Proton Transfer Channels Research Articles

Design and Assembly of Conductive Covalent Organic Frameworks for Proton Exchange Membrane Application

AbstractTo develop proton exchange membranes (PEMs) with robust structure stability and remarkable proton conductivity and explore their application in PEMs fuel cells have significant implications for realizing reduced carbon emission and environmental pollution. Covalent organic frameworks (COFs), as crystalline porous polymer material composed of organic monomers and connected by covalent bond, possess specific framework, inherent porosity, adjustable functional group and eminent thermal/chemical structure stability. Therefore, COFs display prominent superiorities in constructing rigid ordered proton transfer channels and improving fuel cell performance long‐term durability. In this review, the proton conduction properties of extrinsic proton‐conductive COFs (incorporating carriers into the pore), intrinsic proton‐conductive COFs (introducing conductive groups on the backbone) and combined extrinsic/intrinsic proton‐conductive COFs in the form of pressed pellets are discussed in detail. Meanwhile, proton‐conductive COFs related PEMs, including COFs‐related polymer‐based composite membranes, COFs‐based composite membranes and self‐supporting COFs membranes are also systematically summarized. In addition, the existing challenges are analyzed and future outlooks are addressed.

An effective strategy to enhance phosphoric acid retention and proton conductivity stability: Construction of proton transfer channels with starch rather than H3PO4

To enhance the phosphoric acid (PA) retention as well as maintain high proton conductivity of phosphoric acid doped proton exchange membranes at high temperature, we successfully design a series of phosphoric acid doped biobased composite membranes by incorporation of starch and graphene oxide (GO) into poly arylene ether ketones (PAEK). Proton transfer channels should be mainly built through dense hydrogen bonds formed from massive oxygen-containing groups of starch mainchain, which is confirmed by Molecular dynamics (MD) simulation, FT-IR and XRD analysis. The dense hydrogen-bond structure could construct fast proton transfer channels with extreme low doping level (0.00484 molH3PO4). The excellent PA retention properties with almost unchanged proton conductivity at high temperature (200 °C) for 600 min indicates that PA molecules are firmly fixed into membranes. Thus, in this study, we suggest a novel strategy for stablizing proton conductivity at high temperature and improving PA retention properties of PA doped membranes, which is building dense hydrogen-bond structure with low PA doping level.Based on the results in this study and the Grotthuss proton transfer mechanism, dense hydrogen-bonds from oxygen-containing groups in polymer backbones should be more stable than hydrogen-bonds from massive H3PO4 molecules with high acid doping levels to promote proton conduction.

Reasonable construction of low migration energy barrier and continuous proton transfer channels via bimetallic MOFs nanofibers (MIL-(Cr Al)–NH2) for its composite proton exchange membrane

Reasonable construction of low migration energy barrier and continuous proton transfer channel is the key to improving the performance of composite proton exchange membranes. In this paper, we prepared a novel bimetallic MOFs (MIL-(Cr Al)–NH2) nanofibers (MCANs) consisting of Cr and Al elements by electro-blown spinning method (EBS) and hydrothermal synthesis, and subsequently introduced them into non-fluorinated sulfonic acid polymer sulfonated polysulfone (SPSF) matrix to obtain composite proton exchange membranes (MCANs@SPSF). Importantly, MCANs successfully constructed relatively long-range continuous unsaturated metal site proton transfer channels provided by the Cr-based MOFs, thereby reducing the proton migration energy barrier in the MCANs@SPSF, which confirmed by the density functional theory (DFT) calculation. In detail, DFT studies indicated that the migration energy barrier between H+ and MCANs was 1.677 eV, while that of single Al-based MOF (MAN) and single Cr-based MOF (MCN) were 1.879 eV and 1.762 eV respectively. The existence of efficient low migration energy barrier and continuous proton transfer channels made the proton conduction efficiency of MCANs@SPSF greatly improved, that was under the condition of 80 °C and 100 % RH, the proton conductivity of MCANs@SPSF reached the maximum value of 0.358 S cm−1 and the power density reached 104 mW cm−2. Moreover, MCANs@SPSF-5 displayed best DMFC durability performance with only 14.65 % voltage loss under 24h test. In addition, due to the acid-base pair interaction between MCANs and SPSF enhanced the interface binding force between the two phases, meanwhile, the pore structure of MOFs also had a certain adsorption effect on methanol, thus reducing the methanol permeability, and the addition of 5 wt% MCANs in SPSF was tested to reduce the methanol permeability to 5.53 × 10-7 cm2 s−1. Overall, this work provided a new strategy for construction of low migration energy barrier and continuous proton transfer channels, which was beneficial for the development of high performance proton exchange membranes.

