Fast Proton Conductors Research Articles

Uncovering fast solid-acid proton conductors based on dynamics of polyanion groups and proton bonding strength

Cation lattice flexibility and covalent bond lengths serve as good physical descriptors of proton conduction in solid acids and enable the discovery of promising proton conductors beyond traditional chemistries.

Fast proton conductors for low humidity: hydrophilic sulfonated and phosphonated polysilsesquioxanes with Al-O-P crosslinks.

Proton-conducting polysilsesquioxane oligomers with core-shell structures consisting of hydrophilic silica-rich cores surrounded by an organic layer with sulfonic and phosphonic acid groups were synthesized. They were crosslinked by mixing with phosphoric acid and aluminum ions to form a hydrophilic Al-O-P framework. The resulting polymers were clear, uniform, flexible, and exhibited a high proton conductivity above 100 °C in non-humidified low-humidity air (∼22 mS cm-1 at 120 °C and ∼1%RH) because of their high sulfonic acid concentration and high water retention capability.

Coordination polymer design for fast proton conduction: Hybrid atomistic approach based on kinetic Monte Carlo and molecular dynamics methods

Fast proton conductors are important materials for catalysis and energy conversion applications. The glassy coordination polymers are an important class of proton conductors due to their good mechanical moldability; however, their conductivity has been limited to ca. 10 mS cm−1 at 100 °C. The systematic design of coordination polymers with fast proton conduction requires an atomistic simulation method that can describe long-range proton diffusion within an affordable computational time. The methodologies of atomistic simulations are separately limited and cannot fairly describe the long-range proton conduction in non-crystalline materials. In this work, we develop a hybrid approach that combines the molecular dynamics based on a conventional force-field and the kinetic Monte Carlo method, which allows for the large-scale (thousands of atoms) and long time (few nanoseconds) simulation of the long-range ionic diffusion in non-crystalline materials. Based on the developed approach, we propose and confirm a design concept for a fast proton-conducting coordination polymer based on Zn2+ ions and phosphoric acid.

Protonic Conduction in the BaNdInO4 Structure Achieved by Acceptor Doping.

The potential of calcium-doped layered perovskite compounds, BaNd1–xCaxInO4–x/2 (where x is the excess Ca content), as protonic conductors was experimentally investigated. The acceptor-doped ceramics exhibit improved total conductivities that were 1–2 orders of magnitude higher than those of the pristine material, BaNdInO4. The highest total conductivity of 2.6 × 10–3 S cm–1 was obtained in the BaNd0.8Ca0.2InO3.90 sample at a temperature of 750 °C in air. Electrochemical impedance spectroscopy measurements of the x = 0.1 and x = 0.2 substituted samples showed higher total conductivity under humid environments than those measured in a dry environment over a large temperature range (250–750 °C). At 500 °C, the total conductivity of the 20% substituted sample in humid air (∼3% H2O) was 1.3 × 10–4 S cm–1. The incorporation of water vapor decreased the activation energies of the bulk conductivity of the BaNd0.8Ca0.2InO3.90 sample from 0.755(2) to 0.678(2) eV in air. The saturated BaNd0.8Ca0.2InO3.90 sample contained 2.2 mol % protonic defects, which caused an expansion in the lattice according to the high-temperature X-ray diffraction data. Combining the studies of the impedance behavior with four-probe DC conductivity measurements obtained in humid air, which showed a decrease in the resistance of the x = 0.2 sample, we conclude that experimental evidence indicates that BaNd1–xCaxInO4–x/2 is a fast proton conductor.

RETRACTED: Proton transport enabled by a field-induced metallic state in a semiconductor heterostructure.

Tuning a semiconductor to function as a fast proton conductor is an emerging strategy in the rapidly developing field of proton ceramic fuel cells (PCFCs). The key challenge for PCFC researchers is to formulate the proton-conducting electrolyte with conductivity above 0.1 siemens per centimeter at low temperatures (300 to 600°C). Here we present a methodology to design an enhanced proton conductor by means of a Na x CoO2/CeO2 semiconductor heterostructure, in which a field-induced metallic state at the interface accelerates proton transport. We developed a PCFC with an ionic conductivity of 0.30 siemens per centimeter and a power output of 1 watt per square centimeter at 520°C. Through our semiconductor heterostructure approach, our results provide insight into the proton transport mechanism, which may also improve ionic transport in other energy applications.

High oxide ion and proton conductivity in a disordered hexagonal perovskite.

