Activation Energy For Proton Transfer Research Articles

Hydrogen production at intermediate temperatures with proton conducting ceramic cells: Electrocatalytic activity, durability and energy efficiency

Proton conducting ceramic cells (PCCs) are an attractive emerging technology operating in the intermediate temperature range of 500 to 700 °C. In this work, we evaluate the production of hydrogen at intermediate temperatures by proton conducting ceramic cell electrolysis (PCCEL). We demonstrate a high-performance steam electrolysis owing to a composite positrode based on BaGd0.8La0.2Co2O6−δ (BGLC1082) and BaZr0.5Ce0.4Y0.1O3−δ (BZCY541). The high reliability of PCCEL is demonstrated for 1680 h at a current density as high as –0.8Acm−2 close to the thermoneutral cell voltage at 600 °C. The electrolysis cell showed a specific energy consumption ranging from 54 to 66 kW h kg−1 that is comparable to state-of-the-art low temperature electrolysis technologies, while showing hydrogen production rates systematically higher than commercial solid oxide ceramic cells (SOCs). Compared to SOCs, the results verified the higher performances of PCCs at the relevant operating temperatures, due to the lower activation energy for proton transfer comparing with oxygen ion conduction. However, because of the p-type electronic conduction in protonic ceramics, the energy conversion rate of PCCs is relatively lower in steam electrolysis. The faradaic efficiency of the PCC in electrolysis mode can be increased at lower operating temperatures and in endothermic conditions, making PCCEL a technology of choice to valorize high temperature waste heat from industrial processes into hydrogen. To increase the faradaic efficiency by optimizing the materials, the cell design, or the operating strategy is a key challenge to address for future developments of PCCEL in order to achieve even more superior techno-economic merits.

Proton-Conductive and Electrochemical-Sensitive Sensing Behavior of a New Mn(II) Chain Coordination Polymer

In this work, a novel Mn(II) chain coordination polymer (Mn-CP), {[Mn(iip)(2bbpum)0.5(H2O)3]·2H2O}n [H2iip = 5-iodoisophthalic acid, 2bbpum = N,N′-bis(2-pyridinemethyl)oxamide] was synthesized under ambient conditions. Mn-CP further assembles into a 3D supramolecular architecture through hydrogen bonds, halogen bonds, and ππ stacking interactions. The composite Mn-CP/nafion membrane has low activation energy for proton transfer, which results in its temperature-insensitive proton conductivity. The cyclic voltammetry demonstrates that the Mn-CP electrode (Mn-CP/GCE) shows quite different redox properties in 0.1 M H2SO4 and 0.1 M PBS aqueous solutions. The Mn-CP/GCE exhibits a couple of irreversible redox peaks with an oxidation peak potential suitable for the nitrite oxidation in 0.1 M H2SO4. Quite differently, in the PBS aqueous buffer solution, the Mn-CP electrode has a couple of irreversible redox peaks. The reduction peak potential is appropriate for the H2O2 reduction. So, the Mn-CP/GCE can be used not only in the electrocatalytic oxidation of NO2– but also in the electrocatalytic reduction of H2O2. The amperometric response reveals that the Mn-CP/GCE exhibits high-selective and extremely sensitive oxidation sensing of NO2– and reduction sensing of H2O2. The exponential detection range is 0.01–20 mM for NO2– and 0.01–10.5 mM for H2O2. The linear detection range for both H2O2 and NO2– is 0.01–0.10 mM. The detection limit is 1.865 μM for NO2– and 8.57 μM for H2O2. The Mn-CP/GCE exhibits high electrochemical stability and fine reproducibility. Moreover, the strategy can be translated to a portable screen-printed electrode (SPE). The Mn-CP/SPE shows more sensitive sensing to NO2– and H2O2 than Mn-CP/GCE. Moreover, it can detect NO2– and H2O2 efficiently in the real-world samples. So, Mn-CP may be a potential dual nonenzymatic electrochemical sensory material for NO2–via oxidation sensing and H2O2via reduction sensing.