Searching for proton transfer channels in respiratory complex I

We have explored a strategy to identify potential proton transfer channels using computational analysis of a protein structure based on Voronoi partitioning and applied it for the analysis of proton transfer pathways in redox-driven proton-pumping respiratory complex I. The analysis results in a network of connected voids/channels, which represent the dual structure of the protein; we then hydrated the identified channels using our water placement program Dowser++. Many theoretical water molecules found in the channels perfectly match the observed experimental water molecules in the structure; some other predicted water molecules have not been resolved in the experiments. The channels are of varying cross sections. Some channels are big enough to accommodate water molecules that are suitable to conduct protons; others are too narrow to hold water but require only minor conformational changes to accommodate proton transfer. We provide a preliminary analysis of the proton conductivity of the network channels, classifying the proton transfer channels as open, closed, and partially open, and discuss possible conformational changes that can modulate, i.e., open and close, the channels.

Construction of wide-temperature-range proton exchange membrane by regulating proton transfer channels based on phosphate-functionalized carbon nanotubes

Phosphoric acid (PA) doped polybenzimidazole (PBI) high-temperature proton exchange membrane is favored because it can achieve faster proton transfer at 180 °C without additional humidification. However, the PA-PBI membrane is faced with slow PA proton conduction kinetics below 100 °C, and failing rapid start-up of fuel cells. Herein, carbon nanotubes functionalized with phosphate groups (P-CNT) and sulfonated poly [2,2'-(p-hydroxydibenzene)-5,5′-benzimidazole] (SOPBI) composite membranes are prepared as proton exchange membranes with wide temperature range (40–180 °C). Based on the regulation of monophosphate and bisphosphate groups on P-CNT, the bisphosphate group can strengthen the vehicular mechanism of proton transport by accelerating the formation and diffusion of H3O+ at low temperature/high humidity, and the Grotthuss mechanism of proton transport by constructing dense hydrogen bond network at high temperature/low humidity. The 2P-CNT(6 %)/SOPBI composite membrane shows excellent proton conductivity (131.8 mS cm−1 at 80 °C/90%RH, 170.5 mS cm−1 at 180 °C/0%RH) and peak power density (378 mW cm−2 at 80 °C/100 % RH, 531 mW cm−2 at 160 °C/0 % RH) under wide temperature conditions, surpassing SOPBI membrane and other composite membranes. Furthermore, the 2P-CNT(6 %)/SOPBI composite membrane exhibits high proton conduction and fuel cell stability at both 80 °C/100 % RH and 160 °C/0 % RH.

Bubble-water/catalyst triphase interface microenvironment accelerates photocatalytic OER via optimizing semi-hydrophobic OH radical

Photocatalytic water splitting (PWS) as the holy grail reaction for solar-to-chemical energy conversion is challenged by sluggish oxygen evolution reaction (OER) at water/catalyst interface. Experimental evidence interestingly shows that temperature can significantly accelerate OER, but the atomic-level mechanism remains elusive in both experiment and theory. In contrast to the traditional Arrhenius-type temperature dependence, we quantitatively prove for the first time that the temperature-induced interface microenvironment variation, particularly the formation of bubble-water/TiO2(110) triphase interface, has a drastic influence on optimizing the OER kinetics. We demonstrate that liquid-vapor coexistence state creates a disordered and loose hydrogen-bond network while preserving the proton transfer channel, which greatly facilitates the formation of semi-hydrophobic •OH radical and O-O coupling, thereby accelerating OER. Furthermore, we propose that adding a hydrophobic substance onto TiO2(110) can manipulate the local microenvironment to enhance OER without additional thermal energy input. This result could open new possibilities for PWS catalyst design.

