Proton Conductance Pathway Research Articles

Influence of Operating Conditions on the Mass Activity Characteristics and Performance of Carbon-Free Pt Black Electrodes for PEFC

In recent years, the development of polymer electrolyte fuel cells (PEFCs) has been progressing. To expand their application to heavy-duty vehicles (HDVs), enhancing the power density and heat dissipation efficiency of PEFCs through high-temperature operation has become essential. Therefore, there is a need for high durability to enable the high-temperature operation of electrodes. In this context, we focused on carbon-free ionomer-free electrodes utilizing unsupported catalysts such as Pt black and low-Pt yet highly durable platinum nanosheets (Pt-ns). These electrodes offer high durability by avoiding the use of carbon and ionomer, which are factors contributing to performance degradation. Moreover, these electrodes have a different proton conduction pathway compared to conventional electrodes, relying on water on the Pt surface. Therefore, water management becomes crucial, considering the characteristics of carbon-ionomer-free electrodes. This study aims to elucidate the characteristics of the performance in the low-current region of Pt black electrodes and the effect of operating conditions on power generation characteristics.The performance of ionomer-free Pt black electrodes and ionomer-added Pt black electrodes was compared in terms of their IV characteristics. In the high-current region (>1000 mA) where concentration overpotential dominates, ionomer-free Pt black electrodes exhibited a gentler slope, indicating superior gas diffusion compared to ionomer-added Pt black electrodes. Conversely, in the low-current region (<100 mA), ionomer-free Pt black electrodes showed higher power generation performance, attributed to ionomer-induced catalyst poisoning reported in literature.Additionally, the IV characteristics of ionomer-free electrodes using different types of unsupported Pt catalysts, Pt black, and Pt particles were examined. Ionomer-free Pt particle electrodes displayed lower performance across the entire current range compared to ionomer-free Pt black electrodes, with a convex curvature of the IV curve indicating concentration overpotential. Analysis of electrochemical surface area (ECSA) and mass activity (MA) showed that ionomer-free Pt particle electrodes exhibited lower ECSA and MA compared to ionomer-free Pt black electrodes, indicating a lack of effective reaction sites, likely due to limited proton conduction.The IV characteristics of ionomer-free and ionomer-added Pt black electrodes at different relative humidities were investigated. Ionomer-free Pt black electrodes demonstrated consistent performance across relative humidities in the range of 1 to 10 mA, but as humidity increased, the curvature of the IV curve steepened, indicating higher activation overpotential. In the region above 100 mA, the curvature of the IV curve became concave with increasing current under low humidity, suggesting localized current density distribution due to proton conduction resistance as electrolyte water increased.AcknowledgmentsPart of this paper is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).

Synthesis, Thermal Stability, and Proton-Conductivity of the Mixed-Cation Phosphate with One-Dimensional Tunnels Formed by PO4 Tetrahedral Rings