Preparation, characterization, and electrochemical sensing performance of a novel pristine Cd-MOF and its composite with carbon nanotubes.

A novel Cd(II)-organic framework (Cd-MOF) {[Cd(isba)(bbtz)2(H2O)]·H2O}n (1) and its composite with CNTs (Cd-MOF@CNTs), [H2isba = 2-iodo-4-sulfobenzoicacid; bbtz = 1, 4-bis(1,2,4-triazolyl-1-methyl)benzene], were synthesized successfully under ambient conditions. The Cd-MOF is a 2D (4, 4) topological framework, further extending into a two-fold interpenetrated 3D supramolecular network via hydrogen bonding. The activation energy for proton transfer in the composite Cd-MOF@CNT/Nafion membrane is lower than that in pristine Cd-MOF/Nafion, leading to its more temperature-insensitive proton conductivity. Hence, the proton conduction of the composite Cd-MOF@CNT/Nafion membrane was greatly improved. Cyclic voltammograms indicate that the Cd-MOF/GCE only has an oxidation peak with the peak potential suitable for the oxidation of glucose in 0.1 M NaOH. The i-t current response demonstrates that the Cd-MOF/GCE exhibits sensitive and selective oxidative sensing of glucose in the linear range of 0-5 mM with a limit of detection (LOD) of 9.64 μM. Different from the pristine Cd-MOF, Cd-MOF@CNTs shows a pair of irreversible redox peaks with an anodic peak potential appropriate for the glucose oxidation and a cathodic peak potential suitable for the hydrogen peroxide reduction in 0.1 M NaOH. So, the Cd-MOF@CNTs/GCE can be used not only for the electrocatalytic oxidation of glucose, but also for the electrocatalytic reduction of H2O2. The current-time response curve demonstrates that the Cd-MOF@CNTs/GCE exhibits more sensitive and selective oxidative sensing for glucose exponentially in the 0-18.5 mM range with a lower LOD down to 2.63 μM. The reductive sensing of H2O2 increases linearly in the range of 0-14.0 mM (LOD = 33.70 μM). Moreover, the Cd-MOF@CNTs/GCE can detect glucose and H2O2 efficiently in real-world samples. Cd-MOF@CNTs may act as a dual non-enzymatic electrochemical sensory material for glucose and H2O2.

Porous organic polymer with high-density phosphoric acid groups as filler for hybrid proton exchange membranes

Developing sulfonated poly (ether ether ketone) (SPEEK)-based proton exchange membrane with both high proton conductivity and robust mechanical stability is desired for proton exchange membrane fuel cell (PEMFC). Here, a type of phosphoric acid porous organic polymer (PAPOP) with high‐density phosphoric acid (-PO3H2) groups and microporous structure was synthesized and incorporated into SPEEK matrix. The porous skeleton bonding -PO3H2 groups of PAPOP reduced the proton transfer activation energy by providing additional proton conduction pathways. The rigid backbone of PAPOP and its good compatibility with SPEEK increased the mechanical strength of the membrane. The hybrid membrane achieves greatly enhanced proton conductivity up to 417.0 mS cm−1 (80 °C, 100% RH) and high tensile strength of 120.6 MPa. The maximum power density of fuel cell with the hybrid membrane exhibits two times increase compared with the control membrane.

Synthesis and characterization of new proton‐exchange membranes based on poly‐1‐vinyl‐1,2,4‐triazole doped with phenol‐2,4‐disulfonic acid

