Ion Conduction Mechanisms and Thermal Properties of Hydrated and Anhydrous Phosphoric Acids Studied with 1H, 2H, and 31P NMR

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.

Double cross-linked 3D layered PBI proton exchange membranes for stable fuel cell performance above 200 °C

Phosphoric acid doped proton exchange membranes often experience performance degradation above 200 °C due to membrane creeping and phosphoric acid evaporation, migration, dehydration, and condensation. To address these issues, here we present gel-state polybenzimidazole membranes with double cross-linked three-dimensional layered structures via a polyphosphoric acid sol-gel process, enabling stable operation above 200 °C. These membranes, featuring proton-conducting cross-linking phosphate bridges and branched polybenzimidazole networks, effectively anchor and retain phosphoric acid molecules, prevent 96% of its dehydration and condensation, improve creep resistance, and maintain excellent proton conductivity stability. The resulting membrane, with superior through-plane proton conductivity of 0.348 S cm-1, delivers outstanding peak power densities ranging from 1.20-1.48 W cm-2 in fuel cells operated at 200-240 °C and a low voltage decay rate of only 0.27 mV h-1 over a 250-hour period at 220 °C, opening up possibilities for their direct integration with methanol steam reforming systems.

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.

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.

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.

Isotope effects on the dynamics of amorphous ices and aqueous phosphoric acid solutions

The glass transitions of amorphous ices as well as of aqueous phosphoric acid solutions were reported to display very large ¹H/²H isotope effects. Using dielectric spectroscopy, in both types of glassformers for equimolar protonated/deuterated mixtures an almost ideal isotope-mixing behavior rather than a bimodal relaxation is found. For the amorphous ices this finding is interpreted in terms of a glass-to-liquid rather than an orientational glass transition scenario. Based on calorimetric results revealing that major ¹⁶O/¹⁸O isotope effects are missing, the latter scenario was previously favored for the amorphous ices. Considering the dielectric results on ¹⁸O substituted amorphous ices and by comparison with corresponding results for the aqueous phosphoric acid solutions, it is argued that the present findings are compatible with the glass-to-liquid scenario. To provide additional information regarding the deeply supercooled state of ¹H/²H isotopically mixed and ¹⁸O substituted glassformers, the aqueous phosphoric acid solutions are studied using shear mechanical spectroscopy as well, a technique which so far could not successfully be applied to characterize the glass transitions of the amorphous ices.

Molecular dynamics simulations of proton conducting mediums containing phosphoric acid

An anhydrous proton conducting electrolyte is a key material to realise medium-temperature fuel cells that can drastically simplify heat radiation systems in transportation applications. However, practical applications are limited by...

The relationship between charge and molecular dynamics in viscous acid hydrates

Oscillatory shear rheology has been employed to access the structural rearrangements of deeply supercooled sulfuric acid tetrahydrate (SA4H) and phosphoric acid monohydrate, the latter in protonated (PA1H) and deuterated (PA1D) forms. Their viscoelastic responses are analyzed in relation to their previously investigated electric conductivity. The comparison of the also presently reported dielectric response of deuterated sulfuric acid tetrahydrate (SA4D) and that of its protonated analog SA4H reveals an absence of isotope effects for the charge transport in this hydrate. This finding clearly contrasts with the situation known for PA1H and PA1D. Our analyses also demonstrate that the conductivity relaxation profiles of acid hydrides closely resemble those exhibited by classical ionic electrolytes, even though the charge transport in phosphoric acid hydrates is dominated by proton transfer processes. At variance with this dielectric simplicity, the viscoelastic responses of these materials depend on their structural compositions. While SA4H displays a "simple liquid"-like viscoelastic behavior, the mechanical responses of PA1H and PA1D are more complex, revealing relaxation modes, which are faster than their ubiquitous structural rearrangements. Interestingly, the characteristic rates of these fast mechanical relaxations agree well with the characteristic frequencies of the charge rearrangements probed in the dielectric investigations, suggesting appearance of a proton transfer in mechanical relaxation of phosphoric acid hydrates. These findings open the exciting perspective of exploiting shear rheology to access not only the dynamics of the matrix but also that of the charge carriers in highly viscous decoupled conductors.