The Benitoite-type phosphate, KMg1-x H2 x (PO3)3·yH2O, is a promising material as a solid electrolyte for automotive fuel cells due to its high proton conductivity from room temperature to 500°C1). The Benitoite-type KMg1-x H2 x (PO3)3·yH2O has a layered structure stacking the units formed by the units formed by P3O9 ring and those composed of (KO6)(MgO6) octahedral units. The P3O9 ring forms one-dimensional tunnels with a cage large enough to occupy H2O(or H3O+), which would provide proton conduction pathway. However, the correlation between the local structure and proton conductivity of this material has not yet been investigated. In this study, with the aim of further improving proton conductivity, we synthesized Benitoite-type phosphate with new compositions by introducing protons as charge compensation by replacing Mg2+ with Li+.KMg1-x Li x H x (PO3)3·yH2O were synthesized by a coprecipitation method. In the X-ray diffraction patterns for the samples with x = 0 - 0.3, the formation of the single phase of Benitoite-type structure with the space group P-6c2 was confirmed. Both the a and c axes tend to decrease with x in these compositions reflecting the formation of a solid solution. Fig.1 shows FT-IR spectra of KMg1-x Li x H x (PO3)3·yH2O. In the FT-IR spectra, the absorption peaks corresponding to the vibrational modes of POH and OH became relatively stronger for x, suggesting an increase in the proton contents in the samples. The lattice shrinkage by replacing Li+(0.76 Å) with an ionic radius larger than Mg2+(0.72 Å) would be caused by increasing of hydrogen bonds in the structure. In the TG/DTA measurement, the dehydration reactions from room temperature and 150°C were observed, and the water content increased with increasing x. This is suggested that the acidity of the framework is enhanced by the introduced protons, causing water molecules tend to incorporate into the crystal structure as Lewis bases. The shape of the pellet with x > 0.1 was not retained after retained under 80% relative humidity at room temperature.The conductivity of samples was measured by using AC impedance method. The impedance plots for the sample depict a part of semicircle due to the grain boundary and spike caused by the electrode contribution. The total resistance was determined from the intersection of the spike and the real axis, and the conductivity was calculated. The proton conductivity of the sample with x = 0.1 was higher than that of x = 0 under a humidified N2 gas flow (P H2O = 3 kPa). the sample with x = 0.1 showed high proton conductivity exceeding 10-3 S cm-1 from room temperature to 500 ℃, and the highest value of 1.9×10-2 S cm-1 was observed at 200°C under a humidified N2 gas flow( P H2O = 3 kPa). Only the sample with water of crystallization exhibited high conductivity, which suggests domination of proton diffusion via water of crystallization. The conductivity of the Li-substituted Benitoite tends to show higher proton conductivity than Mg-deficiency sample at the same proton content.Reference1) Y. Matsuda, et al, Dalton Trans., 50, 7678 (2021).AcknowledgementThis research was partially supported by Kanto Branch of the Electrochemical Society of Japan, JSPS KAKENHI Grant Number 24K08584 and 21K05246. Figure 1

Chiral Supramolecular Proton Conductors: Harnessing Highly Charged Zirconium-Amino Acid Oxo-Clusters.

Incorporation of amino acid capping molecules (alanine (Ala), methionine (Met), phenylalanine (Phe), tryptophan (Trp), tyrosine (Tyr), and valine (Val)) in their zwitterionic form into archetypal [Zr6(μ3-O)4(μ3-OH)4]12+ clusters creates supramolecular frameworks in which the assembly of these highly charged discrete units with chloride counterions provides a unique combination of porosity, chirality, and proton conductivity. The supramolecular frameworks assembled from these cluster entities (i.e., ZrAla, ZrMet, ZrPhe, ZrTrp, ZrTyr, and ZrVal) are based on the counterbalancing of the cationic hexanuclear entities by chloride anions. The resulting structures provide porous structures (except ZrVal) with variability of chemical and structural stability based on their supramolecular interactions. Among these compounds, ZrPhe and ZrVal remain stable due to the presence of a double chelation-like interaction involving four hydrogen bonds formed between a chloride anion, two ammonium groups, and two coordinated water molecules from two adjacent hexanuclear units. The presence of multiple acidic proton positions and strong hydrogen-bond donor/acceptor groups on the water channels gives rise to easy proton conduction pathways within the structures. In fact, ZrPhe and ZrVal, together with ZrTyr, exhibit high proton conductivity values with varying dependencies on the atmospheric humidity. Finally, the correlation between proton conduction and porosity is discussed.

