The mechanism of proton conduction in phosphoric acid

Phosphate modified zerovalent iron enhances the interfacial hydrogen bond mediated adhesion and inactivation of Escherichia coli

The microbe-surface interaction plays essential roles in various natural and anthropogenic water processes, and it is of great benefit to tailor their interfacial contact. For instance, zero-valent iron (ZVI) holds great promise as an emerging disinfection agent, but its efficiency remains limited due to the insufficient ZVI-bacteria contact. Herein we modified the surface of ZVI with potassium dihydrogen phosphate (KH2PO4) by ball milling (P-ZVIbm) to facilitate the close adhesion and inactivation of Escherichia coli (E. coli). ExDLVO simulation indicated the significantly alleviated Lewis acid-base repulsion between P-ZVIbm and E. coli due to the enhanced hydrogen bonding. ATR-FTIR spectra and DFT calculation suggested that the surface phosphate moiety could form dual hydrogen bonds with the amide domain of bacterial proteins (e.g., membrane proteins, pili, flagella, etc.), significantly shortening the length of hydrogen bonds from 1.784 to 1.781 and 1.646 Å. The enhanced adhesion of E. coli on P-ZVIbm lead to more pronounced oxidative stress and inactivation of the cells. Impressively, P-ZVIbm exhibited superior bactericidal efficiency of 99.2% in batch experiments, far surpassing that of ZVIbm (33.9%, p < 0.001). As a prototype of application, the sand column packed with P-ZVIbm continuously removed E. coli to below 100 CFU/mL from simulated groundwater. This study provided an effective and facile strategy of phosphorylation to modulate the ZVI-bacterium adhesion for contact sterilization, and may shed light on the development of novel disinfection techniques, and the design of biofilm carriers for wastewater treatment, etc.

Water-in-Acid Strategy for Corrosion-Free Proton Storage: Phosphoric Acid Electrolyte Engineering Toward Sustainable Aqueous Batteries

Aqueous proton batteries, leveraging the intrinsic advantages of protons such as minimal hydrated radius, natural abundance, and rapid transport kinetics, have emerged as promising candidates for next-generation energy storage. However, conventional strong acid electrolytes like H2SO4 suffer from critical limitations including electrode dissolution and incompatibility with battery components. To circumvent these challenges, weak acids (e.g., HCOOH and H3PO4) have been strategically selected as alternative electrolytes due to their non-corrosive characteristics. Particularly, the implementation of high-concentration "water-in-acid" (WIA) effectively suppresses undesirable interactions between electrode materials and free water molecules. Through electrolyte engineering, we developed a 9.5 M H3PO4 WIA system that synergizes with a molybdenum trioxide electrode, achieving remarkable electrochemical performance: a high reversible capacity of 229.8 mAh g-1 at 3 A g-1 and exceptional cycling stability with 83.86% capacity retention after 1000 cycles at 5 A g-1, surpassing conventional H₂SO₄-based systems by both capacity and cyclability. This innovative approach establishes a new paradigm for developing high-performance aqueous energy storage systems through acid-dominated electrolyte design.

Confining Phosphoric Acid in Quaternized COF Channels for Ultra-Stable and Fast Anhydrous Proton Transport

Phosphoric acid (H3PO4) doping is a widely employed strategy to facilitate anhydrous proton transport in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, significant H3PO4 leaching during long-term operation poses critical challenges to maintaining membrane stability and proton conductivity. Herein, H3PO4 is incorporated into positively charged nanochannels of quaternized covalent organic framework membranes (QACOFMs), leveraging strong electrostatic interactions and confinement effects to achieve exceptional H3PO4 retention under hydration conditions. Moreover, the shortened hydrogen bond length between H3PO4 (O-H…O <2.7 Å) and the highly interconnected hydrogen bond network in the H3PO4@QACOFMs facilitate ultra-fast anhydrous proton transport. As a result, the H3PO4@QACOFMs exhibit superior anhydrous proton transport in a broader temperature range (60 °C-200 °C) and the highest proton conductivity reaches about 379.7 mS cm-1 at 200 °C.

Investigating the Role of Aniline Structure in Improving Proton Exchange Membranes for High Conductivity

Polybenzothiazole exhibits high proton conductivity; however, its rigid backbone limits its applicability, necessitating processes such as modification or doping. The aniline structure can participate in various reactions, including nucleophilic or electrophilic substitution reactions, salt formation, and acylation reactions. In this experiment, an aniline structure is integrated into the main chain of sulfonated polybenzothiazole to investigate the potential of aniline for enhancing proton exchange membranes. The incorporation of the aniline structure is confirmed through infrared and nuclear magnetic resonance analyses. A comparison of different proportions of aniline revealed that 12.5% aniline increased the tensile modulus to 274.40 MPa and the elongation at break to 6.26%. Furthermore, the water absorption rate reached 65.73%, while the expansion rate remained at 25%. The aniline structure exhibits inherent basicity and utilizes phosphoric acid adsorption to enhance proton conductivity. After aniline adsorbs phosphoric acid, the proton conductivity peaks at 0.157 S cm-1. Additionally, the introduction of amino groups provides further modification potential to the main chain of polybenzothiazole.

Proton transport in liquid phosphoric acid: the role of nuclear quantum effects revealed by neural network potential

Pure phosphoric acid exhibits high proton conductivity and is widely used in modern industry. However, its proton transport mechanism remains less understood compared to that of water, which presents a significant challenge for advancing technologies like phosphoric acid fuel cells. In this study, we utilize machine learning potentials and molecular dynamics (MD) simulations to investigate the proton diffusion mechanisms in liquid phosphoric acid systems. The neural network potentials we developed demonstrate quantum chemical accuracy and stability across a range of temperatures. Our simulations reveal continuous proton hopping between phosphoric acid anions. Moreover, the radial distribution functions and diffusion coefficients obtained from ring polymer MD-a variant of path-integral MD-exhibit improved alignment with experimental values compared to classical MD results, as ring polymer MD inherently accounts for nuclear quantum effects on proton behavior. Additionally, we employed neural networks combined with the charge equilibration method to predict the charge distribution in liquid phosphoric acid, examining the proton transport mechanism through vibrational spectra analysis.

