Acid–base blends of sulfonated polyethersulfone (SPES) with pristine and aminated polyetherimide (APEI) are synthesized. Three blends polyethersulfone (PES)/polyetherimide (PEI), SPES/PEI, and SPES/APEI are prepared and characterized to evaluate their structural, morphological, mechanical, and other properties. Ion exchange capacity (IEC) of SPES/APEI and SPES/PEI blend membranes was determined to be 3.0 and 2.7 meq g−1, which is a substantial improvement over the 1.0 meq g−1 exhibited by unmodified PES/PEI blend. The proton conductivity of 0.093 S cm−1 displayed by SPES/APEI blend is found to be comparable to that of commercial Nafion membrane (0.056 S cm−1) and far superior to conductivities of 0.091 and 0.082 S cm−1 shown by SPES/PEI and PES/PEI blends, respectively. Further, water sorption observed in case of SPES/APEI and SPES/PEI blends was in the range 17–18% over a soaking time period of 12 hrs, which is ideal for proton conduction accompanied by low-membrane swelling. The methanol permeabilities of SPES/APEI and SPES/PEI blends are found to be 2.5 × 10−7 and 3.47 × 10−7 cm2 s−1, respectively. Compared to unmodified PES/PEI blend which revealed a methanol sorption of 12.3%, the modified blends SPES/PEI (9.6%) and SPES/APEI (7.5%) exhibited much lower methanol uptake over a sorption time of 12 hrs, indicating their capacity for low fuel bypass. The results demonstrate the promising potential of polymer blends made by combining a sulfonated polymer with an aminated polymer, such as SPES/APEI for fuel cell (FC) applications.
Nafion 212 membrane was subjected to swelling–dehydration (SD) cycles, as a relevant operation condition for direct methanol fuel cells (DMFCs). The major degradation mechanism due to the treatment was found to be sulfonic group contamination with trace ion, rather than formation of sulfonic anhydride, which is a well-known degradation mechanism for Nafion® membranes under hydrothermal (HT) aging condition. The consequences of the degradation include decreasing water content, thickness, and surface fluoride and increasing resistance, dry weight, and a changed surface morphology. Ion selectivity of the sulfonic group was studied toward different fuel cell relevant conditions. It turned out that the sulfonic groups have much higher selectivity toward cations rather than neighbor sulfonic groups. Trace impurities in the liquid methanol feed in DMFC may therefore represent an important contamination source.
Driven by the search for the highest theoretical efficiency, several studies have investigated in the last years the adoption of fuel cells in the field of power production from natural gas with CO2 capture. Most of the proposed power cycles rely on high temperature fuel cells, namely Solid Oxide Fuel Cells (SOFC) and Molten Carbonate Fuel Cells (MCFC), based on the concept of hybrid fuel cell plus gas turbine cycles. Accordingly, high temperature fuel cells are integrated with a simple or modified Brayton cycle. As far as SOFC are concerned, two main plant solutions can be identified depending on the integration with the natural gas reforming/shift section: (i) systems where natural gas is — partially or totally — internally reformed in the fuel cell and (ii) systems where natural gas is reformed before the fuel cell and the cell is fed with a high hydrogen syngas. In both cases, CO2 can be separated downstream the fuel cell via a range of available technologies, e.g. chemical or physical separation processes, oxy-combustion and cryogenic methods. Following a literature review on very promising plant configurations, this work investigates the advantages and limits of adopting an external natural gas conversion section with respect to the plant efficiency. As a reference plant we considered a power cycle proposed by Adams and Barton [8], whose performance is the highest found in literature for SOFC-based power cycles, with 82% LHV electrical efficiency. It is based on a pre-reforming concept where fuel is reformed ahead the SOFC which thus works with a high hydrogen content fuel. This plant was firstly reproduced considering all the ideal assumptions proposed by the original authors. As second step, the simulations were focused on revising the power cycle, implementing a complete set of assumptions about component losses and more conservative operating conditions about fuel cell voltage, heat exchangers minimum temperature differences, maximum steam temperature, turbomachinery efficiency, component pressure losses and other adjustments. Considering the consequent modifications with respect to the original layout, the net electric efficiency changes to around 66% LHV with nearly complete (95%+) CO2 capture, a still remarkable but less attractive value, while requiring a very complex and demanding heat exchangers network. Detailed results are presented in terms of energy and material balances of the proposed cycles. All the simulations have been carried out with the proprietary code GS, developed by the GECOS group at Politecnico di Milano.
