Polybenzimidazole Research Articles

Synthesis and characterization of 4-pyridine-bridge polybenzimidazolefor proton exchange membranes

A series of polybenzimidazoles (PBIs) incorporated main chain 4-pyridine bridge groups were synthesized from 4,4′-([4,4′-bipyridine]-2,6,-diyl)bis (benzene-1,2-diamine) which reacted with four different diacids like isophthalic acid,4,4′-Oxibis benzoic acid, 5-amino isophthalic acid and 2,5-pyridine dicarboxylic acid using polyphosphoric acid as solvent. A process termed dispersion polymerizationhas been developed to prepare PBIs.For the membranepreparation, 4-pyridine-bridge polybenzimidazole (Py-PBI) productswere re-dissolved in dimethyl sulfoxide and cast. The polymer structure characterization included FT-IR, UV, Powder XRD, Water Uptake, Swelling Ratio, Ion exchange capacity, Acid doping, Acid leaching, Oxidative stability, and Polymer inherent viscosity find out by using Ubbelohde viscometer whilethermal stability assessments via thermogravimetric analysis. The Py-PBI-based polymer electrolyte membranes’ mechanical properties measurement showed that the 4-pyridine-bridge PBIs membranes were flexible, thermally stable, and mechanically strong when compared with conventional PBI. The current-voltage (I-V) characteristics of the 4-Py-PBI membrane show that the conductivity of the 4441P membrane is 0.546 S cm−1.

New nanocomposite membranes based on polybenzimidazole with improved fuel cell performance at high temperatures

In this work, proton exchange membranes based on polybenzimidazole (PBI) with incorporation of acidic Fe3O4@SiO2@RF (resorcinol–formaldehyde)–SO3H nanoparticles are produced. The effects of the core@double-shell nanoparticles on the fuel cell performance of the PBI membrane are examined. The obtained results demonstrate that the proton conductivity of the PBI-Fe3O4@SiO2@RF–SO3H nanocomposite membranes increases. The interactions of Fe3O4@SiO2@RF–SO3H nanoparticles in the PBI matrix (which contains phosphoric acid) have strong effects on proton conductivity. The best proton conductivity of 170 mS cm−1 is obtained in the nanocomposite membrane at 180 °C. The potential for the use of these nanocomposite membranes with improved fuel cell performance in high-temperature applications is confirmed.

Optimization of loose nanofiltration performance of a polybenzimidazole hollow fiber membrane: Air gap and post-crosslinking effects

Current loose nanofiltration membrane materials are not suitable for harsh dye/salt separation environments because they are not resistant to high temperatures, acids and bases, and organic solvents. In this work, a polybenzimidazole (PBI) hollow fiber loose nanofiltration membrane (HFLNM) was prepared by dry-wet spinning and used for dye and salt separation. The membrane showed good heat resistance (25–70 ℃), acid resistance (immerse in10 mol/L HCl solution for 100 h) and solvent resistance (immerse in EtOH, MeOH and IPA for 120 h, respectively). The effects of air gap distance on the membrane structure and properties as well as the effect of cross-linking using K2S2O8 on the solvent resistance of the membrane were investigated. By adjusting the air gap distance to 6 cm, the dense layer of the optimized membrane got thinner. With the help of double layer finger-like pore and strong negative charge, the membrane exhibited outstanding water flux (64.5 L·m−2·h−1), low NaCl salt rejection (∼ 7%), and excellent Congo Red (CR) dye rejection (100%) at the dye/salt mixture solution (100 mg/L Congo red, 1 g/L NaCl) and operating pressure of 0.1 MPa. Moreover, the crosslinked PBI membrane showed good stability after 72 h immersion in polar aprotic solvents (NMP, DMF, and DMAc). Hence, PBI HFLNM has a good application prospect in the field of dye/salt separation in harsh environments.

A Comparative Study of CCM and CCS Membrane Electrode Assemblies for High-Temperature Proton Exchange Membrane Fuel Cells with a CsH5(PO4)2-Doped Polybenzimidazole Membrane.