Anhydrous Solid-State Proton Conduction in Crystalline MOFs, COFs, HOFs, and POMs

Strategic design of solid-state proton-conducting electrolytes for application in anhydrous proton-exchange membrane fuel cells (PEMFCs) has gained burgeoning interest due to a spectrum of advantageous features, including higher CO tolerance and ease in the water management systems. Toward this direction, crystalline materials like metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polyoxometalates (POMs) are emerging PEM materials, offering strategic structural engineering through crystallography, thus enabling ultrahigh anhydrous proton conductivity up to 10-2-10-1 S/cm. This Perspective highlights significant progress achieved thus far with such crystalline platforms in the domain of anhydrous proton conduction across a wide temperature window (sub-zero to above 100 °C). Based on their structural backgrounds, these platforms are categorized into four classes (viz. MOFs, COFs, HOFs, and POMs) with a detailed evolutionary timeline since their emergence early in 2009. Insightful discussions with a key focus on the strategies undertaken to attain anhydrous proton conductivity along with implementation in fuel cell technology through membrane electrode assembly are presented. A section on "Critical Analysis and Future Prospects" provides decisive key viewpoints on those overlooked issues with future endorsement (e.g., performance assessment with CO tolerance analysis and fuel cell test stand) for further development while comparing them with other anhydrous platforms from both academic and industrial perspectives.

Heteroatom-embedded Mellitic Triimido COFs for efficient proton conduction

Ionic covalent organic frameworks (iCOFs) are promising materials for energy storage devices due to their ionic functional groups, which facilitate ion transport, and their highly ordered pores of their frameworks, which provide ideal pathways for long-term ion transport under harsh electrochemical conditions. In this study, we attempted for the first time to synthesize an unprecedented iCOF using a heteroatom-embedded mellitic triimido COF framework that enables practical ion channels on the Angstrom scale. This iCOF was subsequently evaluated as an anhydrous proton-conducting material. The heterocyclic pyridine group of the 2,5-diaminopyridine (DAPy) linker plays an important role, not only as an AB stacking-inducing group but also as a proton acceptor that interacts with impregnated H3PO4. The resulting PA@MTI-DAPy-COF exhibited high proton conductivity of 3.68 × 10-2 S cm-1 at 150oC under anhydrous conditions. This work paves the way for constructing efficient proton-conducting channels by leveraging the stacking structure of COF skeletons.

pKa Matching Enables Quantum Proton Delocalization in Acid-1-Methylimidazole Binary Mixtures

Short hydrogen bonds (SHBs), characterized by donor-acceptor heteroatom separations below 2.7 Å, are prevalent in condensed-phase systems. Recently, we identified SHBs in nonaqueous binary mixtures of acetic acid and 1-methylimidazole (MIm), where electronic and nuclear quantum effects facilitate extensive proton delocalization. In this work, we explore the conditions favoring SHB formation in binary acid-base mixtures and propose that the difference in pKa values between the acid and base, measured in a nonaqueous, aprotic solvent like DMSO, is a key determinant. Using MIm as a model base, we perform electronic structure calculations to systematically analyze pKa matching across 97 acid-MIm pairs in DMSO solutions. Through a combination of first-principles simulations and infrared spectroscopy, we confirm the formation of SHBs and the delocalization of protons in benzoic acid-MIm and salicylic acid-MIm binary mixtures. Our results demonstrate that pKa matching can significantly alter proton behavior in nonaqueous systems, transforming acid-base interactions from conventional proton transfer to quantum mechanical proton delocalization. This work establishes DMSO as a valuable alternative to water for assessing pKa matching and highlights the importance of hydrogen bond networks in modulating these conditions. By elucidating the impact of electronic and nuclear quantum effects, our results provides insights for designing organic mixtures that leverage SHBs for advanced material applications.

Localized Water Restriction in Ternary Eutectic Electrolytes for Ultra-Low Temperature Hydrogen Batteries

Proton batteries are promising candidates for next-generation large-scale energy storage in extreme conditions due to the small ionic radius and efficient transport of protons. Hydrogen gas, with its low working potentials, fast kinetics, and stability, further enhances the performance of proton batteries but necessitates the development of novel electrolytes with low freezing points and reduced corrosion. This work introduces a localized water restriction strategy by incorporating a tertiary component with a high donor number, which forms strong bonds with water molecules. This approach restricts free water molecules and reduces the average hydrogen bond ratio and strength. As-prepared ternary eutectic electrolytes lowered the freezing point to -103 °C, significantly lower than the traditional binary electrolyte (9.5 m H3PO4, -93 °C). This electrolyte is highly compatible with the Cu0.79Co0.21[Fe(CN)6]0.64 ⋅ 4H2O (CoCuHCF) cathode, reducing material dissolution and current collector corrosion. The H2||CoCuHCF battery using this electrolyte demonstrated a high-power density of 23664.3 W kg-1, excellent performance at -80 °C, and stable cyclability over 1000 cycles (>30 days) at -50 °C. These findings provide a framework for proton electrolytes, highlighting the potential of hydrogen batteries in challenging environments.

High Proton Conductivity of Acid Impregnated COFs Stabilized by Post-Oxidation

The investigation of proton conduction processes within artificial nanopores using phosphoric acid (H3PO4) and sulfuric acid (H2SO4) not only sheds light on the mechanisms of proton conduction for these strong acids in confined environments, while also provides critical insights into the proper understanding of biological transmembrane proton transport. However, the synthesis of stable and acid-resistant host frameworks is yet a major challenges. By following that, the present study is conducted with the aim of improving the chemical stability of an imine-based COF (CPOF-10) by converting it into an amide-linked COF (CPOF-11) via a post-oxidative approach. In which, the integration of an appropriate amount of imidazole groups into the framework facilitates the efficient impregnation of liquid proton-conducting acids. The obtained results indicate the ten times greater proton conductivity of H3PO4@CPOF-11 than that of H3PO4@CPOF-10, thereby, successfully achieving 8.63 × 10-2 S cm-1 at 160 °C, under nitrogen (N2) atmosphere. Moreover, the highly stable CPOF-11 tolerated H2SO4 doping, delivering a high proton conductivity of up to 1.70 × 10-1 S cm-1 at 140 °C, with a significantly low activation energy of 0.05 eV. To the best of the knowledge, this activation energy (0.05 eV) of H2SO4@CPOF-11 is found to be one of the lowest value among all the reported proton-conducting materials. Thus, this study will provide new understanding for the fabrication of advanced porous organic materials in fuel cells application.

