Subnanometer Nanowire-Reinforced Construction of COF-Based Membranes with Engineering Biomimetic Texture for Efficient and Stable Proton Conduction

Achieving rapid ion transport through nanochannels is essential for both biological and artificial membrane systems. Covalent organic frameworks (COFs) with well-defined nanostructures hold great promise for addressing the above challenge. However, due to the limited processability and inadequate interlamellar interaction of COF materials, it is extremely difficult to integrate them to prepare high-performance proton conductors. Herein, inspired by the ingenious bio-adhesion strategy in nature, ultrafast proton conduction is achieved by taking advantage of COF membranes where TP-COF nanosheets are linked by subnanometer nanowires/lignocellulosic nanofibrils composites (SNWs/LCNFs) through electrostatic and π-π interactions to form an ordered and robust structure. Notably, the synthesized SNWs exhibited impressive proton conductivity and adhesion capacity due to their inbuilt phosphotungstic acid (HPW) molecules and multidimensional interactions. Therefore, attributed to the synergistic contribution of TP-COFs and SNWs, the composite membrane achieves ultrahigh proton conductivity (0.395 S cm⁻¹ at 80 °C and 100% RH), superior mechanical property (109.8 MPa), exceptional fuel cell performance (71.6 mW cm⁻²), and superior operational stability (OCV decay rate is about 1.5 mV h⁻¹), demonstrating outstanding competitiveness. More importantly, the proposed design concept has the potential to be applied in membranes for various electrochemical devices and molecular separations.     

Superprotonic Conductivity of MOFs and Other Crystalline Platforms Beyond 10−1 S cm−1

The proton-exchange membrane (PEM) is a fundamental module of proton-exchange membrane fuel cells (PEMFCs), permitting proton passage and thus governing the overall performance of PEMFCs. Till now, Nafion has been the extensively used marketable PEM material due to its high protonic conductivity of 10⁻²–10⁻¹ S cm⁻¹ under high relative humidity and 80–85 °C. On the other hand, crystalline materials such as metal-organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), hydrogen-bonded organic framework (HOFs), metalo hydrogen-bonded organic framework (MHOFs), and polyoxometalates (POMs) are emerging as potential PEM materials, where crystallinity has paved the way to study the conduction pathway and associated mechanisms to understand structure-function relationships. However, to date, ultrahigh superprotonic conductivity to the level of 10⁻¹ S cm⁻¹, close to Nafion, is relatively scarce for the crystalline proton conductors. In this review, the discussion is focused on materials that demonstrate a conductivity order of 10⁻¹ S cm⁻¹ and higher for those individual crystalline platforms (to be on the equal footing and superior to nafion, respectively) based on their synthesis approach while highlighting the design norms and key features for attaining such ultrahigh conductivity. While a critical analysis is made, the key issues and future prospects are also addressed.     

Tunable Nanochannels along Graphene Oxide/Polymer Core–Shell Nanosheets to Enhance Proton Conductivity

Simultaneous manipulation of topological and chemical structures to induce ionic nanochannel formation within solid electrolytes is a crucial but challenging task for the rational design of high‐performance electrochemical devices including proton exchange membrane fuel cell. Herein, a novel generic approach is presented for the construction of tunable ion‐conducting nanochannels via direct assembly of graphene oxide (GO)/poly(phosphonic acid) core–shell nanosheets prepared by surface‐initiated precipitation polymerization. Using this simple and rapid approach to engineer GO/polymer nanosheets at the molecular‐level, ordered and continuous nanochannels with interconnected hydrogen‐bonded networks having a favorable water environment can be created. The resulting membranes exhibit proton conductivities up to 32 mS cm^?1 at 51% relative humidity, surpassing state‐of‐the‐art Nafion membrane and all previously reported GO‐based materials.     