Effect of Salt Bridge Formation on Proton Transport in CNTs

Various sectors have initiated technology development projects aimed at realizing a hydrogen-based society with low carbon dioxide (CO2) emissions. A proton (H+) transfer system with high permeability and selectivity is essential for the hydrogen infrastructure and fuel cells that support a hydrogen-based society. Therefore, intensive research is underway to design bioinspired artificial proton channels. Natural proteins achieve high proton selectivity by organizing proton-conducting pathways with scattered ionizable side chains and controlling proton transport through Vehicle and Grotthuss mechanisms. The Hv1 channel is a proton-selective protein that contains the amino acids aspartic acid (Asp) and arginine (Arg) in its pathway. As shown in Figure 1, the proton transport mechanism of Hv1 channels is suggested that Asp and Arg can be protonated, allowing them to transport protons via the Grotthuss mechanism. Additionally, Asp and Arg form salt bridges to close the pathway, thereby inhibiting the transport of other ions via the vehicle mechanism. However, the relationship between Asp and Arg and their impact on the transport mechanism remains unclear. It is important to elucidate the relationship between the inhibition of transport and pore size, as well as to understand the correlation between the inhibition of transport and salt bridges, to install proton transport mechanism of Hv1 channels into artificial proton channels. In this study, carbon nanotubes (CNTs) were used as a model pore for artificial ion channels, and we analyzed the effects different sizes of CNTs modifying with Asp and Arg on ion transport properties using molecular dynamics simulations.We adopted the armchair CNTs model shown in Table 1, using the CNTs chirality (5,5), which is known to spontaneously fill with water at room temperature, as the smallest model. For the molecular models of Asp and Arg, side chain models of Asp(N)–Arg(N) and Asp(-)– Arg(+) pairs, which can form salt bridges, were modified inside CNTs.The results of the probability of hydrogen bond formation are shown in Figure 2. The results confirm that hydrogen bonds are always maintained at a pore radius R = 4.75 Å (CNTs chirality = 7,7) for both conditions. As the distance between Asp and Arg increases with increasing pore radius R above R = 4.75 Å, it appears that the hydrogen bonds cannot be maintained. Even when the pore radius is smaller than R = 4.75 Å, the probability of hydrogen bonding formation decreases. In the poster, we will present further structural analyses, such as the orientations of Asp and Arg and their relation to the hydronium ion conformation.[1] Dudev, T., Musset, B., Morgan, D. et al. Selectivity Mechanism of the Voltage-gated Proton Channel, HV1. Sci Rep 5, 10320 (2015). Figure 1

Construction and Comparative Research of Two MOFs Proton Conducting Materials Containing Nitro Groups.

In field of electrochemistry, there has been a growing interest in the potential applications of proton-conducting metal-organic frameworks (MOFs). Therefore, how to design and synthesize MOFs with high proton conductivity is considered crucial. In this study, two examples of nitro-containing Cd-based MOFs, MOF-1 {[Cd3(TIPE)1.5(NO3)5Cl(H2O)2] ⋅ 17H2O}n and MOF-2 {[Cd(TIPE)0.5(nip)] ⋅ 10H2O}n (TIPE=1,1,2,2-tetrakis(4-(1H-imidazole-1-yl)phenyl)ethene, H2nip=5-Nitroisophthalic Acid), had been successfully designed and synthesized, and their proton-conducting properties were thoroughly investigated. Notably, both materials displayed peak proton conductivity at 98 % RH and 90 °C, exhibiting values of 9.13×10-3 and 3.00×10-3 S cm-1 for MOF-1 and MOF-2, respectively. The plausible proton conduction pathways and mechanisms were elucidated through structural analyses, water vapor adsorption studies, and the determination of activation energy (Ea) values. It was found that the difference in proton conductivity between MOF-1 and MOF-2 was mainly associated with the different water absorption rates of the samples. The uniqueness of this study was that for the first time conducted an in-depth study of the role of nitrate in proton conduction, providing new ideas for designing materials with excellent proton conductivity.

Electrospun Sulfonated Poly(ether ether ketone) and Chitosan/Poly(vinyl alcohol) Bifunctional Nanofibers to Accelerate Proton Conduction at Subzero Temperature.

Multilayered microstructures can accelerate the proton conduction process in proton exchange membranes (PEMs). Herein, we design and construct PEMs with microstructures based on bifunctional nanofibers and sulfonated poly(ether ether ketone) (SPEEK) nanofibers. Specifically, the bifunctional nanofibers composed of poly(vinyl alcohol) and chitosan are prepared and then combined with the electrospun SPEEK nanofibers. The stable microstructure is derived from the compatible interfacial property of nanofibers and the formed hydrogen bonds. The multilayered microstructure consisting of nanofibers accelerates the proton conduction even at subzero temperature because of regulating the proton conduction pathways. Specifically, the (SKNF/CPNF/SKNF)/PA membrane exhibits the proton conductivities of (0.951 ± 0.138) × 10-2 S/cm at -30 °C and (7.32 ± 0.37) × 10-2 S/cm at 160 °C. Additionally, the fine proton conductivity stability is demonstrated by the proton conductivity in the long-term test and the cooling/heating cycle test, such as 1.67 × 10-2 S/cm at -30 °C (after 1000 h), 4.52 × 10-2 S/cm at 30 °C (after 810 h), 1.12 × 10-2 S/cm at -30 °C, and 1.01 × 10-1 S/cm at 30 °C in the cooling/heating process (5 cycles). The single fuel cell possesses an open-circuit voltage of 0.886 V and a peak power density of 0.508 W/cm2 at 130 °C.

