Proton Exchange Membranes For Applications Research Articles

(PEFC&E 1st Place Best Poster Winner) Nanocellulose Crosslinked with Sulfonic Acid As an Alternative Proton Conductive Membrane for Hydrogen Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) using hydrogen as fuel are efficient and clean energy generating devices. Commercialization of fuel cells is however slow, in particular due to high cost of the materials used to fabricate proton exchange membrane (PEM). Nafion®, a benchmark fluorinated polymer used as PEM, contributes significantly to the cost of fuel cell devices and therefore alternative materials are needed. The high proton conductivity of Nafion (i.e. ~100 mS/cm) has made this polymer the main material for PEMs, since its discovery in the late 1960s. However, it has number of disadvantages additionally to high cost, such as degradation under corrosive conditions and difficulty in recycling. Numerous research groups work on synthesis of proton conducting polymers that could compete with Nafion. Among the available techniques sulfonation of hydrocarbon polymers is recognized as an effective approach to create new PEMs [Peckham, T. J. et al. Adv. Mater. 2010, 22, 4667–4690].Nanocellulose (NC) is an environmentally friendly, very low-cost material that can be produced by processing of conventional cellulose, which is plant-derived, renewable, and most abundant, material resource in the world. Research into nanocellulose for various applications is accelerating rapidly. Both cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) are attractive due to their intrinsic properties, such as mechanical strength, high gas barrier, and a chemical structure suitable for various modifications. Being a biodegradable polymer, it has an additional advantage over fluorinated polymers (such as Nafion) The majority of works that considered the use of nanocellulose in PEMFCs have investigated composites with conventional ionomers such as Nafion, with varying results [Jiang, G. P. et al. J. Power Sources 2015, 273, 697–706]. However, the use of non-composited nanocellulose membranes for PEMFCs is a completely novel topic for research.Nanocellulose is an attractive polymer platform for PEMs, however its application for this purpose is not widely explored. The use of nanocellullose membranes in PEMFCs was pioneered by our group and previous works have demonstrated that both CNF and CNC membranes can work as a PEM in fuel cells. However, membranes made of pristine nanocellulose lacked sufficient proton conductivity, and required better stability in high temperature and humid environments. Namely, the inherent proton conductivity of nanocellulose is quite low: i.e. ~0.05 mS/cm for cellulose nanofiber (CNF) and ~4 mS/cm for cellulose nanocrystals (CNC) membrane [Bayer, T. et al., Chem. Mater. 2016, 28, 4805–4814] and thus requires significant improvement. Conventional approaches for nanocellulose modification (e.g. TEMPO-oxidation, backbone sulfonation) can achieve better proton conductivity only by sacrificing other properties (i.e. the mechanical, aqueous, and chemical stability) [Isogai A. et al. Nanoscale, 2011, 3, 71-85]. In contrast, crosslinking of nanocellulose was shown to provide improved mechanical properties and reduced swelling in water [Quellmalz et al. ACS Biomater. Sci. Eng. 2015, 1, 271−276], as well as better proton conductivity when an appropriate crosslinking agent is used (i.e. strong acid and good proton donor) [Seo J. A. et al. Ionics 2009 15, 555–560].In this work we used chemical crosslinking of nanocellulose with sulfonic acid in order to achieve the required properties for PEM applications. Sulfosuccinic acid (SSA) was used as a crosslinking agent. Crosslinking of cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) membranes was performed in a hot-press, to form SSA-x-CNF and SSA-x-CNC membranes. Initial tests resulted in membranes of < 10 µm thickness; significantly decreased swelling; stability in boiling water; and increased through-plane conductivity by 2 orders of magnitude.The findings so far confirm the hypothesis that a proper crosslinking agent can turn nanocellulose into an effective proton conductor as well as improving other relevant properties. Investigation into the effect on mechanical strength, chemical stability and fuel cell performance is still underway and will be also reported.

Understanding the Catalytic Sites of Metal-Nitrogen-Carbon Oxygen Reduction Electrocatalysts.