Nafion Matrix and Ionic Domain Tuning for High-Performance Composite Proton Exchange Membranes

Although proton exchange membranes (PEMs) are widely deployed in an array of commercial applications, limitations linked to their proton conductivity, water retention, and gas permeability still limit ultimate device performance. While ex situ studies have shown additives can enhance membrane stability and mass transport, to date few have demonstrated that these performance enhancements are maintained when tested in commercially relevant electrochemical technologies, such as fuel cells or electrolyzers. Herein, a new multifunctional additive, 2D poly(triazine imide) (PTI), is demonstrated for composite PEMs, which is shown to boost proton conductivity by 37% under optimal high relative humidity (RH) conditions and 67% at low RHs. PTI also enables major improvements (over 55%) in both current and power densities in industrially relevant PEM fuel cells (PEMFCs). Most importantly, in situ and ex situ characterization suggests that the enhanced performance is due to polymer aggregate-PTI clusters that form with increasing 2D character and improved long-range connectivity, while acid-base interactions with pyridinic nitrogen facilitate the critical proton hopping mechanism at all RHs. Hence, this work offers both a new membrane concept with proven benefits for important electrochemical technologies, as well as design principles for future optimization of proton transport and water management within PEMs.     

Zr-MOF-Enabled Controllable Ion Sieving and Proton Conductivity in Flow Battery Membrane

Membrane with ordered channels is the key to controlling ion sieving and proton conductivity in flow batteries. However, it remains a great challenge for finely controlling the nanochannels of polymeric membranes. Herein, two types of acid-stable Zr-metal organic framework (MOF-801 and MOF-808) with variable pore structures and channel properties are introduced as fillers into a non-fluorinated sulfonated poly (ether ether ketone) (SPEEK). The membrane incorporated with MOF-801 of a smaller triangular window (≈3.5 Å) successfully translates the molecular sieving property into the flow battery membrane, resulting in enhanced coulombic efficiency (98.5–99.2%) at 40–120 mA cm⁻² compared with the pristine SPEEK membrane (97.1–98.5%). In contrast, more protophilic internal interconnected channels of MOF-808 yield faster proton highway, leading to a significant increase of voltage efficiency (93.7–84.1%) at 40–120 mA cm⁻² compared with the pristine SPEEK membrane (91.7–78.9%). By regulating the ion sieving and proton conductivity, MOF-801/MOF-808 binary composite membrane exhibits synchronously improved performance in the vanadium redox flow battery system. The revealed structure–property relationship in the Zr-MOFs-based membranes provides a general guideline to design new proton exchange membranes with ordered channels for flow battery application.     

Enhancement of Proton Conduction at Low Humidity by Incorporating Imidazole Microcapsules into Polymer Electrolyte Membranes

Design and fabrication of hierarchically structured membranes with high proton conductivity is crucial to many energy‐relevant applications including proton exchange membrane fuel cell (PEMFC). Here, a series of imidazole microcapsules (IMCs) with tunable imidazole group loading, shell thickness, and lumen size are synthesized and incorporated into a sulfonated poly(ether ether ketone) (SPEEK) matrix to prepare composite membranes. The IMCs play two roles: i) Improving water retention properties of the membrane. The IMCs, similar to the vacuoles in plant cells, can render membrane a stable water environment. The lumen of the IMCs acts as a water reservoir and the shell of IMCs can manipulate water release. ii) They form anhydrous proton transfer pathways and low energy barrier pathways for proton hopping, imparting an enhanced proton transfer via either a vehicle mechanism or Grotthuss mechanism. In particular, at the relative humidity (RH) as low as 20%, the composite membrane exhibits an ultralow proton conductivity decline and the proton conductivity is one to two orders of magnitude higher than that of SPEEK control membrane. The enhanced proton conductivity affords the composite membrane an elevated peak power density from 69.5 to 104.5 mW cm^?2 in a single cell. Moreover, the application potential of the composite membrane for CO₂ capture is explored.     