High ion exchange capacity perfluorosulfonic acid resine proton exchange membrane for high temperature applications in polymer electrolyte fuel cells

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) have attract much attention from academic and industry, which can improve the catalyst activity, reduce Pt loading, enhance CO tolerance, and simplify water and heat management. High-temperature proton exchange membrane (HT-PEM) is the key component for HT-PEMFCs. Here, we provide an investigation of newly developed HT-PEM to evaluate its properties and performances in fuel cells above 90 °C. The HT-PEM exhibits an excellent proton conductivity of 136.1 mS cm−1 at 90 °C and 95% RH, and remarkable power density of 0.95 Wcm−2 at 105 °C and 80% RH. The stability and durability of HT-PEM are studied with varied testing methods. After the continuous operation at open circuit voltage (OCV) for 500 h and continuous dry-wet circulation for 20000 cycles at 90 °C, negligible change of OCV and hydrogen permeation current density are recorded, indicating good chemical and mechanical stability for HT-PEM. The improved property and performance originate from the new PFSA base material and improved morphology, in which the larger ionic clusters and continuous proton-transfer channels contribute to the superior performance, and the high glass transition temperature (Tg∼132 °C) induces good stability.

Reaction dynamics of the methoxy anion CH3O- with methyl iodide CH3I.

Studying larger nucleophiles in bimolecular nucleophilic substitution (SN2) reactions bridges the gap from simple model systems to those relevant to organic chemistry. Therefore, we investigated the reaction dynamics between the methoxy anion (CH3O-) and iodomethane (CH3I) in our crossed-beam setup combined with velocity map imaging at the four collision energies 0.4, 0.7, 1.2, and 1.6 eV. We find the two ionic products I- and CH2I-, which can be attributed to the SN2 and proton transfer channels, respectively. The proton transfer channel progresses in a previously observed fashion from indirect to direct scattering with increasing collision energy. Interestingly, the SN2 channel exhibits direct dynamics already at low collision energies. Both the direct stripping, leading to forward scattering, and the direct rebound mechanism, leading to backward scattering into high angles, are observed.

Ion molecule reactions in the HBr+ + CH4 system: a combined experimental and theoretical study.

Reactions in the system HBr+ + CH4 have been investigated inside a guided ion-beam apparatus under single-collision conditions. The HBr+ is vibrational and rotational state selected in the electronic X2Π1/2 state created by (2+1)-REMPI. Due to the exitation scheme employed different rotational states of the HBr+ are accessible. Four reaction channels have been observed. The cross section, σ, for the exothermic proton transfer channel (PT) decreases with increasing collision energy, steeper than predicted by the Langevin model. The cross section also decreases with increasing rotational energy in the HBr+, with the effect of the rotational energy being stronger than that of translational energy. The cross section for the endothermic charge transfer (CT) increased with increasing collision energy. The energy dependence is well reproduced by a simple line of center (loc) model. Although the bromine transfer (BT) is exothermic the observed cross section increased with increasing collision energy due to an activation barrier on the potential energy surface (PES). Analysis by a modified loc model suggest the relevance of an angle dependence of σ. The cross section for the endothermic hydrogen atom abstraction (HA) exhibits a maximum at 2 eV Ecm. The measured cross sections are rationalized by means of reaction dynamics simulations which show good agreement with the experimental cross sections. The dynamics simulations are carried out with a machine learning potential that is developed and benchmarked with ab initio molecular dynamics simulation. The absolute cross sections predicted by reaction dynamics simulations are well within the same order of magnitude while reproducing the trends over three different collision energies for all four reaction channels. Furthermore, the simulations demonstrate various reaction mechanisms for these reaction channels, including a very interesting HBr+ orientation selectivity for the BT reaction channel.

Paying Comprehensive Attention to the Temperature-Dependent Dual-Channel Excited-State Intramolecular Proton Transfer Mechanism of Fluorescence Ratio Probe BZ-DAM.

