Proton transport of porous triazole-grafted polysulfone membranes for high temperature polymer electrolyte membrane fuel cell

Introduction of porous structure to high temperature polymer electrolyte membranes is one of effective pathways to increase their proton conductivity under elevated temperature. However, the effect of the porous structure on the proton diffusion mechanism of these membranes is still unclear. In this work, the proton transport behaviour of a series of porous triazole-polysulfone (PSf) membranes under elevated temperature is comprehensively investigated. The functional triazole ring in the framework of porous triazole-PSf acts a proton acceptor to form acid-base pair with phosphoric acid (PA). In addition, the proton diffusion coefficient and proton conductivity of PA-doped porous triazole-PSf is an order of magnitude higher than that of the PA-doped dense triazole-PSf membrane. Percolation theory calculation convinces that the high proton conductivity of PA-doped porous triazole-PSf is due to the formation of continuous long-range proton diffusion channels under high pore connectivity and porosity. On the contrary, excessive pore connectivity also results in high gas permeability, leading to decrease of the open circuit voltage and cell performance of the single cell. Consequently, the optimum porosity for the PA-doped porous triazole-PSf membrane is 75% for fuel cell operating with the maximum peak power density of 550 mW·cm^?2 and great durability for 120 h under 140 °C.     

Membrane electrode assemblies doped with H3PO4 for high temperature proton exchange membrane fuel cells

H₃PO₄ content plays a critical role in high temperature proton exchange membrane fuel cells (HT-PEMFC), as it is responsible for the majority of the conductivity of the key components under high temperature operation. The conductivities of commercial AB-PBI membranes doped by immersing in 85 wt.% H₃PO₄ for different times and temperatures are investigated. The effect of H₃PO₄ loading in electrodes, including the AB-PBI polymer and a Pt/C catalyst, is also studied. The as-prepared electrodes and membranes are combined to fabricate a membrane electrode assembly for HT-PEMFCs. The results reveal that AB-PBI membranes doped with 85 wt.% H₃PO₄ at 90 °C for 9 h display a maximum conductivity of 33 mS cm⁻¹. This membrane was selected and combined with electrodes including 15 wt.% AB-PBI and 0.75 mg cm⁻² Pt with different H₃PO₄ loadings. A maximum current density of 260 mA cm⁻² was achieved in the as-prepared MEA (with 5 mg cm⁻² H₃PO₄ in electrodes) operating at 0.6 V and 160 °C, using oxygen and hydrogen.     

Parameter study of high-temperature proton exchange membrane fuel cell using data-driven models

In this paper, a numerical model of high-temperature proton exchange membrane fuel cell (HT-PEMFC) was developed, in which the thermal and electrical properties were treated as temperature dependent. Based on the numerical simulation, the needed training data was acquired and used for the development of data-driven model via the artificial neural network (ANN) algorithm. The developed data-driven model was then used to predict the performance of HT-PEMFC. The simulation results indicated that the deviation of ANN prediction was less than 2.48% compared with numerical simulation. The effects of various influential factors, such as the geometry size of the gas flow channel, the thickness of the membrane and the operating temperature, could be predicted easily by using the ANN model. The ANN model prediction results showed that the more compact fuel cell and the higher operating temperature improved the performance of HT-PEMFC. The proposed ANN model and the parameters study will contribute to the further design and operation of HT-PEMFC.     

Integrated and inherently safe design and operation of a mobile power generation: Process intensification through microreactor reformer and HT-PEMFC

In the present study, due to various advantages of process miniaturization, the integrated design and operation of a mobile power generation system consisting of a microreactor reformer and a proton exchange membrane fuel cell (PEMFC) are investigated. The hydrogen fuel is supplied through autothermal steam-reforming of methanol in a microreactor leading to a safer, as well as more efficient, and economical operation. A high-temperature polymer membrane fuel cell with external accessories is applied for power generation. Then, simulation-optimization programming is applied for simultaneously optimizing the process design and operation at the same level. The result shows that the implemented procedure ensures the economic and flexible operation of the process while satisfying the safety constraints. The maximum gross power and net power generation are 109.3 and 91.9, respectively, while the cost of hydrogen production reduces from 13 to 20 $/kg to 7.7 $/kg. The fuel (methanol) consumption can be as low as 0.42 L/kWh.     