Oxide ion and proton conductors, which exhibit high conductivity at intermediate temperature, are necessary to improve the performance of ceramic fuel cells. The crystal structure plays a pivotal role in defining the ionic conduction properties, and the discovery of new materials is a challenging research focus. Here, we show that the undoped hexagonal perovskite Ba7Nb4MoO20 supports pure ionic conduction with high proton and oxide ion conductivity at 510 °C (the bulk conductivity is 4.0 mS cm-1), and hence is an exceptional candidate for application as a dual-ion solid electrolyte in a ceramic fuel cell that will combine the advantages of both oxide ion and proton-conducting electrolytes. Ba7Nb4MoO20 also showcases excellent chemical and electrical stability. Hexagonal perovskites form an important new family of materials for obtaining novel ionic conductors with potential applications in a range of energy-related technologies.

Arrangement of water molecules and high proton conductivity of tunnel structure phosphates, KMg1-x H2x (PO3)3·yH2O.

A fast proton conductor was investigated in a mixed-valence system of phosphates with a combination of large cations (K+) and small cations (Mg2+), which resulted in a new phase with a tunnel structure suitable for proton conduction. KMg1−xH2x(PO3)3·yH2O was synthesized by a coprecipitation method. A solid solution formed in the range of x = 0–0.18 in KMg1−xH2x(PO3)3·yH2O. The structure of the new proton conductor was determined using neutron and X-ray diffraction measurements. KMg1−xH2x(PO3)3·yH2O has a tunnel framework composed of face-shared (KO6) and (MgO6) chains, and PO4 tetrahedral chains along the c-direction by corner-sharing. Two oxygen sites of water molecules were detected in the one-dimensional tunnel, one of which exists as a coordination water of K+ sites. Multi-step dehydration was observed at 30 °C and 150 °C from thermogravimetric/differential thermal analysis measurements, which reflects the different coordination environments of the water of crystallization. Water molecules are connected to PO4 tetrahedra by hydrogen bonds and form a chain along the c-axis in the tunnel, which would provide an environment for fast proton conduction associated with water molecules. The KMg1−xH2x(PO3)3·yH2O sample with x = 0.18 exhibited high proton conductivity of 4.5 × 10−3 S cm−1 at 150 °C and 7.0 × 10−3 S cm−1 at 200 °C in a dry Ar gas flow and maintained the total conductivity above 10−3 S cm−1 for 60 h at 150 °C under N2 gas atmosphere.

Understanding hydrogen in perovskites from first principles

The behavior of hydrogen in oxides is important to understand their functions; for example, some perovskites are fast proton conductors. Herein we study in detail the geometry, energetics, and chemical bonding of hydrogen in ABO3 perovskites from first principles. We find that hydrogen absorption to a lattice oxygen leads to structure distortion, especially in the (1 0 0) BO2 plane, and ~1.5% volume expansion. Density-of-states and crystal orbital Hamilton population (COHP) analyses indicate that the electron from hydrogen occupies the states at the conduction band minimum which correspond to the BO antibonding states and thereby weakens the BO covalent bonding. More interestingly, oxygen vacancy formation energy (OVFE), the integrated COHP of the BO bond, and the electronegativity difference between A and B are all found to be useful descriptors that correlate with hydrogen absorption energy (HAE).

PH Sensing with Silicon Nanoribbon Devices Modified with Carbon Nanotube Porins

Acidity of the local microenvironments is an important parameter for detection of pathological conditions and understanding of cellular mechanisms. Silicon nanoribbon field effect transistor (FET) has been proved to be a high-throughput and sensitive platform for pH sensing. However, the efficiency of these sensors is limited by the biocompatibility of silicon material and its tendency for bio-fouling. Recent studies have demonstrated sub-2nm diameter carbon nanotube (CNT) porins embedded in lipid bilayers are fast and efficient proton conductors. In this research, we focus on a bioelectronic approach toward designing pH-sensing devices. We will describe an organic/inorganic hybrid device that combines nanoribbon FET with self-assembled CNT/lipid bilayer for pH sensing in physiological conditions. Finally, we will also discuss its potential application in nanometer spatial resolution intracellular biosensing.