The mechanism of fluorescence detection of diethyl chlorophosphate (DCP) based on 2-substituted benzothiazole (BZ-DAM) was studied by a theoretical calculation method. It should not be ignored that both the BZ-DAM and the detection product BZ-CHO have two excited-state intramolecular proton transfer (ESIPT) channels. Density functional theory (DFT) and time-dependent DFT (TDDFT) theory were used to study the photophysical mechanism of two compounds in two channels in (acetonitrile) ACN solvent, and the temperature dependence of the two channels was given. Channel 1 is more likely to exist at low temperatures and channel 2 is more likely to exist at high temperatures. By theoretical analysis of the constructed potential energy curve, the hydrogen bond energy and electron-hole analysis, we confirmed that both molecules undergo ESIPT and intramolecular charge transfer (ICT) processes in channel 1 and ESIPT and twisted intramolecular charge transfer (TICT) coupling processes in channel 2. The formation of product BZ-CHO molecules led to a significant fluorescence blue-shift phenomenon and inhibited the ICT process, which confirmed that BZ-DAM could be used as a fluorescence probe for fluorescence detection. We sincerely hope that this work will not only help to clarify the excited-state dynamics behavior of the BZ-DAM probe but also provide a new idea for designing and optimizing a new chemical dosimeter.

Large Scalable High Ion Conductive and Selective Double Layer Membrane Achieved through Dual Ion Conducting Mechanisms

As the share of sustainable energy sources in the electricity supply rises, large-scale energy storage becomes increasingly important. All vanadium flow batteries, as one of the most promising large-scale energy storage methods, have reached the commercialization stage because of their unique feature of uncoupling power and energy, as well as the advantages of long life, high safety, and low cost. The ion-selective membrane is an essential part of flow batteries, which allows specific ions to pass through to maintain a neutral charge and prevents the crossover of active species to maintain the capacity. However, the high cost and low ion-selectivity of the commercialized ion-selective membranes (i.e., Nafion membranes) hindered the further promotion of the flow batteries. Herein, we developed a large scalable double-layer ion selectivity membrane with a unique bifunctional nanoporous boron nitride (PBN) ion selective layer and a low-cost porous polyetherimide (PEI) ion conductivity and support layer. PBN was synthesized by a large scalable template-free method, which was further decorated with Nafion resin. The nano-sized and tortuous pores of the PBN can effectively block the crossover of vanadium ions and provide excellent ion selectivity based on the pore size exclusion mechanism. Furthermore, the super-acidic sulfonic acid groups of Nafion decorated on the nanoporous structure of PBN provide high-speed proton transfer channels that increase proton conductivity through both Grotthuss and vehicle mechanisms. The low-cost porous PEI ion conductivity and support layer was prepared by the phase separation method. The well-aligned asymmetric finger-like porous structure of the PEI membrane provides excellent ion conductivity. The addition of a hydrophilic polymer additive, polyvinylpyrrolidone, further increased its ion conductivity. The PBN–PEI double layer membrane, prepared by a simple spray coating method, demonstrated an excellent ion selectivity (1.49×108 mS cm−3 min), which is almost 15 times higher than that of the pristine PEI membrane (6.71×106 mS cm−3 min) while maintaining high ion conductivity (64 mS cm−1). The PBN–PEI membrane achieved superior performance (97% of coulombic efficiency, 94% of voltage efficiency, and 91% of energy efficiency at 40 mA cm−2) than the Nafion 115 membrane in the all-vanadium flow battery with higher discharge capacity at all current densities and high stability with a lower capacity fading rate (0.17% per cycle vs. 0.74% per cycle at 100 mA cm−2). The PBN–PEI membrane also demonstrated a stable operation in the all-vanadium flow battery at a current density of 100 mA cm−2 over 700 cycles. The work presented the remarkable performance of the ultra-thin PBN bifunctional layer in terms of ion conductivity and selectivity; the combination with the low-cost high ion-conductive porous PEI membrane and a simple fabrication process demonstrated a great potential for commercialization of the PBN-PEI double-layer membrane.

