Phosphoric Acid Doped Polybenzimidazole Membranes Research Articles

Ionomeric Binders for High Temperature Proton Exchange Membrane Fuel Cells.

High-temperature proton exchange membrane fuel cell (HT-PEMFC) based on phosphoric acid doped polybenzimidazole membrane (PBI/PA) operating at 120-200 °C can provide insensitivity to carbon monoxide (CO) and simplified managements of water and heat and thus attract significant global attention. However, one significant drawback is its low utilization of precious metal catalysts resulted from the PA poisoning and inefficient three-phase boundary. Studies of binder materials in catalyst layers for HT-PEMFC are gradually emerging and there are few literature reviews on this important topic. The purpose of this review is to describe the various types of binders based on their molecular structure and electrochemical properties, with particular emphasis on catalyst layer for fuel cells. Importantly, this review provides a better understanding of relationship between fuel cell performance and the gas permeability and conductivity of different binders. Then, future directions of research and development in binder materials of HT-PEMFC are pointed out.

Core-shell tin pyrophosphate-based composite membrane for fuel cell with durability enhancement at elevated temperatures

Tin pyrophosphate has been employed as a proton conductor in polymer electrolyte membranes (PEMs) at temperatures over 200 °C. However, neither the aggregation of the SnP2O7 nanoparticles nor the proton conduction mechanism of the material has been clearly clarified in PEM. The present work reports a polybenzimidazole (PBI)/SnP2O7 composite membrane with homogeneous distribution of the inorganic phase. The membrane is successfully fabricated by in-situ transformation of SnO2 nanoparticles to core-shell SnP2O7 nanoparticles in the PBI matrix during fuel cell operation. Further in-depth analysis shows that the core-shell SnP2O7 contains a crystalline SnP2O7 core, an amorphous SnP2O7 intermediate layer and a gel-like outer layer mainly containing H4P2O7. In addition, the crystal growth of the SnP2O7 nucleus consumes ions from the amorphous SnP2O7 layer, where the reacted ions are compensated by the diffusion of Sn4+ and P2O74− ions from the gel-like outer layer. Also, the fuel cell with the composite membrane shows superior proton conductivity and durability compared to the pristine phosphoric acid-doped PBI membrane at 240 °C for over 50 h. That is due to the presence of H4P2O7 in the outer layer, which contributes to high and stable proton conductivity of the core-shell SnP2O7 at 220–260 °C.

Investigation of water effects on hydrogen dissolution, diffusion, and reaction in high temperature proton exchange membrane fuel cell

High-temperature proton exchange membrane fuel cell with phosphoric acid-doped polybenzimidazole membrane is typically utilized with a methanol steam reformer due to its high CO tolerance. However, methanol reformate commonly contains 20 % water, and its effect on the high temperature fuel cell is unclear. A comprehensive 3D multiphase non-isothermal model with an enhanced electrochemical kinetics model considering the water influence mechanism is developed. The water influence mechanism includes the proton conductivity, hydrogen dissolution on the pore-ionomer interface, hydrogen diffusion inside the ionomer, and hydrogen surface reaction on the platinum. The sensitivity analysis found that hydrogen dissolution on the pore-ionomer and hydrogen surface reaction on the platinum significantly affect cell performance. The influences of water on ionomer hydrogen concentration, platinum surface hydrogen concentration, and surface reaction rates are investigated under the voltage range of 0.9–0.4 V. The increase of water content can improve the net adsorption rate and benefit electrochemical reaction. Besides, the anode water is better for the uniformity of surface reaction rate distributions while not affecting the hydrogen distributions inside the ionomer and on the platinum surface. Additionally, the positive influence of water on CO poisoning is investigated in terms of surface reaction. The water's beneficial effect on decreasing platinum loading is also discussed. The results show that 20 % water can reduce the platinum loading by 20 %.

