Comprehensive performance enhancement of polybenzimidazole based high temperature proton exchange membranes by doping with a novel intercalated proton conductor

On the study of high temperature proton exchange membrane (HTPEM), the trade-off between proton conductivity and physico-chemical property (such as mechanical strength, dimensional stability and methanol resistance) remained a main obstacle for comprehensive performance enhancement. To address this issue, novel HTPEM was prepared by doping phosphotungstic acid intercalated ferric sulfophenyl phosphate (FeSPP-PWA) into polybenzimidazole (PBI) via hot pressed method. Intense hydrogen bonding network was built between PBI and FeSPP-PWA, rendering construction of proton channels and reinforcement of physico-chemical property. As a novel proton conductor, FeSPP-PWA facilitated formation of efficient proton transfer pathway. The layered morphology and inorganic intrinsity of FeSPP-PWA also improved the mechanical and dimensional stability while reducing the methanol permeability of the PBI/FeSPP-PWA membranes. The composite membrane exhibited good thermal stability up to 200 °C. The proton conductivity of PBI/FeSPP-PWA (30 wt%) reached 110 mS cm^?1 at 170 °C and 100% RH, and was 69.3 mS cm^?1 at 180 °C and 50% RH. The PBI/FeSPP-PWA also showed low methanol permeability and high membrane selectivity for application in direct methanol fuel cells.     

Metal tri-phosphonates containing aliphatic tertiary amines for medium-temperature proton conduction

New medium-temperature proton conductors (MATMP) were synthesized from amino trimethylene phosphonic acid (ATMP) and metal ions (Fe²⁺, Ce⁴⁺ and Zr⁴⁺) with varying molar ratios. The high content of phosphonic acid groups of ATMP endowed MATMP with good ion exchange capacity (IEC, 0.58～3.45 meq·g^?1) and proton conductivity. The aliphatic tertiary amine in ATMP improved proton conduction via ionic and hydrogen bonds. The proton conductivity of FeATMP(1:2) reached 0.124, 0.079 and 0.038 S cm^?1 at 100%, 50% and 0 RH at 180 °C, respectively. MATMP exhibited good thermal and oxidative stability, demonstrating promising applications in fuel cell technology.     

Modeling of high temperature proton exchange membrane fuel cells with novel sulfonated polybenzimidazole membranes

In this study, a three-dimensional, steady-state, non-isothermal numerical model of high temperature proton exchange membrane fuel cells (HT-PEMFCs) operating with novel sulfonated polybenzimidazole (SPBI) membranes is developed. The proton conductivity of the phosphoric acid doped SPBI membranes with different degrees of sulfonation is correlated based on experimental data. The predicted conductivity of SPBI membranes and cell performance agree reasonably with published experimental data. It is shown that a better cell performance is obtained for the SPBI membrane with a higher level of phosphoric acid doping. Higher operating temperature or pressure is also beneficial for the cell performance. Electrochemical reaction rates under the ribs of the bipolar plates are larger than the values under the flow channels, indicating the importance and dominance of the charge transport over the mass transport.     

High-temperature water-free proton conducting membranes based on poly(arylene ether ketone) containing pendant quaternary ammonium groups with enhanced proton transport

Poly(arylene ether ketone) containing pendant quaternary ammonium groups (QPAEKs) are anion-conducting polymers synthesized from benzylmethyl-containing poly(arylene ether ketone)s (PAEK-TM). Then QPAEK membranes doped with different concentrations of H₃PO₄ are prepared and evaluated as high temperature proton exchange membranes. The H₃PO₄ doping ability of quaternary ammonium groups in QPAEK system is found to be stronger than that of imidazole groups in polybenzimidazole system. The doping level of resulting QPAEK/H₃PO₄ composite membranes increases with both the concentration level of soaking H₃PO₄ solution and the ion exchange capacity. For example, the highest doping level of composite membranes is 28.6, which is derived from QPAEK-5 with an ion exchange capacity of 2.02 mmol g⁻¹ saturated with concentrated phosphoric acid. A strong correlation between the doping level and the proton conductivity is observed for all the membranes. Besides their low cost, novel high temperature proton exchange membranes, QPAEK/H₃PO₄, show really high proton conductivity and possess excellent thermal and mechanical stability, suggesting a bright future for applications in high temperature fuel cell.     