Nitrogen‐Rich Rigid Polybenzimidazole With Phosphoric Acid Shows Promising Electrochemical Activity and Stability for High‐Temperature Proton Exchange Membrane Fuel Cells

AbstractPolybenzimidazoles (PBIs) are the most promising binders for the catalyst layer (CL) in high‐temperature proton exchange membrane fuel cells (HT‐PEMFC). However, traditional commercial PBIs are not applied in binders because they do not enhance the electrochemical performance and because the related solvents are not environmentally friendly. In addition, proton transfer channels in PBIs are not investigated at the microscopic and atomic scales to date. In this study, a nitrogen‐rich rigid PBI binder containing pyridine, diazofluorene, and partially grafted nitrile (PBPBI‐3CN) is prepared with a functionalized structure, good thermal stability, and good solubility in an environmentally friendly solvent. A membrane electrode assembly (MEA) is fabricated with the PBPBI‐3CN binder, providing a high peak power density, low resistance, and good stability. The protonation, hydrogen bond networks, and platform for proton transfer are confirmed in the CLs. The protonation of PBPBI‐3CN occurs in two steps. First, some phosphoric acid (PA) molecules bind to nitrogen‐containing acidophilic groups via preliminary protonation; second, multiple PA molecules then interact with nitrogen‐containing acidophilic groups via further protonation. With protonation as the foundation, a sufficient amount of PA molecules form a hydrogen bond network, and proton transfer channels are established.

Nanosized Proton Conductor Array with High Specific Surface Area Improves Fuel Cell Performance at Low Pt Loading.

The use of ordered catalyst layers, based on micro-/nanostructured arrays such as the ordered Nafion array, has demonstrated great potential in reducing catalyst loading and improving fuel cell performance. However, the size (diameter) of the basic unit of the most existing ordered Nafion arrays, such as Nafion pillar or cone, is typically limited to micron or submicron sizes. Such small sizes only provide a limited number of proton transfer channels and a small specific area for catalyst loading. In this work, the ordered Nafion array with a pillar diameter of only 40 nm (D40) was successfully prepared through optimization of the Nafion solvent, thermal annealing temperature, and stripping mode from the anode alumina oxide (AAO) template. The density of D40 is 2.7 × 1010 pillars/cm2, providing an abundance of proton transfer channels. Additionally, D40 has a specific area of up to 51.5 cm2/cm2, which offers a large area for catalyst loading. This, in turn, results in the interface between the catalyst layer and gas diffusion layer becoming closer. Consequently, the peak power densities of the fuel cells are 1.47 (array as anode) and 1.29 W/cm2 (array as cathode), which are 3.3 and 2.9 times of that without array, respectively. The catalyst loading is significantly reduced to 17.6 (array as anode) and 61.0 μg/cm2 (array as cathode). Thus, the nanosized Nafion array has been proven to have high fuel cell performance with low Pt catalyst loading. Moreover, this study also provides guidance for the design of a catalyst layer for water electrolysis and electrosynthesis.

Incorporating functionalized graphene oxide in green material-based membrane for proton exchange membrane fuel cell application

A novel organic-inorganic nanocomposite membrane with green material based is prepared by incorporating sulfonated holey graphene oxide (SHGO) into nanocrystalline cellulose/polyvinyl alcohol (NCC/PVA) matrix as polymer electrolyte membrane (PEMs) via solution casting method. Introducing sulfonic acid group in graphene oxide (GO) yielded sulfonated graphene oxide (SGO) and the hole effect on the graphitic plane of GO for SHGO on NCC/PVA significantly improves the proton conductivity of membranes by creating more interconnected proton transfer channels and promoting proton transfer across the membrane. Highest proton conductivity values of 1.1 × 10-2 S cm-1 is obtained by NCC/PVA-SHGO-1.0 at 80o C under 100% relative humidity (RH). Additionally, mechanical and chemical stability tests indicate an enhancement in those properties. However, hydrogen permeation has slightly increase due to the hole effect. Increasing SHGO loading in NCC/PVA has improve its properties and performance. At 80o C, under 100% RH, NCC/PVA-SHGO-1.0 exhibit the highest power density and current density of 31.4 mW/cm2 and 60.2 mA/cm2, respectively which is three times higher compared to pristine NCC/PVA. This work summarise that incorporating SHGO into NCC/PVA membranes can significantly improves its properties, performance and membrane durability which entitle them as a potential candidate for PEMs.