The development of low-cost catalysts containing earth-abundant elements as alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) is crucial for the large-scale commercial application of proton exchange membrane fuel cells (PEMFCs). Nonprecious metal-nitrogen-carbon (M-N-C) materials represent the most promising candidates to replace Pt-based catalysts for PEMFCs applications. However, the high-temperature pyrolysis process for the preparation of M-N-C catalysts frequently leads to high structural heterogeneity, that is, the coexistence of various metal-containing sites and N-doped carbon structures. Unfortunately, this impedes the identification of the predominant catalytic active structure, and thus, the further development of highly efficient M-N-C catalysts for the ORR. This Minireview, after a brief introduction to the development of M-N-C ORR catalysts, focuses on the commonly accepted views of predominant catalytic active structures in M-N-C catalysts, including atomically dispersed metal-Nx sites, metal nanoparticles encapsulated with nitrogen-doped carbon structures, synergistic action between metal-Nx sites and encapsulated metal nanoparticles, and metal-free nitrogen-doped carbon structures.

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.

Preparation and Application of Aromatic Polymer Proton Exchange Membrane with Low-Sulfonation Degree

4,4′-Dichlorodiphenylsulfone-3,3′-disulfonic acid (disodium) salt and 4,4′-difluorodiphenylsulfone were used as sulfonated monomer. 4,4′-Fluorophenyl sulfones were used as the nonsulfonated monomer. 4,4′-Dihydroxy diphenyl ether or 4,4′-thiodibenzenethiol was used as the comonomer. The sulfonated poly (aryl ether sulfone) (SPES) and sulfonated poly (arylene thioether sulfone) (SPTES) with sulfonation degree of 30% and 50% were successfully prepared by nucleophilic polycondensation. Two kinds of aromatic polymer proton exchange membranes were prepared by using sulfonated poly phthalazinone ether ketone (SPPEK) material and fluidization method. The performance of the prepared aromatic polymer proton exchange membrane was researched by the micromorphology, ion exchange capacity, water absorption and swelling rate, oxidation stability, tensile properties, and proton conductivity. Experimental results show that there is no agglomeration in the prepared aromatic polymer proton exchange membrane. The ion exchange capacity is 0.76–1.15 mmol/g. The water absorption and swelling rate increase with the increase of sulfonation degree. The sulfonated poly (aryl ether sulfone) membrane shows better oxidation stability than sulfonated poly (aryl sulfide sulfone). They have good mechanical stability. The prepared aromatic polymer proton exchange membrane with low sulfonation degree has good performance, which can be widely used in portable power equipment, electric vehicles, fixed power stations, and other new energy fields.

High methanol resistant polyelectrolyte membrane based on semi-crystalline Poly(ether ketone) with densely sulfonated side chain for direct methanol fuel cell

Different from the widely developed amorphous polyelectrolyte membrane materials, semi-crystalline sulfonated poly(ether ketone) (PEK-SPx) membranes composed of crystalline poly(ether ketone) main chain and densely sulfonated side chain are prepared. Wide-angle X-ray diffraction analysis indicates that these membranes manifest crystalline nature. The dimensional stability and methanol resistance of PEK-SPx membranes are greatly enhanced by introduction of semi-crystalline structure. The dimensional change in pure methanol at 60 °C and methanol permeability of PEK-SP20 membrane is only 25.9% and 36 × 10−8 cm2 s−1, respectively. In addition, coupled with excellent thermal stability, oxidative stability, good mechanical property, satisfactory proton conductivity and distinct micro-phase separation, PEK-SP20 membrane exhibits considerable single cell performance. Overall, semi-crystalline PEK-SPx membranes are a promising new type of proton exchange membranes for application in direct methanol fuel cells.

A mass-producible integrative structure Pt alloy oxygen reduction catalyst synthesized with atomically dispersive metal-organic framework precursors