Effect of Ca doping on the electrical conductivity of the high temperature proton conductor LaNbO4

The sintering properties, crystal structure and electrical conductivity of La_1−xCa_xNbO_4−δ (x = 0, 0.005, 0.01, 0.015, 0.02 and 0.025), prepared by a solid-state reaction, have been investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and electrochemical impedance spectroscopy (EIS). In 2.5% Ca-doped samples, a small amount of impurities Ca₂Nb₂O₇ were observed from the XRD patterns. Impedance spectra show that the grain boundary resistance increases with increasing Ca content, while the bulk resistance remains essentially constant below 550 °C. Despite the higher degree of grain growth observed for higher Ca doping levels, the total conductivity of the La_1−xCa_xNbO_4−δ series decreases with increasing Ca content from 0.5 to 2.0 mol%. The activation energy for the total conductivity decreases with increasing Ca content from 0.71 eV (x = 0) to 0.54 eV (x = 0.01) for the high temperature tetragonal phase, then it increases to 0.60 eV for x = 0.02. For the monoclinic phase, the activation energy exhibits similar trend except La_0.995Ca_0.005NbO_4−δ shows the lowest value of 1.26 eV. The Ca and Nb content present at the grain boundaries for La_0.99Ca_0.01NbO_4−δ are much higher than that on the grain surface, as determined from the EDS analysis. These results imply that the solubility of CaO in LaNbO₄ is in the range from 0.5 to 1.0 mol%. By increasing the sintering temperature from 1500 °C to 1550 °C, the proton conductivity of the Ca-doped LaNbO₄ was improved with enlarged grain size due to a reduction in the resistive grain boundary contribution.     

Branched polybenzimidazole / polymeric ionic liquid cross-linked membranes with high proton conductivity and mechanical properties for HT-PEM applications

Branched polymers have unique three-dimensional dendritic structures, so they have received a lot of attention in the application of high temperature proton exchange membranes. In this work, we synthesize BOPBI-X (X = 3%, 6%, 9%) membranes with different branched ratios based on the synthesis of OPBI, due to the introduction of a rigid triazine structure with a larger free volume, the membranes could absorb more phosphoric acid (PA) while maintaining sufficient mechanical strength. Among them, the BOPBI-6% membrane obtains splendid comprehensive performance, when the PA doping level (ADL) is 9.55, it achieves a proton conductivity of 99.2 mS cm^?1 at 180 °C, which is 1.8 times higher than OPBI (54.8 mS cm^?1), and it performs well in long-term oxidation stability test after 144 h, it still has a mass retention rate of 90%. For the sake of further boost the performance of the membrane, cross-linkable polymeric ionic liquid (PIL) is introduced to the system. Among them, BOPBI-PIL-30% membrane has sufficient mechanical properties (5.50 ± 0.8 MPa), and the proton conductivity (146.9 mS cm^?1) at 180 °C is also excellent, so BOPBI-PIL-30% membrane is expected to be a promising candidate as HT-PEMs.     

Functionalized titania nanotube composite membranes for high temperature proton exchange membrane fuel cells

In this study, functionalized titania nanotubes (F-TiO₂-NT) were synthesized by using 3-mercaptopropyl-tri-methoxysilane (MPTMS) as a sulfonic acid functionalization agent. These F-TiO₂-NT were investigated for potential application in high temperature hydrogen polymer electrolyte membrane fuel cells (PEMFCs), specifically as an additive to the proton exchange membrane. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results confirmed that the sulfonic acid groups were successfully grafted onto the titania nanotubes (TiO₂-NT). F-TiO₂-NT showed a much higher conductivity than non-functionalized titania nanotubes. At 80 °C, the conductivity of F-TiO₂-NT was 0.08 S/cm, superior to that of 0.0011 S/cm for the non-functionalized TiO₂-NT. The F-TiO₂-NT/Nafion composite membrane shows good proton conductivity at high temperature and low humidity, where at 120 °C and 30% relative humidity, the proton conductivity of the composite membrane is 0.067 S/cm, a great improvement over 0.012 S/cm for a recast Nafion membrane. Based on the results of this study, F-TiO₂-NT has great potential for membrane applications in high temperature PEMFCs.     