Experimental Analysis of Catalyst Layer Operation in a High-Temperature Proton Exchange Membrane Fuel Cell by Electrochemical Impedance Spectroscopy

High-temperature proton exchange membrane fuel cells (HT-PEMFC) directly convert hydrogen and oxygen to produce electric power at a temperature significantly higher than conventional low-temperature fuel cells. This achievement is due to the use of a phosphoric acid-doped polybenzimidazole membrane that can safely operate up to 200 °C. PBI-based HT-PEMFCs suffer severe performance limitations, despite the expectation that a higher operating temperature should positively impact both fuel cell efficiency and power density, e.g., improved ORR electrocatalyst activity or absence of liquid water flooding. These limitations must be overcome to comply with the requirements in mobility and stationary applications. In this work a systematic analysis of an HT-PEMFC is performed by means of electrochemical impedance spectroscopy (EIS), aiming to individuate the contributions of components, isolate physical phenomena, and understand the role of the operating conditions. The EIS analysis indicates that increases in both the charge transfer and mass transport impedances in the spectrum are negatively impacted by air humidification and consistently introduce a loss in performance. These findings suggest that water vapor reduces phosphoric acid density, which in turn leads to liquid flooding of the catalyst layers and increases the poisoning of the electrocatalyst by phosphoric acid anions, thus hindering performance.

Construction of high-density proton transport channels in phosphoric acid doped polybenzimidazole membranes using ionic liquids and metal-organic frameworks

Introducing metal-organic frameworks (MOFs) into phosphoric acid (PA) doped polybenzimidazole (PBI) membranes to form proton transfer channels is an effective strategy to reduce the dependence of proton conductivity on PA. However, excessive MOF loading inevitably leads to reduced mechanical properties and damaged membrane structure. Ionic liquids (ILs) are reported as good plasticizers that can obviously improve the interface performance. Along this line, IL is introducing into the branched poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (BOPBI) cross-linked by KH-560. The interface performance between the MOF and PBI in the cross-linked BOPBI (CBOPBI)@MOF composite system is improved. The high MOF loading (50 wt%) composite membranes are prepared with acceptable mechanical properties for the first time. High density proton transport channels are constructed based on the high MOF loading in the PA doped CBOPBI@MOF membranes. For instance, the CBOPBI@MOF50%-IL30 shows an excellent proton conductivity (0.135 S cm−1), and a power density about a 26.9% increase (736 mW cm−2) compared with that of the CBOPBI@MOF40% membrane. Furthermore, the load voltage of the membrane with 50 wt% MOF is also well maintained. The membranes with high density proton transport channels exhibit high potential application prospects as high-temperature proton exchange membranes (HT-PEMs).

Low Pt loading for high-performance fuel cell electrodes enabled by hydrogen-bonding microporous polymer binders.

A key challenge for fuel cells based on phosphoric acid doped polybenzimidazole membranes is the high Pt loading, which is required due to the low electrode performance owing to the poor mass transport and severe Pt poisoning via acid absorption on the Pt surface. Herein, these issues are well addressed by design and synthesis of effective catalyst binders based on polymers of intrinsic microporosity (PIMs) with strong hydrogen-bonding functionalities which improve phosphoric acid binding energy, and thus preferably uphold phosphoric acid in the vicinity of Pt catalyst particles to mitigate the adsorption of phosphoric acid on the Pt surface. With combination of the highly mass transport microporosity, strong hydrogen-bonds and high phosphoric acid binding energy, the tetrazole functionalized PIM binder enables an H2-O2 cell to reach a high Pt-mass specific peak power density of 3.8 W mgPt-1 at 160 °C with a low Pt loading of only 0.15 mgPt cm-2.

Two-Dimensional Model of a High-Temperature Proton Exchange Membrane Hydrogen Pump for Efficient, Single-Stage Separation of Fuel Cell Quality Hydrogen Gas from Hydrogen-Enriched Natural Gas