The Pt–Co alloying effect on the performance and stability of high temperature PEMFC cathodes

High temperature (HT) PEMFC technology offers a number of advantages over its low temperature counterpart, including high tolerance to fuel impurities, system simplifications and generation of high-quality waste heat. Nevertheless, the operating temperature and the presence of phosphoric acid necessitate the use of high amounts of platinum metal at the electrodes, especially the cathodic one. In this work, we report the facile preparation of a Pt–Co alloy supported on multi-wall carbon nanotubes. Using the rotating disk electrode method in HClO₄, the activity of this catalyst towards the oxygen reduction reaction, as well as its tolerance to phosphoric acid is evaluated. A comparison is made with two electrocatalysts with similar characteristics (support, nanoparticle size and spatial distribution), where one is based on Pt and the other is a physical mixture of the aforementioned metals (Pt and Co). The superior behavior of the alloyed electrocatalyst urged its electrochemical characterization in-situ, at the cathode of a HT-PEMFC, where performance and, very importantly, stability are thoroughly evaluated and discussed. A comparison with a commercial state-of-the-art electrocatalyst shows the potential to decrease the metal loading of HT-PEMFC electrodes without compromising performance.     

Maximizing the efficiency of a HT-PEMFC system integrated with glycerol reformer

The efficiency and output power density of an integrated high temperature polymer electrolyte fuel cell system and glycerol reformer are studied. The effects of reformer temperature, steam to carbon ratio (S/C), fuel cell temperature, and anode stoichiometric ratio are examined. An increase in anode stoichiometric ratio will reduce CO poisoning effect at cell’s anode but cause lower fuel utilization towards energy generation. High S/C operation requires large amount of the energy available, however, it will increase anode tolerance to CO poisoning and therefore will lead to enhanced cell performance. Consequently, the optimum gas composition and flow rate is very dependent on cell operating current density and temperature. For example, at low current densities, similar efficiencies were obtained for all the S/C ratio studied range at cell temperature of 423.15 K, however, at cell temperature of 448.15 K, low S/C ratio provided higher efficiency in comparison to high S/C ratio. High S/C is essential when operating the cells at high current densities where CO has considerable impact on cell performance. Optimal conditions that provide the maximum power density at a given efficiency are reported.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

Polybenzimidazole containing ether units as electrolyte for high temperature proton exchange membrane fuel cells

A rapid method to synthesize poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI) through a solution polycondensation under microwave irradiation is explored. Synthesis parameters affecting the molecular weight (M_w) of OPBI, including the mass ratio of solvent to P₂O₅, the monomer concentration, and reaction time, are optimized. The main characteristics of OPBI are studied, and the corresponding membrane is prepared through a solvent casting process. A series of sulfuric acid doped OPBI (H₂SO₄/OPBI) hybrid membranes with different acid doping levels (ADLs) are developed. The effects of H₂SO₄ on microstructure, ADL and electrochemical properties of these membranes are explored. Herein, the hybrid membrane shows high proton conductivity (190 mS cm⁻¹) at elevated temperature (160 °C) and anhydrous conditions, high ADL (18.73 mol of H₂SO₄ for OPBI per repeat unit, i.e., ADL = 18.73 mol PRU⁻¹) and excellent dimensional stability (40.3%). All these properties demonstrated that H₂SO₄/OPBI hybrid membrane can be used as an alternative membrane for high temperature proton exchange membrane fuel cells (HT-PEMFCs).     

Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells

Graphite oxide/polybenzimidazole synthesized by 3, 3′-diaminobenzidine and 5-tert-butyl isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI) composite membranes were prepared for high temperature polymer proton exchange membrane fuel cells (PEMFCs). All membranes were loaded with different content of phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the chemical stability of BuIPBI membrane and support a higher acid content. The proton conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and 0.027 S/cm, respectively, at 140 °C and without humidity.     

Utilizing coke oven gases as a fuel for a cogeneration system based on high temperature proton exchange membrane fuel cell; energy,exergy and economic assessment

Based on a high temperature proton exchange membrane fuel cell (HT-PEMFC), a cogeneration system is proposed to produce heat and power. The system includes a coke oven gas steam reformer, a water gas shift reactor, and an afterburner. The system is analyzed in detail considering the energy, exergy and economic viewpoints. The analyses reveal the importance of HT-PEMFC in the system and according to the results, 9.03 kW power is generated with energy and exergy efficiencies of 88.2% and 26.2%, respectively and the total product unit cost is calculated as 91.8 $/GJ. Through a parametric study the effects on system performance are studied of such variables as the current density, fuel cell and reformer operating temperatures, and cathode stoichiometric ratio. It is found that an increase in the fuel cell temperature and/or a decrease in the reformer temperature enhance the exergy efficiency. The exergy efficiency is also maximized at the cathode stoichiometric ratio of 2.4. By performing a two-objective optimization using genetic algorithm, the best operating point is determined at which the exergy efficiency is (32.86%) and the total product unit cost is (78.68 $/GJ).