Hierarchically Macro–Microporous Covalent Organic Frameworks for Efficient Proton Conduction

Assembling molecular proton carriers into crosslinked networks is widely used to fabricate proton conductors, but they often suffer losses in conduction efficiency and stability accompanied by unclear causes. Covalent organic frameworks (COFs), with well-defined crystal frameworks and excellent stability, offer a platform for exploring the proton transfer process. Herein, a strategy to construct proton conductors that induce conductivity and stability by introducing bottom-up hierarchical structure, mass transport interfaces, and host–guest interactions into the COFs is proposed. The proton-transport platforms are designed to possess hierarchically macro–microporous structure for proton storage and mass transport. The protic ionic liquids, with low proton dissociation energies investigated by DFT calculation, are installed at open channel walls for faster proton motion. As expected, the resultant proton conductors based on a covalent organic framework (PIL_0.5@m-TpPa-SO₃H) with hierarchical pores increase conductivity by approximately three orders of magnitude, achieving the value of 1.02 × 10⁻¹ S cm⁻¹ (90 °C, 100% RH), and maintain excellent stability. In addition, molecular dynamics simulations reveal the mechanism of “hydrogen-bond network” for proton conduction. This work offers a fresh perspective on COF-based material manufacturing for high-performance proton conductors via a protocol of macro-micropores.     

Alloyed Pt Single-Atom Catalysts for Durable PEM Water Electrolyzer

The high cost of noble metals is one of the key factors hindering the large-scale application of proton exchange membrane (PEM) water electrolyzer for hydrogen production. Recently, single-atom catalysts (SACs) with a potential of maximum atom utilization efficiency enable lowering the metal amount as much as possible; unfortunately, their durability remains a challenge under PEM water electrolyzer working conditions. Herein, a highly-stable alloyed Pt SAC is demonstrated through a plasma-assisted alloying strategy and applies to a PEM water electrolyzer. In this catalyst, single Pt atoms are firmly anchored onto a Ru support via a robust metal–metal bonding strength, as evidenced by these complementary characterizations. This SAC is used in a PEM water electrolyzer system to achieve a cell voltage as low as 1.8 V at 1000 mA cm⁻². Impressively, it can operate over 1000 h without obvious decay, and the catalyst is present in the form of individual Pt atoms. To the knowledge, this will be the first SAC attempt at a cell level toward long-term PEM. This work paves the way for designing durable SACs employed in the actual working condition in the PEM water electrolyzer.     

Fractal Inorganic−Organic Interfaces in Hybrid Membranes for Efficient Proton Transport

A facile method for preparing highly conductive hybrid organic?inorganic membranes is reported. These membranes are synthesized using an electrospinning process with a sol?gel‐based solution containing PVDF?HFP (polyvinylidenefluoride‐hexafluoropropylene), functionalized or not functionalized silicon alkoxides, and additives. Proton conduction measurements highlight that these hybrid membranes exhibit conductivity value of 101 mS/cm at 120 °C under 80% RH (relative humidity), comparable to the best Nafion measured under the same conditions. These membranes have a proton conductivity‐humidity variation close to Nafion and a modulus value higher than that for Nafion above 80 °C. Their proton conductivity value is about 15 mS/cm under 50% RH, and it constitutes one of the highest values reported. These interesting properties are related to the microstructure of the electrospun membranes that have been characterized using field emission scanning electron microscopy (FE‐SEM) and small angle neutron scattering (SANS). The electrospun membranes are made composed of a bundle of fibers surrounded by a functionalized silica network. The bundle of fibers corresponds to the assembly of small polymer fibers surrounded by small anisotropic functionalized silica domains. Coupling the reactive chemistry of the sol–gel‐based process with electrospinning allows the design of hybrid membranes with fractal hydrophobic/hydrophilic interfaces exhibiting different length scales.     