Achievement of flow uniformity among cells of a fuel cell stack continues to be an issue in fuel cell design and can affect performance and longevity. While many studies have sought to examine the effects of manifold and cell geometries on stack pressure drops and current density, few have provided detailed mapping of the manifold flowfield or examined the effect of reactant supply pipe bends on this flow, as these bends can introduce flow asymmetries within the pipe downstream of the bend. A simplified scaled up model of a proton exchange membrane (PEM) fuel cell was fitted with different inlet flow configurations, including straight piping and piping containing a 90 deg bend and 180 deg bend prior to entering the manifold. Particle image velocimetry (PIV) was used to obtain mean and fluctuating velocity statistics within the manifold and in individual cells. These distributions were compared with previous results using a partially developed square inlet profile, as well as available experimental and computational data in the literature. The presence of pipe bends resulted in highly skewed flow within the manifold, which also affected the flow distribution among individual cells.
This paper presents a power control strategy for a marine power system made up of a hybrid diesel generator, a fuel cell, and an energy storage unit. For this purpose, a self-tuning fuzzy control is designed to manage the power generation between power sources during different maneuverings and voltage disturbances (both balanced and unbalanced) in an AC system. As a solution, a current control strategy using a voltage source converter is presented. Simulation results show the response of the whole system under a test driving cycle and this variety of voltage disturbance conditions. They illustrate the performance, including power flow control and voltage disturbance ride-through capability, of the proposed control strategy.
An optimal or near to optimal design and operation of a direct liquid fuel cell (DLFC) stack requires an understanding of the relevant physical phenomena across length scales in the stack. In particular, perturbations between cells can arise due to external manifold design as well as variations in material and design parameters between cells. In this work, we seek to derive closed-form analytical expressions that capture the global stack performance, as well as individual cell behavior such as cell potential, current density, and methanol distribution. This approach allows for the simulation of large stacks with near to negligible computational overhead. Finally, the solutions are demonstrated for a stack subjected to perturbations in the anode inlet velocity of each cell.
Polymer electrolyte membrane (PEM) fuel cells are well suited for automotive applications compared to other types of fuel cells owing to their faster transient response and low-temperature operation. Due to rapid change in loads during automotive applications, study of dynamic behavior is of paramount importance. This study focuses on elucidating the transient response of a PEM fuel cell for specified changes in operating parameters, namely, voltage, pressure, and stoichiometry at the cathode and the anode. Transient numerical simulations are carried out for a single-channel PEM fuel cell to illustrate the response of power as the operating parameters are subjected to specified changes. These parameters are also optimized with an objective to match the power requirements of an automotive drive cycle over a certain period of time.
The United Arab Emirates (UAE) has been exploring several renewable and green technologies to help reduce the increasing pollution rates. However, its coarse climate might impose some limitations toward the types of green technologies that can be effectively deployed in the region. In the first part of this work, an improved dynamic model of the Nexa 1.2 kW PEM fuel cell is developed using particle swarm optimization (PSO), and validated using experimental data. The developed model is then used to analyze the effect of the severe climate conditions of the UAE on the performance of the system to evaluate its operational compatibility with the region.
Well-dispersed and low Pt content Pt–Ru/C nanoparticles were prepared by a developed impregnating reduction method with sodium citrate as stabilizer. The as-prepared Pt–Ru/C catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray (EDX) analysis. TEM showed that the Pt–Ru particles were quite uniformly distributed on the surface of the carbon with an average particle size of 3.5–4.5 nm. The effect of pH values on methanol electro-oxidation reaction was examined by cyclic voltammetry (CV) and chronoamperometry (CA). Pre-adsorbed CO monolayer stripping was used to evaluate the antipoisoning ability. The results showed that Pt–Ru/C (pH = 8) catalyst had the highest catalytic activity and stability toward the oxidation of methanol. Finally, comparing Pt–Ru/C (Pt–Ru 20 wt.%, Pt/Ru = 1:1) catalysts with Pt/C (Pt 20 wt.%), the onset potential was 200 mV lower and electrochemical active area was much bigger.