Membrane electrode assemblies (MEAs) are critical components in influencing the electrochemical performance of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). MEA manufacturing processes are mainly divided into the catalyst-coated membrane (CCM) and the catalyst-coated substrate (CCS) methods. For conventional HT-PEMFCs based on phosphoric acid-doped polybenzimidazole (PBI) membranes, the wetting surface and extreme swelling of the PA-doped PBI membranes make the CCM method difficult to apply to the fabrication of MEAs. In this study, by taking advantage of the dry surface and low swelling of a CsH5(PO4)2-doped PBI membrane, an MEA fabricated by the CCM method was compared with an MEA made by the CCS method. Under each temperature condition, the peak power density of the CCM-MEA was higher than that of the CCS-MEA. Furthermore, under humidified gas conditions, an enhancement in the peak power densities was observed for both MEAs, which was attributed to the increase in the conductivity of the electrolyte membrane. The CCM-MEA exhibited a peak power density of 647 mW cm-2 at 200 °C, which was ~16% higher than that of the CCS-MEA. Electrochemical impedance spectroscopy results showed that the CCM-MEA had lower ohmic resistance, which implied that it had better contact between the membrane and catalyst layer.

In situ etching strategy for the preparation of high-temperature proton-exchange membranes with continuous porous proton-transport channels

Phosphoric acid (PA)-doped polybenzimidazole (PBI) is widely used in high-temperature (HT) proton-exchange membrane fuel cells (PEMFCs). The efficiency of proton conduction is intricately linked to the performance of cell. Thus, constructing a continuous proton-transport channel in the PBI matrix is an effective approach to enhance the performance of PA-doped PBI. In this study, for the first time, a segmented block copolyamide with a microphase-separated structure was used as a template to fabricate a range of HT-proton-exchange membranes (PEMs) with continuous porous proton-transport channels using an in situ etching strategy, which used polyamide as a sacrificial template hydrolyzed under high-temperature PA conditions. The HT-PEMs with continuous porous proton-transport channels exhibit excellent cell performances. The membranes with 30% segmented block copolyamides possess an ultra-high PA doping of 420.7% and high in-plane conductivity of 117 mS cm−1 at 160 °C without humidification. Therefore, the resulting membrane has an excellent cell performance at 160 °C in H2/O2 (680.02 mW cm−2), which is considerably higher than that of the OPBI membranes under the same conditions. The results indicate that continuous porous proton-transport channels can comprehensively improve the performance of membranes. This efficient, simple, and environment-friendly in situ etching strategy is expected to become a routine method for preparing PA-doped HT-PEMs.

NMR investigation of proton transport in polybenzimidazole/polyphosphoric acid membranes prepared via novel synthesis route

Here we present a molecular-level view of the structural changes in polybenzimidazole (PBI) membranes doped with phosphoric acid (PA) generated by a novel membrane fabrication technique. The modified PA doped membranes displayed unprecedented ionic conductivity at elevated temperatures, comparable to the starting PBI gel membranes prepared by the commonly used PPA process, while also exhibiting enhanced mechanical properties. To elucidate the cause of these effects, we used multi-nuclear (1H, 13C, 31P) 1D PFG, MAS and CPMAS, and 2D HETCOR magnetic resonance (NMR) techniques to characterize the structure and dynamics of the modified film and the original gel PBI membranes. 1H diffusivity measurements show significantly enhanced proton diffusivity, both in magnitude and in lower activation energy, which were consistent with its high ionic conductivity despite the lower PA content compared to the original gel membrane. CPMAS experiments further substantiated that the location of these distinct phosphate environments as being close to the polymer network. Finally, the phosphate groups in the modified membrane were revealed to be strongly bounded to each other and to the PBI polymer backbone in contrast to the combination of weakly and strongly bounded groups of the original gel membrane.

