High Temperature Polymer Electrolyte Membrane Research Articles

Poly(arylene piperidine)s with phosphoric acid doping as high temperature polymer electrolyte membrane for durable, high-performance fuel cells

The properties of high temperature polymer electrolyte membranes (HT-PEMs) are determined directly by the polymer matrix with phosphoric acid (PA) doping. Here, new polymer materials based on poly (arylene piperidine)s (PAPs) are prepared by one-step polymerization as PA absorbing matrix for HT-PEM fuel cells (HT-PEMFCs) application. The PAPs show excellent thermal stability and high glass transition temperature. Among PAPs, PA doped poly (N-methyl-piperidine-co-p-terphenyl) (PPT/PA) shows a high proton conductivity of 96.0 mS cm−1 at 180 °C without humidification and an excellent tensile strength of 12 MPa, which is much better than that of PA doped commercial poly [2,2’-(p-oxydiphenylene)-5,5′-benzimidazole] (OPBI/PA). The enhancement on conductivity of PPT/PA is attributed to the formation of micro-phase separation. A single cell based on PPT/PA membrane presents outstanding performance with a maximum power density of 1220.2 mW cm−2 with H2/O2 feeding under 0.15 MPa backpressure in cathode at 180 °C, which is 1.85 times of that with OPBI/PA membrane (660.8 mW cm−2). Furthermore, a H2/air single cell with PPT/PA shows excellent stability at a constant current density of 120 mAcm−2 and 150 °C for 1600 h. This work indicates PAPs (especially, PPT) are a promising PA doping polymer matrix for high temperature polymer electrolyte membrane fuel cells application.

α-ZrP Nanoreinforcement Overcomes the Trade-Off between Phosphoric Acid Dopability and Thermomechanical Properties: Nanocomposite HTPEM with Stable Fuel Cell Performance.

In recent times, high-temperature polymer electrolyte membranes (HTPEMs) have emerged as viable alternatives to the Nafion-based low-temperature-operated polymer electrolyte membrane fuel cells. This is owing to their higher tolerance to fuel impurities, efficient water management, and higher cathode kinetics. However, the most efficacious HTPEMs such as poly(benzimidazole) (PBI) or 2,5-poly(benzimidazole) (ABPBI), which rely on the extent of phosphoric acid (PA) doping level for fuel cell performance, suffer from poor mechanical properties at higher acid doping levels and dopant leaching during continuous operation. To overcome these issues, we report the synthesis of ABPBI membranes and fabrication of ABPBI-zirconium pyrophosphate (α-ZrP)-based nanocomposite membranes by an ex situ methodology using methane sulfonic acid as the solvent. The incorporation of hydrophilic α-ZrP into the membrane resulted in higher dopability of PA (6.5 mol) and proton conductivity (46 mS/cm) of the membranes (10 wt % of α-ZrP) as against the corresponding values of 3.6 mol and 27 mS/cm, respectively, for the pristine membrane. More remarkably, these property improvements could be achieved while simultaneously augmenting the thermomechanical properties and oxidative stability of the membranes. The unit-cell tests showed a marked improvement in the maximum power density for the nanocomposite membrane (335 mW/cm2 at 10 wt % α-ZrP content) over the pristine ABPBI membrane (200 mW/cm2). We also report for the first time the feasibility of a 100 W HTPEM fuel cell (HTPEMFC) stack operated with the nanocomposite membrane with an active area of 39 cm2. The HTPEMFC stack delivered a stable voltage and power output, with a voltage drop rate of 0.84 μV/h over a run time of 730 h.