Oxygen reduction reaction (ORR) catalyst is one of the most significant influential factors for the application of proton exchange membrane fuel cells. This work introduces a mass-producible high-performance PtZn alloy integrative structure ORR catalyst, synthesized with the atomically dispersive metal-organic framework precursors. This PtZn catalyst displays excellent catalytic activity with the onset reduction potential of 1.0 VRHE@ 0.16 mA cm−2 (Reversible hydrogen electrode; RHE) and the half-wave potential of 0.934 VRHE for the ORR catalysis. The calculated specific activity and mass activity at 0.9 V are 9.44 A m−2 and 544 A gPt−1, respectively, which are 5.62 times and 5.77 times as high as the commercial Pt/C. The mass activity is remarkably higher than the target put forward by the Department of Energy (DOE; 440 A gPt−1). Furthermore, this PtZn catalyst also exhibits outstanding stability after the 10,000 potential cyclic degeneration test. The ORR current is much higher than Pt/C in the whole potential range not only before but also after the 10,000 potential cycles with identical Pt loading. This catalyst has a multifarious active-site catalytic structure with PtZn alloyed particles and atomically dispersive metal-N active sites on the N-doped graphited carbon matrix, exhibiting appealing ORR catalytic activity and sound stability for the application and scalable production of fuel cell catalysts.

TiO2-Si- or SrTiO3-Si-impregnated PVA–based low-cost proton exchange membranes for application in microbial fuel cell

Proton exchange membranes (PEMs) were synthesized in polyvinyl alcohol (PVA) matrix by impregnating TiO2-Si or SrTiO3-Si as proton exchanger for microbial fuel cells (MFCs). These PEMs were physically characterized, which approved the presence of hygroscopic oxides like SiO2 to increase the proton transfer due to the formation of Si-H along with the water channelling through TiO2 or SrTiO3 induced microstructural modifications. Chemical characterization techniques further illustrated their comparable performance to the commercially available Nafion-117 and superior performance than bare PVA membrane. Polarization curves for MFCs using TiO2-Si- or SrTiO3-Si-based PVA membranes exhibited a power density of 6.16 ± 0.31 W m−3 and 5.39 ± 0.27 W m−3, respectively. Power density of MFC with TiO2-Si membrane was comparable to that of the MFC using Nafion-117 (6.50 ± 0.33 W m−3). Thus, comparable properties and 30-folds less synthesis cost of the fabricated TiO2-Si-impregnated PVA membrane in comparison to Nafion, demonstrate its potential for real-life applications in MFC.

Nitrogen, Sulfur Co-Doped Hierarchically Porous Carbon as a Metal-Free Electrocatalyst for Oxygen Reduction and Carbon Dioxide Reduction Reaction.

High-cost and low-efficiency electrocatalysts have hindered oxygen reduction reaction (ORR) in fuel cells and CO2 reduction reaction (CO2RR) for producing fuels and value-added chemicals. Here, a low-cost metal-free electrocatalyst of a N, S co-doped hierarchically porous carbon (NSHPC) for efficient ORR and CO2RR is reported. The NSHPC is prepared by pyrolysis of glucosamine hydrochloride and thiocyanuric acid precursors using SiO2 as hard templates. The N, S co-doping effectively enhances catalytic activity and selectivity, and the hierarchically porous structure largely exposes abundant active sites to reaction species and facilitates electrolyte transport, thereby leading to significantly increased catalytic activities for the NSHPC. The resultant NSHPC exhibits excellent electrocatalytic activities toward ORR in both acidic and alkaline electrolytes and also shows application in proton exchange membrane fuel cells (PEMFCs). More importantly, the NSHPC enables CO2 reduction to CO with 87.8% maximum Faraday efficiency (FE) in aqueous electrolytes. This work offers a novel insight into the development of multifunctional electrocatalysts for producing electricity, fuels, and value-added chemicals.

A Power Allocation Method for Multistack PEMFC System Considering Fuel Cell Performance Consistency

The purpose of this work is to design a power allocation method (PAM) for multistack fuel cell system (MFCS). Due to the limitations of technology, large-scale application of proton exchange membrane fuel cell (PEMFC) in the field of rail transportation has not been implemented. Therefore, this study presents an MFCS to promote the use of PEMFCs in the high-power energy market. During the operation of the MFCS, since the output power of the FC varies dynamically with different operation conditions, the performance of each stack is usually inconsistent. A method for evaluating the degree of performance degradation (DOPD) of stacks is presented in this study. In addition, a fuel cell (FC) semiempirical model is also adopted to simulate the effects of aging on the stacks’ performance. In order to maintain the uniform performance of stacks and enhance the lifespan of the MFCS, a PAM that considers the DOPD of each stack is presented. Besides, in order to facilitate system expansion, this article proposes a virtual droop control method to realize power splitting among the stacks. Also, a voltage control strategy is presented to compensate for the bus voltage drop caused by the droop control. The effectiveness and practicability of the presented PAM and MFCS are verified on the hardware-in-the-loop test bench constructed by RT-LAB.