A key component of the emerging hydrogen energy economy is the distribution system which facilitates the movement of hydrogen energy between production and end-use. There is interest in valorization of existing gas systems, which today are dedicated to natural gas movement, to facilitate the co-transportation of hydrogen via blending of hydrogen gas into these gas systems at concentrations up to 20% H2 by volume [1]. Fuel cell quality hydrogen, at a purity of 99.97%> H2, is required for the most valuable energy end-uses of hydrogen gas and thus the ability to efficiently extract H2 gas at such high purity is critical to the adoption of natural gas infrastructure as a means of co-transportation and distribution.Electrochemical hydrogen pump (EHP) utilizes a proton conducting membrane to separate hydrogen from mixtures by driving the electrochemical process of oxidation of hydrogen at an anode and subsequent evolution of hydrogen gas at a cathode, while other gaseous impurities are ideally unable to permeate through the membrane. HT-PEM EHP based on polybenzimidazole (PBI) membranes doped with phosphoric acid exhibit high proton conductivities at temperatures in the range of 160C Celsius while also exhibiting characteristics that are complimentary to their application in gas separation processes. These characteristics include a limited tolerance to common sources of platinum catalyst inhibition such as CO [1] and the presence of an aqueous phosphoric acid phase inhibits gas cross-over, providing better product purity although at the cost of performance in the form of catalyst inhibition.A two-dimensional model based on an HT-PEM system employing a phosphoric acid doped PBI membrane and free phosphoric acid as the proton conducting phase of the catalyst layer is developed to better understand the underlying processes governing the performance of the EHP. The model is validated with experimental measurements with mixtures as low as 2% H2 by volume in methane. In-operando micro-CT imaging of an HT-PEM EHP cell taken at the LBNL Advanced Light Source (ALS) is used to further validate physical parameters and assumptions of the model.The influence of differential pressure, relative humidity of the feed, and concentration of feed gases on separation performance are investigated. Losses due to back-permeation of hydrogen under pressure are accounted for. Power-loss voltage breakdown analysis indicates appreciable losses due to proton transport in the relatively thick catalyst layer, and the dependence of these losses on the volume fraction of acid present (Figure 1). The influence of CO2 as a catalyst inhibiting contaminant as was previously observed in low temperature EHP [3] is examined. The model and experiments show that a specific energy of separation of 5.1 kWh/kg H2 at a hydrogen recovery factor (HRF) of 50% can be achieved in a single stage with the EHP, producing 99.995% > H2 from a 2% H2 in CH4 blend, while pressurizing the product H2 at a product pressure ratio of 1.3 relative to feed pressure. Bibliography [1] C. J. Quarton and S. Samsatli, "Power-to-gas for injection into the gas grid: What can we learn from real-life projects, economic assessments and systems modelling?," Renewable and Sustainable Energy Reviews, vol. 98, pp. 302-316, 2018. [2] K. Perry and B. B. Eisman G.A., "Electrochemical hydrogen pumping using a high-temperature polybenzimidazole (PBI) membrane," Journal of Power Sources, pp. 478-484, 2008. [3] N. e. al., "Effect of CO2 on the performance of an electrochemical hydrogen compressor," Chemical Engineering Journal, vol. 329, 2020. Figure 1

Advances in polybenzimidazole based membranes for fuel cell applications that overcome Nafion membranes constraints

High-temperature proton exchange membrane fuel cells (PEMFCs) are becoming more appealing to researchers around the world due to several advantages such as high energy conversion efficiency, lightweight due to the use of polymeric materials, low noise due to the lack of mechanical components, zero or low emission of harmful greenhouse gases, simple design, first reaction kinetics, and simple water management. The performance, limitations, and possibilities for the practical implementation of different high-temperature proton exchange membranes (PEMs) utilized in fuel cells are discussed in this review paper. A suitable PEM must be developed to improve the performance of membrane electrode assemblies for high-temperature PEMFCs (working temperature above 100 °C). Various PEMs such as sulfonated poly(ether ether ketone) (SPEEK), sulfonated polystyrene (SPS), sulfonated polyimide (SPI), sulfonated polysulfone (SPSU), phosphoric acid doped polybenzimidazole (PA-PBI), perfluorosulfonic acid (PFSA) polymer-based membranes have been investigated as PEMs for fuel cell applications by several research groups. Among the various PEMs, PA-PBI membranes are the most promising high-temperature PEM for fuel cell applications because of their higher proton conductivity at high temperatures and anhydrous conditions, good chemical and thermal, mechanical stability, and high durability. However, the performance of high-temperature PEMs based on pristine PA-PBI is insufficient to fulfill the need for practical implementation of high-temperature PEMFCs, which needs to be enhanced further before they can be used commercially. Therefore, PA-PBI composite membranes containing various multifunctional inorganic, organic, and hybrid fillers are being actively explored to produce high-temperature PEMs. The PA-PBI composite membranes’ proton conductivity increases with rising temperature and maintains conductivity in anhydrous conditions, as well as chemical, thermal, and oxidative stability under operating conditions and long-term performance stability.