Synergistic effect of organic metal chalcogenides acid-base pairs for enhancing proton conduction

Acid-base pairs synergistic strategy is one of the most effective ways to improve the proton conductivity of materials at low humidity. However, the random distribution of acid or base functional groups on the surface of the materials are difficult to form long-range ordered proton transfer channels, which hinders the proton conductivity. Herein, two organic metal chalcogenides (OMCs), Ag(SPh-NH2) and Ag(SPh-COOH) nanosheets were self-assembled into composite membranes by vacuum filtration. Benefiting from the long-range ordered acid-base functional groups on the surface of OMCs, the proton conductivity was significantly enhanced. As a result, the proton conductivity of the Ag(SPh-NH2)/Ag(SPh-COOH) composite membrane is greatly improved at a low humidity condition of 45% RH, reaching the level of the single membrane at high humidity. In addition, the proton conductivity of Ag(SPh-NH2)/Ag(SPh-COOH) composite membrane is 47 times higher than that of Ag(SPh-NH2) membrane and 10 times higher than that of Ag(SPh-COOH) membrane under high-temperature hydration state. Collectively, this study broadens the avenue for designing proton-conducting materials that can work under low humidity conditions.

Construction of high-density proton transport channels in phosphoric acid doped polybenzimidazole membranes using ionic liquids and metal-organic frameworks

Introducing metal-organic frameworks (MOFs) into phosphoric acid (PA) doped polybenzimidazole (PBI) membranes to form proton transfer channels is an effective strategy to reduce the dependence of proton conductivity on PA. However, excessive MOF loading inevitably leads to reduced mechanical properties and damaged membrane structure. Ionic liquids (ILs) are reported as good plasticizers that can obviously improve the interface performance. Along this line, IL is introducing into the branched poly[2,2′-(p-oxydiphenylene)-5,5′-benzimidazole] (BOPBI) cross-linked by KH-560. The interface performance between the MOF and PBI in the cross-linked BOPBI (CBOPBI)@MOF composite system is improved. The high MOF loading (50 wt%) composite membranes are prepared with acceptable mechanical properties for the first time. High density proton transport channels are constructed based on the high MOF loading in the PA doped CBOPBI@MOF membranes. For instance, the CBOPBI@MOF50%-IL30 shows an excellent proton conductivity (0.135 S cm−1), and a power density about a 26.9% increase (736 mW cm−2) compared with that of the CBOPBI@MOF40% membrane. Furthermore, the load voltage of the membrane with 50 wt% MOF is also well maintained. The membranes with high density proton transport channels exhibit high potential application prospects as high-temperature proton exchange membranes (HT-PEMs).

A study on reaction mechanism and kinetics of CO2 and MEA/DEA-tertiary amines in non-aqueous and water-lean solutions

The kinetic behavior of the reaction of CO2 with MEA-TREA in ethanol solution was studied using the stopped-flow technique. The results indicate that the zwitterion mechanism successfully correlates the experimentally measured pseudo first-order reaction constant (k0) and that the TREA participated in the deprotonation step of MEA-zwitterion as an alkaline substance after the formation of MEA-zwitterion. Then, the applicability of this proposed mechanism was investigated by using the reaction kinetics of CO2 with MEA-DMEA, MEA-DEEA, MEA-MDEA, DEA-TREA, DEA-DMEA blended amines in non-aqueous ethanol solution, as well as MEA-TREA, MEA-DMEA, DEA-TREA, DEA-DMEA blended amines in water-lean solutions. It was found that the mechanism of the reaction of CO2 with these mixed amines can be expressed by:k0=KA-Az[A]2+KA-Bz[A][B]+K1z[A]. The ethanol and water molecules which contribute to K1z can be used as proton transfer channels to accelerate proton transfer, but they do not directly participate in the reaction. In addition, the alkalinity and the steric hindrance effect controlled by the molecular structure of tertiary amines affected the reaction rate at the same time. The activation energies in MEA-EtOH and MEA-TREA-EtOH system were estimated by fitting the second-order rate constants to the Arrhenius expression, indicating that TREA can reduce the reaction activation energy and make the reaction more kinetically favorable.

Nuclear wave-packet-propagation-based study of the electron-coupled, proton-transfer process in the charge-transfer state of FHCl exhibiting three electronic states in full-dimensional space.