A crystalline imidazole-carboxylic acid-based hydrogen-bonded organic framework with high proton conductivity

A hydrogen-bonded organic framework (HOF-SZU-1) was successfully synthesized from an organic linker containing both imidazole and carboxylic acid groups and structurally characterized. The ordered hydrogen bond networks formed by imidazolium, carboxylic acid, and carboxylate groups were favorable to construct continuous proton-conductive pathways in the crystal structure. Therefore, at 100 °C and 98 % relative humidity, HOF-SZU-1 manifested a prominent proton conductivity of up to 4.0 × 10−3 S cm−1. It was also worth noting that HOF-SZU-1 exhibited excellent water/acid/alkali resistance, and structural integrity after the electrochemical test, indicating its extraordinary structural and chemical stability. Finally, the proton conduction process in the framework of the HOF was studied minutely.

Preparation of high temperature proton exchange membrane through covalent organic framework doped polyvinylidene fluoride nanofibers

Covalent organic framework (COF) exhibits the great potential to promote proton conduction under low relative humidity for proton exchange membranes (PEMs). In this research, quick and stable proton conduction has been achieved since the prepared COF and polyvinylidene fluoride (PVDF) nanofibers are believed to constitute successive proton conduction channels. The PVDF-COF nanofibers membrane is prepared through the in-situ growth of COF along PVDF nanofibers. The fine compatibility can drive the stable combination of COF with the PVDF nanofibers in PVDF-COF nanofibers. Most importantly, the fibrous PVDF nanofibers accelerate proton conduction through confining the proton conduction pathways. Additionally, ionic liquids of 1-butyl-3-methylimidazolium chloride (BmimCl) and 1-butyl-3-methylimidazole hexafluorophosphate (BmimPF6) as proton conduction carriers have been introduced to accelerate proton conduction in the prepared PVDF-COF/BmimCl/PA and PVDF-COF/BmimPF6/PA membranes. Phosphoric acid (PA) molecules are combined by COF and ionic liquid cations with the formation of intermolecular hydrogen bonds. As a result, the PVDF-COF/BmimPF6/PA membrane exhibits the proton conductivity of (1.81 ± 0.07) × 10−1 S/cm at 160 °C. The long-term proton conductivity stability is determined, deriving from the proton conductivities of 1.77×10−2 S/cm at 80 °C and 1.42×10−2 S/cm at 110 °C in the 400 h non-stop measurement. The single fuel cell equipped with the PVDF-COF/BmimPF6/PA membrane represents the open circuit voltage of 0.927 V and the peak power density of 178.8 mW/cm2 at 100 °C, 0.907 V and 326.5 mW/cm2 at 120 °C.

Rational Tuning the Proton Conductivity and Stability of Hydrogen-Bonded Organic Frameworks.

In the development of proton conductors, it is crucial to regulate proton conduction pathways and enhance structural stability. In this study, we designed and constructed three hydrogen-bonded organic frameworks (HOFs), namely, NKM-HOF-9, NKM-HOF-10, and NKM-HOF-11, with different dimensional hydrogen-bonding pathways using 4,4'-sulfonyldibenzoic acid and various bases. They are cost-effective and easy to synthesize, allowing for their large-scale production at room temperature. By purposefully altering the ammonium ions, we achieved enhancements in the conductivity and stability of these HOFs. Proton conductivity studies at different humidities and temperatures revealed that at 85 °C and 98% relative humidity, the proton conductivity of NKM-HOF-10 reached 1.7 × 10-3 S cm-1, surpassing that of NKM-HOF-9 by 1 order of magnitude. This improvement was accomplished by increasing the number of proton donors from the base, which resulted in a transition of the hydrogen bond network from discontinuous to continuous, thereby enhancing the proton conduction performance. Moreover, stability tests showed that raising the base's pKa could improve the stability of these frameworks. NKM-HOF-11, which features the highest pKa, demonstrated superior stability by maintaining its structural integrity even at 450 °C.

Proton Conducting Metal-Organic Frameworks (MOFs) via Post Synthetic Transmetallation and Water Induced Structural Transformations.