Speciation of the proton in water-in-salt electrolytes

Water-in-salt (WiS) electrolytes are promising systems for a variety of energy storage devices. Indeed, they represent a great alternative to conventional organic electrolytes thanks to their environmental friendliness, non-flammability, and good electrochemical stability. Understanding the behaviour of such systems and their local organisation is a key direction for their rational design and successful implementation at the industrial scale. In the present paper, we focus our investigation on the 21 m bis(trifluoromethanesulfonyl)imide (LiTFSI) WiS electrolyte, recently reported to have acidic pH values. We explore the speciation of an excess proton in this system and its dependence on the initial local environment using ab initio molecular dynamics simulations. In particular, we observe the formation of HTFSI acid in the WiS system, known to act as a superacid in water. This acid is stabilised in the WiS solution for several picoseconds thanks to the formation of a complex with water molecules and a neighboring TFSI- anion. We further investigate how the excess proton affects the microstructure of WiS, in particular, the recently observed oligomerisation of lithium cations, and we report possible notable perturbations of the lithium nanochain organisation. These two phenomena are particularly important when considering WiS as electrolytes in batteries and supercapacitors, and our results contribute to the comprehension of these systems at the molecular level.

Nitrogen-Rich Covalent Organic Frameworks Composited High-Temperature Proton Exchange Membranes with Ultralow Volume Expansion and Reduced Phosphoric Acid Leakage

Phosphoric acid (PA) leakage and volume expansion are critical factors limiting long-term stable operation of PA-doped polybenzimidazole (PBI) for high-temperature proton exchange membrane fuel cells. Enhancing the interaction between the polymer matrix and PA provides an effective way to minimize PA loss and inhibit excessive membrane swelling. The covalent organic frameworks (COFs) are helpful in improving the performance of PA-PBI membranes due to the robust frameworks, adjustable structures, and good compatibility with polymers. Here, in this work, we synthesized porous COFs named TTA-DFP containing triazine rings and pyridine groups at room temperature for as short as 2 h without oxygen isolation. TTA-DFP was then blended with commercial poly[2,2'-(p-oxidiphenylene)-5,5'-benzimidazole] (OPBI) to prepare composite membranes. The abundant alkaline N sites in TTA-DFP exhibit strong interactions with PA and OPBI, which not only provide more proton transport pathways to promote proton conduction but also immobilize PA in acidophilic micropores to reduce PA leakage. The composite membranes exhibit a much lower volume swelling ratio than that of the OPBI membrane. The PA retention of the composite membrane after 120 h of treatment at 80 °C and 40% relative humidity can reach as high as 84.6%. Particularly, the proton conductivity of the composite membrane doped with 15 wt% TTA-DFP achieves 0.112 S cm-1 at 180 °C without humidification with a swelling ratio of 24.1%. In addition, it has an optimal peak power density of 824.4 mW cm-2 at 180 °C, which is 1.7 times that of the OPBI membrane. The stability of the composite membrane is much better than that of OPBI at a current density of 0.3 A cm-2 at 140 °C for 120 h.

Proton Transport Scenarios in Sulfuric Acid Explored via Ab Initio Molecular Dynamics Simulations

Sulfuric acid (H2SO4), a highly reactive reagent containing intrinsic protonic charge carriers, has been studied via ab initio molecular dynamics simulations. Specifically, we explore the solvation shell structure of the protonic defects, H1SO4- and H3SO4+, as well as the underlying proton transport mechanisms in both the neat and hydrated H2SO4 solutions. Our findings reveal a significant contraction of the dynamic hydrogen-bonded network around the protonic defects, which resembles features seen in water. The simulations provide estimates of the structural relaxation time scales for proton release from both the covalent O-H bonds (∼23 ps) and the hydrogen bonds (∼0.4 ps). In contrast to water, our analysis of the proton transfer scenarios in sulfuric acid reveals correlated events mediated by the formation of longer (up to four) hydrogen-bonded Grotthuss chains.

Phosphoric acid salts of amino acids as a source of oligopeptides on the early Earth

Because of their unique proton-conductivity, chains of phosphoric acid molecules are excellent proton-transfer catalysts. Here we demonstrate that this property could have been exploited for the prebiotic synthesis of the first oligopeptide sequences on our planet. Our results suggest that drying highly diluted solutions containing amino acids (like glycine, histidine and arginine) and phosphates in comparable concentrations at elevated temperatures (ca. 80 °C) in an acidic environment could lead to the accumulation of amino acid:phosphoric acid crystalline salts. Subsequent heating of these materials at 100 °C for 1-3 days results in the formation of oligoglycines consisting of up to 24 monomeric units, while arginine and histidine form shorter oligomers (up to trimers) only. Overall, our results suggest that combining the catalytic effect of phosphate chains with the crystalline order present in amino acid:phosphoric acid salts represents a viable solution that could be utilized to generate the first oligopeptide sequences in a mild acidic hydrothermal field scenario. Further, we propose that crystallization could help overcoming cyclic oligomer formation that is a generally known bottleneck of prebiotic polymerization processes preventing further chain growth.

Melanin: Nature's 4th bioorganic polymer

The pigments known as the melanins are widely recognized for their responsibility in the coloration of human skin, eyes, hair, and minimising the harmful effects of solar ultraviolet radiation. But specialists are aware that the melanins are present in all living kingdoms, barring viruses, and have functionality that extends beyond neutralizing ionising radiation. The ubiquitous presence of melanin in almost all human organs, recognized in recent years, as well as the presence of melanin in organisms that are evolutionarily distant from each other, indicate the fundamental importance of this class of material for all life forms. In this review, we argue for the need to accept melanins as the fourth primordial class of biological polymers, along with nucleic acids, proteins and polysaccharides. We consistently compare the properties of these canonical biological polymers with the properties of melanin and highlight key features that fundamentally distinguish melanins, their function and its mysteries.

Frustration of H-Bonding and Frustrated Packings in a Hexamorphic Crystal System with Reversible Crystal-Crystal Transitions

The study of transitions between polymorphic phases is a less investigated chapter of the widely studied book of polymorphism. In this paper, we discuss the phase behavior of a new compound that has been rationally designed to show frustration of H-bonds for the strong amide N-H donor, which cannot be involved in H-bonding nor in van der Waals interactions. The compound (ImB) is a showcase of almost all possible cases of transitions between polymorphs [monotropic/enantiotropic, fast/slow, diffusive/displacive, and single-crystal-to-single-crystal (SCSC)] and of relation between polymorphs with different Z'. Six crystal phases (I, II, III, IV, V, and VI) were identified for it with five crystal-crystal transitions. Two transitions are reversible/SCSC/fast. Of the three monotropic transitions, all non-SCSC, one is slow, and the others are fast. Of the two enantiotropic SCSC transitions, one does not exhibit undercooling, while the other shows strong undercooling. Phase III, with Z' = 6, is stable at room temperature between phase II (Z' = 1), stable at high temperature, and phase IV (Z' = 2), stable at low temperature. All six polymorphs are based on the same O-H···O═C H-bonding synthon, which produces infinite chains in five polymorphs and ring tetramers in one. The sequence of reversible SCSC transitions IV ⇆ III ⇆ II involves a remarkable ping pong of the symmetry rules by which H-bonded chains are built. Based on all of this, a possible roadmap for prediction of SCSC transitions in crystals is shortly outlined.