Achieving over 1,000 mW cm−2 Power Density Based on Locally High-Density Cross-Linked Polybenzimidazole Membrane Containing Pillar[5]arene Bearing Multiple Alkyl Bromide as a Cross-Linker

Cross-linking is widely accepted as an effective method to improve the mechanical strength and durability of phosphoric acid (PA) doped polybenzimidazole (PBI) membranes. However, the cross-linked membranes generally exhibit compromised overall performance since their compact network structures decrease the free volumes of membranes, leading to poor proton conductivity. In this study, a locally high-density cross-linked polybenzimidazole network based on pillar[5]arene bearing multiple alkyl bromide is constructed for the first time to achieve high proton conductivity, desired mechanical properties, and excellent fuel cell performance. The pillar[5]arene-cross-linked network considerably enhances the mechanical strength of membrane (14.6 MPa), particularly with high PA uptake, and provides loose PBI chain segment packing to retain PA (315.9%). Surprisingly, the pillar[5]arene-cross-linked PBI membrane displays a high-power density of 1,084.1 mW cm⁻² at 180 °C and 0.6 mg cm⁻² Pt loading without backpressure and humidification, that is the highest value reported in cross-linked membranes for high-temperature proton exchange membrane fuel cells.     

Porphyrin Helical Nanochannel-Assembled Polybenzimidazole Membranes Doped with Phosphoric Acid for Fuel Cells Operating in a Temperature Range of 25–200 °C

High-temperature proton-exchange membrane fuel cells (HT-PEMFCs) fabricated with phosphoric acid (PA)-doped polybenzimidazole (PBI) show apparent technical advantages. In practical automotive applications, achieving cold start-up capability is crucial. In this work, a kind of branched block proton exchange membrane (PEM) based on PBI with a low content of porphyrin ring (<1 mol.%) is reported as a branched monomer. Self-assembly into high-density helical nanochannels under the synergistic effect of phase separation and porphyrin π−π stacking, thus the PEM can maintain a high level of PA doping. Specifically, the PA/1.8TCPP-BrPy-OPBI membrane shows a proton conductivity of 0.169 and 0.071 S cm⁻¹, as well as an H₂-O₂ fuel cell peak power density of 1077 and 357 mW cm⁻² at 180 and 80 °C without humidification and backpressure, respectively. The membrane electrode assembly (MEA) can exhibit good fuel cell stability, with a voltage decay rate of only 7.0 µV h⁻¹ at 80 °C. Furthermore, it maintains a peak power density of 93% even after 150 start-up/shut-down cycles at 25 °C. This work expands the operating temperature range of conventional PBI membranes between 25 and 200 °C and thus provides a novel strategy for high-performance PBI-based HT-PEMFCs.     

Cation Exchange Membranes with Bi-Functional Sites Induced Synergistic Hydrophilic Networks for Selective Proton Transport

Acid recycling via cation exchange membranes (CEMs) has attracted considerable attention from traditional industries and advanced manufacturing because of the economic and environmental advantages. However, current polymeric CEMs merely have constant ion channels by the fixed groups in the matrix and lack the synergy of bi-functional sites. Herein, a series of dibenzo-18-crown-6 (DB18C6) functionalized sulfonated poly(biphenyl alkylene) membranes is reported. The resultant membranes form phase separation and ordered ion channels by the electrostatic interaction between DB18C6-H⁺ complexes and the  SO₃⁻ anionic sites, constructing a low-swelling synergistic hydrophilic network. The prepared membranes have high proton permeation rates of 2.98-4.85 mol m⁻² h^-1 and extremely low ferrous ion permeabilities, leading to a high H⁺/Fe²⁺ selectivity of ≈3153 at the current density of 10 mA cm^-2, which is one order of magnitude higher than the commercial and previously reported membranes via the electrodialysis. These results provide strategies for designing bi-functional ion exchange membranes for selective ion transport via utilizing crown ether/cation complexes.     