Multi-functionalized acid-base double-shell nanotubes are incorporated into the proton exchange membrane to cope with low humidity conditions

Proton exchange membranes (PEM) with high proton conductivity and water retention are critical to the commercial application of proton exchange membrane fuel cells (PEMFC). In this study, acid-base double-shell nanotubes with carboxylate inner shell and an imidazole outer shell (DSNT-A@B) are synthesized via continuous distillation-precipitation polymerization using halloysite nanotubes (HNTs) as seeds. Then, it is incorporated into sulfonated poly (ether ether ketone) matrix to prepare composite membranes. The carboxylic inner shell can increase the content of combined water, thereby giving the composite membrane higher water retention. The imidazole shell acts as basic shell to create acid-base pairs with the membrane and inner shell to promote proton conductivity following the Grotthuss mechanism. The results show that when the blending amount is 5 wt%, the proton conductivity of the composite membrane reaches 0.336 S/cm at 80 °C and 100% relative humidity (RH), which is twice as high as that of the original membrane. In particular, the water loss of SPEEK/DSNT-A@B-10 composite membrane is only 54.55% at 40 °C and 20% RH, which is 32.77% lower than the SPEEK membrane. Therefore, this DSNT-A@B/SPEEK composite membrane can be used as a potential candidate for high temperature and low humidity fuel cells.

3D Network Structural Poly (Aryl Ether Ketone)-Polybenzimidazole Polymer for High-Temperature Proton Exchange Membrane Fuel Cells

Poor mechanical property is a critical problem for phosphoric acid-doped high-temperature proton exchange membranes (HT-PEMs). In order to address this concern, in this work, a 3D network structural poly (aryl ether ketone)-polybenzimidazole (PAEK-cr-PBI) polymer electrolyte membrane was successfully synthesized through crosslinking reaction between poly (aryl ether ketone) with the pendant carboxyl group (PAEK-COOH) and amino-terminated polybenzimidazole (PBI-4NH2). PAEK-COOH with a poly (aryl ether ketone) backbone endows superior thermal, mechanical, and chemical stability, while PBI-4NH2 serves as both a proton conductor and a crosslinker with basic imidazole groups to absorb phosphoric acid. Moreover, the composite membrane of PAEK-cr-PBI blended with linear PBI (PAEK-cr-PBI@PBI) was also prepared. Both membranes with a proper phosphoric acid (PA) uptake exhibit an excellent proton conductivity of around 50 mS cm-1 at 170°C, which is comparable to that of the well-documented PA-doped PBI membrane. Furthermore, the PA-doped PAEK-cr-PBI membrane shows superior mechanical properties of 17 MPa compared with common PA-doped PBI. Based upon these encouraging results, the as-synthesized PAEK-cr-PBI gives a highly practical promise for its application in high-temperature proton exchange membrane fuel cells (HT-PEMFCs).

Selection of proton exchange membrane fuel cell for transportation

This work evaluates the success of the application of proton-exchange membrane fuel cells for transportation with a focus on issues and areas for possible developments. Fuel cell technology when applied in road, air and water transportation and all the instances had contributed to the provision of a cleaner environment. The new technology, if developed, would meet the economic prospect of the people. If the cost of materials, labour and overhead could be reduced to make it affordable for the transportation sector. Other factors that could make the technology accessible to customers, such as performances and durability, infrastructural development, human factor and government policy, were critically analysed. This paper will provide comprehensive details and discussions on the above issue to show the current application and future developments of the PEM fuel cell in transportation.