Exceptional Anhydrous Proton Conduction in Covalent Organic Frameworks

Covalent organic frameworks (COFs) offer an irreplaceable platform for mass transport, as they provide aligned one-dimensional channels as pathways. Especially, proton conduction is of great scientific interest and technological importance. However, unlike proton conduction under humidity, anhydrous proton conduction remains a challenge, as it requires robust materials and proceeds under harsh conditions. Here, we report exceptional anhydrous proton conduction in stable crystalline porous COFs by integrating neat phosphoric acid into the channels to form extended hydrogen-bonding networks. The phosphoric acid networks in the pores are stabilized by hierarchical multipoint and multichain hydrogen-bonding interactions with the 3D channel walls. We synthesized five hexagonal COFs that possess different pore sizes, which are gradually tuned from micropores to mesopores. Remarkably, mesoporous COFs with a high pore volume exhibit an exceptional anhydrous proton conductivity of 0.31 S cm-1, which marks the highest conductivity among all examples reported for COFs. We observed that the proton conductivity is dependent on the pore volume, pore size, and content of phosphoric acid. Increasing the pore volume improves the proton conductivity in an exponential fashion. Remarkably, changing the pore volume from 0.41 to 1.60 cm3 g-1 increases the proton conductivity by 1150-fold. Interestingly, as the pore size increases, the activation energy barrier of proton conduction decreases in linear mode. The mesopores enable fast proton hopping across the channels, while the micropores follow sluggish vehicle conduction. Experiments on tuning phosphoric acid loading contents revealed that a well-developed hydrogen-bonding phosphoric acid network in the pores is critical for proton conduction.

Achieving 1060 mW cm-2 with 0.6 mg cm-2 Pt Loading Based on Imidazole-Riched Semi-Interpenetrating Proton Exchange Membrane at High-Temperature Fuel Cells

Enhancing phosphoric acid (PA) doping in polybenzimidazole (PBI) membranes is crucial for improving the performance of high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, excessive PA uptake often leads to drawbacks such as PA loss and compromised mechanical properties when surpassing PA capacity of PBI basic functionality. Herein, a new strategy that integrates high PA uptake, mechanical strength, and acid retention is proposed by embedding linear PBI chains into a crosslinked poly(N-vinylimidazole) (PVIm) backbone via in-situ polymerization. The imidazole (Im)-riched semi-interpenetrating polymer network (sIPN) membrane with high-density nitrogen moieties, significantly enhancing the PA doping degree to 380% shows an excellent conductivity (0.108 S cm-1). Meanwhile, the crosslinking structure in the sIPN membrane ensures adequate mechanical properties, low hydrogen permeability, and a relatively low swelling ratio. As a result, the single cell based on the membrane achieves the highest power density of 1060 mW cm-2 with a low Pt loading (0.6 mg cm-2) up to now and exhibits excellent fuel cell stability.

Formation of hydrogen bonding network of methane sulfonic acid at low degree of hydration (MSA)m·(H2O)n (m = 1-2 and n = 1-5)

This study employs ab initio calculations based on density functional theory (DFT) to investigate the structural properties, 1H-NMR spectra, and vibrational spectra of methane sulfonic acid (MSA) at low degree of hydration. The findings reveal that energetically stable structures are formed by small clusters consisting of one or two MSA molecules (m = 1 and 2) and one or two water molecules in (MSA)m·(H2O)n (m = 1-2 and n = 1-5).These stable structures arise from the formation of strong cyclic hydrogen bonds between the proton of the hydroxyl (OH) group in MSA and the water molecules. However, clusters containing three or more water molecules (n > 2) exhibit proton transfer from MSA to water, resulting in the formation of ion-pairs composed of CH3SO3- and H3O+species. The measured 1H-NMR spectra demonstrate the presence of hydrogen-bonded interactions between MSA and water, with a single MSA molecule interacting with water molecules. This interaction model accurately represents the hydrogen bonding network, as supported by the agreement between the experimental and calculated NMR chemical shift results.

Advances in ionogels for proton-exchange membranes

To ensure the long-term performance of proton-exchange membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) have stringent requirements at high temperatures and humidities, as they may lose proton carriers. This issue poses a serious challenge to maintaining their proton conductivity and mechanical performance throughout their service life. Ionogels are ionic liquids (ILs) hybridized with another component (such as organic, inorganic, or organic-inorganic hybrid skeleton). This design is used to maintain the desirable properties of ILs (negligible vapor pressure, thermal stability, and non-flammability), as well as a high ionic conductivity and wide electrochemical stability window with low outflow. Ionogels have opened new routes for designing solid-electrolyte membranes, especially PEMs. This paper reviews recent research progress of ionogels in proton-exchange membranes, focusing on their electrochemical properties and proton transport mechanisms.

Investigating the phase diagram-ionic conductivity isotherm relationship in aqueous solutions of common acids: hydrochloric, nitric, sulfuric and phosphoric acid

The relationship between phase diagram features around the solid-liquid equilibrium region and ionic conductivity in aqueous solutions is not well understood over the whole concentration range as is the case for acidic aqueous solutions. In this work, we have studied the ionic conductivity (κ) as a function of molar fraction (x) and temperature (T) for four acid/water solutions namely, monoprotic hydrochloric acid (HCl) and nitric acid (HNO3), diprotic sulfuric acid (H2SO4) and triprotic phosphoric acid (H3PO4) along with their binary phase diagrams. The connection between the main features of the phase diagrams and the trends in the ionic conductivity isotherms is established with a new insight on the two pertinent dominant conductivity mechanisms (hopping and vehicular). Ionic conductivity at different temperatures were collected from literature and fitted to reported isothermal (κ vs. x) and iso-compositional (κ vs. T) equations along with a novel semi-empirical equation (κ = f (x, T)) for diprotic and triprotic acids. This equation not only has the best fit for acids with different valency; but also contains four parameters, less than any other similar equation in literature. This work is one of few that advances the understanding of the intricate relationship between structure and ionic transport in various acidic aqueous solutions.