Poly-1-vinyl-1,2,4-triazole (PVT) was synthesized in isolated yield by free radical polymerization of 1-vinyl-1,2,4-triazole. The number average molecular weight and weight average molecular weight of the synthesized PVT, determined by gel permeation chromatography, was 88 567 and 153 309 Da, respectively. PVT had a unimodal molecular weight distribution with a polydispersity coefficient of 1.7. Mixing PVT with phenol-2,4-disulfonic acid (PDSA) in the presence of a polyvinyl alcohol cross-linked with oxalic acid produces proton exchange membranes having different PVT:PDSA ratios. The composition and structure of the membranes were established by elemental analysis, energy-dispersive X-ray spectroscopy (EDS), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance spectroscopy (NMR). FT-IR and 15N NMR studies have shown that the interaction of PVT with PDSA was accompanied by the formation of acid-base complexes due to proton transfer from PDSA to the triazole rings of PVT. Formation of acid-base complexes PVT-PDSA additionally stabilizes membranes and increases their thermal stability. The SEM study showed that the membrane surface was homogeneous. The properties of membranes, such as thermal and oxidative stability, ion-exchange capacity, proton conductivity, water uptake, and their mechanical properties, have been studied. The commercial membrane Nafion 212 was used as a comparison. The proton conductivity of membranes increases with an increase in the content of PDSA in their composition and for membranes 1-3 was 59.80, 7.30, 6.23 mS/cm, respectively. The activation energies for proton transfer across the membranes were 19.5, 21.6, and 38.2 kJ/mol for the membranes 1-3, respectively. Water uptake, ion-exchange capacity, and proton conductivity depend on the content of PDSA in the membrane. The membranes obtained were tested in a test cell of an ElectroChem hydrogen fuel cell (USA) at a temperature of 25°C with the supply of humidified hydrogen and air.

Quantum Calculations on the Role of Two Amino Acid Groupings that Include Three Positive, Two Negative Charged Residues on the Transport of Protons in the Voltage Sensing Domain of the KV1.2 Potassium Channel

The upper part of the voltage sensing domain (VSD) of the Kv1.2 channel is very similar to the upper part of the Hv1 proton channel, so the transport of protons in the VSD is possible. Our calculations have shown how two sets of five charged amino acids, each with two acid and three basic residues, hence with net one positive charge each, plus three polar but uncharged residues, are critical elements of the proton pathway, together with a limited number of water molecules. The upper group consists of (using numbering from the 3Lut pdb structure) R297,T184,E183,R300,Y266, R303,E226,S176; the lower section includes D259,K306,R309, R240,E236, W232,H310,S169. F233 is an additional critical residue: but rotating it facilitates the motion of a proton through what would otherwise seem a hydrophobic block. An electric field is needed to start the proton cascade from the upper section, which drives protons through the lower section, then along the cytoplasmic surface of the membrane, to the gate. A mutation, F233W, left shifts the gating current curve (MacKinnon and coworkers). In addition to finding the path through the VSD, our calculations explain this left shift, as well as the right shift found with F233E. The calculations show minimal shifts in structure with the field, of the order of 1 Å, but these are enough to produce electrostatic effects that lower the barrier to the proton's progress. Calculations with the protons in several positions have shown the relative energy, and the barriers, to proton transitions along the path. In particular the F233W mutant lowers the activation energy of proton transfer from K306 to D259.

Quantum Calculations on the Transport of Protons in the Voltage Sensing Domain of the Kv1.2 Potassium Channel: Multiple Paths, Charged Groups, and a Switch

The upper part of the voltage sensing domain (VSD) of the Kv1.2 potassium channel resembles the upper part of the Hv1 proton channel, suggesting that transport of protons in the VSD is possible. Our calculations have shown how two sets of amino acids, each with two acid and three basic residues, hence with net one positive charge each, plus three polar but uncharged residues, and a limited number of water molecules, are critical elements of the proton pathway. The upper group consists of (numbering from the 3Lut pdb structure) R297:T184:E183:R300:Y266:R303:E226:S176; the lower section includes D259:K306:R309:R240:E236:W232:H310:S169, in each case, plus water. F233 is an additional critical residue: rotating it facilitates the motion of a proton through what would otherwise seem a hydrophobic block; in effect it acts as a switch. An electric field starts the proton cascade from the upper section, which drives protons through the lower section; the cascade continues, in a nearly field-free environment, along the cytoplasmic surface of the membrane, to the gate. A mutation within the VSD, F233W, left shifts the gating current curve (MacKinnon and coworkers). In addition to finding the path through the VSD, our calculations explain this left shift by lower activation energy of proton transfer from K306 to D259, then on to H310; we also explain the right shift found with F233E. The calculations show minimal shifts in structure with the field, of the order of 1 Å, but these are enough to produce electrostatic effects that lower the barrier to the proton's progress. Calculations with the protons in several positions have shown the relative energy, and the barriers, to proton transitions along the path.