A high-temperature proton exchange membrane (PEM) fuel cell using H3PO4-doped poly benzimidazole (PBI) as solid polymer electrolyte has been developed and tested. The influences of operating temperature (between 130 and 170 °C), operating pressure (between 0 and 2 bar), and air flow rate on the performances of the fuel cell have been measured. A maximum power density of ca. 200 mW/cm2 has been measured. The existence of an optimum air flow rate (expressed in oxygen stoichiometric ratio) has been put into evidence. It allows an increase of the fuel cell voltage from 250 mV up to ca. 400 mV at 0.4 A/cm2.
In a proton exchange membrane fuel cell (PEMFC), the hydrogen feed into the anode in a periodical pressure swing, so-called hydrogen pressure pulsation feed (HPPF), significantly affects the transport phenomena of hydrogen and water in the anode flow field. HPPF could adjust the distribution of the back diffusion water and the hydrogen partial pressure along the anode flow channels, improve hydrogen mass transfer in the anode flow field, and enhance the diffusion of hydrogen in the porous medium (anode diffusion layer). On the other hand, HPPF technique could mitigate the anode flooding issue caused by water back diffusion from the cathode, improve the fuel cell performance. In this work, the principle of HPPF technique was introduced and analyzed by a mathematic approach. Some of the important parameters used in HPPF technique, such as amplitude of pulsation pressure, pulsating frequency, etc., were experimentally investigated on dead-end mode PEMFC stack. The experimental results showed that the amplitude of pressure pulsation, pulsating frequency, and position applied for HPPF highly affected the performance of the PEMFC stack. It can be seen that higher the frequency and/or amplitude of pressure pulsation, the better the performance of PEMFC stack.
This paper presents a novel startup approach for solid oxide fuel cell (SOFC) hybrid systems (HSs) based on recompression technology. This startup approach shows a novel method of managing a complete plant to obtain better performance, which is always also a difficult task for equipment manufactures. The research activities were carried out using the HS emulator rig located in Savona (Italy) and developed by the Thermochemical Power Group (TPG) of the University of Genoa. The test rig consists of three integrated technologies: a 100 kWe recuperated microturbine modified for external connections, a high temperature modular vessel necessary to emulate the dimensions of an SOFC stack, and, for air recompression, a turbocharger necessary to increase fuel cell pressure (using part of the recuperator outlet flow) as required for efficiency increase and to manage the cathodic recirculation. It was necessary to develop a theoretical model in order to prevent abnormal plant startup conditions as well as motivated by economic considerations. This transient model of the emulator rig was developed using Matlab®-Simulink® environment to study the time-dependent (including the control system aspects) behavior during the entire system (emulator equipped with the turbocharger) startup condition. The results obtained were able to demonstrate that the HS startup phase can be safely managed with better performance developing a new control logic. In detail, the startup phase reported in this paper shows that all important parameters were always inside acceptable operating zones (surge margin kept above 1.1, turbine outlet temperature (TOT), and fuel flow maintained lower than 918.15 K and 7.7 g/s, respectively).
Degradation tests of two phosphoric acid (PA) doped polybenzimidazole (PBI) membrane based high temperature polymer electrolyte membrane (HT-PEM) fuel cells were reported in this paper to investigate the effects of start/stop and the presence of methanol in the fuel to the performance degradation. Continuous tests with H-2 and simulated reformate which was composed of H-2, water steam and methanol as the fuel were performed on both single cells. 12-h-startup/12-h-shutdown dynamic tests were performed on the first single cell with pure dry H-2 as the fuel and on the second single cell with simulated reformate as the fuel. Along with the tests electrochemical techniques such as polarization curves and electrochemical impedance spectroscopy (EIS) were employed to study the degradation mechanisms of the fuel cells. Both single cells showed an increase in the performance in the H-2 continuous tests, because of a decrease in the oxygen reduction reaction (ORR) kinetic resistance probably due to the redistribution of PA between the membrane and electrodes. EIS measurement of first fuel cell during the start/stop test showed that the mass transfer resistance and ohmic resistance increased which can be attributed to the corrosion of carbon support in the catalyst layer and degradation of the PBI membrane. During the continuous test with simulated reformate as the fuel the ORR kinetic resistance and mass transfer resistance of both single cells increased. The performance of the second single cell experienced a slight decrease during the start/stop test with simulated reformate as the fuel.