Dual-Proton Conductor for Fuel Cells with Flexible Operational Temperature

The properties of proton conductors determine the operating temperature range of fuel cells. Typically, phosphoric acid (PA) proton conductors exhibit excellent proton conductivity owing to their high proton dissociation and self-diffusion abilities. However, at low temperatures or high current densities, water-induced PA loss causes rapid degradation of cell performance. Maintaining efficient and stable proton conductivity within a flexible temperature range can significantly reduce the start-up temperature of PA-doped proton exchange membrane fuel cells. In this study, a dual-proton conductor composed of an organic phosphonic acid (ethylenediamine tetramethylene phosphonic acid, EDTMPA) and an inorganic PA is developed for proton exchange membranes. The proposed dual-proton conductor can operate within a flexible temperature range of 80-160 °C, benefiting from the strong interaction between EDTMPA and PA, and the enhanced proton dissociation. Fuel cells with the EDTMPA-PA dual-proton conductor showed excellent cell stability at 80 °C. In particular, under the high current density of 1.5 A cm-2 at 160 °C, the voltage decay rate of the fuel cell with the dual-proton conductor is one-thousandth of that of the fuel cell with PA-only proton conductor, indicating excellent stability.

Exploiting Cation Structure and Water Content in Modulating the Acidity of Ammonium Hydrogen Sulfate Protic Ionic Liquids

In this paper, we investigated the effect of cation structure and water content on proton dissociation in alkylammonium [HSO4]- protic ionic liquids (ILs) doped with 20 wt % water and correlated this with experimental Hammett acidities. For pure systems, increased cation substitution resulted in a reduction in the number of direct anion-anion neighbors leading to larger numbers of small aggregates, which is further enhanced with addition of water. We also observed spontaneous proton dissociation from [HSO4]- to water only for primary amine-based protic ILs, preceded by the formation of an anion trimer motif. Investigation using DFT calculations revealed spontaneous proton dissociation from [HSO4]- to water can occur for each of the protic ILs investigated; however, this is dependent on the size of the anion aggregates. These findings are important in the fields of catalysis and lignocellulosic biomass, where solvent acidity is a crucial parameter in biomass fractionation and lignin chemistry.

1H, 31P NMR, Raman and FTIR spectroscopies for investigating phosphoric acid dissociation to understand phosphate ion kinetics in body fluids

The study investigates the formation and transportation of ionic charge carriers in phosphoric acid-water system. This investigation encompasses an analysis of 1H and 31P NMR chemical shifts, self-diffusion coefficients, spin-lattice relaxation rates, spin-spin relaxation rates, activation energies, dissociation constants, electrical conductivity, and Raman shifts, along with FTIR spectra across various water concentrations. Significantly, the maxima observed in these curves at around 0.8 water molar fraction predominantly from the unique molecular arrangement between phosphoric acid and water molecules, influenced by a hydrogen bonding network. These findings yield valuable insights into phosphate ion kinetics within body fluids, covering essential aspects like hydrogen bonding networks, ionization processes, and the energy kinetics of phosphoric dissociation. A customized semiempirical model is applied to calculate dissociated species (water, phosphoric acid, and hydronium ion) at different water contents within a wide range of water mole fraction. Furthermore, this investigation extends to the dissociation of phosphoric acid in DMEM cell culture media, offering a more precise model for phosphate ionic kinetics within body fluids, especially at nominal phosphate concentrations of approximately 1:700μL.

Acidified Nitrogen Self-Doped Porous Carbon with Superprotonic Conduction for Applications in Solid-State Proton Battery

Solid proton electrolytes play a crucial role in various electrochemical energy storage and conversion devices. However, the development of fast proton conducting solid proton electrolytes at ambient conditions remains a significant challenge. In this study, a novel acidified nitrogen self-doped porous carbon material is presented that demonstrates exceptional superprotonic conduction for applications in solid-state proton battery. The material, designated as MSA@ZIF-8-C, is synthesized through the acidification of nitrogen-doped porous carbon, specifically by integrating methanesulfonic acid (MSA) into zeolitic imidazolate framework-derived nitrogen self-doped porous carbons (ZIF-8-C). This study reveals that MSA@ZIF-8-C achieves a record-high proton conductivity beyond 10-2  S cm-1 at ambient condition, along with good long-term stability, positioning it as a cutting-edge alternative solid proton electrolyte to the default aqueous H2 SO4 electrolyte in proton batteries.

Recent advances in sustainable preparation of cellulose nanocrystals via solid acid hydrolysis: A mini-review

As a green and renewable nanomaterial, cellulose nanocrystals (CNC) have received numerous attention due to the unique structural features and superior physicochemical properties. Conventionally, CNC was isolated from lignocellulosic biomass mostly depending on sulfuric or hydrochloric acid hydrolysis. Although this approach is effective, some critical issues such as severe equipment corrosion, excessive cellulose degradation, serious environmental pollution, and large water usage are inevitable. Fortunately, solid acid hydrolysis is emerging as an economical and sustainable CNC production technique and has achieved considerable progress in recent years. Herein, the preparation of CNC by solid acid hydrolysis was summarized systematically, including organic solid acids (citric, maleic, oxalic, tartaric, p-toluenesulfonic acid) and inorganic solid acids (phosphotungstic, phosphoric, and Lewis acid). The advantages and disadvantages of organic and inorganic solid acid hydrolysis methods were evaluated comprehensively. Finally, the challenges and opportunities in the later exploitation and application of solid acid hydrolysis to prepare CNC in the industrial context are discussed. Considering the future development of this technology in the large-scale CNC production, much more efforts should be made in lowering CNC processing cost, fabricating high-solid-content and re-dispersible CNC, developing value-added applications of CNC, and techno-economic analysis and life cycle assessment on the whole process.

A highly proton conductive perfluorinated covalent triazine framework via low-temperature synthesis

Proton-conducting materials are essential to the emerging hydrogen economy. Covalent triazine frameworks (CTFs) are promising proton-conducting materials at high temperatures but need more effective sites to strengthen interaction for proton carriers. However, their construction and design in a concise condition are still challenges. Herein, we show a low temperature approach to synthesize CTFs via a direct cyclotrimerization of aromatic aldehyde using ammonium iodide as facile nitrogen source. Among the CTFs, the perfluorinated CTF (CTF-TF) was successfully synthesized with much lower temperature ( ≤ 160 °C) and open-air atmosphere. Due to the additional hydrogen-bonding interaction between fluorine atoms and proton carriers (H3PO4), the CTF-TF achieves a proton conductivity of 1.82 × 10-1 S cm-1 at 150 °C after H3PO4 loading. Moreover, the CTF-TF can be readily integrated into mixed matrix membranes, displaying high proton conduction abilities and good mechanical strength. This work provides an alternative strategy for rational design of proton conducting media.

Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers

Ionomeric binders in catalyst layers, abbreviated as ionomers, play an essential role in the performance of polymer-electrolyte membrane fuel cells and electrolyzers. Due to environmental issues associated with perfluoroalkyl substances, alternative hydrocarbon ionomers have drawn substantial attention over the past few years. This review surveys literature to discuss ionomer requirements for the electrodes of fuel cells and electrolyzers, highlighting design principles of hydrocarbon ionomers to guide the development of advanced hydrocarbon ionomers.

Catalyst Development for High-Temperature Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) Applications

A constant increase in global emission standard is causing fuel cell (FC) technology to gain importance. Over the last two decades, a great deal of research has been focused on developing more active catalysts to boost the performance of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFC), as well as their durability. Due to material degradation at high-temperature conditions, catalyst design becomes challenging. Two main approaches are suggested: (i) alloying platinum (Pt) with low-cost transition metals to reduce Pt usage, and (ii) developing novel catalyst support that anchor metal particles more efficiently while inhibiting corrosion phenomena. In this comprehensive review, the most recent platinum group metal (PGM) and platinum group metal free (PGM-free) catalyst development is detailed, as well as the development of alternative carbon (C) supports for HT-PEMFCs.

Coexistence of Cationic and Anionic Phosphate Moieties in Solids: Unusual but Not Impossible

Phosphoric acid is commonly known either as a neutral molecule or as an anion (phosphate). We theoretically confirm by ab initio molecular dynamics simulations (AIMD) that a cationic form H4PO4+ coexists with the anionic form H2PO4- in the same salt. This paradoxical situation is achieved by partial substitution of Cs+ by H4PO4+ in CsH2PO4. Thus, HnPO4 acts simultaneously as both the positive and the negative ion of the salt. We analyze the dynamical protonation pattern within the unusual hydrogen bond network that is established between the ions. Our AIMD simulations show that a conventional assignment of protonation states of the phosphate groups is not meaningful. Instead, a better description of the protonation situation is achieved by an efficiently fractional assignment of the strongly hydrogen-bonded protons to both its nearest and next-nearest oxygen neighbors.

PAF-6 Doped with Phosphoric Acid through Alkaline Nitrogen Atoms Boosting High-Temperature Proton-Exchange Membranes for High Performance of Fuel Cells

High-temperature proton-exchange-membrane fuel cells (HT-PEMFCs) can offer improved energy efficiency and tolerance to fuel/air impurities. The high expense of the high-temperature proton-exchange membranes (HT-PEMs) and their low durability at high temperature still impede their further practical applications. In this work, a phosphoric acid (PA)-doped porous aromatic framework (PAF-6-PA) is incorporated into poly[2,2'-(p-oxydiphenylene)-5,5'-benzimidazole] (OPBI) to fabricate novel PAF-6-PA/OPBI composite HT-PEMs through solution-casting. The alkaline nitrogen structure in PAF-6 can be protonated with PA to provide proton hopping sites, and its porous structure can enhance the PA retention in the membranes, thus creating fast pathways for proton transfer. The hydrogen bond interaction between the rigid PAF-6 and OPBI can also enhance the mechanical properties and chemical stability of the composite membranes. Consequently, PAF-6-PA/OPBI exhibits an optimal proton conductivity of 0.089 S cm-1 at 200 °C, and peak power density of 437.7 mW cm-2 (Pt: 0.3 mg cm-2 ), which is significantly higher than that of the OPBI.   The PAF-6-PA/OPBI provides a novel strategy for the practical application of PBI-based HT-PEMs.

A Critical Review of Electrolytes for Advanced Low- and High-Temperature Polymer Electrolyte Membrane Fuel Cells

In the 21st century, proton exchange membrane fuel cells (PEMFCs) represent a promising source of power generation due to their high efficiency compared with coal combustion engines and eco-friendly design. Proton exchange membranes (PEMs), being the critical component of PEMFCs, determine their overall performance. Perfluorosulfonic acid (PFSA) based Nafion and nonfluorinated-based polybenzimidazole (PBI) membranes are commonly used for low- and high-temperature PEMFCs, respectively. However, these membranes have some drawbacks such as high cost, fuel crossover, and reduction in proton conductivity at high temperatures for commercialization. Here, we report the requirements of functional properties of PEMs for PEMFCs, the proton conduction mechanism, and the challenges which hinder their commercial adaptation. Recent research efforts have been focused on the modifications of PEMs by composite materials to overcome their drawbacks such as stability and proton conductivity. We discuss some current developments in membranes for PEMFCs with special emphasis on hybrid membranes based on Nafion, PBI, and other nonfluorinated proton conducting membranes prepared through the incorporation of different inorganic, organic, and hybrid fillers.

Rationalizing and Adapting Water-Accelerated Reactions for Sustainable Flow Organic Processes

Water-accelerated reactions, wherein at least one organic reactant is not soluble in water, are an important class of organic reactions, with a potentially pivotal impact on sustainability of chemical manufacturing processes. However, mechanistic understanding of the factors controlling the acceleration effect has been limited, due to the complex and varied physical and chemical nature of these processes. In this study, a theoretical framework has been established to calculate the rate acceleration of known water-accelerated reactions, giving computational estimations of the change to ΔG^‡ which correlate with experimental data. In-depth study of a Henry reaction between N-methylisatin and nitromethane using our framework led to rationalization of the reaction kinetics, its lack of dependence on mixing, kinetic isotope effect, and different salt effects with NaCl and Na₂SO₄. Based on these findings, a multiphase flow process which includes continuous phase separation and recycling of the aqueous phase was developed, and its superior green metrics (PMI-reaction = 4 and STY = 0.64 kg L^-1 h^-1) were demonstrated. These findings form the essential basis for further in silico discovery and development of water-accelerated reactions for sustainable manufacturing.