High Temperature Polymer Electrolyte Membrane Achieved By Grafting Poly(1-vinylimidazole) on Polysulfone for Fuel Cell Applications

High temperature polymer electrolyte membranes with phosphoric acid doping are crucial materials of high temperature fuel cells. However, the evolution of the type membrane is suffering from the trade-off between proton conductivity and mechanical strength, because high proton conductivity requires high phosphoric acid doping level, which will reduce the mechanical properties due to the plasticization of phosphoric acid on polymer backbone. Here, a new strategy is employed to respond to the unresolved challenges by grafting poly(1-vinylimidazole) as phosphoric acid doping sites on polysulfone backbone via atom transfer radical polymerization. Then poly(1-vinylimidazole) grafted polysulfone membrane with different length of side chains is obtained and soaked in phosphoric acid to work as high temperature polymer electrolyte membrane. The tapping-mode AFM images and 1-D SAXS result of PA doped membrane suggest the formation of micro-phase separated structures, and the ionomer peaks shift from 1.34 nm-1 to 1.19 nm-1 with increasing the length of side chain, corresponding to d-spacing of 4.67 nm and 5.28 nm, respectively. Therefore, the prepared phosphoric acid doped membranes possess excellent proton conductivity of 127 mS cm-1 under 160 °C due to the structure, while mechanical properties are retained due to the reduction of plasticizing effect caused by the separation of phosphoric acid adsorption sites and polymer backbone, presenting an outstanding tensile strength of 7.94 MPa. Meanwhile, the impressive fuel cell performance with the membranes is gained, reaching a maximum peak power density of 559 mW cm-2 under 160 oC. More importantly, the work provides a new sight to solve the trade-off between proton conductivity and mechanical strength for phosphoric acid doped high temperature polymer electrolyte membranes. Figure 1

Effect of Inner Catalyst Layer with PTFE Binder on Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells

Recently, high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have attracted much attention due to their tolerance to carbon monoxide and simple water management in the system level. HT-PEMFCs mostly use polybenzimidazole membrane doped with phosphoric acid (PA) as electrolyte. Therefore, uniform distribution of PA through a whole MEA plays a critical role in determining fuel cell performance. Distribution of PA in a MEA does not only depend only on properties of membrane such as a doping level of PA in membrane but also on gas diffusion layer(GDE) characteristics, especially the structure of the catalyst layer (CL). At present, the CL for HT-PEMFCs is typically composed of platinum catalyst and a polymeric binder. The polymeric binder maintaining the integrity of CL has an influence on mechanical stability of CL and the PA distribution within MEA. There have been many studies to explore the characteristics of different binders such as polytetrafluoroethylene(PTFE), polybenzimidazole(PBI) and polyvinylidenefluoride(PVDF). As previously described by Huaneng Su et al., PVDF has a good affinity with PA which leads to quick distribution of PA, on the other hand hydrophobic PTFE reduce PA leaching.[1] In this study, dual structured CLs with two different binders(PVDF and PTFE) by controlling the contents in catalyst layer are fabricated to investigate the influence of the wettability of CL surface in terms of fuel cell perfoemance. The properties of these CL were evaluated by structure characterization and electrochemical analysis. At ambient pressure and 150oC, the GDEs with PTFE binder exhibit the highest power density (383mW/cm2) with H2/air which may result from highly hydrophobic CL, leading to improved mass transport. It was suggested that the change in ratio of two binders with different wettabilities respectively had an impact on small arc on EIS. Keywords :high temperature proton exchange membrane fuel cell, catalyst layer, Membrane electrode assembly, poly(2,5-benzimidazole), hydrophobicity, Pt loading, gas diffusion electrode References S. Huaneng, J. Power Sources, 246, 63 (2014).

High temperature polymer electrolyte membrane achieved by grafting poly(1-vinylimidazole) on polysulfone for fuel cells application

Phosphoric acid (PA)-doped high temperature polymer electrolyte membranes (HT-PEMs) are crucial materials for HT-PEM fuel cells (HT-PEMFCs). However, the development of HT-PEMs suffers from the trade-off between proton conductivity and mechanical strength. High proton conductivity requires a high doping level of PA, and PA acts as a plasticizer that reduces the mechanical properties. Here, a new strategy is employed to address the unresolved challenges; the strategy is to graft poly(1-vinylimidazole) as PA doping sites on the polysulfone backbone. This is achieved via atom transfer radical polymerization. High proton conductivity is achieved because of the formation of micro-phase separated structures, and the mechanical properties are retained because of the reduced plasticizing effect, which is caused by the separation of PA adsorption sites and the polymer backbone. The prepared PA-doped membranes have excellent proton conductivity of 127 mS cm−1 at 160 °C and outstanding tensile strength of 7.94 MPa. Meanwhile, single H2-O2 cell performance with the optimized membrane is impressive, reaching a peak power density of 559 mW cm−2 at 160 °C. More importantly, this work provides new insight into solving the trade-off between proton transport and mechanical strength for PA-doped HT-PEMs.