Reactive end-group polybenzimidazoles for high temperature thermosets

Polybenzimidazoles (PBI) are heterocyclic polymers that possess high thermal and oxidative stability. The inherent strength and chemical and thermal stability of PBI make it ideal for use in extreme environments, however, the current processing of PBI molded parts makes the process practically and economically unfavorable. This work explores the improvement of PBI processability using a reactive oligomer approach. An easily processable PBI resin was synthesized, characterized, and tested in molding studies. The resin consists of PBI oligomers containing the p,p-OPBI chemistry (-O- linkage) with reactive end groups that crosslink at high temperature. The use of oligomers decreases the melt processing temperature of PBI, allowing for processability in less extreme conditions. The addition of reactive end groups allows for crosslinking during a post molding thermal cure, which imparts the high mechanical properties associated with PBI. The p,p-OPBI molded and cured articles exhibited tensile strengths of greater than 60 MPa, decreased water uptake and possessed high chemical and thermal stability.

Soluble polybenzimidazoles incorporating Tröger’s base for high-temperature proton exchange membrane fuel cells

In this work, a novel dicarboxylic acid (TB–COOH) containing Tröger’s base (TB) was synthesized, and newly TB-based polybenzimidazole (PBI) membranes were prepared as electrolytes for high-temperature proton exchange membrane (HT-PEM) fuel cells. The TB-based PBIs, upon treatment with triethylammonium salts, showed good processability. The as-prepared TB-based PBI membranes exhibit superior tensile strength above 126.6 MPa and high decomposition temperatures at 5% weight loss ranging from 374 °C to 411 °C. The residual weights of the TB-based PBI membranes remained at above 80% in Fenton’s reagent at 80 °C for 48 h. Notably, with extra nitrogen atoms in the TB-based PBIs, their phosphoric acid uptakes show obvious improvement over the corresponding sulfonated polybenzimidazole (sPBI) without TB units, achieving the highest proton conductivity (93.2 mS cm−1) at 160 °C, 2.3 times that of 41.2 mS cm−1 from the sulfonated PBI (sPBI) membrane. The highest peak power density of the H2/air fuel cell reached 225.5 mW cm−2 under non-humidity conditions at 160 °C, much higher than the value of the sPBI membrane. Based on their excellent comprehensive performance, incorporating TB into chain backbones optimized the PBI electrolytes for HT-PEM applications.

Thermal-Tolerant CdPS3-Polybenzimidazole Membranes with High Proton Conductivity

Thermal-tolerant proton-exchange membranes are highly demanded for updating hydrogen–electricity conversion technologies, since the high-temperature operation is expected to suppress the catalyst poisoning of fuel cells or to increase the efficiency of water (steam) electrolysis. But it is still challenged by both stability and conductivity. Herein, two-dimensional CdPS3 nanosheets with atomically in-plane defects were synthesized and employed for the preparation of proton-conductive and thermal-tolerant membranes with polybenzimidazole (PBI). Ultrathin and Cd vacancy-containing CdPS3 two-dimensional (2D) nanolayers were successfully obtained from bulk CdPS3 crystals by ion treatments. The resultant CdPS-PBI hybrid membranes exhibit reliable long-term conductivity stability for 170 h at 180 °C and a maximum proton conductivity of 539 mS cm–1 at 200 °C, which is 1 order of magnitude higher than PBI membranes and the reported polymeric membranes. The proton-transport performance over hybrid membranes was ascribed to abundant proton donor centers, easy proton desorption, and excellent hydration of Cd vacancies on CdPS nanolayers. Therefore, the CdPS-PBI membranes with well-proven chemical/mechanical/thermal stabilities and high proton conductivity would meet the requirements of proton-exchange membranes (PEMs) for high-temperature operations.

Study of Innovative GO/PBI Composites as Possible Proton Conducting Membranes for Electrochemical Devices.

The appeal of combining polybenzimidazole (PBI) and graphene oxide (GO) for the manufacturing of membranes is increasingly growing, due to their versatility. Nevertheless, GO has always been used only as a filler in the PBI matrix. In such context, this work proposes the design of a simple, safe, and reproducible procedure to prepare self-assembling GO/PBI composite membranes characterized by GO-to-PBI (X:Y) mass ratios of 1:3, 1:2, 1:1, 2:1, and 3:1. SEM and XRD suggested a homogenous reciprocal dispersion of GO and PBI, which established an alternated stacked structure by mutual π-π interactions among the benzimidazole rings of PBI and the aromatic domains of GO. TGA indicated a remarkable thermal stability of the composites. From mechanical tests, improved tensile strengths but worsened maximum strains were observed with respect to pure PBI. The preliminary evaluation of the suitability of the GO/PBI X:Y composites as proton exchange membranes was executed via IEC determination and EIS. GO/PBI 2:1 (IEC: 0.42 meq g-1; proton conductivity at 100 °C: 0.0464 S cm-1) and GO/PBI 3:1 (IEC: 0.80 meq g-1; proton conductivity at 100 °C: 0.0451 S cm-1) provided equivalent or superior performances with respect to similar PBI-based state-of-the-art materials.