Synthesis of 2,2′-hindered pyridine containing semifluorinated polytriazoles and investigation for low-temperature proton exchange membrane application with enhanced oxidative stability

A series of new sulfonated copolytriazoles were prepared from a novel fluorinated diazide monomer with pyridine moiety to enhance both the oxidative stability and proton conductivity of proton exchange membranes. These proton donor–acceptor based copolymers and homopolymers were synthesized by click cycloaddition polymerization of the dialkyne monomer 1,2,4,5-tetrafluoro-3,6-bis(prop-2-yn-1-yloxy)benzene (TFAK) with or without the pyridine containing diazide monomer 2,2′-bis(4-(4-azidophenoxy)-3-(trifluoromethyl)phenyl)pyridine (PYFAZ) and the sulfonated diazide monomer 4,4′-diazido-2,2′-stilbenedisulfonic acid disodium salt (DSSAZ). 1H, 13C, and 19F NMR, in addition to FTIR analyses, confirm the formation of polytriazoles with a graded degree of sulfonation between 70% and 100%. The solution cast membranes were flexible and showed high chemical, thermal and mechanical properties. Inclusion of pyridine moiety proved to play a significant role in increasing peroxide stability by quenching hydroxyl and hydroperoxide radicals and forming pyridine-N-oxide, which was analyzed by EPR and FTIR spectroscopy. The presence of pyridine moiety also enhanced proton conductivity owing to the presence of the weakly basic nitrogen in pyridine that acts as a proton hopping site. AFM and HR-TEM images of the membranes indicated the formation of phase-separated morphology and that became more pronounced with the increase in the degree of sulfonation. The membranes obtained from PYFTSH-90 copolymer exhibited proton conductivity of 125 mS cm−1 at 80 °C, which is comparable to Nafion-117. The proton conductivity of PYFTSH-90 was further increased to 139 mS cm−1 at 90 °C. Thus, three significant objectives have been fulfilled in this work, namely (i) high proton conductivity, (ii) high hydrolytic stability, and (iii) high peroxide stability. These properties demonstrate that these copolymer membranes are potentially highly suitable for application in low-temperature proton exchange membrane fuel cell (PEMFC).

Sulfonated poly(arylene ether sulfone) composite membrane having sulfonated polytriazole grafted graphene oxide for high-performance proton exchange membrane fuel cells

We investigated the effect of graphene oxide (GO) and sulfonated polytriazole grafted graphene oxide (SPTA-GO) on sulfonated poly(arylene ether sulfone) (SPAES) based polymer electrolytes for application in proton exchange membrane fuel cells. GO was fabricated by the chemical oxidation of graphite and SPTA-GO was prepared via the click reaction using copper (Ⅰ) catalyzed azide-alkyne 1,3-dipolar cycloaddition between ethynyl-terminated sulfonated polytriazole (E-SPTA) and azide functionalized GO (N3-GO). All the SPAES based composite membranes with GO and SPTA-GO revealed better physical/chemical stabilities and mechanical strength than the pristine SPAES membrane, while noticeable improvement of proton conductivity was only observed in the SPAES composite membrane with SPTA-GO since the SPTA moiety attached on GO has both sulfonic acid and triazole groups that can introduce the additional proton conducting channels into the SPAES matrix. The maximum power density of the membrane electrode assembly employing the SPAES composite membrane with SPTA-GO reached up to 1.58 W cm−2 at 80 °C and 100% RH condition, which is larger than that employing commercial Nafion 212 (1.33 W cm−2).

Atomic Level Dispersed Metal–Nitrogen–Carbon Catalyst toward Oxygen Reduction Reaction: Synthesis Strategies and Chemical Environmental Regulation

For development and application of proton exchange membrane fuel cell (PEMFC) energy transformation technology, the cost performance must be elevated for the catalyst. At present, compared with noble metal‐based catalysts, such as Pt‐based catalysts, atomically dispersed metal–nitrogen–carbon (M–N–C) catalysts are popularity and show great potential in maximizing active site density, high atom utilization and high activity, making them the first choice to replace Pt‐based catalysts. In the preparation of atomically dispersed metal–nitrogen–carbon catalyst, it is difficult to ensure that all active sites are uniformly dispersed, and the structure system of the active sites is not optimal. Based on this, we focus on various approaches for preparing M–N–C catalysts that are conducive to atomic dispersion, and the influence of the chemical environmental regulation of atoms on the catalytic sites in different catalysts. Therefore, we discuss the chemical environmental regulation of the catalytic sites by bimetals, atom clusters, and heteroatoms (B, S, and P). The active sites of M–N–C catalysts are explored in depth from the synthesis and characterization, reaction mechanisms, and density functional theory (DFT) calculations. Finally, the existing problems and development prospects of the current atomic dispersion M–N–C catalyst are proposed in detail.