Effect of vinylphosphonic acid and polymer binders with phosphate groups on performance of high-temperature polymer electrolyte membrane fuel cell

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) are operated between 120 °C and 180 °C. As a result, they cannot employ Nafion® as an electrode ionomer because the proton conductivity of Nafion® is very low without water. In this study, new polymers (poly(phenylene oxide), PPO) with a phosphate group including methyl phosphonic acid (PPO-MPA) or 6-oxohexyl phosphonic acid (PPO-HPA), which show good proton conductivities, are applied as ionomers and binders to form an electrode. In addition, vinylphosphonic acid (VPA) is investigated as an electrode additive. The PPO-MPA electrode shows a lower wetting angle than that of the PPO-HPA electrode. After the addition of VPA, the PPO-MPA electrode becomes more hydrophobic, whereas the PPO-HPA electrode becomes more hydrophilic. In the FT-IR spectra of the PPO-MPA electrode, the peak associated with the PO32− asymmetric stretch is more blue-shifted than the peak of the polyvinylidene difluoride electrode at a high temperature due to increased amounts of released protons from PPO-MPA, indicating that the binder with a phosphate group improved the proton conductivity of the ionomer in the HT-PEMFC. At 0.2 A/cm2, the performance of the membrane electrode assembly (MEA) using the PPO-MPA cathode increases from 0.55 V to 0.59 V by the introduction of VPA, whereas that using the PPO-HPA cathode decreases from 0.61 V to 0.57 V, which exhibits the same propensity as the hydrophobicity change. The value of charge transfer resistance at 0.6 V estimated by fitting of the electrochemical impedance spectra follows the same trend as the MEA performance. This is attributed to a change in catalyst coverage by phosphoric acid in the electrode by the addition of VPA and not a change in proton conductivity. The possibility of new polymer binders for the HT-PEMFC is demonstrated, and the MEA performance is shown to be influenced by the incorporation of VPA, which is accounted for the change in hydrophobicity of the electrode.

Composite Membranes for High Temperature PEM Fuel Cells and Electrolysers: A Critical Review.

Polymer electrolyte membrane (PEM) fuel cells and electrolysers offer efficient use and production of hydrogen for emission-free transport and sustainable energy systems. Perfluorosulfonic acid (PFSA) membranes like Nafion® and Aquivion® are the state-of-the-art PEMs, but there is a need to increase the operating temperature to improve mass transport, avoid catalyst poisoning and electrode flooding, increase efficiency, and reduce the cost and complexity of the system. However, PSFAs-based membranes exhibit lower mechanical and chemical stability, as well as proton conductivity at lower relative humidities and temperatures above 80 °C. One approach to sustain performance is to introduce inorganic fillers and improve water retention due to their hydrophilicity. Alternatively, polymers where protons are not conducted as hydrated H3O+ ions through liquid-like water channels as in the PSFAs, but as free protons (H+) via Brønsted acid sites on the polymer backbone, can be developed. Polybenzimidazole (PBI) and sulfonated polyetheretherketone (SPEEK) are such materials, but need considerable acid doping. Different composites are being investigated to solve some of the accompanying problems and reach sufficient conductivities. Herein, we critically discuss a few representative investigations of composite PEMs and evaluate their significance. Moreover, we present advances in introducing electronic conductivity in the polymer binder in the catalyst layers.