Polybenzimidazole-reinforced polyethylene oxide-based polymer-in-salt electrolytes enabling excellent structural stability and superior electrochemical performance for lithium metal batteries

Polyethylene oxide (PEO)-based solid polymer electrolyte (SPEs) is full of attraction due to its exceptional lithium ion dissolubility and strong resistance toward lithium metal reduction. Nevertheless, it still suffers from the unfulfilling room temperature ionic conductivity and poor mechanical properties. Herein, a flexible and robust polymer-in-salt electrolyte based on composite of polybenzimidazole (PBI) and PEO via a facile preparation approach has been developed for room-temperature lithium metal batteries (LMBs). The rigid framework structure of PBI and dense hydrogen bonds formed between PBI and PEO jointly reinforce the structural stability of SPEs at high lithium salt concentration, which makes the SPEs possess sufficient mechanical properties to resist the lithium dendrites growth. The thermal stability and fire resistance of SPEs have also been improved relying on the distinctive flame retardancy and incombustibility of PBI. Besides, PBI is prone to forming hydrogen bonds with lithium salt anions to limit their mobility, while the rich N atoms on PBI chains can promote the dissociation of lithium salt through electrostatic attraction interaction. Their synergistic effects make the PEO-based polymer-in-salt electrolytes achieve the high room temperature ionic conductivity of 5.7 × 10−4 S cm−1, wide electrochemical window of 4.45 V, and large lithium transference number of 0.639. When applied to LMBs with LiFePO4 or LiNi0.88Co0.06Mn0.06O2 cathode, both coin cells exhibit improved cycling performance and rate capability. Furthermore, the LiNi0.88Co0.06Mn0.06O2/SiOx-C pouch cells using the SPEs demonstrate remarkable flexibility and safety, clarifying that the PEO-based polymer-in-salt electrolytes possess a promising application prospect for safe and high-performance LMBs.

Fabrication and performance evaluation of graphene-supported PtRu electrocatalyst for high-temperature electrochemical hydrogen purification

The main aim of this study is to investigate the high-temperature electrochemical hydrogen purification (HT-ECHP) performances of graphene nanoplatelet (GNP) support material decorated with platinum (Pt) and platinum-ruthenium (PtRu) nanoparticles prepared by microwave irradiation technique. Prepared catalysts coupled to the phosphoric acid doped polybenzimidazole (PBI) membrane for HT-ECHP application. The structural and electrochemical properties of the catalysts were examined by thermogravimetric analysis (TGA), X-Ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Transition electron microscopy (TEM) and cyclic voltammetry (CV) analyses. The characterization results indicate that the catalysts provided the necessary properties for HT-ECHP application. The HT-ECHP performances are investigated with reformate gas mixture containing hydrogen (H2), carbon dioxide (CO2) and carbon monoxide (CO) in the range of 140–180 °C. The results show that the electrochemical purification performances of the catalysts increase with increasing operating temperature. The highest H2 purification performance is obtained with PtRu/GNP catalyst. The high electrochemical H2 purification performance of the PtRu/GNP catalyst can be attributed to the strong synergistic interactions between Pt and Ru particles decorated on the GNP. These results advocate that the PtRu/GNP catalyst is a hopeful catalyst for HT-ECHP application.