Non-conservation of activation energy barriers in the same chemical process: a cooperative (effect) proton transfer on (HF)n molecular aggregates

The formation of (HF)n aggregates with n = 2, 3, 4, 5 and 6 and concerted proton transfer processes in these aggregates were systematically analyzed. It was verified that, by a cooperative effect, the barrier associated with the proton transfer process decreases for aggregates with a larger number of molecules, indicating that the activation energy for proton transfer depends on the molecularity of the process. Natural bond orbital and quantum theory of atoms in molecules were used to characterize the strength of the hydrogen bonds established in the aggregates, which verified a general increase in the delocalization energy as a function of increasing aggregate size. A deformed Eyring (d-Eyring) equation was used to calculate the proton transfer rate constants, where the d-Eyring equation adequately described the proton transfer kinetics. Analysis of the rate constants showed that proton transfer became faster as the cluster size increased. Arrhenius and d-Arrhenius plots showed a decrease in the dependence of the rate constants on temperature, particularly for the tetramer, pentamer, and hexamer. The d-Arrhenius plots, for which the d parameter was included in the Eyring equation, suggest non-Arrhenius behavior for proton transfer in the HF aggregates at low temperatures.

Fibril growth captured by electrical properties of amyloid-β and human islet amyloid polypeptide.

The aggregation of amyloid-β (Aβ) and human islet amyloid polypeptide (hIAPP) proteins have attracted considerable attention because of their involvement in protein misfolding diseases. These proteins have mostly been investigated using atomic force microscopy, transmission electron microscopy, and fluorescence microscopy to study the directional growth of fibrils both perpendicular to and along the fibril axis. Here, we demonstrate the real-time monitoring of the directional growth of fibrils in terms of activation energy of proton transfer using an impedance spectroscopy technique. The activation energy is used to quantify the sensitivity of proton conduction to the different stages of protein aggregation. The decrement (increment) in activation energy is related to the fibril growth along (perpendicular to) the fibril axis in intrinsic protein aggregation. The entire aggregation process shows different phases of the directional growth for Aβ and hIAPP, indicating different pathways for their aggregation. The activation energy for hIAPP is found to be smaller than the activation energy of Aβ during the aggregation process. The oscillatory behavior of the activation energy of hIAPP reflects a rapid change in the directional growth of the protofilaments of hIAPP. The results indicate higher aggregation propensity of Aβ than hIAPP. In the presence of resveratrol, hIAPP exhibits slower aggregation compared to Aβ. Methods of this study may in general be used to reveal the modulated aggregation pathway of proteins in the presence of different ligands.

Studying Proton Mobility in Zeolites by Varying Temperature Infrared Spectroscopy

We report a varying temperature infrared spectroscopic (VTIR) study with partial deuterium isotopic exchange as a method for characterizing proton mobility in acidic materials. This VTIR technique permits the estimation of activation energies for proton diffusion. Different acidic materials comprising classical proton-conducting materials, such as transition metal phosphates and sulfonated solids, as well as different zeolites, are tested with this new method. The applicability of the method is thus extended to a vast library of materials. Its underlying principles and assumptions are clearly presented herein. Depending on the temperature ranges, different activation energies for proton transfer are observed irrespective of the different materials. In addition to the well-studied transition metal phosphates, Si-rich zeolites appear to be promising proton-transfer materials (with Eact < 40 kJ mol−1) for application in high-temperature (>150 °C) PEM fuel cells. They significantly outperform Nafion and sulfonated silica, which exhibit higher activation energies with Eact ~ 50 and 120 kJ mol−1, respectively.