An effective strategy to enhance dimensional-mechanical stability of phosphoric acid doped polybenzimidazole membranes by introducing in situ grown covalent organic frameworks

Poor dimensional-mechanical stability and severe acid leakage have been the crucial issues restraining the practical applications of phosphoric acid-doped polybenzimidazole (PA−PBI) as high-temperature proton exchange membranes (HT-PEMs). In view of their robust, ordered, and tunable porous structures, chemically stable covalent organic frameworks (COFs) show great promise to enhance the performance of PA-doped PBI membrane. In this work, the in situ synthesized COF sheets are introduced to the PBIs for the first time and form a unique composite structure. The composite structure comprising soft PBIs matrix and rigid COF sheets can obviously improve dimensional-mechanical stability of the membranes under high acid doping level. Importantly, the rigid COF sheets help to construct proton transport channel in the membrane, resulting in high proton conductivity and excellent fuel cell performance. Among them, the 40%–COF–OPBI membrane shows the best overall performance, such as excellent proton conductivity (177.7 mS cm−1), desirable mechanical tensile strength (12.1 MPa), and high power density (774.7 mW cm−2) at 160 °C without humidification in H2/O2 fuel cell tests. These results suggest that introducing COFs to PBI is an effective strategy for fabricating high-performance proton exchange membranes for fuel cell applications.

Thermodynamic Modeling and Performance Analysis of Vehicular High-Temperature Proton Exchange Membrane Fuel Cell System.

Since the high temperature proton exchange membrane fuel cells (HT-PEMFC) stack require a range of auxiliary equipments to maintain operating conditions, it is necessary to consider operation of related components in the design of HT-PEMFC systems. In this paper, a thermodynamic model of a vehicular HT-PEMFC system using phosphoric acid doped polybenzimidazole membrane is developed. The power distribution and exergy loss of each component are derived according to thermodynamic analysis, where the stack and heat exchanger are the two components with the greatest exergy loss. In addition, ecological functions and improvement potentials are proposed to evaluate the system performance better. On this basis, the effects of stack inlet temperature, pressure, and stoichiometric on system performance are analyzed. The results showed that the energy efficiency, exergy efficiency and net output power of the system achieved the maximum when the inlet gases temperature is 406.1 K. The system performance is better when the cathode inlet pressure is relatively low and the anode inlet pressure is relatively high. Moreover, the stoichiometry should be reduced to improve the system output performance on the basis of ensuring sufficient gases reaction in the stack.

An Effective Strategy to Enhance Dimensional-Mechanical Stability of Phosphoric Acid Doped Polybenzimidazole Membranes by Introducing in Situ Grown Covalent Organic Frameworks

An Effective Strategy to Enhance Dimensional-Mechanical Stability of Phosphoric Acid Doped Polybenzimidazole Membranes by Introducing in Situ Grown Covalent Organic Frameworks

Improvement of Performance and Lifetime of Polybenzimidazole Membranes in High Temperature Fuel Cells by Incorporation of Muscovite