Porous Proton Exchange Membrane with High Stability and Low Hydrogen Permeability Realized by Dense Double Skin Layers Constructed with Amino tris (methylene phosphonic acid)

Porous proton exchange membranes (PEMs) with abundant porous structures show enhanced phosphoric acid (PA) doping levels and proton transport capability. However, the high PA loss rate and serious hydrogen cross-over lead to poor membrane stability. Enhancing the stability of PA-doped porous PEMs is therefore crucial for obtaining high-performance proton exchange membrane fuel cells. Herein, a porous polybenzimidazole membrane with dense double skin layers is reported using amino tris (methylene phosphonic acid) (ATMP) constructed. This membrane effectively alleviates hydrogen permeation and PA loss in a water/anhydrous environment and exhibits enhanced stability. Surprisingly, as an organic proton conductor, ATMP has strong hydrogen bonding with PA, leading to the formation of more continuous proton transport channels. Due to the dense double skin layers protection and the synergistic mass transfer of ATMP and PA, the porous membrane shows excellent proton conductivity (0.112 S cm⁻¹) and a H₂-O₂ fuel cell peak power density of 0.98 W cm⁻² at 160 °C. Moreover, it presents excellent fuel cell stability, with a voltage decay rate of only 5.46 µV h⁻¹. In addition, the porous membrane surpasses the traditional working temperature range, operating in the range of 80–220 °C. This study provides new insight into developing high-performance porous PEMs.     

Pre-Protonated Vanadium Hexacyanoferrate for High Energy-Power and Anti-Freezing Proton Batteries

Proton batteries have been considered as an innovative energy storage technology owing to their high safety and cost-effectiveness. However, the development of fast-charging proton batteries with high energy/power density is greatly limited by feasible material selection. Here, the pre-protonated vanadium hexacyanoferrate (H-VHCF) is developed as a proton cathode material to alleviate the capacity loss of proton-free electrode materials during electrochemical tests. The pre-protonation process realizes fast and long-distance transport of protons by shortening diffusion path and reducing migration barriers. Benefitting from the enhanced hydrogen bonding network combined with dual redox reactions of V and Fe in protonated H-VHCF cathode, a high energy density of 74 Wh kg⁻¹ at 1.1 kW kg⁻¹, and a maximum power density of 54 kW kg⁻¹ at 65 Wh kg⁻¹ is achieved for the asymmetric proton batteries coupling with MoO₃/MXene anode. Proton transport and double oxidation-reduction center are verified by theoretical calculations and ex situ experimental measurements. Considering the anti-freezing availability of proton batteries, 82.5% of its initial capacity is maintained after 10000 cycles under −40 °C at 0.5 A g⁻¹. As a proof-of-concept, flexible device fabricated by optimized electrodes and hydrogel electrolytes can power up a light-emitting diode even under a bent state.     

Spray Layer‐by‐Layer Electrospun Composite Proton Exchange Membranes

Polymer electrolyte films are deposited onto highly porous electrospun mats using layer‐by‐layer (LbL) processing to fabricate composite proton conducting membranes. By simply changing the assembly conditions for generation of the LbL film on the nanofiber mat substrate, three different and unique composite film morphologies can be achieved in which the electrospun mats provide mechanical support; the LbL assembly produces highly conductive films that coat the mats in a controlled fashion, separately providing the ionic conductivity and fuel blocking characteristics of the composite membrane. Coating an electrospun mat with the LbL dipping process produces composite membranes with “webbed” morphologies that link the fibers in‐plane and give the composite membrane in‐plane proton conductivities similar to that of the pristine LbL system. In contrast, coating an electrospun mat using the spray‐LbL process without vacuum produces a uniform film that bridges across all of the pores of the mat. These membranes have methanol permeability similar to free‐standing poly(diallyl dimethyl ammonium chloride)/sulfonated poly(2,6‐dimethyl 1,4‐phenylene oxide) (PDAC/sPPO) thin films. Coating an electrospun mat with the vacuum‐assisted spray‐LbL process produces composite membranes with conformally coated fibers throughout the bulk of the mat with nanometer control of the coating thickness on each fiber. The mechanical properties of the LbL‐coated mats display composite properties, exhibiting the strength of the glassy PDAC/sPPO films when dry and the properties of the underlying electrospun polyamide mat when hydrated. By combining the different spray‐LbL fabrication techniques with electrospun fiber supports and tuning the parameters, mechanically stable membranes with high selectivity can be produced, potentially for use in fuel cell applications.     