Properties of Pr and In-doped BaZrCeY-based electrolyte for Proton Conducting Fuel Cell systems

Proton conducting oxides are potential electrolyte candidates for intermediate-temperature solid oxide fuel cell (SOFC) because of their attractive advantages such as excellent ionic conductivity and moderate activation energy for proton transfer. At present, many investigated protons conducting oxides are BaCeO3 and BaZrO3-based materials. In this study, BaCeZrY-based electrolyte doped with Pr and In was synthesised by microwave-assisted glycine nitrate process. The synthesised powders showed a single cubic perovskite phase without any additional phases and thus have potential application in Proton Conducting Fuel Cell systems.

Ion exchange membranes based on silica and sulfonated copolymers of styrene with allyl glycidyl ether

Proton-conducting membranes have been obtained by sol–gel synthesis involving tetraethyl orthosilicate and sulfonated suspension copolymers of styrene with allyl glycidyl ether. The membranes are gels consisting of a polymer matrix, in which silica particles are uniformly distributed. The synthesized membranes are characterized by the proton conductivity of up to 4.21 × 10–2 S/cm at 343 K and 75% humidity, an ion exchange capacity of 3.5 meq/g, a proton-transfer activation energy of 25.2 ± 2.6 kJ/mol, thermal stability of up to 130°C, and mechanical strength (tensile modulus of elasticity, 322 MPa).

31P, 1H NMR Relaxation and Molecular Mobility in Layered α-Zirconium Phosphate: Variable-Temperature NMR Experiments

Internal rotations in HPO4– groups and proton transfer between them have been characterized in static α-zirconium phosphate by variable-temperature measurements of 31P T1 and 1H T1ρ times. The 1H T1ρ times have been obtained by the kinetic of phosphorus–proton cross-polarization treated with the I-I*-S model. The calculations of the temperature T1 relaxation dependences have resulted in the rotational activation energy of 2.6 kcal/mol (or 3.9 kcal/mol in account for the τC distribution) and the proton transfer activation energy of 5.5 kcal/mol. Thus, two types of motions have been found in α-zirconium phosphate: the low-frequency proton transfer with the correlation time τc of 1.5 × 10–5 s at 295 K and the high-frequency rotation in P–OH groups with the correlation time τc of 2.2 × 10–8 s at 295 K.

Preparations of an inorganic-framework proton exchange nanochannel membrane

In this work, a proton exchange membrane composed of straight and aligned proton conducting nanochannels is developed. Preparation of the membrane involves the surface sol-gel method assisted with a through-hole anodic aluminum oxide (AAO) template to form the framework of the PEM nanochannels. A monomolecular layer (SO3H(CH2)3Si(OCH3)3) is subsequently added onto the inner surfaces of the nanochannels to shape a proton-conducting pathway. Straight nanochannels exhibit long range order morphology, contributing to a substantial improvement in the proton mobility and subsequently proton conductivity. In addition, the nanochannel size can be altered by changing the surface sol-gel condition, allowing control of the active species/charge carrier selectivity via pore size exclusion. The proton conductivity of the nanochannel membrane is reported as high as 11.3 mS cm−1 at 70 °C with a low activation energy of 0.21 eV (20.4 kJ mol−1). First-principle calculations reveal that the activation energy for proton transfer is impressively low (0.06 eV and 0.07 eV) with the assistance of water molecules.

