Proton Conductance Pathway Research Articles

Acid-Functionalized Polysilsesquioxane−Nafion Composite Membranes with High Proton Conductivity and Enhanced Selectivity

A series of new Nafion-based composite membranes have been prepared via an in situ sol-gel reaction of 3-(trihydroxylsilyl)propane-1-sulfonic acid and solution casting method. The morphological structure, ion-exchange capacity, water uptake, proton conductivity, and methanol permeability of the resulting composite membranes have been extensively investigated as functions of the content of sulfopropylated polysilsesquioxane filler, temperature, and relative humidity. Unlike the conventional Nafion/silica composites, the prepared membranes exhibit an increased water uptake and associated enhancement in proton conductivity compared to unmodified Nafion. In particular, considerably high proton conductivities at 80 and 120 degrees C under 30% relative humidity have been demonstrated in the composite membranes, which are over 2 times greater than that of Nafion. In addition to a remarkable improvement in proton conductivity, the composite membranes display lower methanol permeability and superior electrochemical selectivities in comparison to the pure Nafion membrane. These unique properties could be exclusively credited to the presence of pendant sulfonic acid groups in the filler, which provides fairly continuous proton-conducting pathways between filler and matrix in the composite membranes and thus facilitates the proton transport without the anticipated trade-off between conductivity and selectivity. This work opens new opportunities of tailoring the properties of Nafion-the benchmark fuel cell membrane-to obviate its limitations and enhance the conductive properties at high temperature/low humidity and in direct methanol fuel cells.

Structural Changes in the N and N′ States of the Bacteriorhodopsin Photocycle

The bacteriorhodopsin transport cycle includes protonation of the retinal Schiff base by Asp 96 (M→N reaction) and reprotonation of Asp 96 from the cytoplasmic surface (N→N′ reaction). We measured distance changes between pairs of spin-labeled structural elements of interest, and in general observed larger overall structural changes in the N state compared with the N′ state. The distance between the C-D loop and E-F interhelical loops in A103R1/M163R1 increased ∼6 Å in the N state and ∼3 Å in the N′ state. The opposite trend of distance changes in V101R1/A168R1 and L100R1/T170R1 supports counterclockwise rotation of helix F in the N but not the N′ state. Small distance increases were observed in S169R1/S226R1, but little change was seen in G106R1/G155R1. Taking earlier published EPR data into account, we suggest that structural changes of the E-F loop occur first, and then helices F and G begin to move together in the late M state. These motions then reach their maximum amplitude in the N state, evidently to facilitate the release of a proton from Asp 96 and the formation of a proton-conduction pathway from Asp 96 to the Schiff base. The structural changes reverse their directions and decay in the N′ state.

Functional Reconstitution of Purified Human Hv1 H+ Channels

Voltage-dependent H+ (Hv) channels mediate proton conduction into and out of cells under the control of membrane voltage. Hv channels are unusual compared to voltage-dependent K+, Na+, and Ca2+ channels in that Hv channel genes encode a voltage sensor domain (VSD) without a pore domain. The H+ currents observed when Hv channels are expressed heterologously suggest that the VSD itself provides the pathway for proton conduction. In order to exclude the possibility that the Hv channel VSD assembles with an as yet unknown protein in the cell membrane as a requirement for H+ conduction, we have purified Hv channels to homogeneity and reconstituted them into synthetic lipid liposomes. The Hv channel VSD by itself supports H+ flux.

Conformational Transitions and Proton Conduction in the Multidrug Efflux Pump AcrB

The increasing problem of multi-drug resistance (MDR) in cancer therapy or bacterial infections is to a large degree caused by multi-drug efflux pumps in pathogenic cells. In Escherichia coli, a major resistance mechanism against antibiotics is based on a tripartite multi-drug export complex comprising the inner membrane translocase AcrB, the membrane-fusion protein AcrA and the outer-membrane channel TolC. AcrB functions as the engine of this complex, using proton motive force to expel a wide variety of unrelated toxic compounds such as antibiotics, disinfectants or detergents. The molecular mechanisms of how proton conduction through AcrB is coupled to drug expulsion are not fully understood yet. Here we report a combination of normal mode analysis (NMA) and molecular dynamics (MD) simulation to investigate conformational transitions occurring in the AcrB reaction cycle and to identify residues crucial for proton conduction. In the crystallographic structure of AcrB each monomer is trapped in a different conformation, representing consecutive states in the transport mechanism. Applying the elastic network NMA variant of minimum action pathway (Kim et al. 2002), we computed transitions between these states. The resulting c-alpha trajectories were then converted back to all atom in an approach of steered energy minimization. We also performed multi-copy MD simulations of AcrB embedded in a phospholipid/water environment using the GroMACS simulation package. Mapping the proton conduction pathway was done on the basis of protein-internal water dynamics and monitoring their frequency of forming hydrogen bonds to adjacent residues.