Effect of Inner Catalyst Layer with PTFE Binder on Performance of High Temperature Polymer Electrolyte Membrane Fuel Cells

Herein, gas diffusion electrodes (GDEs) structured with a double catalyst layer were fabricated by introducing an inner catalyst layer containing PTFE. The Pt loading of the inner catalyst layer with Polytetrafluoroethylene (PTFE) was varied over 0.1–0.9 mg·cm-2. The fabricated GDEs were applied to the membrane electrolyte assembly (MEA) as a cathode to evaluate the single cell performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Peak power density improved by 33%, from 266 mW·cm-2 to 355 mW·cm-2 for 0.1 mg·cm-2 of Pt loading of the inner catalyst layer. This substantial enhancement in single cell performance even with a small amount of Pt loading of the inner catalyst layer results from highly hydrophobic catalyst layers leading to improved mass transport. As the Pt loading of the inner catalyst layer increases, the MEA performance in fuel cells improved; however, the performance saturated at Pt loading of 0.5 mg·cm-2 and above.

Thermally crosslinked sulfonated polybenzimidazole membranes and their performance in high temperature polymer electrolyte fuel cells

The degradation pathway of phosphoric acid doped polybenzimidazole membranes in high temperature polymer electrolyte membrane fuel cells depends on the acid contents. If it is high, creep is discussed as the main reason. If it is low (membranes prepared by solvent evaporation and post-doping), the main cause may be loss of acid due to evaporation. The net transport of acid to the anode side at high current densities should also lead to local softening of the membrane, which could be mitigated by crosslinking the membrane.Here we show that sulfonated para-polybenzimidazole membranes can be stabilized by curing at 350 °C. In contrast to meta-polybenzimidazole and sulfonated para-polybenzimidazole, crosslinked sulfonated para-polybenzimidazole is insoluble in dimethylacetamide at room temperature and phosphoric acid at 160 °C. At 160 °C and 5% relative humidity the conductivity of crosslinked sulfonated para-polybenzimidazole and meta-polybenzimidazole is 214 mS cm−1 and 147 mS cm−1, respectively. At 600 mA cm−2, the voltage decay rate is 16 μV h−1, much lower than published for commercial meta-polybenzimidazole (308 μV h−1). Furthermore, the average voltage at 600 mA cm−2 is 523 mV, while a previously published cured meta-polybenzimidazole membrane only reaches 475 mV.

Hydrogen mass transport resistance changes in a high temperature polymer membrane fuel cell as a function of current density and acid doping

High temperature polymer electrolyte membrane fuel cells (HT-PEMFC) have phosphoric acid doped membranes. Acid in the membrane is mobile and tends to move out of the membrane depending on the acid doping. The migration of acid (when the doping is high) towards the anode at high current density >0.4Acm−2 causes gas diffusion layer (GDL) and catalyst flooding which thereby results higher hydrogen transport resistance. Thus, it is important to determine the acid doping level, which is optimal. In this study, transient changes in hydrogen mass transport is investigated as a function of doping level and current density. Three doping levels 11, 8.3 and 7 molecules of H2PO4 per PBI repeat unit are investigated. Electrochemical impedance spectroscopy (EIS) was modified to a single frequency measurement and time constant are calculated for resistance change with current density using a linear fit. The time constants are 2.0 ± 0.5, 3.4 ± 0.3, 8.2 ± 0.2 min for low and 2.5 ± 0.8, 4.9 ± 0.3 and 4.5 ± 0.2 min for high current densities, for the respective doping levels. The resistance decreases at high and increases at low current densities for all the doping levels with a varying time constant. This change in time constant is attributed to low doping level having lower capillary pressure to push the acid from reaching GDL pores from the membrane and/or catalyst layer.