Post Synthetic Modification (PSM) of Metal-Organic Frameworks (MOFs) is a crucial strategy for developing new MOFs with enhanced functional properties compared to their parent one. PSM can be accomplished through various methods:1) modification of organic linkers; 2) exchange of metal ions or nodes; and 3) inclusion or exchange of solvent/guest molecules. Herein, PSM of bimetallic and monometallic MOFs containing biphenyl dinitro-tetra-carboxylates (NCA) are demonstrated. The tetra carboxylate NCA, produces monometallic Cd-MOF-1 and Cu-MOF-1 and bimetallic CoZn-MOF in solvothermal reactions with the corresponding metal salts. The CoZn-MOF undergoes post-synthetic transmetallation with Cd(NO3)2 and Cu(NO3)2 in aqueous solution to yield Cd-MOF-2 and Cu-MOF-2, respectively. Additionally, green crystals of Cu-MOF-1 found to undergo a single-crystal-to-single-crystal (SCSC) transformation to blue crystals of Cu-MOF-3 upon dipped into water at room temperature. These MOFs demonstrate notable proton conductivities ranging from 10-3 to 10-4 S cm-1 under variable temperatures and humidity levels. Among them, Cu-MOF-3 achieves the highest proton conductivity of 1.36×10-3 S cm-1 at 90 °C and 98 % relative humidity, attributed to its continuous and extensive hydrogen bonding network, which provides effective proton conduction pathways within the MOF. This work highlights a convenient strategy for designing proton-conducting MOFs via post-synthetic modification.

Tuning Proton Conduction by Staggered Arrays of Polar Preyssler-Type Oxoclusters.

Polyoxometalates (POMs), anionic nanosized oxoclusters that can be considered as fragments of metal oxides, have been extensively studied for their diverse composition and structure, showing promise in various fields such as catalysis and electronics. Proton conduction, relevant to catalysis and electronics, has attracted interest in materials chemistry, and POM anions are advantageous in terms of their proton carrier density and mobility. Recently, polar POMs have attracted attention for their unique ferroelectric behaviors, yet they have been little studied with regard to proton conduction, as their polarity has generally been believed to have a negative impact. Here, we propose that polar POMs can be used to align polar proton carriers, such as H2O and polymers, to construct efficient proton-conducting pathways. In this study, we present ionic crystals composed of polar Preyssler-type POMs ([Xn+(H2O)P5W30O110](15-n)-, Xn+ = Ca2+, Eu3+) and K+ exhibiting ultrahigh proton conductivity surpassing 10-2 S cm-1, which is required for practical applications. In contrast, ionic crystals with nonpolar Preyssler-type POMs show an order of magnitude lower proton conductivity. Structural and spectroscopic studies combined with theoretical calculations reveal that proton carriers align with the aid of staggered arrays of polar POMs, forming a hydrogen-bonding network favorable for proton conduction. This study integrates molecular chemistry by the design of POMs and solid-state chemistry by exploring long-range proton conduction mechanisms, offering novel insights for future materials design.

Prominent Intrinsic Proton Conduction in Two Robust Zr/Hf Metal-Organic Frameworks Assembled by Bithiophene Dicarboxylate.

To date, developing crystalline proton-conductive metal-organic frameworks (MOFs) with an inherent excellent proton-conducting ability and structural stability has been a critical priority in addressing the technologies required for sustainable development and energy storage. Bearing this in mind, a multifunctional organic ligand, 3,4-dimethylthiophene[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), was employed to generate two exceptionally stable three-dimensional porous Zr/Hf MOFs, [Zr6O4(OH)4(DTD)6]·5DMF·H2O (Zr-DTD) and [Hf6O4(OH)4(DTD)6]·4DMF·H2O (Hf-DTD), using solvothermal means. The presence of Zr6 or Hf6 nodes, strong Zr/Hf-O bonds, the electrical influence of the methyl group, and the steric effect of the thiophene unit all contribute to their structural stability throughout a wide pH range as well as in water. Their proton conductivity was fully examined at various relative humidities (RHs) and temperatures. Creating intricate and rich H-bonded networks between the guest water molecules, coordination solvent molecules, thiophene-S, -COOH, and -OH units within the framework assisted proton transfer. As a result, both MOFs manifest the maximum proton conductivity of 0.67 × 10-2 and 4.85 × 10-3 S·cm-1 under 98% RH/100 °C, making them the top-performing proton-conductive Zr/Hf-MOFs. Finally, by combining structural characteristics and activation energies, potential proton conduction pathways for the two MOFs were identified.