Boosting the oxygen reduction reaction of a nonprecious metal Fe–Nx/C electrocatalyst by integrating tube-terminated edges into the basal plane of Fe- and N-codoped carbon bubbles

Abstract The preparation of a highly efficient and economical oxygen reduction reaction (ORR) catalyst is one of the greatest challenges for the application of proton exchange membrane fuel cells (PEMFCs). Herein, we integrated tube-terminated edges into the basal plane of Fe- and N-codoped carbon bubbles to obtain an excellent Fe-Nx/C ORR electrocatalyst. The synthesis of the uniformly distributed Fe- and N-codoped tube-terminated edge-dominated carbon bubble (Fe-TTNCB) was based on a new self-sacrificing template strategy. The unique micro-meso/macromorphological design endowed the Fe-TTNCB catalysts with abundant surface-terminated features to facilitate the construction of edge-hosted Fe-Nx/C active sites, thus improving the mass transport properties and promoting the accessibility of oxygen to the active sites. Due to the synergistic effects among all of the above characteristics, the Fe-TTNCB catalysts had excellent ORR catalytic activity in 0.1 M KOH electrolyte with a half-wave potential of 0.905 V (E1/2, V vs. a reference hydrogen electrode (RHE)), which is 62 mV higher than that of commercial 20% Pt/C (0.843 V vs. RHE). Moreover, by integrating tube-terminated edges into the basal plane of Fe- and N-codoped carbon bubbles, the kinetic current density (Jk) of the nonprecious metal Fe–Nx/C electrocatalyst increased from 15.38 mA cm−2 (Fe- and N-codoped carbon bubble, Fe-NCB) and 32.25 mA cm−2 (Fe- and N-codoped hybrid carbon bubble, Fe-HNCB to 56.18 mA cm−2 (Fe-TTNCB) at 0.8 V, and E1/2 was boosted from 0.87 V (Fe- and N-codoped carbon bubble, Fe-NCB) and 0.88 V (Fe-HNCB) to 0.905 V (Fe-TTNCB).

Sulfonated polybenzimidazole/amine functionalized titanium dioxide (sPBI/AFT) composite electrolyte membranes for high temperature proton exchange membrane fuel cells usage

The novel sulfonated polybenzimidazole (sPBI)/amine functionalized titanium dioxide (AFT) composite membrane is devised and studied for its capability of the application of high temperature proton exchange membrane fuel cells (HT-PEMFCs), unlike the prior low temperature AFT endeavors. The high temperature compatibility was actualized because of the filling of free volumes in the rigid aromatic matrix of the composite with AFT nanoparticles which inhibited segmental motions of the chains and improved its thermal stability. Besides, amine functionalization of TiO2 enhanced their dispersion character in the sPBI matrix and shortened the interparticle separation gap which finally improved the proton transfer after establishing interconnected pathways and breeding more phosphoric acid (PA) doping. In addition, the appeared assembled clusters of AFT flourished a superior mechanical stability. Thus, the optimized sPBI/AFT (10 wt%) showed 65.3 MPa tensile strength; 0.084 S·cm−1 proton conductivity (at 160 °C; in anhydrous conditions), 28.6% water uptake and PA doping level of 23 mol per sPBI repeat unit. The maximum power density peak for sPBI/AFT-10 met the figure of 0.42 W·cm−2 at 160 °C (in dry conditions) under atmospheric pressure with 1.5 and 2.5 stoichiometric flow rates of H2/air. These results affirmed the probable fitting of sPBI/AFT composite for HT-PEMFC applications.