Polybenzimidazole (PBI) is a basic aromatic heterocyclic polymer with excellent thermal and chemical stability. The phosphoric acid (PA) doped PBI and its derivative membranes have high proton conductivity, low gas permeability, good fuel impurity tolerance, and excellent oxidative and thermal stability at temperatures up to 200 °C [1], hence, PA doped PBI is a promising candidate for high-temperature proton exchange membrane fuel cells. However, PA doped PBI membranes also has some limitations, including the dehydration of phosphoric acid above 140 °C, PA leaching out of the membranes, and deterioration of membrane mechanical strength especially at high acid doping level.The incorporation of clay minerals into membranes attract a great interest due to their hygroscopicity, high surface area and cheap cost [2]. The hygroscopic properties of clay minerals can increase the humidity inside the membranes, interact with PA molecules and maintain a strong proton conductivity. Here we present work on the incorporation of a cheap muscovite into the PBI membrane. As the muscovite is highly hygroscopic and has hydroxyl groups on the surface, an optimal cell performance was observed when the loading of muscovite is 1 wt.%. The PBI membrane with 1 wt.% muscovite showed the membrane resistance of 189 mohm cm2 and peak power density of 612 mW cm-2 at 150 °C, 26% higher than the pure PBI membrane with the membrane resistance of 235 mohm cm2 and the power density of 485 mW cm-2. Furthermore, the durability of inorganic-organic composite membranes was tested by an accelerated stress test (AST) operating the membrane electrode assembly between 0.6 A cm−2 and 1.0 A cm−2 [3]. After running around 50 h AST, the composite PBI membrane with 1 wt.% muscovite showed much lower degradation (9.3% at 0.6 A cm−2 and 12.8% at 1.0 A cm−2) than the pure PBI membrane (11.2% at 0.6 A cm−2 and 22.6% at 1.0 A cm−2). The improved durability of the composite membranes might be explained by the muscovite forming hydrogen bonding with PA molecules and polymer chains, which alleviates the degradation of polymer chains and PA leaching out of the membranes. The PA distribution and retention properties of the composite membranes were also characterized by energy dispersive X-ray spectroscopy and electron probe micro-analysis.

Durability Studies on PBI Based High Temperature PEMFC

HT-PEMFC based on phosphoric acid-doped polybenzimidazole membranes are a technology characterized by simplified construction and operation along with methanol reformers. Durability issues including acid loss, platinum sintering and carbon corrosion are recognized for both steady state and start-stop cycling operations. This work reports experimental studies on the degradation of PBI-based fuel cells operating with synthetic reformate fuel and air. Degradation stressors include elevated temperatures, pressures, current densities, and start-stop cycles. An average degradation rate of 9.3 mV/h is observed for continuous operation at 0.4 A/cm2 and 160 ˚C for 12,000 hours. High pressure (1.5 bara) operation at 170 oC and 0.8 A/cm2 shows an average degradation rate of 12.6 µV/h during a period of 2,000 hours. A start-stop test from 50 oC consisting of 240 cycles between temperatures of 165 and 175 oC and current density of 0.31 and 0.55 A/cm2 reveals a performance decay by 0.48-0.58 mV/cycle.

Effect of type and stoichiometry of fuels on performance of polybenzimidazole-based proton exchange membrane fuel cells operating at the temperature range of 120–160 °C

Herein, a proton exchange membrane fuel cell (PEMFC) equipped with phosphoric acid-doped polybenzimidazole (PA-PBI) membrane was exploited to determine the effects of changing type and stoichiometry of feed gas at operating temperature from 120 to 160 °C. Results show that maximum power density of proposed system increases as increasing temperature, and varying the type and stoichiometry of feed gas. For example, a typical power density of 0.254, 0.299 and 0.389 W/cm2 was obtained when operating PEMFC at 120, 140 and 160 °C respectively with pure hydrogen (H2) as feed gas. By contrast, power density of only 0.128, 0.194 and 0.243 W/cm2 was achieved when operating the PEMFC under identical condition with reformed H2 as feed gas. On the other hand, when varying oxygen (O2) stoichiometry from 2 to 6, power density of PEMFC vary from 0.330 to 0.472 W/cm2 at 160 °C. At high temperature and high O2 diffusion rate, reaction kinetics of electrodes and membrane were boosted, resulting lower mass-transfer resistance and higher PEMFC performance. In addition, we conducted long-term operation of PEMFC at 160 °C for 500 h to examine durability of PA-PBI. PA-PBI membrane was not lose open circuit voltage (OCV) significantly, indicating its good PEMFC durability.