Polybenzimidazole membrane based aqueous redox flow batteries using anthraquinone-2,7-disulfonic acid and vanadium as redox couple

New cost-effective aqueous redox flow batteries (ARFBs) using anthraquinone-2,7-disulfonic acid (2,7-AQDS) and vanadium oxide sulfate (VOSO4) as anolyte and catholyte are suggested with polybenzimidazole (PBI) membranes replacing the conventionally used Nafion membranes. To evaluate the conduction mechanism of PBI and Nafion membranes, the permeability of protons and hydrogen sulfate ions is measured. Permeability for charge balancing ions is 2100 (proton, Nafion), 0.78 (hydrogen sulfate, Nafion), 180 (proton, PBI) and 2 (hydrogen sulfate, PBI) 10−14 m2 s−1, indicating that PBI has much lower cation/anion selectivity than Nafion. The low permeability of 2,7-AQDS and VOSO4 in PBI membrane ensures the stable discharge capacity of ARFBs. ARFBs using 15, 30 and 45 μm thick PBI membranes show the capacity fade rate of less than 3% even after 100 cycles. Conclusively, 30 μm thick PBI is optimal because this exhibits the balanced ohmic resistance between membrane thickness and has few voids between electrodes and membrane. When ARFB using 30 μm thick PBI membrane is operated, this shows excellent performances, such as energy efficiency of 70.1% and discharge capacity of 10.6 Ah L−1 during 100 cycles.

Effect of the annealing temperature of polybenzimidazole membranes in high pressure and high temperature H2/CO2 gas separations

The effect of the annealing temperature of polybenzimidazole (PBI) membranes on H2/CO2 gas separations was investigated. Membranes annealed from 250 °C to 400 °C were tested for gas permeation with pure H2, CO2, and N2 gases and a H2:CO2 (1:1) mixture at 35 °C, 100 °C, 200 °C, and 300 °C and at pressures up to 45 bar. Gas permeation data show that permeability and selectivity of the membranes is significantly impacted by the annealing temperature, the presence of adsorbed water, and remaining casting solvent (DMAc). At a testing temperature of 35 °C, ideal H2/CO2 selectivities of 50, 49, and 66 with pure H2 permeabilities of 1.5, 0.8, and 1.5 Barrer were obtained for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. At this temperature, high gas mixture H2/CO2 selectivities of 41, 73, and 47 with H2 permeabilities of 1.03, 0.26, and 0.50 Barrer were also obtained for these membranes. At testing temperatures of 300 °C, both the ideal and gas mixture H2/CO2 selectivities dropped to 44, 28, and 30 (ideal, H2 = 45, 45, 44 Barrer) and to 19, 22, and 23 (mixture, H2 = 41, 43, and 44 Barrer) for membranes annealed at 250 °C, 300 °C, and 400 °C, respectively. As water was removed from the membranes at temperatures greater than 100 °C during permeation cycles, where the testing temperature was increased from 35 °C to 300 °C, the permselectivity properties of the membranes annealed at 400 °C became more reproducible. Permeabilities at 35 °C from a second permeability cycle increased, but H2/CO2 selectivities decreased to 21 for gas mixtures (H2 = 1.4 Barrer) and to 34 for pure gases (H2 = 2.2 Barrer). The results suggest that high annealing temperatures may induce changes in the configuration and conformation of the polymer chains, imparting distinctive permselectivity properties to the membranes. Activation energies of permeability for H2, CO2, and N2 from pure gases and H2:CO2 mixtures correlated with these changes as well.

Aliphatic Polybenzimidazoles: Synthesis, Characterization and High-Temperature Shape-Memory Performance

A series of aliphatic polybenzimidazoles (PBIs) with methylene groups of varying length were synthesized by the high-temperature polycondensation of 3,3'-diaminobenzidine (DAB) and the corresponding aliphatic dicarboxylic acid in Eaton's reagent. The influence of the length of the methylene chain on PBIs' properties was investigated by solution viscometry, thermogravimetric analysis, mechanical testing and dynamic mechanical analysis. All PBIs exhibited high mechanical strength (up to 129.3 ± 7.1 MPa), glass transition temperature (≥200 °C) and thermal decomposition temperature (≥460 °C). Moreover, all of the synthesized aliphatic PBIs possess a shape-memory effect, which is a result of the presence of soft aliphatic segments and rigid bis-benzimidazole groups in the macromolecules, as well as strong intermolecular hydrogen bonds that serve as non-covalent crosslinks. Among the studied polymers, the PBI based on DAB and dodecanedioic acid has high adequate mechanical and thermal properties and demonstrates the highest shape-fixity ratio and shape-recovery ratio of 99.6% and 95.6%, respectively. Because of these properties, aliphatic PBIs have great potential to be used as high-temperature materials for application in different high-tech fields, including the aerospace industry and structural component industries.