Energetics of the H-Bond Network in Exiguobacterium sibiricum Rhodopsin.

Exiguobacterium sibiricum rhodopsin (ESR) functions as a light-driven proton pump utilizing Lys96 for proton uptake and maintaining its activity over a wide pH range. Using a combination of methodologies including the linear Poisson-Boltzmann equation and a quantum mechanical/molecular mechanical approach with a polarizable continuum model, we explore the microscopic mechanisms underlying its pumping activity. Lys96, the primary proton uptake site, remains deprotonated owing to the loss of solvation in the ESR protein environment. Asp85, serving as a proton acceptor group for Lys96, does not form a low-barrier H-bond with His57. Instead, deprotonated Asp85 forms a salt-bridge with protonated His57, and the proton is predominantly located at the His57 moiety. Glu214, the only acidic residue at the end of the H-bond network exhibits a pKa value of ∼6, slightly elevated due to solvation loss. It seems likely that the H-bond network [Asp85···His57···H2O···Glu214] serves as a proton-conducting pathway toward the protein bulk surface.

Tunnel-Structured Phosphate Exhibiting High Proton Conductivity and Thermal Stability over a Wide Intermediate Temperature Range.

For the practical application of fuel cells in vehicles, it is a challenge to develop a proton solid electrolyte that coexhibits thermal stability and high proton conductivity at wide intermediate temperatures. Here, we report on the tunnel structured phosphate KNi1-xH2x(PO3)3·yH2O, which exhibits high proton conductivity at room temperature up to 500 °C, with the conductivity value reaching 1.7 × 10-2 S cm-1 at 275 °C for x = 0.18. This material, composed of the smallest cations that form the tunnel framework with face-shared (KO6) and (NiO6) chains and PO4 tetrahedral chains, retained the rigid framework up to 600 °C. Two oxygen sites of water molecules located adjacent to each other along the PO4 tetrahedral chains in the tunnel provided the proton conduction pathway. The sample maintained a conductivity of 5.0 × 10-3 S cm-1 for 10 h at 150 °C while changing the measurement atmosphere to a N2 gas flow, a 4% H2-96% Ar gas flow, and an O2 gas flow. The conductivity value at x = 0.18 obtained from the DC measurement was in the order of 10-6 S cm-1, close to the instrument's measurement limit. These results demonstrate that tunnel phosphate has potential as a proton solid electrolyte for next-generation fuel cells.

Constructing sandwich-like microstructure based on multi-nanofibers to accelerate proton conduction at subzero temperature

The well-ordered microstructure has been demonstrated to accelerate the proton conduction process through reducing proton conduction resistance. In this research, the proton exchange membranes (PEMs) with sandwich-like microstructure have been constructed based on Kevlar nanofibers and bifunctional nanofibers of chitosan (CS) with polyvinyl alcohol (PVA). The CS/PVA bifunctional nanofibers (CPNF) layer has been prepared with electrospinning process, which is stably adhered to the Kevlar nanofibers layer owing to compatible interfacial property. The fine structure stability without layer separation is achieved even with phosphoric acid (PA) molecules doping in the prepared PA doped composite membranes. The fibrous microstructure accelerats proton conduction through regulating proton conduction pathways. The objective of accelerating proton conduction at subzero temperature has been realized, reflecting in high and stable proton conductivities. For instance, the (Kevlar/CPNF/Kevlar)/PA membrane exhibits proton conductivities of 9.75 × 10−3 S/cm at −30 °C and 8.22 × 10−2 S/cm at 30 °C. Most importantly, the fine proton conductivity stability is identified by the results of the cycle test and long-term test. Specifically, the proton conductivities are respectively 8.62 × 10−3 S/cm at −30 °C and 8.07 × 10−2 S/cm at 30 °C after a five heating/cooling cycle process. The proton conductivities remain 2.97 × 10−2 S/cm at −25 °C and 7.19 × 10−2 S/cm at 30 °C after a 1032 h non-stop test. Additionally, the single proton exchange membrane fuel cell with the (Kevlar/CPNF/Kevlar)/PA membrane exhibits the peak power densities of 279.5 mW/cm2 at 100 °C and 352.7 mW/cm2 at 130 °C.