Functionality of Single-Cysteine Mutants in Subunit III of Rhodobacter sphaeroides Cytochrome c Oxidase

Cytochrome c oxidase (COX) catalyzes the reduction of oxygen to water using ferrocytochrome c and conserves the energy of this reaction by translocating protons across the bacterial or inner-mitochrondrial membrane. COX from Rhodobacter sphaeroides is a four subunit transmembrane protein that serves as a model for the mitochondrial enzyme. Subunit I and II contain the redox centers and proton pathways necessary for redox chemistry and proton translocation. The indispensable role of subunit III is an area still being investigated. This work examines the functionality of three mutant forms of COX - one in which all cysteines have been removed from the enzyme (CA1CS3), and two in which single cysteines are reintroduced into CA1CS3 at specific locals in subunit III (A4C, S187C). The single cysteine mutants provide a means to specifically target thiol-reactive probes to areas of interest in COX subunit III. The A4C mutant allows for a probe to be placed at the mouth of the D-channel - an important proton-conducting pathway necessary for the pumping and redox activities of COX. Bioconjugation of S187C would place a probe on an exterior loop which is thought to undergo redox-linked transient conformational changes. All three mutants were expressed and purified, and their absorbance spectra are identical to wildtype, indicating that the heme active centers are unperturbed. SDS-PAGE gels show that all three mutants retain wildtype subunit composition. The oxygen reduction activity of the mutants are also comparable to wildtype, with values between 1200-1600 e−/s∗mol at pH 7.4. In conclusion, these results indicate that the cysteine-free mutant and two mutants in which single cysteines are reintroduced at non-conserved locations retain wildtype functionality, indicating that cytochrome c oxidase subunit III is a candidate for cysteine scanning-mutagenesis studies utilizing thiol-reactive probes.

On the conduction pathway for protons in nanocrystalline yttria-stabilized zirconia

In this communication we elucidate a microstructural picture of proton conduction in nano-crystalline yttria-stabilized zirconia at low temperatures (Kim et al. Adv. Mater., 2008, 20, 556). Based on careful analysis of electrical impedance spectra obtained from samples with grain sizes of approximately 13 and approximately 100 nm under both wet and dry atmospheres over a wide range of temperatures (room temperature-500 degrees C), we were able to identify the pathway for proton conduction in this material. It was found that the grain boundaries in nano-crystalline yttria-stabilized zirconia are highly selective for ion transport, being conductive for proton transport but resistive for oxygen-ion transport.

Transport properties of composite membranes containing silicon dioxide and Nafion ®

Silicon dioxide (SiO 2) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO 2 membranes were fabricated with SiO 2 content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO 2 content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO 2 content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO 2 content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO 2 nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.

Electron and proton transfer in the ba 3 oxidase from Thermus thermophilus

The ba(3)-type cytochrome c oxidase from Thermus thermophilus is phylogenetically very distant from the aa(3)-type cytochrome c oxidases. Nevertheless, both types of oxidases have the same number of redox-active metal sites and the reduction of O(2) to water is catalysed at a haem a(3)-Cu(B) catalytic site. The three-dimensional structure of the ba(3) oxidase reveals three possible proton-conducting pathways showing very low homology compared to those of the mitochondrial, Rhodobacter sphaeroides and Paracoccus denitrificans aa(3) oxidases. In this study we investigated the oxidative part of the catalytic cycle of the ba( 3 )-cytochrome c oxidase using the flow-flash method. After flash-induced dissociation of CO from the fully reduced enzyme in the presence of oxygen we observed rapid oxidation of cytochrome b (k congruent with 6.8 x 10(4) s(-1)) and formation of the peroxy (P(R)) intermediate. In the next step a proton was taken up from solution with a rate constant of approximately 1.7 x 10(4) s(-1), associated with formation of the ferryl (F) intermediate, simultaneous with transient reduction of haem b. Finally, the enzyme was oxidized with a rate constant of approximately 1,100 s(-1), accompanied by additional proton uptake. The total proton uptake stoichiometry in the oxidative part of the catalytic cycle was approximately 1.5 protons per enzyme molecule. The results support the earlier proposal that the P(R) and F intermediate spectra are similar (Siletsky et al. Biochim Biophys Acta 1767:138, 2007) and show that even though the architecture of the proton-conducting pathways is different in the ba(3) oxidases, the proton-uptake reactions occur over the same time scales as in the aa(3)-type oxidases.