Sub-2-nm Channels within Covalent Triazine Framework Enable Fast Proton-Selective Transport in Flow Battery Membrane

Ion conductive membranes (ICMs) with robust sub-2-nm channels show high proton transport rate in flow battery, but it remains a great challenge to precisely control the ion sieving of the membranes. Herein, as a promising proton-selective carrier, sulfonated piperazine covalent triazine framework (s-pCTF) with the channel size of ≈1.5 nm and abundant fast proton hopping sites is introduced into sulfonated poly(ether ether ketone) (SPEEK) to fabricate advanced ICM for vanadium flow battery (VFB) application. The interior protoplasmic channels of s-pCTF demonstrate significant Donnan exclusion effect, resulting in a high proton/vanadium ion selectivity in theory (6.22 × 10⁵). Meanwhile, the nitrogen-rich sub-2-nm channels yield fast proton highway, and exterior-grafted sulfonic acid groups further facilitate the proton transfer. By regulating the ion sieving and proton conductivity, the optimal hybrid membrane exhibits synchronously improved battery performance with an enhanced energy efficiency (92.41% to 78.53% at 40–200 mA cm⁻²) and long-term stability for 900 cycles over 400 h (EE: 87.2–85% at 120 mA cm⁻²), outperforming pure SPEEK and Nafion212 membranes. This study validates the applicability of organic porous CTF with sub-2-nm channels and desired functionality in ICMs for high-performance VFB application.     

Proton Turnover Dominated Cascade Route for CO2 Photoreduction

Photoreduction carbon dioxide (CO₂) and water (H₂O) into valuable chemicals is a huge potential to mitigate immoderate CO₂ emissions and energy crisis. To date, tremendous attention is concentrated on the improvement of independent CO₂ reduction or H₂O oxidation behaviors. However, the simultaneous control of efficient electron and hole utilization is still a huge challenge due to the complex cascade redox reactions. Here, a proton turnover exists in the whole CO₂ photoreduction process is discovered, which is defined as the pivot to concatenate the hole and electron behaviors. As a demonstration of the concept, the efficient activated hydrogen (*H) production centers of copper (Cu) and rapid hydrogenation centers of nickel (Ni) are coupled by an alloying strategy, and the proton turnover behaviors could be directly determined by adjustment of the molar ratios of Cu_xNi_y. Moreover, Cu₃Ni₁–TiO₂ exhibits the highest electron selectivity of 93.7% for methane (CH₄) production with a rate of 175.9 µmol g⁻¹ h⁻¹, while Cu₁Ni₅–TiO₂ reaches up to the highest carbon monoxide (CO) electron selectivity and generation rate at 84.4% and 164.6 µmol g⁻¹ h⁻¹, respectively. Consequently, the experimental and theoretical analysis all clarify the predominate proton turnover effect during the overall CO₂ photoreduction process, which directly determines the categories and generated efficiency of C-based products by regulating variable reaction pathways. Therefore, the revelation of the proton turnover pivot could broaden the new sights by bidirectional optimization of dynamics during the overall CO₂ photoreduction system, which favors the efficient, selective, and stable photocatalytic CO₂ reduction with H₂O.     