First approaches for hydrogen production by the depolarized electrolysis of SO2 using phosphoric acid doped polybenzimidazole membranes

Renewable energy storage and conversion is nowadays a major target for the scientific community. Their conversion into hydrogen is a clear and clean alternative for their storage. This work shows, for the first time, the results of the SO2 depolarized electrolysis for hydrogen production at high temperature (120–170 °C) using phosphoric acid doped polybenzimidazole (PBI) membranes. A standard and a thermally cure PBI membrane doped with phosphoric acid were used for manufacturing the MEA of two electrolyzers. The benefit of the temperature was demonstrated but an unexpected behavior occurs at voltages higher than 0.8 V when temperature increases. Moreover, the thermally cured membrane shows a superior performance as compared with the standard one. Production of sulfur by reduction of SO2 becomes an important drawback and advices not operating above 130 °C. Results show that PBI membranes doped with phosphoric acid are suitable for high temperature operation for the sulfur dioxide depolarized electrolysis. Increasing temperature is beneficial up to a certain value of potential, showing a considerable influence in the charge transfer resistance of the system.

NMR Investigation of Transport in Polybenzimidazole/Polyphosphoric Acid Membranes Prepared Via Novel Synthesis Route

Previous work by the some of us has demonstrated enhanced electrochemical performance of polybenzimidazole (PBI) membranes with high phosphoric acid content prepared by the so-called PPA process.1 Methods of PBI synthesis that involve casting from an organic solvent cannot incorporate the more rigid PBIs characteristic of the PPA process due to their limited solubility. Recently, a carefully controlled drying method has been discovered in which a gel PBI membrane, made in the PPA process, is transformed into a dense PBI film. This method provides a synthetic route to PBI films without the use of any organic solvents. The dense PBI films with repeat unit para-PBI (Fig.1), were originally developed for flow battery applications. However they also can be re-doped in phosphoric acid to be used as a membrane in high-temperature fuel cells.The re-doped PBI films have displayed unprecedented ionic conductivity at elevated temperatures, similar to the starting PBI gel membranes prepared by the PPA process1, while also exhibiting enhanced mechanical properties. Temperature-dependent conductivity data for two of these films are displayed below. Sample A is the dried, densified, and re-acidified version of sample B, with a much lower acid content, ~18 mol PA/PBI repeat unit compared to 30 mol PA/PBI RU, respectively, but exhibits comparable conductivity (Fig. 2). Thus, in high-temperature fuel cell operation the electrochemical performance is similar, but performance degradation due to mechanical creep is reduced, leading to greater durability of the Version A PBI.In attempting to understand the enhanced proton conductivity of the modified PBI membrane despite its lower acid content, we performed both 1H and 31P nuclear magnetic resonance (NMR) measurements. Proton self-diffusion coefficients for four membranes two (A,C) prepared by the modified process and two (B,D) by the original PPA process, were measured by pulsed gradient NMR and plotted in Fig. 3.The results show significantly enhanced proton diffusivity, both in magnitude and in lower activation energy for sample A, which is consistent with its high conductivity despite lower acid content compared to the original PPA-processed membranes (B, D). We have also performed solid state 1D 31P and 2D 31P{1H} HETCOR NMR measurements, which yield additional information on structural differences between the membranes and how these differences may affect transport properties. Multinuclear NMR Study of the Effect of Acid Concentration on Ion Transport in Phosphoric Acid Doped Polybenzimidazole Membranes, N. Suarez, N.K.A.C. Kodiweera, P. Stallworth, S.G. Greenbaun, S. Yu, and B. Benicewicz, Journal of Physical Chemistry B, 116, 12545-51(2012). Figure 1

Effect of phosphoric acid-doped polybenzimidazole membranes on the performance of H+-ion concentration cell

H+-ion concentration cell, which can harvest thermal energy to generate electricity by hydrogen concentration difference principle with a fuel cell structure, is an innovative thermoelectric conversion device. In this system, phosphoric acid-doped polybenzimidazole (PA-doped PBI) membrane is a key component influencing the power generation performance of the cell. Herein, 30, 45, 60, 75, and 90 μm thick PBI membranes are successfully synthesized and doped with phosphoric acid. To achieve a good compromise between the proton conductivity and durability, the properties of PA-doped PBI membranes are experimentally evaluated to clarify the effect of the acid doping time and membrane thickness on cell performance. The results indicate that the higher the acid doping level, the worse the dimensional stability of the membrane. Also the thinner the PBI membrane, the smaller the membrane resistance to ions motion, while the poorer the stability. Upon reaction at 170 °C, this cell can boast a power density from 3.0 to 8.0 W m−2, which results in a thermoelectric conversion efficiency of 5.97–14.32%. This study potentially boosts the practical application of thermal-to-electrical conversion technology.