Structural and conductivity study of the proton conductor BaCe(0.9−x)ZrxY0.1O(3−δ) at intermediate temperatures

The perovskite BaCe_(0.9−x)Zr_xY_0.1O_(3−δ) is prepared by solid-state reaction at 1400 °C and sintering at 1700 °C. It is characterised using X-ray diffraction, Raman spectroscopy and electrical measurements. A distortion from the cubic structure at room temperature is noticeable in the Raman spectra for 0.2 < x < 0.8, but not in the X-ray diffraction patterns. This work points out the rhombohedral nature of this distortion. Phase transitions are studied up to 600 °C. The direct current conductivity is measured as a function of oxygen partial pressure, and at a water vapour partial pressure of 0.015 atm. The total conductivity is resolved into an ionic and a p-type component using a fitting procedure appropriate to the assumed defect model. The first contribution is useful for estimating the proton transport number, while the value of the second one should not be too high not to deteriorate the electrodes performance.     

Phase stability and electrical conductivity of Ca-doped LaNb1−xTaxO4−δ high temperature proton conductors

The electrical conductivity, crystal structure and phase stability of La_0.99Ca_0.01Nb_1−xTa_xO_4−δ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5, δ = 0.005), a potential candidate for proton conductor for solid oxide fuel cells (SOFCs), have been investigated using AC impedance technique and in situ X-ray powder diffraction. Partially substituting Nb with Ta elevates the phase transition temperature (from a monoclinic to a tetragonal structure) from ∼520 °C for x = 0 to above 800 °C for x = 0.4. AC conductivity of the La_0.99Ca_0.01Nb_1−xTa_xO_4−δ both in dry and wet air decreased slightly with increasing Ta content above 750 °C, while below 500 °C, it decreased by nearly one order of magnitude for x = 0.4. It was also determined that the activation energy for the total conductivity increases with increasing Ta content from 0.50 eV (x = 0) to 0.58 eV (x = 0.3) for the tetragonal phase, while it decreases with increasing Ta content from 1.18 eV (x = 0) to 1.08 eV (x = 0.4) for the monoclinic phase. By removing the detrimental structural phase transition from the intermediate-temperature range, consequently avoiding the severe thermal expansion problem up to 800 °C, partial substitution of Nb with Ta brings this class of material closer to its application in electrode-supported thin-film intermediate-temperature SOFCs.     

Transport properties of proton conducting phosphate glass: An electrochemical hydrogen pump enabling the formation of dry hydrogen gas

In this study, a proton conducting phosphate glass, which was fabricated by alkali-proton substitution, was clearly demonstrated to transport protons over a wide oxygen partial pressure in air and hydrogen atmospheres, and in both dry and wet conditions. The proton transport number was confirmed to be unity, regardless of whether the glass was exposed to air or hydrogen atmospheres, in both hydrogen concentration and water vapor concentration cells. The dry hydrogen can be formed by electrochemical hydrogen pumping without current leakage as a result of the presence of other charge carriers. These properties derive from a proton incorporation mechanism, which is non-dependent on defect equilibria, unlike acceptor-doped perovskite-type proton conducting oxides. The advantages of applying this proton conducting glass electrolyte to fuel cells and steam electrolysis is also discussed.     