Transient Potential Induced Dissolution of Metal Biporlar Plates Used in Proton Exchange Membrane Fuel Cell

Corrosion of metallic bipolar plates (BPPs) under the aggressive working conditions is the major issue that impedes its application in proton exchange membrane fuel cells (PEMFCs). Transient potential is generated during the different operating conditions of PEMFCs such as start-up/shut-down(SU/SD), and dynamic loading stages. Transient potential-induced corrosion is a serious problem encountered by BPPs. To investigate the influence of transient potentials on corrosion of BPPs, potential pulses with different amplitudes are applied to specimens. Also, a novel zirconium oxynitride coating is prepared via atomic layer deposition to improve the stability of BPPs.Results show that transient potentials generated at passive and transpassive potential regions are both detrimental to the stability of BPPs. For the uncoated samples, a larger potential fluctuation in the passive region could induce a much higher frequency of passive film breakdown than that generated by a smaller potential fluctuation during the cyclic pulse tests. In the transpassive region, upon increasing transient potentials, the passive film formed on surface enters the potential dependent dissolution region, leading to severe intergranular corrosion after a certain cycles of pulse tests.The zirconium oxynitride coated samples present improved performance in both normal potentiostatic polarization and transient potential conditions. The enhanced corrosion resistance of coated samples is attributed to the continuous formation of protective oxide layer which has a wider band gap than passive films formed on uncoated samples. Therefore, the coated sample enters the transpassive potential region at a more positive potential and thus exhibits much lower dissolution rate. Moreover, the Mott-Schottky plots suggests that the defect density of the coated samples is much lower than the uncoated counterpart, and the deterioration of uncoated sample performance under different transient potentials is related to the passive film disorder degree.From our study, transient potentials would accelerate the degradation of BPPs, and the development of bipolar plates coated with dense protective surface films could help mitigate the dissolution induced by the transient potentials, thus improving stability and durability of BPPs. Specially, a layer of coating with a wider band gap between Fermi level and valance band is beneficial to reduce the dissolution in the transpassive potential region.

Evaluation criterion of different flow field patterns in a proton exchange membrane fuel cell

Flow field plays an important role in the design and application of proton exchange membrane fuel cell (PEMFC). The superiority of flow field is commonly evaluated by comparing the performance improvement of PEMFC. However, it’s necessary to decrease the flow resistance to reduce the parasitic power and to obtain more uniform water and gas distributions during operation. As a consequence, the combined effect of performance and resistance should be considered during the optimal design of flow channel in PEMFC. A new criterion of efficiency evaluation criterion (EEC) using Sherwood number and Euler number has been proposed to evaluate the relationship between the mass transfer and the pressure drop in this study. The performances of PEMFCs with parallel, single-serpentine, and pressured parallel flow fields are investigated in detail. The results show that PEMFC with single-serpentine flow field has a better performance under normal atmospheric pressure, while the pressure drop in cathode is much larger than that in the parallel flow field. The pressured parallel flow field presents better performance and uniform gas distribution and it is thus proposed as the flow channel in the application of PEMFC.

Sequential Synthesis and Active‐Site Coordination Principle of Precious Metal Single‐Atom Catalysts for Oxygen Reduction Reaction and PEM Fuel Cells

AbstractCarbon‐supported precious metal single‐atom catalysts (PM SACs) have shown promising application in proton exchange membrane fuel cells (PEMFCs). However, the coordination principle of the active site, consisting of one PM atom and several coordinating anions, is still unclear for PM SACs. Here, a sequential coordination method is developed to dope a large amount of PM atoms (Ir, Rh, Pt and Pd) into a zeolite imidazolate framework (ZIF), which are further pyrolyzed into nitrogen‐coordinated PM SACs. The PM loadings are as high as 1.2–4.5 wt%, achieving the highest PM loadings in ZIF‐derived SACs to date. In the acidic half‐cell, Ir1‐N/C and Rh1‐N/C exhibit much higher oxygen reduction reaction (ORR) activities than nanoparticle catalysts Ir/C and Rh/C. In the contrast, the activities of Pd1‐N/C and Pt1‐N/C are considerably lower than Pd/C and Pt/C. Density function theory (DFT) calculations reveal that the ORR activity of PM SAC depends on the match between the OH* adsorption on PM and the electronegativity of coordinating anions, and the stronger OH* adsorption is, the higher electronegativity is needed for the coordinating anions. PEMFC tests confirm the active‐site coordination principle and show the extremely high atomic efficiency of Ir1‐N/C. The revealed principle provides guidance for designing future PM SACs for PEMFCs.