Compatible ionic crosslinking composite membranes based on SPEEK and PBI for high temperature proton exchange membranes

Highly sulfonated poly (ether ether ketone) (SPEEK)/polybenzimidazole (PBI) composite high temperature proton exchange membranes were prepared. To enhance the compatibility of SPEEK and PBI, SPEEK was transformed into SPEEK-Na and the prepared SPEEK-Na/PBI was immersed in dilute acid to obtain desired ionic crosslinking SPEEK/PBI membrane. Scanning electron microscope results showed that PBI formed a dense structure. Energy dispersive X-ray spectroscopy of the cross-section of composite membranes and X-ray diffraction declared that PBI were uniformly distributed in SPEEK without phase separation. Fourier-transform infrared spectroscopy indicated the intense ionic bonds between SO3H and N-basic group. Because of the intense interaction, SPEEK/PBI composite membranes could not only significantly improve the mechanical strength and oxidation resistance, but also reduce the swelling degree and methanol crossover. Thermogravimetric analysis results revealed that the membranes could be used below 200 °C. The proton conductivity of the SPEEK/PBI (20 wt%) composite membranes could reach 0.1985 S cm−1 at 170 °C and 100% relative humidity (RH). Proton transfer activation energy was 49.31 kJ mol−1. At 50% RH, the conductivity was 0.099 S cm−1 at 180 °C and it remained the value of 0.0084 S cm−1 at 180 °C in dry oven. The methanol permeability of SPEEK/PBI (20 wt%) membrane was 3.96 × 10−8 cm2 s−1 at 70 °C. The composite membrane appeared to be potential for usage in direct methanol fuel cells.

Intercalation of Lamellar Mineral Kaolinite with 2‐Picolinic Acid: Facile Preparation, Crystal Structure Optimization, and Proton Conductivity

AbstractKaolinite intercalated with 2‐picolinic acid was prepared and characterized by scanning electron microscopy, powder X‐ray diffraction, infrared spectroscopy, and thermogravimetric analysis. The proton conductivity (σ) of the material is 1.85 × 10–10 S cm–1 at 353 K and gradually increases to 1.7 × 10–7 S cm–1 at 393 K under N2 (anhydrous environment) with an activation energy of Ea = 1.93 eV, whereas σ = 1.56 × 10–7 S cm–1 under 80 % relative humidity at 353 K with Ea = 0.91 eV. The crystal structure optimized with the Cambridge Sequential Total Energy Package discloses the absence of a dense H‐bond network in the layer space of kaolinite, which explains the low proton conductivity and high proton transfer activation energy of the intercalated proton conductor.

Dissociation of equimolar mixtures of aqueous carboxylic acids in ionic liquids: role of specific interactions.

Hammett acidity function observes the effect of protonation/deprotonation on the optical density/absorbance of spectrophotometric indicator. In this work, the Hammett acidity, H0, of equimolar mixtures of aqueous HCOOH, CH3COOH, and CH3CH2COOH was measured in 1-methylimidazolium-, 1-methylpyrrolidinium-, and 1-methylpiperidinium-based protic ionic liquids (PILs) and 1-butyl-3-methylimidazolium-based aprotic ionic liquid (AIL) with formate (HCOO(-)) anion. Higher H0 values were observed for the equimolar mixtures of aqueous carboxylic acids in protic ionic liquids compared with those of the aprotic ionic liquid because of the involvement of the stronger specific interactions between the conjugate acid of ionic liquid and conjugate base of carboxylic acids as suggested by the hard-soft acid base (HSAB) theory. The different H0 values for the equimolar mixtures of aqueous carboxylic acids in protic and aprotic ionic liquids were noted to depend on the activation energy of proton transfer (Ea,H(+)). The higher activation energy of proton transfer was obtained in AIL, indicating lower ability to form specific interactions with solute than that of PILs. Thermodynamic parameters determined by the "indicator overlapping method" further confirmed the involvement of the secondary interactions in the dissociation of carboxylic acids. On the basis of the thermodynamic parameter values, the potential of different ionic liquids in the dissociation of carboxylic acids was observed to depend on the hydrogen bond donor acidity (α) and hydrogen bond acceptor basicity (β), characteristics of specific interactions.