The charge-transfer (CT) excited state of FHCl (F+H-Cl-), generated by the photodetachment of an electron from its precursor anion (FHCl-) by a photon energy of ∼9.5eV, is a realistic prototype of two bidirectional-coupled reaction pathways, namely the proton-transfer (PT) and electron-transfer (ET) channels, that produce F + HCl and FH + Cl combinations, respectively. The early-time dynamics of the CT was studied via the time-dependent propagations of nuclear wave packets comprising three nonadiabatically coupled electronic states defined within a three-dimensional space. The detailed analyses of the early-time dynamics revealed an interesting phenomenon in which the onset of PT was ∼80fs earlier than that of ET, indicating that PT dominated ET in this case. A more significant finding was that the proper adjustment of the electronic-charge distribution for the onset of ET was obtained ∼80fs after the onset of PT; this adjustment was mediated by the initial movement of the H atom, i.e., the F-H vibration mode. To avail experimental observables, the branching ratio, χ = PT/(PT + ET), and absorption spectrum generating the neutral FHCl molecule from its precursor anion were also simulated. The results further demonstrated the dependences of the χs and spectrum on the change in the initial vibration level of the precursor anion, as well as the isotopic substitution of the connecting H atom with deuterium, tritium, and muonium.

Are complete metal-organic frameworks really responsible for improving the performance of high-temperature proton exchange membranes?

Constructing continuous proton transfer channels used metal-organic frameworks (MOFs), which can effectively improve proton conductivity of proton exchange membrane, have recently attracted a lot of attentions. MOFs have relatively harsh operating environment in phosphoric acid-doped (PA-doped) high-temperature proton exchange membranes (HTPEMs). However, there are few reports on the stability and state of MOFs in HTPEMs after PA doping. In this work, a series of MOFs (UIO-66, UIO-66-COOH, UIO-66-NH2, UIO-66-SO3H, MIL-101(Cr), and MIL-53(Al)) are selected to investigate their stability via simulating the operating environment for the first time. Composite membranes based on the MOFs are prepared to explore the influence of the stability and state of MOFs on HTPEMs properties. These results indicate that proton transfer channels are constructed in two different styles. After soaking in PA of UIO-66, UIO-66-COOH, MIL-101(Cr), and MIL-53(Al) at 160 °C, metal ions leave the ligands and dissolve, while the ligands are kept in the membranes. These ligands can provide proton transport sites in the membranes and help to construct proton transfer channels. UIO-66-NH2 and UIO-66-SO3H are dissolved completely in PA, leading to continuous nanopores. The proton transfer channels are constructed using the nanopores. From the results, we can infer that constructing proton transfer channels is an effectively method to improve the membranes performance, but the transmission mechanism needs to be revealed carefully.

Sulfonated graphene oxide doped sulfonated polybenzothiazoles for proton exchange membrane fuel cells

A series of sulfonated polybenzothiazoles (sPBT-E) were synthesized by direct polycondensation of 3,3′-disulfonate-4,4′-dicarboxylbiphenyl (DSBP), 4,4′-dicarboxylic diphenyl ether (DCPE) and 2,5-diamino-1,4-benzenedithiol dihydrochloride (DABDT) in polyphosphoric acid (PPA). The sPBT-E series showed considerable solubility in common organic solvents due to the presence of flexible ether groups in the polymer backbone. Sulfonated graphene oxide (SGO) with different loadings (1, 2, 3, and 4 wt%) was doped into the sPBT-E matrix with the sulfonated monomer ratio of 62.5% to prepare composite PEMs (denoted as sPBT-E62.5/SGO) for the first time. sPBT-E membranes with different sulfonated monomer ratios (55, 57.5, 60, and 62.5) were also prepared for comparison. sPBT-E and sPBT-E62.5/SGO series membranes exhibit excellent dimensional stability, appropriate water uptake, high thermal stabilities, and mechanical properties. Additionally, SGO can provide additional ion-transporting sites (resulting in continuous proton transfer channels) to enhance proton conduction of the sPBT-E62.5/SGO membranes. Therefore, the proton conductivity and power density of sPBT-E62.5/SGO3 were up to 0.139 S cm−1 and 519.9 mW cm−2 at 80 °C and 100% RH, which are superior to sPBT-E series and Nafion 212 (0.133 S cm−1, 413.2 mW cm−2). After 20 h continuous operation, the output voltage of sPBT-E62.5/SGO3 only decreased by 13.2%, which is better than sPBT-E62.5 (22.7%). Hence, the sPBT-E62.5/SGO3 membrane exhibits excellent potential as an alternative PEM for PEMFCs.