Constructing amino-functionalized flower-shaped ZnO@Al2O3 nanofibers composite membrane (ZANFs/SPSF) with hierarchical proton conductive pathways and excellent methanol resistance

Traditional nanofiber-hybrid proton exchange membranes (PEMs) are limited in their further application due to differences in proton conductivity between vertical and horizontal orientations. In this study, we design and synthesize a novel type aminated flower-shaped ZnO@Al2O3 nanofibers and then filling its three-dimensional interstices with sulfonated polysulfone (SPSF) to obtain a nanofiber composite proton exchange membrane (ZANFs/SPSF). The Al2O3 nanofibers provide long-range and stable proton transport nanochannels in plane, while the flower-shaped ZnO on its surface provide additional vertical transport channels for protons. This combination of 1D and 2D conductive pathways forms a typical 3D hierarchical proton conductive configuration, optimizing the proton transfer pathway. Specifically, the amino groups (-N-H) on the nanofibers surface not only form acid-base pairs with sulfonic acid groups (-SO3H), providing a rich hydrogen bonding network for proton hopping, but also have a similar structure to the polyether sulfone groups (–SO2–NH-), which significantly improves the compatibility between the nanofibers and SPSF. Additionally, the flower-like pleated surface of the nanofibers has a high contact area with the SPSF, providing more proton hopping sites, improving the utilization efficiency of proton carriers, and also resulting in a more stable structure. Moreover, the crosslinked network of nanofibers serves as a barrier to further inhibit methanol permeation. The ZANFs/SPSF exhibits excellent parallel proton conductivity of 348 mS cm−1 at 80 °C and 100% RH, which is 2.7 times higher than that of SPSF under the same conditions, and achieves a peak power density of 405.7 mW cm−2. This approach presents a unique perspective for the preparation of high-performance PEMs for methanol fuel cells.

Maximizing the Potential of Electrically Conductive MOFs.

ConspectusElectrically conductive metal-organic frameworks (EC-MOFs) have emerged as a compelling class of materials, drawing increasing attention due to their unique properties facilitating charge transport within porous structures. The synergy between electrical conductivity and porosity has opened a wide range of applications, including electrocatalysis, energy storage, chemiresistive sensing, and electronic devices that have been underexplored for their insulating counterparts. Despite these promising prospects, a prevalent challenge arises from the predominant adoption of two-dimensional (2D) structures by most EC-MOFs. These 2D frameworks often show modest surface areas and short interlayer distances, hindering molecular accessibility, which deviates from the inherent characteristics of conventional MOFs. Furthermore, the quest for efficient charge transport imposes design constraints, leading to a restricted selection of functional building blocks. Additionally, there is a lack of established functionalization methods within EC-MOFs, limiting their functional diversity. Thus, these challenges have impeded EC-MOFs from reaching their full potential.In this Account, we summarize and discuss our group's efforts aimed at enhancing molecular accessibility and deploying the functional diversity of EC-MOFs. Our focus on enhancing molecular accessibility involves several strategies. First, we employed macrocyclic ligands with intrinsic pockets as the building blocks for EC-MOFs. The integrated intrinsic pockets in the frameworks supplement surface areas and additional pores to enhance molecular accessibility. The resulting macrocyclic ligand-based EC-MOFs exhibit exceptionally high surface areas and confer advantages in electrochemical performances. Second, our efforts extend to addressing the structural limitations, frequently associated with EC-MOFs' 2D structures. Through the pillar insertion strategy, we transformed a 2D EC-MOF platform into a three-dimensional (3D) structure, thereby achieving higher porosity and enhanced molecular accessibility. In pursuing functional diversity, we have delved into molecular-level tuning of EC-MOF building blocks. We demonstrated that electron-rich alkyne-based pockets in the macrocyclic ligands can host transition metals and alkali ions, enabling ion selectivity and showcasing diverse use of EC-MOFs. We utilized a postsynthetic approach to further functionalize metal nodes on the molecular level within an EC-MOF framework, introducing a proton-conducting pathway while preserving its electrical conductivity.We aspire for this Account to provide practical insights and strategies to surmount structural and functional diversity limitations in the realm of EC-MOFs. By integrating enhanced molecular accessibility and diverse functionality, our endeavor to propel the utility of these materials will inspire further rational development for future EC-MOFs and unlock their full potential.