High Purity Hydrogen Separation with HT-PBI Based Electrochemical Pump Operation at 120 °C

Electrochemical Hydrogen Pumps (EHP) provide a unique highly efficient means of separating and compressing hydrogen with continuous steady-state operation. In this paper, we demonstrate the performance of a commercially available, polybenzimidazole (PBI) membrane based platform as a benchmark for ultra-high efficiency performance. A primary gas mixture of CO2 and H2 with a ratio of 4:1 respectively was selected to demonstrate the performance of EHPs with near theoretical Faradaic efficiency with negligible CO poisoning due to reverse water gas shift reaction (RWGS). It was found that humidification of the feed gas at room temperature improved polarization performance while also improving energy efficiency, thus reducing the need for a tightly controlled relative humidity of feed gas. A new perspective on EHP energy efficiency calculation methodology is also provided by including the cell heating requirement in the calculation. In this manner, an overall improvement to the energy efficiency of nearly 20% was realized by dropping the cell temperature to 120 °C while paying no significant penalty to electrochemical performance. Nearly 99.99% pure H2 and 99.93% pure CO2 were produced with a hydrogen yield of 99.34%.

Influence of the PBI structure on PBI/CsH5(PO4)2 membrane performance for HT-PEMFC application

Conventional high-temperature proton exchange membranes use phosphoric acid (PA) as a proton conductor. However, PA loss, poor mechanical stability, and catalyst poisoning represent major limitations. Solid acids are an alternative to PA that does not require humidification and is less prone to leaching. In this study, we prepared two composite membranes by high-temperature impregnation to investigate the effects of the structure of polybenzimidazole (PBI) on the anchoring of solid acids: a laboratory-synthesized m-PBI/CsH5(PO4)2 and the commercial OPBI/CsH5(PO4)2. Compared to PA-doped membranes, CsH5(PO4)2-doped membranes showed excellent mechanical properties, particularly m-PBI/CsH5(PO4)2. Scanning electron microscopy and energy dispersive spectroscopy images showed that CsH5(PO4)2 was uniformly dispersed on the surface and cross-section of both membranes, conducive to the formation of proton transport channels following the CsH5(PO4)2 melting in both samples. The characterization of the physical structures proved the interaction between the imidazole ring and solid acid. Furthermore, the proton conductivity without humidification was similar in m-PBI/CsH5(PO4)2 and OPBI/CsH5(PO4)2, demonstrating the suitability of m-PBI for fuel cells production. To test this, we assembled a m-PBI/CsH5(PO4)2 cell that showed competitive performances, with a peak power density of 361.28 mW cm−2 at 160 °C in H2/O2 atmosphere.

Finely tuning the microporosity in phosphoric acid doped triptycene-containing polybenzimidazole membranes for highly permselective helium and hydrogen recovery

High-performance polymer membranes with well-defined microporosity and size-sieving ability are especially attractive for helium and hydrogen recovery. Here we report novel macromolecular engineering of polybenzimidazole (PBI) membranes that integrate hierarchical triptycene units for high permeability and polyprotic acid doping for size sieving via controllable manipulation of microporous architecture. The triptycene moieties disrupt chain packing and introduce additional configurational free volume, leading to significantly boosted He and H2 permeabilities compared to previously reported PBI membranes. The acid doping resulted in crosslinked PBI membranes via hydrogen bonding and proton transfer with dramatically enhanced gas selectivities. Via adjusting the H3PO4-doping level, triptycene-based polybenzimidazole (TPBI) composite membranes (TPBI-(H3PO4)x) exhibit the highest gas selectivities for He enrichment (i.e., α(He/CH4) = 7052 ± 156) and H2 purification (i.e., α(H2/CH4) = 5128 ± 110) among existing polymeric gas separation membranes. Additionally, under mixed-gas conditions at 150 °C, the TPBI-(H3PO4)0.98 membrane displays a H2 permeability of 46.7 Barrer and a H2/CO2 selectivity of 16, far beyond the Robeson's 2008 upper bound for H2/CO2 separation. The facile and diverse tunability and excellent gas separation performance make TPBI-(H3PO4)x membranes highly attractive for helium and hydrogen separation.