Highly enhanced proton conductivity of single-step-functionalized graphene oxide/nafion electrolyte membrane towards improved hydrogen fuel cell performance

Acidic group functionalized graphene oxide (GO) as a filler to the state-of-art Nafion electrolytes are regarded as potential materials towards next-generation fuel cell application. However, the tedious synthesis process for GO functionalization, and aggravated chemical durability at high temperatures demands the scientific community to design suitable Nafion-based functionalized GO electrolytes with superior proton conductivity and power density at actual fuel cell conditions i.e., 80 °C and 100% relative humidity (RH). Herein, a potential single-step-phosphorylated graphene oxide (sPGO) modified Nafion (sPGO/NF) is introduced to simultaneously multifold the proton conductivity, chemical durability, and power density of Nafion. Under actual fuel cell conditions, the sPGO/NF exhibits maximum proton conductivity (0.306 Scm⁻¹) which is 1.7-fold and 1.6-fold higher than that of rNF and GO/NF, respectively. Moreover, sPGO/NF achieves the maximum power density of 0.652 Wcm⁻² (80 °C, 100% RH), much higher than the rNF (0.51 Wcm⁻²) and GO/NF (0.53 Wcm⁻²) at same condition.     

Influence of operating temperature on cell performance and endurance of high temperature proton exchange membrane fuel cells

The relation between high temperature proton exchange membrane fuel cell (HT-PEMFC) operation temperature and cell durability was investigated in terms of the deterioration mechanism. Long-term durability tests were conducted at operational temperatures of 150, 170, and 190 °C for a HT-PEMFC with phosphoric acid-doped polybenzimidazole electrolyte membranes. Higher cell temperatures were found to result in a higher cell voltage, but decrease cell life. The reduction in cell voltage of approximately 20 mV during the long-term tests was considered to be caused both by aggregation of the electrode catalyst particles in the early stage of power generation, in addition to the effects of crossover due to the depletion of phosphoric acid in the terminal stage, which occurs regardless of cell temperature. It is expected that enhanced long-term durability for practical applications can be achieved through effective management of phosphoric acid transfer.     

Ion track grafting: A way of producing low-cost and highly proton conductive membranes for fuel cell applications

Proton conductive individual channels through a poly(vinyl di-fluoride) PVDF matrix have been designed using the ion track grafting technique. The styrene molecules were radiografted and further sulfonated leading to sulfonated polystyrene (PSSA) domains within PVDF. The grafting process all along the cylindrical ion tracks creates after functionalisation privileged paths perpendicular to the membrane plane for proton conduction from the anode to the cathode when used in fuel cells. Such ion track grafted PVDF-g-PSSA membranes have low gas permeation properties against H₂ and O₂. A degree of grafting (Y_w) of 140% was chosen to ensure a perfect coverage of PSSA onto PVDF-g-PSSA surface minimizing interfacial ohmic losses with the active layers of the Membrane Electrolyte Assembly (MEA). A three-day fuel cell test has been performed feeding the cell with pure H₂ and O₂, at the anode and cathode side respectively. Temperature has been progressively increased from 50 to 80 °C. Polarisation curves and Electrochemical Impedance Spectroscopy (EIS) at different current densities were used to evaluate the MEA performance. From these last measurements, it has been possible to determine the resistance of the MEA during the fuel cell tests and, thus the membrane conductivity. The proton conductivities of such membranes estimated during fuel cell tests range from 50 mS cm⁻¹ to 80 mS cm⁻¹ depending on the operating conditions. These values are close to that of perfluorosulfonated membrane such as Nafion^® in similar conditions.     

Ir-Pt/C composite with high metal loading as a high-performance anti-reversal anode catalyst for proton exchange membrane fuel cells