A Rheo-Impedance investigation on the interparticle interactions in the catalyst ink and its impact on electrode network formation in a proton exchange membrane fuel cell

The performance of a proton exchange membrane fuel cell is highly dependent on the microstructure of the catalyst layer. The catalyst layer consists of a network of platinum supported by carbon particles mixed with an ion-conductive ionomer. Optimum coverage of ionomer on catalyst is essential in providing proton conductive pathways while maintaining minimal oxygen transport resistance. Considering that most of the catalyst and ionomer interactions occur as a result of ink formulation and evolution during the high-shear coating process, a new methodology involving a combination of ink flow properties and electrochemical impedance spectroscopy is introduced to study interparticle interactions in catalyst ink. Five ink samples with ionomer-to-carbon ratios ranging from 0 to 1 are used to investigate the interplay between ionomer and catalyst. The flow and impedance properties of the ink are measured simultaneously to investigate the agglomerate characteristics and the percolation of conductive catalyst in the catalyst ink, while fixing the carbon to solvent ratio. This study successfully demonstrate the effectiveness of the newly developed characterization tools, which can be used for evaluating new materials and processing parameters. The insights gained from this study can be applied to optimize the triple-phase-boundaries and form efficient electrode structure with low Pt loading to enhance overall fuel cell performance.

Electrospinning-assisted construction of rapid proton conduction channels in halloysite nanotube-encapsulated ionic liquid-embedded sulfonated poly(ether ether ketone) proton exchange membranes

The most fatal drawback of traditional sulfonated polymer proton-exchange membranes (PEMs) is their heavy dependence on water molecules to conduct protons, which causes a sharp decrease in their proton conductivity in low-humidity environments. Here, composite membranes were fabricated by electrostatically spinning halloysite nanotube-encapsulated ionic-liquids (IL@HNTs) incorporated into sulfonated poly(ether ether ketone) (SPEEK) (SPEEK/IL@HNTs). Using electrospinning, one-dimensional IL@HNTs ionogels with good water absorption were axially and uniformly aligned along the fiber filament direction. This formed long-range uninterrupted proton conduction pathways that greatly improved the proton conductivity of the composite membrane under various humidity environments. The axially-aligned structure facilitated crystallization, which improved the mechanical properties and thermal stability of the composite membrane. The SPEEK/6IL@HNTs membrane exhibited a through-plane proton conductivity of 139.2 mS cm−1 at 90 °C/98 % RH, representing a significant increase compared with the intrinsic SPEEK membrane. The SPEEK/6IL@HNTs membrane demonstrated a maximum power density of 732 mW cm−2, which exceeded that of the Nafion115 membrane (328 mW cm−2) by a factor of 2.2. It exhibited a single-cell performance equivalent to that of the Nafion212 membrane (723 mW cm−2), demonstrating that this provides a feasible approach to operating fuel cells in low-humidity environments without additional humidification.

Improved performance of lanthanide-doped UIO-66/Nafion hybrid proton exchange membrane for water electrolyzer

Incorporating metal-organic frameworks (MOFs) especially the ones with protic functional groups into proton exchange membrane (PEM) can effectively improve its proton conductivity. Lanthanide MOFs (Ln-MOFs) have been recognized as more potential candidates exhibiting proton dissociation capability. However, the performance of PEM upon integrating Ln-MOFs in water electrolyzer has yet to be investigated. In this study, Nafion composites with different loadings of lanthanide Ce-UiO-66 were prepared, characterized and tested by comparing with the non-lanthanide Zr-UiO-66. It was revealed that the proton conductivity of Ce-UiO-66/Nafion composite membrane was higher than pristine Nafion and Zr-UiO-66/Nafion membrane, which was attributed to the fact that the Ce-UiO-66 doping contributed to the construction of the hydrogen-bonded network to enhance the proton conductivity. At 3 wt% Ce-UiO-66 doping, the proton conductivity of the composite membrane can reach 124.45 mS/cm, which is significantly improved by 17 % compared with Zr-UiO-66/Nafion. Such a trend is mainly contributed by the lower proton affinity and proton transfer capability of Ce atoms in Ce-UiO-66, and the higher number of water molecules around Ce atoms favors the reduced tortuosity of the proton conduction pathway in membranes. Single electrolytic cell tests demonstrated the enhancement of electrolysis efficiency upon Ce-UiO-66 doping was improved from 58.77 % to 65.37 % at a current density of 2 A cm−2.