using carbon paper GDL, the gradient of polarization curve became steep in the high current density region. These phenomena are generally considered to be caused by mass transport, and in this experiment it is thought that this is probably due to the effect of bubbles in the anode GDL. Therefore, in order to investigate the influence of bubbles in the anode GDL, visualization was performed. Fig. 1 shows the CO2 bubble distribution in the anode GDL at high current density operation using each GDL. These figures are three-dimensional representation of the bubble distribution in the anode GDL averaged over 15 minutes of visualization time. Furthermore, these are top views seen from the vertically upward direction of the GDL plane, the white part indicates that bubbles exist. As shown in Fig. 1, using carbon paper GDL, bubbles were present under most of the rib and also present under the channel, which was thought to cause the power generation performance degradation due to the mass transport. On the other hand, with the carbon cloth GDL, there were almost no bubbles under the channel, and bubbles and voids were concentrated under the rib. It is considered that in the carbon cloth, the bubbles were concentrated in part and the discharge path of the bubbles was formed, so that the bubbles were efficiently discharged and the power generation performance did not decrease even in the high current density region. Reference 1) Y. Zhu, Z. Khan, R.I. Masel, Journal of Power Sources, 139, 15-20 (2005) Figure 1

Thin film electrodes from Pt nanorods supported on aligned N-CNTs for proton exchange membrane fuel cells

The enhanced performance of carbon nanotubes (CNTs) over carbon black as a catalyst support and the outstanding catalytic activities of one-dimensional (1D) Pt nanostructures endow them big potential for applications in fuel cells. However, the research has been mainly focused on the materials, and a combination of both 1D Pt nanostructures and CNTs to fabricate practical high power performance fuel cell electrodes still remains a challenge. In this work, we demonstrate catalyst electrodes from Pt nanorods grown on aligned nitrogen doped CNTs for proton exchange membrane fuel cell (PEMFC) applications. Short Pt nanorods are grown on CNTs deposited directly on 16 cm2 carbon paper gas diffusion layers (GDLs) via plasma enhanced chemical vapour deposition (PECVD) and nitrided using active screen plasma (ASP) treatment, which are directly employed as cathodes for H2/air PEMFCs. The thin open catalyst layer effectively enhances mass transfer performance and, with a less than half of the Pt loading, 1.23 fold power density is achieved as compared with that from commercial Pt/C catalysts. A better durability is also confirmed which can be attributed to the good structure stability of nanorods and the enhancement effects from the N-CNT support.