Synthesis and Characterization of Composite Membrane with Three-Dimensionally Ordered Macroporous Polyimide Matrix for DMFC

A composite membrane consisting of three-dimensionally ordered macroporous (3DOM) polyimide matrix and a proton-conducting gel polymer was prepared for direct methanol fuel cells (DMFCs). The matrix effect on the physico-electrochemical properties of the composite membrane was investigated in order to compare 3DOM silica composite membrane containing the same proton-conducting polymer. From the comparison of 3DOM polyimide matrix with the 3DOM silica matrix reported in our previous paper, it was confirmed that the polyimide matrix has some structural advantages. In particular, the high uniformity of 3DOM structure provides fairly continuous proton-conducting pathways in the composite membrane, resulting in considerably high proton conductivity of at under 90% relative humidity, which is about three times greater than that obtained with the 3DOM silica composite membrane. Furthermore, high mechanical strength of 3DOM polyimide suppressed polymer swelling in the composite membrane, which resulted in low methanol permeability of in methanol solution. Consequently, was obtained as transfer selectivity of the composite membrane, which was about ten times larger than that of Nafion membrane.

Potential contribution of a voltage-activated proton conductance to acid extrusion from rat hippocampal neurons

We examined the potential contribution of a voltage-gated proton conductance ( g H +) to acid extrusion from cultured postnatal rat hippocampal neurons. In neurons loaded with Ca 2+- and/or pH-sensitive fluorophores, transient exposures to 25–139.5 mM external K + (K + o) or 20 μM veratridine in the presence of 2 mM Ca 2+ o (extracellular pH (pH o) constant at 7.35) caused reversible increases and decreases in intracellular free calcium concentration ([Ca 2+] i) and intracellular pH (pH i), respectively. In contrast, under external Ca 2+-free conditions, the same stimuli failed to affect [Ca 2+] i but caused an increase in pH i, the magnitude of which was related to the [K +] o applied and the change in membrane potential. Consistent with the properties of g H +s in other cell types, the magnitude of the rise in pH i observed in the absence of external Ca 2+ was not affected by the removal of external Na + but was sensitive to external Zn 2+ and temperature and was dependent on the measured transmembrane pH gradient (ΔpH memb). Increasing ΔpH memb by pretreatment with carbonylcyanide- p-trifluoromethoxyphenylhydrazone augmented both the high-[K +] o-evoked rise in pH i and the Zn 2+-sensitive component of the rise in pH i, suggestive of increased acid extrusion via a g H +. The inhibitory effect of Zn 2+ at a given ΔpH memb was further enhanced by increasing pH o from 7.35–7.8, consistent with a pH o-dependent inhibition of the putative g H + by Zn 2+. Under conditions designed to isolate H + currents, a voltage-dependent outward current was recorded from whole-cell patch-clamped neurons. Although the outward current appeared to show some selectivity for protons, it was not sensitive to Zn 2+ or temperature and the H +-selective component could not be separated from a larger conductance of unknown selectivity. Nonetheless, taken together, the results suggest that a Zn 2+-sensitive proton conductive pathway is present in rat hippocampal neurons and contributes to H + efflux under depolarizing conditions.