Pyrazine-Functionalized Donor-Acceptor Covalent Organic Frameworks for Enhanced Photocatalytic H2 Evolution with High Proton Transport

The well-defined 2D or 3D structure of covalent organic frameworks (COFs) makes it have great potential in photoelectric conversion and ions conduction fields. Herein, a new donor-accepter (D-A) COF material, named PyPz-COF, constructed from electron donor 4,4',4″,4'″-(pyrene-1,3,6,8-tetrayl)tetraaniline and electron accepter 4,4'-(pyrazine-2,5-diyl)dibenzaldehyde with an ordered and stable π-conjugated structure is reported. Interestingly, the introduction of pyrazine ring endows the PyPz-COF a distinct optical, electrochemical, charge-transfer properties, and also brings plentiful CN groups that enrich the proton by hydrogen bonds to enhance the photocatalysis performance. Thus, PyPz-COF exhibits a significantly improved photocatalytic hydrogen generation performance up to 7542 µmol g^-1 h^-1 with Pt as cocatalyst, also in clear contrast to that of PyTp-COF without pyrazine introduction (1714 µmol g^-1 h^-1 ). Moreover, the abundant nitrogen sites of the pyrazine ring and the well-defined 1D nanochannels enable the as-prepared COFs to immobilize H₃ PO₄ proton carriers in COFs through hydrogen bond confinement. The resulting material has an impressive proton conduction up to 8.10 × 10^-2 S cm^-1 at 353 K, 98% RH. This work will inspire the design and synthesis of COF-based materials with both efficient photocatalysis and proton conduction performance in the future.

Highly stable Ru-complex-based metal-covalent organic frameworks as novel type of electrochemiluminescence emitters for ultrasensitive biosensing

Developing novel types of high-performance electrochemiluminescence (ECL) emitters is of great significance for constructing ultrasensitive ECL sensors. Herein, a highly stable metal-covalent organic framework (MCOF), termed Ru-MCOF, has been devised and synthesized by employing a classic ECL luminophore, tris(4,4'-dicarboxylicacid-2,2'-bipyridyl)ruthenium(II) (Ru(dcbpy)₃²⁺), as building unit and applied as a novel ECL probe to construct an ultrasensitive ECL sensor for the first time. Impressively, the topologically ordered and porous architectures of the Ru-MCOF not only allow Ru(bpy)₃²⁺ units to precisely locate and homogeneously distribute in the skeleton via strong covalent bonds but also facilitate the transport of co-reactants and electrons/ions in channels to promote the electrochemical activation of both external and internal Ru(bpy)₃²⁺ units. All these features endow the Ru-MCOF with excellent ECL emission, high ECL efficiency, and outstanding chemical stability. As expected, the constructed ECL biosensor based on the Ru-MCOF as a high-efficiency ECL probe accomplishes the ultrasensitive detection of microRNA-155. Overall, the synthesized Ru-MCOF not only enriches the MCOF family but also displays excellent ECL performance and thus expands the application of MCOFs in bioassays. Considering the structural diversity and tailorability of MCOFs, this work opens a new horizon to design and synthesize high-performance ECL emitters, therefore paving a new way to develop highly stable and ultrasensitive ECL sensors and motivating further research on MCOFs.

Ultrafast Proton Transfer Pathways Mediated by Amphoteric Imidazole

Imidazole, being an amphoteric molecule, can act both as an acid and as a base. This property enables imidazole, as an essential building block, to effectively facilitate proton transport in high-temperature proton exchange membrane fuel cells and in proton channel transmembrane proteins, enabling those systems to exhibit high energy conversion yields and optimal biological function. We explore the amphoteric properties of imidazole by following the proton transfer exchange reaction dynamics with the bifunctional photoacid 7-hydroxyquinoline (7HQ). We show with ultrafast ultraviolet-mid-infrared pump-probe spectroscopy how for imidazole, in contrast to expectations based on textbook knowledge of acid-base reactivity, the preferential reaction pathway is that of an initial proton transfer from 7HQ to imidazole, and only at a later stage a transfer from imidazole to 7HQ, completing the 7HQ tautomerization reaction. An assessment of the molecular distribution functions and first-principles calculations of proton transfer reaction barriers reveal the underlying reasons for our observations.

Development of a potassium-based soil washing solution using response surface methodology for efficient removal of cesium contamination in soil

The overuse of chelating soil washing agents for removal of heavy metal can release soil nutrients and negatively affect organisms. Therefore, developing novel washing agents that can overcome these shortcomings is necessary. In this study, we tested potassium as a main solute of novel washing agent for cesium-contaminated field soil, owing to the physicochemical similarities between potassium and cesium. Response surface methodology was combined with a four-factor, three-level Box-Behnken design to determine the superlative washing conditions of the potassium-based solution for the removal of cesium from the soil. The parameters that were considered were the following: potassium concentration, liquid-to-soil ratio, washing time, and pH. Twenty-seven sets of experiments were conducted using the Box-Behnken design, and a second-order polynomial regression equation model was obtained from the results. Analysis of variance proved the significance and goodness of fit of the derived model. Three-dimensional response surface plots displayed the results of each parameter and their reciprocal interactions. The washing conditions that achieved the highest cesium removal efficiency (81.3%) in field soil contaminated at 1.47 mg/kg were determined to be the following: a potassium concentration of 1 M, a liquid-to-soil ratio of 20, washing time of 2 h, and a pH of 2.

Search for a Grotthuss mechanism through the observation of proton transfer

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between 'chains' of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5-0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Proton Transport in Perfluorinated Ionomer Simulated by Machine-Learned Interatomic Potential

Polymers are a class of materials that are highly challenging to deal with using first-principles methods. Here, we present an application of machine-learned interatomic potentials to predict structural and dynamical properties of dry and hydrated perfluorinated ionomers. An improved active-learning algorithm using a small number of descriptors allows to efficiently construct an accurate and transferable model for this multielemental amorphous polymer. Molecular dynamics simulations accelerated by the machine-learned potentials accurately reproduce the heterogeneous hydrophilic and hydrophobic domains formed in this material as well as proton and water diffusion coefficients under a variety of humidity conditions. Our results reveal pronounced contributions of Grotthuss chains consisting of two to three water molecules to the high proton mobility under strongly humidified conditions.

2D Covalent Organic Frameworks Based on Heteroacene Units

Covalent organic frameworks (COFs) are a unique new class of porous materials that arrange building units into periodic ordered frameworks through strong covalent bonds. Accompanied with structural rigidity and well-defined geometry, heteroacene-based COFs have natural advantages in constructing COFs with high stability and crystallinity. Heteroacene-based COFs usually have high physical and chemical properties, and their extended π-conjugation also leads to relatively low energy gap, effectively promoting π-electron delocalization between network units. Owing to excellent electron-withdrawing or -donating ability, heteroacene units have incomparable advantages in the preparation of donor-acceptor type COFs. Therefore, the physicochemical robust and fully conjugated heteroacene-based COFs solve the problem of traditional COFs lacking π-π interaction and chemical stability. In recent years, significant breakthroughs are made in this field, the choice of various linking modes and building blocks has fundamentally ensured the final applications of COFs. It is of great significance to summarize the heteroacene-based COFs for improving its complexity and controllability. This review first introduces the linkages in heteroacene-based COFs, including reversible and irreversible linkages. Subsequently, some representative building blocks are summarized, and their related applications are especially emphasized. Finally, conclusion and perspectives for future research on heteroacene-based COFs are presented.