Solid oxide fuel cells (SOFCs) can be operated on a wide range of fuels, including hydrocarbons. Such a fuel supply includes the risk of carbon formation on the catalytically active nickel centers within the porous anodic substrate. Coking is very critical for the reliability and durability of the SOFCs. Thus, within this study, coking propensity of the most prominent carbon containing fuels was analyzed by thermodynamic equilibrium calculations for two fundamentally different types of carbon and detailed transient numerical simulations based on heterogeneous reforming kinetics. This approach is new to the literature and reveals the strengths and weaknesses of both fundamentally different approaches. It was shown that in thermodynamic equilibrium calculations, carbon formation is most likely due to pure methane. Carbon monoxide will form the least amounts of carbon in typical SOFC temperature ranges. Furthermore, based on a validated computational fluid dynamics (CFD) simulation model, detailed heterogeneous reaction kinetics were used to directly simulate elementary carbon adsorbed to the reactive substrate surface. The amounts, spatial and temporal distribution, of carbon atoms within the porous structure were identified between 600 °C and 1000 °C for a broad steam-to-carbon ratio range of 0.5–2. It was shown that most carbon is formed at the beginning of the substrate. A key finding was that steady-state results differ greatly from results within the first few seconds of fuel supply. An increment in temperature causes a significant increase in the amount of carbon formed, making the highest temperatures the most critical for the SOFC operation. Moreover, it was shown that mixtures of pure methane deliver the highest amounts of adsorbed elementary carbon. Equimolar mixtures of CH4/CO cause second highest surface coverages. Pure carbon monoxide blends led to least significant carbon formations. This work has shown the important contribution that thermodynamic equilibrium calculation results, as well as the outcomes of the detailed CFD simulations, allow to identify suitable operating conditions for the SOFC systems and to minimize the risk of coking on porous anodes.
Thin metallic bipolar plates (BPPs) fabricated by stamping technology are regarded as promising alternatives to traditional graphite BPPs in proton exchange membrane (PEM) fuel cell. However, during the stamping process, dimensional error in terms of the variation in channel height is inevitable, which results in performance loss for PEM fuel cell stack. The objective of this study is to investigate the effect of dimensional error on gas diffusion layer (GDL) pressure characteristics in the multicell stacks. At first, parameterized finite element (FE) model of metallic BPP/GDL assembly is established, and the height of channels is considered as varying parameters of linear distribution according to measurements of actual BPPs. Evaluation methods of GDL contact pressure are developed by considering the pressure distribution in the in-plane and through-plane directions. Then, simulation of the assembly process for a series of multicell stacks is performed to explore the relation between dimensional error and contact pressure based on the evaluation methods. Influences of channel number, cell number, and clamping force on the constitutive relation are discussed. At last, experiments are conducted and pressure sensitive films are used to obtain the actual GDL contact pressure. The numerical results show the same trend as experimental results. This study illustrates that contact pressure of each cell layer is in severely uneven distribution for the in-plane direction, and pressure change is unavoidable for the through-plane direction in the multicell stack, especially for the first several cells close to the endplate. The methodology developed is beneficial to the understanding of the dimensional error effect, and it can also be applied to guide the assembling of PEM fuel cell stack.