Fabrication and investigation of phosphoric acid doped imidazolium siloxane crosslinked poly(2,6‐dimethyl‐1,4‐phenylene oxide) for high temperature polymer electrolyte membranes

AbstractA series of high temperature polymer electrolyte membranes were fabricated based on imidazolium poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) using methylimidazole (MeIm) and triethoxysilylpropyldihydroimidazole (SiIm) as quaternization reagents via the SN2 nucleophilic substitution. Meanwhile SiIm was also employed as a crosslinking agent and the crosslinked Si–O–Si network was constructed through a hydrolysis procedure of SiIm in an acid medium. Compared with the PPO‐100%MeIm membrane without the crosslinking structure, the imidazolium siloxane crosslinked PPO‐x%SiIm‐y%MeIm membranes exhibited increased acid doping contents, enhanced dimensional stabilities, improved mechanical properties and higher conductivities. The PPO‐30%SiIm‐70%MeIm/(198 wt% phosphoric acid) membrane displayed a conductivity of 0.08 S cm−1 at 180 °C without humidifying and a tensile strength of 6.4 MPa at room temperature. © 2019 Society of Chemical Industry

New Gas-Diffusion Electrode Based on Heterocyclic Microporous Polymer PIM-1 for High-Temperature Polymer Electrolyte Membrane Fuel Cell

Polymer of intrinsic microporosity PIM-1 was used for electrospun polymer nanofiber producing. After pyrolysis, the obtained nanofibers, in a form of entire mat, were used as a support for cathode electrocatalyst for high-temperature polymer electrolyte membrane fuel cell on polymer polybenzimidazole membrane. The material was characterized by the methods of standard contact porosimetry, Raman spectroscopy and scanning electron microscopy. The I-V curves for membrane-electrode assembly suggest a possibility of using the carbon material for electrodes in a fuel cell on polymer membrane.

Durability and degradation of vapor-fed direct dimethyl ether high temperature polymer electrolyte membrane fuel cells

Dimethyl ether (DME) combines high energy density with easy handling and low toxicity and is therefore an attractive fuel. The absence of carbon-carbon bonds allows for electro-oxidation with good kinetics and it is therefore particularly interesting for use in fuel cells. This work presents the first durability studies of vapor-fed direct dimethyl ether fuel cells with phosphoric acid doped polybenzimidazole membranes as electrolytes. Fuel cells are operated in direct DME mode at 160 and 200 °C and the cell voltage at a constant current load of 100 mA cm−2 is recorded over more than 200 h. Regular electrochemical impedance spectroscopy and polarization data are used as diagnostic measures to monitor the cell characteristics. It is shown that the cell performance deteriorates severely within 200 h of operation at 160 or 200 °C. The degradation is connected to different modes that ultimately result in both increasing polarization resistance and increasing area specific resistance, which may be connected to the chemical incompatibility between the fuel and the electrolyte.

Energetic and exergetic analyses of a combined system consisting of a high-temperature polymer electrolyte membrane fuel cell and a thermoelectric generator with Thomson effect

A combined system model consisting of a high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC), a regenerator and a thermoelectric generator (TEG) is proposed, where the TEG is applied to harness the generated waste heat in the HT-PEMFC for extra electricity production. The TEG considers not only the Seebeck effect and Peltier effect but also the Thomson effect. The mathematical expressions of power output, energy efficiency, exergy destruction rate and exergy efficiency for the proposed system are derived. The energetic and exergetic performance characteristics for the whole system are revealed. The optimum operating ranges for some key performance parameters of the combined system are determined using the maximum power density as the objective function. The combined system maximum power density and its corresponding energy efficiency and exergy efficiency allow 19.1%, 12.4% and 12.6% higher than that of a stand-alone HT-PEMFC, while the exergy destruction rate density is only increased by 8.6%. The system performances are compared between the TEG with and without the Thomson effect. Moreover, the impacts of comprehensive parameters on the system performance characteristics are discussed. The obtained results are helpful in developing and designing such an actual combined system for efficient and clean power production.

Performance assessment of a combined system consisting of a high-temperature polymer electrolyte membrane fuel cell and a thermoelectric generator

A new combined system consisting of a high-temperature proton exchange membrane fuel cell (HT-PEMFC), a regenerator and a thermoelectric generator (TEG) is developed. The mathematical relationship between the HT-PEMFC operating current density and the TEG dimensionless current is derived, and the operating current density range of HT-PEMFC in which the TEG allowed to work is determined. Power output and efficiency of the combined system are formulated under different operating conditions. Compared with the stand-alone HT-PEMFC, the proposed combined system allows the equivalent power density to increase by 21%. The optimum criteria and general performance characteristics for the complete system are specified. Moreover, the effects of the operating current density, doping level, relative humidity, operating temperature, heat conductivity and figure of merit of the thermoelectric materials on the combined system performance characteristics are revealed. The obtained results may provide some theoretical insights into the design and integration of such an actual combined system.