Tuning proton conductivity and energy barriers for proton transfer

Proton transport is critical for many technologies and for a variety of biochemical and biophysical processes. Proton transfer between molecules (via structural diffusion) is considered to be an efficient mechanism in highly proton conducting materials. Yet, the mechanism and what controls energy barriers for this process remain poorly understood. It was shown that mixing phosphoric acid (PA) with lidocaine leads to an increase in proton conductivity at the same liquid viscosity. However, recent simulations of mixtures of PA with various bases, including lidocaine, suggested no decrease in the proton transfer energy barrier. To elucidate this surprising result, we have performed broadband dielectric spectroscopy to verify the predictions of the simulations for mixtures of PA with several bases. Our results reveal that adding bases to PA increases the energy barriers for proton transfer, and the observed increase in proton conductivity at a similar viscosity appears to be related to the increase in the glass transition temperature (T_g) of the mixture. Moreover, the energy barrier seems to increase with T_g of the mixtures, emphasizing the importance of molecular mobility or interactions in the proton transfer mechanism.

Insights into the hydrogen bond network topology of phosphoric acid and water systems

Phosphoric acid and its mixtures with water are some of the best proton conducting materials known to science. Although the proton conductivity in pure phosphoric acid decreases upon external doping with excess H+ or OH-, the addition of water improves it substantially. A number of experimental and theoretical studies indicate that these systems form a very special case of hydrogen bond networks which not only facilitate fast proton transport but also show a number of other interesting properties such as glass forming ability. In this work, we present the molecular dynamics simulation results of the H3PO4-H2O system over the entire concentration range. The hydrogen bond networks were analyzed in terms of conventional microscopic as well as topological properties based on graph and network theory. The results show that the hydrogen bond network of H3PO4 is fundamentally different from that of H2O. On average, each phosphoric acid molecule tends to form more and stronger hydrogen bonds than water which leads to a much more connected and clustered network showing small-world properties which are absent in pure water. Moreover, these hydrogen bond network properties persist in the H3PO4-H2O mixtures as well, even at relatively high water contents. Finally, many of the physical properties such as molecular diffusion coefficients seem to be also intimately related to the network topological properties and follow similar trends with respect to system content. These results strongly indicate that many important properties such as proton transport in phosphoric acid and its aqueous systems are fundamentally related to their hydrogen bond network topology and might hold the key for their ultimate molecular understanding.

Confining H3PO4 network in covalent organic frameworks enables proton super flow

Development of porous materials combining stability and high performance has remained a challenge. This is particularly true for proton-transporting materials essential for applications in sensing, catalysis and energy conversion and storage. Here we report the topology guided synthesis of an imine-bonded (C=N) dually stable covalent organic framework to construct dense yet aligned one-dimensional nanochannels, in which the linkers induce hyperconjugation and inductive effects to stabilize the pore structure and the nitrogen sites on pore walls confine and stabilize the H₃PO₄ network in the channels via hydrogen-bonding interactions. The resulting materials enable proton super flow to enhance rates by 2–8 orders of magnitude compared to other analogues. Temperature profile and molecular dynamics reveal proton hopping at low activation and reorganization energies with greatly enhanced mobility.

Development of porous proton-transporting materials combining stability and high performance has remained a challenge. Here, the authors report a stable covalent organic framework with excellent proton conductivity in which nitrogen sites on pore walls confine and stabilize a H3PO4 network in the channels via hydrogen-bonding interactions.