The creep and failure probability of a planar solid oxide fuel cell (SOFC) through a duty cycle is calculated by finite element method (FEM) and Weibull method, respectively. Two sealing methods, namely, rigid seal and bonded compliant seal (BCS), are compared. For the rigid seal, failure is predicted in the glass ceramic because of a failure probability of 1 and maximum creep strain. For the BCS design, the foil can absorb part of thermal stresses in the cell by its own elastoplastic deformation, which considerably decreases failure probability and creep strain in the SOFC. The creep strength of BCS method is achieved by sealing foil with excellent creep properties. Temperature fluctuation during the operating stage leads to the increase in thermal stress and failure probability. In particular, temperature change from low-power to high-power state results in a considerable increase in the creep strain, leading to creep failure for the rigid seal. A failure probability of 1 is generated during start-up and shut-down stages. Therefore, temperature fluctuation should be controlled to ensure structural integrity, and lowering the operating temperature can decrease failure probability and creep failure.
Most single-phase inverters, being sourced by fuel cell stacks (FCSs), subject the stacks to reflected low-frequency (120 Hz) current ripples that ride on average dc currents. The ripple current impacts the sizing and efficiency of the FCS. As such and typically, a passive or active filter is required at the input of the inverter (or output of the FCS) to mitigate the ripple current. Toward that end, this paper outlines a guideline to choose the optimum size of a passive input-filter capacitor for a fuel-cell-based power system from the standpoints of the overall system energy density and cost. Detailed case-specific simulation results, based on an analytical approach, are provided to illustrate key issues for both unity power factor as well as harmonic loads.
The objective of this study is to examine the effect of heat treatment at various temperatures on the corrosion behavior of electroless silver-coated SS 304 in a simulated proton exchange membrane (PEM) fuel cell environment. The corrosion properties of this material were studied using a potentiodynamic polarization technique. X-ray diffraction (XRD) patterns, polarization curves, and scanning electron microscopy (SEM) of coated and heat-treated specimens obtained in various heating temperatures were also utilized. It was found that the corrosion potential of the coated and heat-treated specimens shift toward a noble potential, and a significant decrease in corrosion current density was also observed. The corrosion current density decreased by a factor of about 1/500 for the heat-treated sample of 600 °C compared to the substrate. The heat-treated specimens displayed greater corrosion resistance than unheated and bare ones. According to the polarization studies and SEM images, the heat-treated specimen at 600 °C shows excellent corrosion resistance with a homogeneous dense surface morphology. These results demonstrated the coatings were suited for fuel cell applications in the proton exchange membrane fuel cell (PEMFC) environment.
This paper focuses on the fabrication of micro-coplanar interdigitated single chamber solid oxide fuel cell (μ-SC-SOFC) using a combination of micropatterning technique and thin-film deposition technology. Photolithography was used to generate the micro-interdigitated photoresist patterns on the substrates. Pulsed laser deposition (PLD) method was used to deposit thin films of microstructured electrolytes yttrium stabilized zirconia (YSZ) and electrodes (anode: YSZ + NiO and cathode: lanthanum strontium ferrous cobalt (LSCF)). Process parameters were optimized to obtain consistent functional microstructure and crystal morphology. This research shows good potential for combinatorial manufacturing methods to fabricate high quality and repeatable micro fuel cell components.
Proper water management is required for the operation of polymer electrolyte fuel cells (PEFCs), in order to maintain the critical balance between adequate membrane hydration and prevention of water flooding in the catalyst layer. In PEFCs, the membrane electrode assembly (MEA) is sandwiched between two gas diffusion layers (GDLs). In addition, a microporous layer (MPL) is generally applied to the GDL substrates for better water removal from the cathode catalyst layer. This paper is the first to report on an ex situ characterization method for water flooding in GDLs. As the humidity of O-2 gas on the substrate side of the GDL was increased in incremental steps, O-2 gas began to diffuse into the MPL side of the GDL. When the O-2 relative humidity exceeded the dew point, water flooding was observed on the surface of the MPL and the O-2 concentration dropped sharply because the O-2 diffusion was suppressed by the produced liquid water. When comparing to the estimated mass transfer loss based on the actual polarization curves of an MEA using the GDL, it was found that the decrease in the O-2 concentration on the MPL side of the GDL can be used as an index of water flooding in the PEFC.