Effect of Surfactant for PTFE Binder Dispersion of Electrodes Prepared By Bar Coating Method in High Temperature Polymer Electrolyte Membrane Fuel Cell

Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most promising energy devices for both mobile and stationary applications due to their high energy conversion efficiency and low environment pollution. However, there are some problems to be solved. PEMFCs operating at low temperatures (below 80℃) based on Nafion require humidification and have low CO tolerance. To overcome this problem, high-temperature PEMFCs that operate up to 180℃ are being studied. High temperature PEMFCs does not require humidification, which simplifies the system and is highly resistant to CO. It also has the advantage of using waste heat. However, the cell performance of high-temperature PEMFCs is still much lower than that of low-temperature PEMFCs and needs to be increased for practical applications The reason for this is slow Oxygen Reduction Reaction (ORR) and limited transport of protons and reactants, especially in the presence of phosphoric acid in the catalyst layer. In the high-temperature PEMFCs, phosphoric acid acts as a proton conduction as an electrolyte and is a necessary element for electrochemical reactions at the triple phase boundaries. Therefore, if the amount of phosphoric acid in the catalyst layer is too small, it exhibits low electrochemical activity. If it is too much, the pore inside the catalyst layer is blocked to restrict the movement of the fuel, thereby reducing the number of triple phase boundaries and reducing the performance. Therefore, the amount and distribution of phosphoric acid and the formation of structural network pores in the catalyst layer are important. Thus, several polymeric binders have been introduced to perform this role. Among them, PTFE is known as the most promising binder. PTFE has hydrophobicity and high chemical stability. However, PTFE easily agglomerates and sinks immediately in the solvent due to its high hydrophobicity. Therefore, if an electrode is made of a catalyst slurry that does not properly disperse PTFE, uniform PTFE distribution in the catalyst layer cannot be expected. So, in the catalyst layer, there may be more or less parts of the specific part than the desired PTFE content. Therefore, it is not only difficult to form a proper phosphoric acid distribution and structural network of the catalyst layer, but also may affect the reproducibility of electrode manufacturing. In this study, the catalytic electrode was prepared by introducing a surfactant to disperse PTFE in the preparation of catalyst slurry. Single cell test was carried out using membrane electrode assembly made from this electrode. And the electrochemical and structural characterizations were compared. The details will be discussed in presentation.

Deterministic and stochastic economic analysis based on historical natural gas and CO2 allowance prices for steam reforming of methanol

As an appropriate hydrogen supply system for high temperature polymer electrolyte membrane fuel cells, steam reforming of methanol (SRM) is proposed because reformate gas including low quality H2 with some impurities can be directly used as the fuel. In this study, we compare two SRM systems with and without the use of tail gas from pressure swing adsorption and comprehensive economic analysis is performed for the SRM process with the use of tail gas to estimate a unit H2 production cost (CUHP) based on historical data of natural gas and the CO2 allowance prices for Korea, Europe, Western Climate Initiative (WCI), and Regional Greenhouse Gas Initiative (RGGI). From deterministic economic analysis with a H2 production capacity of 700 m3 h−1, the CUHP values are 6.88 $ kgH2−1 for Korea, 6.84 $ kgH2−1 for Europe, 6.50 $ kgH2−1 for WCI, and 6.54 $ kgH2−1 for RGGI, respectively. Furthermore, additional two scenarios, pessimistic and optimistic ones (Scenario 1 and 2, respectively), are investigated to evaluate the current level of CUHP values for H2 production capacities from 30 to 700 m3 h−1. With a Monte-Carlo simulation method, stochastic economic analysis is carried out to predict the ranges of potential CUHP values for each region. The CUHP values for WCI and RGGI show the narrow distributions meaning lower change of CUHP; however, a wide range of distribution for Korea and Europe suggest that there is considerably higher variation, demonstrating the necessity of stochastic uncertainty analysis when assessing the economic feasibility.