Proton generation and transport in the fuel cell environment: atomistic computer simulations

Hydrogen atoms in direct methanol fuel cells are produced ’in situ’ by dissociation of methanol on precious metal catalysts (Pt, Pt/Ru) in an aqueous environment. The abstraction of the first hydrogen atom via C–H bond cleavage is generally considered to be the rate-limiting step of dissociative methanol adsorption on the catalyst surface. This oxidation reaction on platinum particles in a fuel cell is investigated by means of a combined approach of classical molecular dynamics (MD) simulations and ab initio calculations in order to obtain an understanding of the role of the solvent for the stabilization of intermediates and for the enhancement of proton desorption from the catalyst surface and subsequent transfer into the nearby polymer electrolyte membrane (PEM). The anodically generated protons need to migrate efficiently through the membrane to the cathode were they are consumed. At the same time water and methanol (in a direct methanol fuel cell) transport should be slow. Humidified PEMs are considered to consist of a nanometer-scale phase-separated bicontinuous network of polymer regions providing structural integrity, and of aqueous regions providing the pathways for proton conduction. MD simulations provide a powerful theoretical tool for the investigation and clarification of the relationship between molecular structure and these transport phenomena. In order to atomistically model larger fractions of a humidified PEM, a coarse-grained model of humidified polymer electrolyte membranes has been developed.

Influence of silica content in crosslinked PVA/PSSA_MA/Silica hybrid membrane for direct methanol fuel cell (DMFC)

In the present study, crosslinked poly(vinyl alcohol) (PVA) membranes were prepared at different temperatures using poly(styrene sulfonic acid-co-maleic acid) (PSSA_MA) (PVA:PSSA_MA = 1:9). The hybrid membranes were prepared by varying the TEOS content between 5 and 30 wt%. The PSSA_MA was used both as a crosslinking agent and the hydrophilic group donor (-SO3H and/or -COOH). The proton conductivity increased with up to 20 wt% TEOS, but decreased above this level, although the water content decreased with increasing TEOS content. This result suggests that the silica doped into the membrane improved the formation of proton-conduction pathways due to the absorption of molecular water. The PVA/PSSA_MA/Silica containing TEOS 20% showed both high proton conductivity (0.026 S/cm at 90°C) and low methanol permeability (5.55×10-7 cm2/s).

Comparison of proton conduction in KTaO3 and SrZrO3

In the present paper, the authors focus on proton conduction pathways in a cubic perovskite KTaO(3) and an orthorhombic perovskite SrZrO(3). Density functional theory with a generalized gradient approximation is used to find proton binding sites. The nudged elastic band method is used to find transition states between minima. With this potential energy map of binding and transition states, adjacency matrices and their analogs identify four types of conduction paths in KTaO(3). Distortions from these paths are seen in SrZrO(3). In both cases, the lowest energy path has an intraoctahedral transfer rate-limiting barrier. A Fourier analysis of the OH stretch in ab initio molecular dynamics simulations revealed a strongly redshifted OH stretch in SrZrO(3) relative to KTaO(3). Hence, an orthorhombic system with a lowest energy conduction path limited by an intraoctahedral barrier can exhibit a redshifted OH stretch.

Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.

Spontaneous formation of hierarchical proton‐conductive structures in sulfonated poly(p‐phenylene terephthalamide) copolymer films

AbstractTwo partially sulfonated copolymers of poly(p‐phenylene terephthalamide) were studied; the sulfonated diamine to nonsulfonated diamine ratios were x = 1 and x = 2. Polymer solutions in water demonstrated lyotropic liquid‐crystalline behavior, with the critical concentration for nematic phase formation being around 0.7 wt %. Films of these copolymers could be considered for fuel‐cell applications. The in‐plane proton conductivities were of the order of 10−3 to 10−2 S cm−1 between 20 and 90 °C. Increasing the sulfonation level resulted in a more conductive material. Spontaneous alignment of the polymer occurred during film formation, as revealed by X‐ray diffraction. Scattering along the polymer backbone was observed perpendicular to the film, implying that the polymer chains were homeotropically aligned with respect to the film. The average degree of alignment was determined to be 0.66 and 0.77 for x = 1 and x = 2, respectively. Evidence of secondary layering within the plane of the film was seen in SEM images. These layers could provide a pathway for proton conduction to occur within the plane of the film. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 666–676, 2007

The United States Department of Energy's high temperature, low relative humidity membrane program

The U.S. Department of Energy (USDOE) Hydrogen Program works with industry, academia, and National Laboratories through research and development to overcome technical barriers of fuel cell and hydrogen production, delivery, and storage technologies. Two of the major challenges in the advancement of fuel cell technology are cost and durability of the polymer electrolyte membranes used for proton conduction in the fuel cell. To address these challenges, DOE initiated new membrane research and development projects to design membranes that meet its 2010 technical targets and will lead to membranes that operate in a fuel cell system that performs as well and costs as little as internal combustion engines. Three strategies are employed in the program: implementation of phase segregation in the membrane to create proton conduction pathways, use of non-aqueous proton conductors for operation under dry conditions, and hydrophilic additives to retain water at low relative humidity.