Voltage reversal induced by hydrogen starvation can severely corrode the anode catalyst support and deteriorate the performance of proton exchange membrane fuel cells. A material-based strategy is the inclusion of an oxygen evolution reaction catalyst (e.g., IrO₂) in the anode to promote water electrolysis over harmful carbon corrosion. In this work, an Ir-Pt/C composite catalyst with high metal loading is prepared. The membrane-electrode-assembly (MEA) with 80 wt% Ir-Pt(1:2)/C shows a first reversal time (FRT) of up to 20 hours, which is about ten times that of MEA with 50 wt% Ir-Pt(1:2)/C does. Furthermore, the MEA with 80 wt% Ir-Pt(1:2)/C exhibits a minimum cell voltage loss of 6 mV@1 A/cm² when the FRT is terminated in 2 hours, in which the MEA with 50 wt% Ir-Pt(1:2)/C exhibits a voltage loss of 105 mV@1 A/cm². Further physicochemical and electrochemical characterizations demonstrate that the destruction of anode catalyst layer caused by the voltage reversal process is alleviated by the use of the composite catalyst with high metal loading. Hence, our results reveal that the combination of OER catalyst on the Pt/C with high metal loading is a promising approach to alleviate the degradation of anode catalyst layer during the voltage reversal process for PEMFCs.     

Development of a one-dimensional and semi-empirical model for a high temperature proton exchange membrane fuel cell

High temperature proton exchange membrane fuel cells (HT-PEMFC), which operate between 160 °C and 200 °C, can be generally used in portable and stationary power generation applications. In this study, a one-dimensional, semi-empirical, and steady-state model of a HT-PEMFC fed with a gas mixture consisting of hydrogen and carbon monoxide is developed. Some modeling parameters are adjusted using empirical data, which are obtained conducting experiments on a HT-PEMFC for different values of Pt loading and cell temperature. For adjusting these parameters, the total summation of the square of the difference between the cell voltages found using the experimental and theoretical methods is minimized using genetic algorithm. After finding the values of the adjusted parameters, the effects of different cell temperature, Pt loading, phosphoric acid (PA) percentage, and different binders (PBI and PVDF) on the performance of the fuel cell are examined. It was found that, the performance of the fuel cell using PVDF binder exhibited better performance as compared to that using PBI binder.     

Stack performance of phosphotungstic acid functionalized mesoporous silica (HPW-meso-silica) nanocomposite high temperature proton exchange membrane fuel cells

In this paper, a series of short stacks with 2-cell, 6-cell and 10-cell employing phosphotungstic acid functionalized mesoporous silica (HPW-meso-silica) nanocomposite proton exchange membranes (PEMs) have been successfully fabricated, assembled and tested from room temperature to 200 °C. The effective surface area of the membrane was 20 cm² and fabricated by a modified hot-pressing method. With the 2-cell stack, the open circuit voltage was 1.94 V and it was 5.01 V for the 6-cell stack, indicating a low gas permeability of the HPW-meso-silica membranes. With the 10-cell stack, a maximum power density of 74.4 W (equivalent to 372.1 mW cm⁻²) occurs at 150 °C in H₂/O₂, and the stack produces a near-constant power output of 31.6 W in H₂/air at 150 °C without external humidification for 50 h. The short stack also displays good performance and stability during startup and shutdown cycling testing for 8 days at 150 °C in H₂/air. Although the stack test period may be too short to extract definitive conclusions, the results are very promising, demonstrating the feasibility of the new inorganic HPW-meso-silica nanocomposites as PEMs for fuel cell stacks operating at elevated temperatures in the absence of external humidification.     

Development and performance evaluation of a high temperature proton exchange membrane fuel cell with stamped 304 stainless steel bipolar plates

In this work, a high temperature proton exchange membrane fuel cell (HT-PEMFC) with stamped SS304 bipolar plates is successfully developed. Its performance was evaluated under two types of gaskets at different assembly torques and air stoichiometric ratios. The rates of pressure loss at a torque of 7 N-m with 50 Shore A hardness gaskets was 2.0 × 10^?3 MPa min^?1, which is acceptable. The best performance of the developed HT-PEMFC with stamped SS304 bipolar plates was 228.33 mW cm^?2, which approaches the performance of HT-PEMFCs with graphite bipolar plates. The optimal air stoichiometric ratio for the HT-PEMFC with stamped SS304 bipolar plates was 4.0, which is higher than that for proton exchange membrane fuel cells with CNC milled graphite bipolar plates. This is probably because of the deformation of the flow channels under the assembly compression force, which causes an elevated gas-diffusion drag in the flow channels. After the test, it was observed that some products of corrosion reaction formed on the surface of the SS304 bipolar plate. This phenomenon may lead to a decrease in the operating life of the HT-PEMFC.     