Achieving high Pt utilization and superior performance of high temperature polymer electrolyte membrane fuel cell by employing low-Pt-content catalyst and microporous layer free electrode design

Reducing the platinum (Pt) use is of significance to popularize high temperature polymer electrolyte membrane fuel cell (HT-PEMFC) since its average Pt loading in a typical gas diffusion electrode (GDE) normally reaches 0.7 mg cm−2. In this work, an attempt to lower the Pt loading for the electrodes of HT-PEMFC is made by employing low-Pt-content catalysts (20 wt% and 10 wt% Pt/C) with the combination of microporous layer (MPL)-free electrode structure design, by which high Pt utilization and minimum ohmic/mass transfer resistances can be simultaneously maintained. Voltage loss mechanism and the catalyst layer (CL) morphologies of the GDEs are analyzed by polarization curve, electrochemistry impedance spectroscopy, cyclic voltammetry and scanning electron microscopy. The results show that the electrode Pt loading can be lowered to 0.2 mg cm−2 by this strategy but it still demonstrates a maximum Pt-specific performance of 1.6 W mgPt−1 and an area-specific power density of 0.32 W cm−2, which is a considerable improvement on developing HT-PEMFC with low Pt loading. There is a tradeoff between reducing Pt loading and increasing Pt utilization to maintain a superior CL quality, then ensuring that the fuel cell performance is fit for practical applications.

Validation of a pseudo 2D analytical model for high temperature PEM fuel cell impedance valid at typical operative conditions

High temperature polymer electrolyte membrane fuel cells (HT-PEMFC) are a promising technology for carbon-free, combined heat and power generation. Analytical impedance models have the potential to support the creation of effective control logics with short response time. In this work an exact pseudo 2D model for HT-PEMFC impedance is developed, in which the anode has been neglected. The model considers losses due to kinetics and oxygen transport across the gas diffusion layer. Moreover, the cathode catalyst layer is assumed totally flooded and an additional oxygen transport loss is attributed to diffusion inside spherical agglomerates with a simplified approach. The model is tested against experiments on HT-PEMFC at different cathode stoichiometries, oxygen molar concentrations, and current densities. A simple fitting procedure is described to use the model. Moreover, sensitivity analysis is performed to assess the contribution of the physical phenomena to impedance spectra. Finally, the effect of the channel at different stoichiometries is discussed and a zero-frequency resistance is derived.

Iron Single Atoms on Graphene as Nonprecious Metal Catalysts for High-Temperature Polymer Electrolyte Membrane Fuel Cells.

Iron single atom catalysts (Fe SACs) are the best‐known nonprecious metal (NPM) catalysts for the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), but their practical application has been constrained by the low Fe SACs loading (<2 wt%). Here, a one‐pot pyrolysis method is reported for the synthesis of iron single atoms on graphene (FeSA‐G) with a high Fe SAC loading of ≈7.7 ± 1.3 wt%. The as‐synthesized FeSA‐G shows an onset potential of 0.950 V and a half‐wave potential of 0.804 V in acid electrolyte for the ORR, similar to that of Pt/C catalysts but with a much higher stability and higher phosphate anion tolerance. High temperature SiO2 nanoparticle‐doped phosphoric acid/polybenzimidazole (PA/PBI/SiO2) composite membrane cells utilizing a FeSA‐G cathode with Fe SAC loading of 0.3 mg cm−2 delivers a peak power density of 325 mW cm−2 at 230 °C, better than 313 mW cm−2 obtained on the cell with a Pt/C cathode at a Pt loading of 1 mg cm−2. The cell with FeSA‐G cathode exhibits superior stability at 230 °C, as compared to that with Pt/C cathode. Our results provide a new approach to developing practical NPM catalysts to replace Pt‐based catalysts for fuel cells.