Nano-Structure Controlled Polymer Electrolyte Membranes for Fuel Cell Applications Prepared by Ion Beam Irradiation

Fluoropolymer-based electrolyte membranes for fuel cells were prepared by using heavy ion beams from the cyclotron accelerator of the Takasaki Ion Accelerators for Advanced Radiation Application (TIARA), Japan Atomic Energy Agency (JAEA). The preparation method for these so-called nano-scale structure-controlled membranes involves (i) the swift heavy ion irradiation of polyvinylidene fluoride (PVDF) films and subsequent chemical etching to obtain cylindrical pores with a diameter of 100 nm, and (ii) the filling of proton-conducting polymer chains into the etched pores by gamma-ray-induced graft polymerization. The proton transport only in the thickness direction was observed for the resulting membranes with controlled ion exchange capacities, indicating the formation of one-dimensional straight proton-conducting pathways parallel to the ion-beam incident axis. The membranes exhibited a lower water uptake and reduced methanol permeability compared to commercially-available Nafion probably due to the restricted structures.

Relaxation of Proton Conductivity and Stress in Proton Exchange Membranes Under Strain

The stress relaxation and proton conductivity of Nafion 117 membrane (N117-H) and sulfonated poly(arylene ether sulfone) copolymer membrane with 35% sulfonation (BPSH35) in acid forms were investigated under uniaxial loading conditions. The results showed that when the membranes were stretched, their proton conductivities in the direction of the strain initially increased compared to the unstretched films. The absolute increases in proton conductivities were larger at higher temperatures. It was also observed that proton conductivities relaxed exponentially with time at 30°C. In addition, the stress relaxation of N117-H and BPSH35 films under both atmospheric and an immersed (in deionized water) condition was measured. The stresses were found to relax more rapidly than the proton conductivity at the same strains. An explanation for the above phenomena is developed based on speculated changes in the channel connectivity and length of proton conduction pathway in the hydrophilic channels, accompanied by the rotation, reorientation, and disentanglements of the polymer chains in the hydrophobic domains.

The basal proton conductance of mitochondria depends on adenine nucleotide translocase content

The basal proton conductance of mitochondria causes mild uncoupling and may be an important contributor to metabolic rate. The molecular nature of the proton-conductance pathway is unknown. We show that the proton conductance of muscle mitochondria from mice in which isoform 1 of the adenine nucleotide translocase has been ablated is half that of wild-type controls. Overexpression of the adenine nucleotide translocase encoded by the stress-sensitive B gene in Drosophila mitochondria increases proton conductance, and underexpression decreases it, even when the carrier is fully inhibited using carboxyatractylate. We conclude that half to two-thirds of the basal proton conductance of mitochondria is catalysed by the adenine nucleotide carrier, independently of its ATP/ADP exchange or fatty-acid-dependent proton-leak functions.

The effect of octahedral tilting on proton binding sites and transition states in pseudo-cubic perovskite oxides

Proton conducting oxide ceramics have shown potential for use in fuel cell technologies. Understanding the energy pathways for proton conduction could help us design more efficient fuel cell materials. This paper describes how octahedral tilting affects the relative energies of proton binding sites, transition states, and conduction pathways in cubic and pseudo-cubic perovskites. First, the structure for cubic and pseudo-cubic forms of BaTiO(3), BaZrO(3), CaTiO(3), and CaZrO(3), is found. Even when cubic symmetry is enforced, CaTiO(3), and CaZrO(3) exhibit octahedral tilting distortions characteristic of orthorhombic phases while BaTiO(3) and BaZrO(3) remain undistorted. Octahedral tilting gives rise to proton binding sites facilitating inter- and intra-octahedral proton transfer while the proton binding sites of undistorted perovskites facilitate only intra-octahedral proton transfer. The nudged elastic band method is used to find minimum energy paths between the proton binding sites. As distortions increase, inter-octahedral proton transfer barriers decrease while intra-octahedral proton transfer barriers increase. Concurrently, rotational barriers from oxygens facilitating inter-octahedral proton transfer increase while rotational barriers from oxygens facilitating intra-octahedral proton transfer decrease. Intra-octahedral transfer is the rate-limiting step to the lowest energy extended proton conduction pathway in all the perovskites considered.