In Operando Neutron Radiography Analysis of a High-Temperature Polymer Electrolyte Fuel Cell Based on a Phosphoric Acid-Doped Polybenzimidazole Membrane Using the Hydrogen-Deuterium Contrast Method

In order to characterize high temperature polymer electrolyte fuel cells (HT-PEFCs) in operando, neutron radiography imaging, in combination with the deuterium contrast method, was used to analyze the hydrogen distribution and proton exchange processes in operando. These measurements were then combined with the electrochemical impedance spectroscopy measurements. The cell was operated under different current densities and stoichiometries. Neutron images of the active area of the cell were captured in order to study the changeover times when the fuel supply was switched between hydrogen and deuterium, as well as to analyze the cell during steady state conditions. This work demonstrates that the changeover from proton to deuteron (and vice versa) leads to local varying media distributions in the electrolyte, independent of the overall exchange dynamics. A faster proton-to-deuteron exchange was re-discovered when switching the gas supply from H2 to D2 than that from D2 to H2. Furthermore, the D2 uptake and discharge were faster at a higher current density. Specifically, the changeover from H to D takes 5–6 min at 200 mA cm−2, 2–3 min at 400 mA cm−2 and 1–2 min at 600 mA cm−2. An effect on the transmittance changes is apparent when the stoichiometry changes.

On the nanosecond proton dynamics in phosphoric acid–benzimidazole and phosphoric acid–water mixtures

Combining ¹H-NMR, ¹⁷O-NMR, and high-resolution backscattering QENS hydrodynamic and structural proton transport in phosphoric acid is separated. The rate limiting steps for structural proton diffusion in mixtures of acid with Brønsted bases are found to occur below the nanosecond timescale.

Dynamics of structural diffusion in phosphoric acid hydrogen-bond clusters

For protonated H₃PO₄ clusters, the Eigen–Zundel–Eigen mechanism is enhanced by fluctuations in the H-bond chain length and local-dielectric environment, and can proceed without the reorientation of H₃PO₄ molecules as in the case of neat liquid H₃PO₄.

Why do proton conducting polybenzimidazole phosphoric acid membranes perform well in high-temperature PEM fuel cells?

This ¹H-NMR, ³¹P-NMR, thermo-gravimetrical analysis, and conductivity study elucidates how hygroscopicity, acidity, and proton transport of phosphoric acid are affected by acid–base interactions with (benz)imidazole present in proton conducting high-temperature PEM fuel cell membranes.

Proton conduction mechanisms in the phosphoric acid–water system (H4P2O7–H3PO4·2H2O): a 1H, 31P and 17O PFG-NMR and conductivity study

The exceptionally high structural proton conductivity in neat phosphoric acid (H₃PO₄), which is closely related to the topology of its frustrated hydrogen bond network, is a singularity in that its contribution to the total ionic conductivity decreases with both increasing and decreasing water content.

Proton Conductivity in Phosphoric Acid: The Role of Quantum Effects

Phosphoric acid has one of the highest intrinsic proton conductivities of any known liquids, and the mechanism of this exceptional conductivity remains a puzzle. Our detailed experimental studies discovered a strong isotope effect in the conductivity of phosphoric acids caused by (i) a strong isotope shift of the glass transition temperature and (ii) a significant reduction of the energy barrier by zero-point quantum fluctuations. These results suggest that the high conductivity in phosphoric acids is caused by a very efficient proton transfer mechanism, which is strongly assisted by quantum effects.

High proton conductivity and spectroscopic investigations of metal-organic framework materials impregnated by strong acids

Strong toluenesulfonic and triflic acids were incorporated into a MIL-101 chromium(III) terephthalate coordination framework, producing hybrid proton-conducting solid electrolytes. These acid@MIL hybrid materials possess stable crystalline structures that do not deteriorate during multiple measurements or prolonged heating. Particularly, the triflic-containing compound demonstrates the highest 0.08 S cm(-1) proton conductivity at 15% relative humidity and a temperature of 60 °C, exceeding any of today's commercial materials for proton-exchange membranes. The structure of the proton-conducting media, as well as the long-range proton-transfer mechanics, was unveiled, in a certain respect, by Fourier transform infrared and (1)H NMR spectroscopy investigations. The acidic media presumably constitutes large separated droplets, coexisting in the MIL nanocages. One component of proton transfer appears to be related to the facile relay (Grotthuss) mechanism through extensive hydrogen-bonding interactions within such droplets. The second component occurs during continuous reorganization of the droplets, thus ensuring long-range proton transfer along the porous structure of the material.