Sulfonated SBA-15 mesoporous silica-incorporated sulfonated poly(phenylsulfone) composite membranes for low-humidity proton exchange membrane fuel cells: Anomalous behavior of humidity-dependent proton conductivity

Sulfonated SBA-15 mesoporous silica (SM-SiO₂)-incorporated sulfonated poly(phenylsulfone) (SPPSU) composite membranes are fabricated for potential application in low-humidity proton exchange membrane fuel cells (PEMFCs). The SM-SiO₂ particles are synthesized using tetraethoxy silane (TEOS) as a mechanical framework precursor, Pluronic 123 triblock copolymer as a mesopore-forming template, and mercaptopropyl trimethoxysilane (MPTMS) as a sulfonation agent. A distinctive feature of the SM-SiO₂ particles is the long-range ordered 1-D skeleton of hexagonally aligned mesoporous cylindrical channels bearing sulfonic acid groups. Based on a comprehensive characterization of the SM-SiO₂ particles, the effect of SM-SiO₂ (as a functional filler) addition on the proton conductivity of the SPPSU composite membrane is examined as a function of temperature and relative humidity. An intriguing finding is that the proton conductivity of the SPPSU composite membrane exhibits a strong dependence on the relative humidity of measurement conditions. This anomalous behavior is further discussed with an in-depth consideration of the characteristics and dispersion state of SM-SiO₂ particles, which affect the tortuous path for proton movement, water uptake, and state of water. Notably, at low-humidity conditions, the SM-SiO₂ particles in the SPPSU composite membrane serve as an effective water reservoir to tightly retain water molecules and also as a supplementary proton conductor, whereas they behave as a barrier to proton transport at fully hydrated conditions.     

Effects of inlet relative humidity (RH) on the performance of a high temperature-proton exchange membrane fuel cell (HT-PEMFC)

A high temperature-proton exchange membrane fuel cells (HT-PEMFC) based on phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane is able to operate at elevated temperature ranging from 100 to 200 °C. Therefore, it is evident that the relative humidity (RH) of gases within a HT-PEMFC must be minimal owing to its high operating temperature range. However, it has been continuously reported in the literature that a HT-PEMFC performs better under higher inlet RH conditions. In this study, inlet RH dependence on the performance of a HT-PEMFC is precisely studied by numerical HT-PEMFC simulations. Assuming phase equilibrium between membrane and gas phases, we newly develop a membrane water transport model for HT-PEMFCs and incorporate it into a three-dimensional (3-D) HT-PEMFC model developed in our previous study. The water diffusion coefficient in the membrane is considered as an adjustable parameter to fit the experimental water transport data. In addition, the expression of proton conductivity for PA-doped PBI membranes given in the literature is modified to be suitable for commercial PBI membranes with high PA doping levels such as those used in Celtec^® MEAs. Although the comparison between simulations and experiments shows a lack of agreement quantitatively, the model successfully captures the experimental trends, showing quantitative influence of inlet RH on membrane water flux, ohmic resistance, and cell performance during various HT-PEMFC operations.     

Research progress of proton exchange membrane fuel cells utilizing in high altitude environments

The unconventional working conditions bring more challenges to the operation of proton exchange membrane fuel cells in high altitude environments. One is vibration caused by road turbulence when fuel cell vehicles are in the process of driving, the others are harsh operating environment of low pressure, cathode air starvation, and low ambient temperature etc. at a high altitude. Furthermore, the occurred literature displays that these abnormal working conditions caused at high altitude environments exhibit significant impact on the working state of proton exchange membrane fuel cells. Thus, we review the research progress of proton exchange membrane fuel cells affected by various unconventional conditions caused by high altitude climate environments, including vibration, low pressure, cathode air starvation, and low ambient temperature conditions, respectively. And at the end, we anticipate further research directions of fuel cells in high altitude climate environments.