Anhydrous Superprotonic Conductivity in the Zirconium Acid Triphosphate ZrH5 (PO4 )3

The development of solid-state proton conductors with high proton conductivity at low temperatures is crucial for the implementation of hydrogen-based technologies for portable and automotive applications. Here, we report on the discovery of a new crystalline metal acid triphosphate, ZrH₅ (PO₄ )₃ (ZP3), which exhibits record-high proton conductivity of 0.5-3.1×10^-2  S cm^-1 in the range 25-110 °C in anhydrous conditions. This is the highest anhydrous proton conductivity ever reported in a crystalline solid proton conductor in the range 25-110 °C. Superprotonic conductivity in ZP3 is enabled by extended defective frustrated hydrogen bond chains, where the protons are dynamically disordered over two oxygen centers. The high proton conductivity and stability in anhydrous conditions make ZP3 an excellent candidate for innovative applications in fuel cells without the need for complex water management systems, and in other energy technologies requiring fast proton transfer.

Dynamics and Proton Conduction of Heterogeneously Confined Imidazole in Porous Coordination Polymers

The nanoconfinement of proton carrier molecules may contribute to the lowing of their proton dissociation energy. However, the free proton transportation does not occur as easily as in liquid due to the restricted molecular motion from surface attraction. To resolve the puzzle, herein, imidazole is confined in the channels of porous coordination polymers with tunable geometries, and their electric/structural relaxations are quantified. Imidazole confined in a square-shape channels exhibits dynamics heterogeneity of core-shell-cylinder model. The core and shell layer possess faster and slower structural dynamics, respectively, when compared to the bulk imidazole. The dimensions and geometry of the nanochannels play an important role in both the shielding of the blocking effect from attractive surfaces and the frustration filling of the internal proton carrier molecules, ultimately contributing to the fast dynamics and enhanced proton conductivity.

Hydrogen Bond Activation by Pyridinic Nitrogen for the High Proton Conductivity of Covalent Triazine Framework Loaded with H3 PO4

Under high temperature anhydrous conditions, it is still a formidable challenge to improve the performance of proton-conducting materials based on H₃ PO₄ and elucidate its proton conduction mechanism. Herein, a highly stable covalent triazine frameworks (CTFs) based on H₃ PO₄ is reported. The more pyridinic nitrogen CTFs contain, the higher proton conductivity is. Compared with H₃ PO₄ @CTF-L with less pyridinic nitrogen, H₃ PO₄ @CTF-H has a higher proton conductivity of 1.6×10^-1  S cm^-1 at 150 °C under anhydrous conditions, which does not decay after about 18 months exposure in air. The high proton conductivity is associated with the formation and breaking of the activated N_triazine ⋯H⁺ ⋯H₂ PO₄ ^- pairs by pyridinic nitrogen of CTFs. The outstanding long-term stability is mainly attributed to the ultra-strong triazine skeleton structure of CTFs.

Conduction Mechanism in Graphene Oxide Membranes with Varied Water Content: From Proton Hopping Dominant to Ion Diffusion Dominant

Proton conductors, particularly hydrated solid membranes, have various applications in sensors, fuel cells, and cellular biological systems. Unraveling the intrinsic proton transfer mechanism is critical for establishing the foundation of proton conduction. Two scenarios on electrical conduction, the Grotthuss and the vehicle mechanisms, have been reported by experiments and simulations. But separating and quantifying the contributions of these two components from experiments is difficult. Here, we present the conductive behavior of a two-dimensional layered proton conductor, graphene oxide membrane (GOM), and find that proton hopping is dominant at low water content, while ion diffusion prevails with increasing water content. This change in the conduction mechanism is attributable to the layers of water molecules in GOM nanosheets. The overall conductivity is greatly improved by forming one layer of water molecules. It reaches the maximum with two layers of water molecules, resulting from creating a complete hydrogen-bond network within GOM. When more than two layers of water molecules enter the GOM nanosheets, inducing the breakage of the ordered lamellar structure, protons spread in both in-plane and out-of-plane directions inside the GOM. Our results validate the existence of two conduction mechanisms and show their distinct contributions to the overall conductivity. Furthermore, these findings provide an optimization strategy for the design of realizing the fast proton transfer in materials with water participation.

Facile, Direct, De Novo Synthesis of an Alkyl Phosphoric Acid-Decorated Covalent Organic Framework

The development and understanding of proton conductors based on phosphoric acid are critical for the field of chemistry, biology, and energy. Covalent organic frameworks (COFs), featuring highly crystalline structures and controllable pore sizes, are suitable for constructing phosphoric acid-based proton conductors. However, because of tedious and intricate synthesis, how to develop COFs based on phosphoric acid remains a substantial challenge. Herein, we contributed a side-chain decorated strategy to construct a phosphoric acid-functionalized, imine-linked COF by de novo synthesis. The phosphoric acid side chains with vigorous motion integrating with 1D nanochannels endow the resulting COF with intrinsic proton conductivity. This work expectantly provides a competitive alternative for producing phosphoric acid-functionalized COFs with high intrinsic proton conductivity. This article is protected by copyright. All rights reserved.

Speciation and Proton Conductivity of Phosphoric Acid Confined in Mesoporous Silica

Phosphoric acid (PA) confined in a commercial mesoporous silica (CARIACT G) with porous size in the range of 3 to 10 nm was studied in relation to its coordination with the silanol groups on the silica surface as a function of temperature, up to 180 °C, using 31P and 29Si MAS NMR spectroscopy. As the temperature increases, the coordination of Si and P in the mesopores depends on the pore size, that is, on the area/volume ratio of the silica matrix. In the mesoporous silica with the higher pore size (10 nm), a considerable fraction of PA is nonbonded to the silanol groups on the surface, and it seems to be responsible for its higher conductivity at temperatures above 120 °C as compared to the samples with a smaller pore size. The electrical conductivity of the functionalized mesoporous silica was higher than that reported for other silico-phosphoric composites synthesized by sol-gel methods using soft templates, which require high-temperature calcination and high-cost reagents and are close to that of the best PA-doped polybenzimidazole membranes used in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). The rate of PA release from the mesoporous silica matrix when the system is exposed to water has been measured, and it was found to be strongly dependent on the pore size. The low cost and simplicity of the PA-functionalized mesoporous silica preparation method makes this material a promising candidate to be used as an electrolyte in HT-PEMFCs.