Permanently Mechanically Adjustable Photothermal Catalytic Spontaneous Double Cross-Linking Network Enables Durable and Stretchable Plant Skin-Like Materials

In cellulose-based plastics, as a type of thermoplastic and thermosetting materials, the excellent balance of mechanical strength and ductility poses a large challenge. To tackle this problem, a novel approach is devised to introduce reversible non-covalent ester cross-linking into dynamic covalent hydrogen-bonded polymer networks. However, the formation of ester bonds typically requires excess reactants and dehydrating agents, which is energy-intensive, environmentally harmful, and costly. To address these concerns, inspired by polyester-rich plant bark, a supramolecular composite material is developed. It can be dissolved and regenerated using a binary solvent system (choline hexanoate/choline chloride-oxalic acid). In water, this supramolecular composite material underwent self-healing and ester exchange reactions to form double-cross-linked networks, interfaced with photo-thermal catalysis promoting the reaction due to its high photo-thermal conversion efficiency (86.7%) and water evaporation rate (1.38 kg m⁻² h⁻¹). This enables the rapid and repeatable construction of durable and stretchable biomaterials. The mechanical properties of the supramolecular plastic can be adjusted by solar photo-thermal conditions of the synthesis environment. These materials exhibit high performance in solar water evaporation and have self-healing properties and are degradable, recyclable, and capable of eliminating their own adhesions.     

Proton Conducting Phase‐Separated Multiblock Copolymers with Sulfonated Poly(phenylene sulfone) Blocks for Electrochemical Applications: Preparation,Morphology, Hydration Behavior,and Transport

A family of multiblock copolymers consisting of alternating fully sulfonated hydrophilic poly(phenylene sulfone) and hydrophobic poly(phenylene ether sulfone) segments are prepared and characterized. The multiblock copolymers are formed by the coupling of preformed hydrophilic and hydrophobic blocks using a specially designed coupling agent. The block lengths (degree of polymerization) of both segment types were varied in order to control the ion exchange capacity. Solution cast films show spontaneous nanophase separation leading to distinct bicontinuous morphologies with correlation lengths around 15 nm. The hydrophobic phase gives the membranes their advantageous viscoelastic properties even at high temperatures under both wet and dry conditions, while proton conductivity takes place within the hydrophilic phase. Since the properties of fully sulfonated poly (phenylene sulfone)s are locally preserved within the hydrophilic domain, the membranes show very high proton conductivity and high hydrolytic stability. The very high degree of water dispersion within the hydrophilic domains leads to very low electro‐osmotic water drag. Because of their superior transport and stability properties these multiblock copolymers have a great potential for use as a substitute for perfluorosulfonic acid membranes which are used as separator materials in electrochemical applications such as polymer electrolyte membrane (PEM) fuel cells and redox flow batteries.     

Redox-Active Polymer Integrated with MXene for Ultra-Stable and Fast Aqueous Proton Storage

Proton is a charge carrier with the smallest ionic size and quickest kinetics, making aqueous proton batteries (APBs), a promising technology for safe and profitable energy storage systems. Despite being potential electrode materials, organic compounds have not yet been fully investigated in terms of proton storage properties and APB applications due to their low capacity and unstable cycle life in aqueous electrolytes. Herein, a novel redox-active polymer (PDPZ) with diquinoxalino-phenazine as the structural unit has been designed, which is further integrated with MXene nanosheets to construct a flexible PDPZ@MXene electrode material with a rapid and ultra-stable proton storage behavior. In-operando monitoring techniques, i.e., in situ Raman and in situ FTIR, demonstrate the highly reversible redox reaction between CN and C N/N H bonds in electro-active PDPZ molecule with the strong proton absorption ability. Theoretical calculation further proves the electron transfer from MXene to PDPZ promotes the redox reaction of the PDPZ@MXene electrode. As a result, a flexible APB device is developed with a considerable energy density (64.3 mWh cm⁻³), a supercapacitor-level power density (6000 mW cm⁻³), and a record lifespan with ≈98.2% capacity retention over 10 000 cycles, revealing its potential applications in satisfying the various requirements of energy storage systems.