Constructing proton-conductive highways within an ionomer membrane by embedding sulfonated polymer brush modified graphene oxide

Sulfonated polymer brush modified graphene oxide (SP-GO) fillers with controllable brush length are synthesized via the facile distillation–precipitation polymerization, and then incorporated into sulfonated poly(ether ether ketone) (SPEEK) matrix to fabricate composite membranes. The influences of SP-GO upon the microstructures, including thermal and mechanical properties, water uptake/swelling, proton conduction, H2 permeability and single PEMFC performances of composite membranes are intensively investigated. It is found that the SP-GO fillers are uniformly dispersed and tend to lie perpendicularly to the cross-section surface of the whole membrane, which allow SP-GO fillers creating inter-connected and broad ionic pathways through the sulfonic acid groups in polymer brushes. Meanwhile, the SP-GO fillers connect the ionic clusters in SPEEK matrix via interfacial interactions. In such a way, proton-transfer highways are constructed along the SPEEK/SP-GO interface, which lower the proton transfer activation energy and enhance the proton conductivities of the composite membranes under both hydrated and anhydrous conditions. Furthermore, elevating the brush length on SP-GO could further enhance the proton conductivity. Compared to SPEEK control membrane, a 95.5% increase in hydrated conductivity, an 178% increase in anhydrous conductivity and a 37% increase in maximum power density are obtained for the optimal composite membrane.

Insight into Proton Transfer in Phosphotungstic Acid Functionalized Mesoporous Silica-Based Proton Exchange Membrane Fuel Cells

We have developed for fuel cells a novel proton exchange membrane (PEM) using inorganic phosphotungstic acid (HPW) as proton carrier and mesoporous silica as matrix (HPW-meso-silica) . The proton conductivity measured by electrochemical impedance spectroscopy is 0.11 S cm(-1) at 90 °C and 100% relative humidity (RH) with a low activation energy of ∼14 kJ mol(-1). In order to determine the energetics associated with proton migration within the HPW-meso-silica PEM and to determine the mechanism of proton hopping, we report density functional theory (DFT) calculations using the generalized gradient approximation (GGA). These DFT calculations revealed that the proton transfer process involves both intramolecular and intermolecular proton transfer pathways. When the adjacent HPWs are close (less than 17.0 Å apart), the calculated activation energy for intramolecular proton transfer within a HPW molecule is higher (29.1-18.8 kJ/mol) than the barrier for intermolecular proton transfer along the hydrogen bond. We find that the overall barrier for proton movement within the HPW-meso-silica membranes is determined by the intramolecular proton transfer pathway, which explains why the proton conductivity remains unchanged when the weight percentage of HPW on meso-silica is above 67 wt %. In contrast, the activation energy of proton transfer on a clean SiO2 (111) surface is computed to be as high as ∼40 kJ mol(-1), confirming the very low proton conductivity on clean silica surfaces observed experimentally.

Probing the Acidity of Carboxylic Acids in Protic Ionic Liquids, Water, and Their Binary Mixtures: Activation Energy of Proton Transfer

Acidity functions were used to express the ability of a solvent/solution to donate/accept a proton to a solute. The present work accounts for the acidity determination of HCOOH, CH3COOH, and CH3CH2COOH in the alkylimidazolium-based protic ionic liquids (PILs), incorporated with carboxylate anion, water, and in a binary mixture of PIL and water using the Hammett acidity function, H0. A reversal in the acidity trend was observed, when organic acids were transferred from water to PIL. It was emphasized that an increased stabilization offered by PIL cation toward the more basic conjugate anion of organic acid was responsible for this anomalous change in acidity order in PILs, which was absent in water. The greater stabilization of a basic organic anion by PIL cation is discussed in terms of the stable hard–soft acid base (HSAB) pairing. A change in the H0 values of these acids was observed with a change in temperature, and a linear correlation between the ln H0 and 1/T was noted. This relationship points toward the activation energy of proton transfer (E(a,H+)), a barrier provided by the medium during the proton transfer from Brønsted acid to indicator. The H0 function in binary mixtures points to the involvement of pseudosolvent, the behavior of which changes with the nature and concentration of acid. The presence of the maxima/minima in the H0 function is discussed in terms of the synergetic behavior of the pseudosolvent composed of the mixtures of aqueous PILs.