Kinetics of Oxygen Reduction Reaction of Polymer-Coated MWCNT-Supported Pt-Based Electrocatalysts for High-Temperature PEM Fuel Cell

Sluggish oxygen reduction reaction (ORR) of electrodes is one of the main challenges in fuel cell systems. This study explored the kinetics of the ORR reaction mechanism, which enables us to understand clearly the electrochemical activity of the electrode. In this research, electrocatalysts were synthesized from platinum (Pt) catalyst with multi-walled carbon nanotubes (MWCNTs) coated by three polymers (polybenzimidazole (PBI), sulfonated tetrafluoroethylene (Nafion), and polytetrafluoroethylene (PTFE)) as the supporting materials by the polyol method while hexachloroplatinic acid (H2PtCl6) was used as a catalyst precursor. The oxygen reduction current of the synthesized electrocatalysts increased that endorsed by linear sweep voltammetry (LSV) curves while increasing the rotation rates of the disk electrode. Additionally, MWCNT-PBI-Pt was attributed to the maximum oxygen reduction current densities at −1.45 mA/cm2 while the minimum oxygen reduction current densities of MWCNT-Pt were obtained at −0.96 mAcm2. However, the ring current densities increased steadily from potential 0.6 V to 0.0 V due to their encounter with the hydrogen peroxide species generated by the oxygen reduction reactions. The kinetic limiting current densities (JK) increased gradually with the applied potential from 1.0 V to 0.0 V. It recommends that the ORR consists of a single step that refers to the first-order reaction. In addition, modified MWCNT-supported Pt electrocatalysts exhibited high electrochemically active surface areas (ECSA) at 24.31 m2/g of MWCNT-PBI-Pt, 22.48 m2/g of MWCNT-Nafion-Pt, and 20.85 m2/g of MWCNT-PTFE-Pt, compared to pristine MWCNT-Pt (17.66 m2/g). Therefore, it can be concluded that the additional ionomer phase conducting the ionic species to oxygen reduction in the catalyst layer could be favorable for the ORR reaction.

Effect of cooling surface temperature difference on the performance of high-temperature PEMFCs

Internal temperature distribution of the high-temperature proton exchange membrane fuel cell (HT-PEMFC) is affected by the cooling temperature, heat generation and reactant gas flow. Reasonable temperature control is helpful to improve the fuel cell performance and durability. In this work, a three-dimensional model that couples the reactant flows, species transport, heat transfer, charge transfer, and electrochemical reaction, was developed to simulate the HT-PEMFC operation. A solid mechanics model was established to analyze the stress distribution of the fuel cell. The polarization curves, distributions of temperature, membrane proton conductivity, current density and stress are investigated for different cooling surface temperature. Furthermore, the effect of assembly temperature on the stress of phosphoric acid-doped polybenzimidazole (PBI) membrane is discussed. Results reveals that the peak power density and uniformity of current density decrease with the increase of cooling surface temperature difference. The peak power density decreases by 9.14% when the temperature difference increases from 0 K to 40 K. The cooling surface temperature difference of less than 10 K and the voltage range in 0.5–0.7 V can achieve better current density uniformity and smaller current density change rate. In addition, the membrane in fuel cell has the highest stress, and increasing the assembly temperature is helpful to reduce the membrane stress. When the assembly temperature increases from 293.15 K to 343.15 K, the max and min compressive stresses in membrane in-plane decrease from 39.436 MPa to 31.416 MPa–24.934 MPa and 17.369 MPa at the temperature difference of 30 K, which decreases by 36.77% and 44.7%, respectively.