Preparation and molecular simulation of grafted polybenzimidazoles containing benzimidazole type side pendant as high-temperature proton exchange membranes

Phosphoric acid-doped polybenzimidazole membranes have exhibited great potential in high-temperature proton exchange membrane applications, but proton conductivity and stability should be further improved for high performance fuel cells. Herein, a series of grafted polybenzimidazoles containing benzimidazole type side pendant were successfully prepared by facile N-substitution reaction without any catalyst. These grafted polybenzimidazole membranes with side chains of benzimidazole group exhibit excellent properties in the doping level of phosphoric acid, retention stability of phosphoric acid and proton conductivity under certain mechanical properties, oxidation resistance and thermal stability. Importantly, the phosphoric acid (PA) retention stability and proton conductivity can be preciously controlled by adjusting the length of side chain and the number of benzimidazole groups. With the acid doping level of 19.3, the grafted membrane exhibits a high conductivity of 101 mS cm−1 and a great power density of 305 mW cm−2 at 160 °C without humidification. The molecular dynamics simulation show that the improved performance of the grafted membrane is mainly due to the increase in the free volume of the polymer membrane, the enhancement of the binding energy and the densification of the hydrogen bond network structure between the polymer membrane and phosphoric acid.

Polybenzimidazole-Based High-Temperature Polymer Electrolyte Membrane Fuel Cells: New Insights and Recent Progress

High-temperature proton exchange membrane fuel cells based on phosphoric acid-doped polybenzimidazole membranes are a technology characterized by simplified construction and operation along with possible integration with, e.g., methanol reformers. Significant progress has been achieved in terms of key materials, components and systems. This review is devoted to updating new insights into the fundamental understanding and technological deployment of this technology. Polymers are synthetically modified with basic functionalities, and membranes are improved through cross-linking and inorganic–organic hybridization. New insights into phosphoric acid along with its interactions with basic polymers, metal catalysts and carbon-based supports are recapped. Recognition of parasitic acid migration raises acid retention issues at high current densities. Acid loss via evaporation is estimated with respect to the acid inventory of membrane electrode assembly. Acid adsorption on platinum surfaces can be alleviated for platinum alloys and non-precious metal catalysts. Binders have been considered a key to the establishment of the triple-phase boundary, while recent development of binderless electrodes opens new avenues toward low Pt loadings. Often ignored microporous layers and water impacts are also discussed. Of special concern are durability issues including acid loss, platinum sintering and carbon corrosion, the latter being critical during start/stop cycling with mitigation measures proposed. Long-term durability has been demonstrated with a voltage degradation rate of less than 1 μV h−1 under steady-state tests at 160 °C, while challenges remain at higher temperatures, current densities or reactant stoichiometries, particularly during dynamic operation with thermal, load or start/stop cycling.

In Situ Preparation of Ionomer as a Tool for Triple‐Phase Boundary Enhancement in 3D Graphene Supported Pt Catalyst

AbstractFor improving the performance of platinum electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs), it is important to enhance the Pt utilization level in the catalyst systems. A high performing electrocatalyst (Pt/3DNG) is developed for PEMFC applications by using nitrogen‐doped 3D graphene (3DNG) as the support material and an in situ grafted active “triple‐phase boundary” to more precisely control the formation of the proton conducting ionomer interface at the active sites. Considering the 3D morphology of the system, during the electrode fabrication for realistic single‐cell evaluation, the concept of in situ generation of the proton conducting‐ionomer based “active triple‐phase boundary” is introduced, which could potentially replace the conventional method of using Nafion ionomer for the electrode preparation. The monomers owing to their small‐size can access the pores and inner regions of the 3DNG support, which on UV‐curing, undergo polymerization and transform into an ionomer with an extended interfacial network into the nanoregimes of 3DNG. Single cell evaluation of the membrane electrode assembly in a high‐temperature PEMFC by using phosphoric acid doped polybenzimidazole membrane demonstrates the utility of the present strategy.