A novel proton conductor of imidazole–aluminium phosphate hybrids in the solid state

Anhydrous proton transport at temperatures above 100 °C has attracted considerable attention in the development of fuel cells that operate at intermediate temperatures. Liquid-state imidazole (ImH) is known to be a fast anhydrous proton conductor above 100 °C; however, evaporation and severe conductivity drops above and below its melting point (∼90 °C), respectively, are major drawbacks to ImH. In this paper, we report a novel solid-state anhydrous ImH-Al(H(2)PO(4))(3) (AlP) hybrid material prepared via a simple synthesis using mechanical milling. This solid-state hybrid exhibits relatively a high ionic conductivity of ∼0.1 mS cm(-1) at 100 °C and remarkably a small activation energy of 0.23 eV. In addition, the ImH-AlP hybrid material provides a means of overcoming both temperature-dependent drawbacks to pure ImH: (1) the ImH-AlP hybrid is thermally stable up to 130 °C, and (2) the hybrid material maintains high ionic conductivity below the melting point of ImH.

Synthesis of calcium phosphate hydrogel from waste incineration fly ash and its application to fuel cell

Waste incineration fly ash was successfully recycled to calcium phosphate hydrogel, a type of fast proton conductor. The crystallized hydrogel from incineration fly ash had a lower electric conductivity and a lower crystallinity than that from calcium carbonate reagent. However, the difference in electric conductivity between these crystallized hydrogels decreases with temperature. This was due to the presence of potassium in the incineration fly ash. The fuel cell with a membrane electrode assembly (MEA) using the calcium phosphate hydrogel membrane prepared from incineration fly ash was observed to generate electricity. The performance of this fuel cell was almost equal to that of a mixture of K 2CO 3 and CaCO 3 reagents; further, the performance of the former was superior to the fuel cell with a perfluorosulfonic polymer membrane at temperatures greater than approximately 85 °C.

Performance of fuel cell using calcium phosphate hydrogel membrane prepared from waste incineration fly ash and chicken bone powder

Waste incineration fly ash and bone powder could be successfully recycled to calcium phosphate hydrogel, a type of fast proton conductor. The electric conductivity of the crystallized hydrogel from them was compared with that from calcium carbonate reagent. It was found that the conductivity of the hydrogel from bone powder is almost equal to that from calcium carbonate reagent, which is higher than that from incineration fly ash. Because the crystallized hydrogel from incineration ash has a lower crystallinity than that from bone powder and calcium carbonate reagent. However, the difference of the conductivity among them can be hardly observed above 100 °C. The fuel cell with membrane electrode assembly (MEA) using the calcium phosphate hydrogel membrane prepared from incineration fly ash and bone powder was observed to generate electricity. The performance of fuel cells having the hydrogel membrane obtained from all raw materials increases with the cell temperature, and the fuel cell containing the hydrogel membrane from incineration fly ash has the highest dependence of the fuel cell performance. For this reason, the difference in the cell performance among them can be hardly observed above 120 °C. This tendency agrees with the change in the electric conductivity with the temperature. Further, the performance of all fuel cells with the hydrogel membrane is superior to that of the fuel cell with perfluorosulfonic polymer membrane at temperatures greater than approximately 85 °C.

Synthesis of calcium phosphate hydrogel from waste incineration fly ash and bone powder

Waste incineration fly ash and bone powder could be successfully recycled into calcium phosphate hydrogel, a type of fast proton conductor. Various properties of the intermediate and calcium phosphate hydrogel from them were characterized and compared with that from calcium carbonate reagent. It was found that the intermediate from the incineration fly ash and calcium phosphate glass was more brittle than that from bone powder and calcium carbonate reagent. The electric conductivity of crystallized hydrogel obtained from all raw materials increases exponentially with temperature. However, the crystallized hydrogel from incineration fly ash has lower electric conductivity and lower crystallinity than that from bone powder and the reagent. Moreover, the difference in electric conductivity between these crystallized hydrogels decreases with temperature. Compared with using the reagent as a raw material, bone powder provides a 25% reduction in the usage of H 3PO 4 to acquire the crystallized hydrogel which has the highest conductivity. These experimental results suggest that the incineration fly ash and bone powder are useful calcium sources for the synthesis of calcium phosphate hydrogel.

Fast proton conductor under anhydrous condition synthesized from 12-phosphotungstic acid and ionic liquid

In this study, we synthesized a molecular hybrid conductor electrolyte using PWA ([H 3PW 12O 40· nH 2O]) and [1-butyl-3-methylimidazole][bis-(fluoromethanesulfonyl) amide] ([BMIM][TFSI]) ionic liquid. The [BMIM][TFSI] ionic liquid can interact with the strongly acidic PWA. The hybrids were hydrophilic, and included some water molecules in the structure of the hybrids. The water molecules remained up to 80 °C, contributing to improve conductivity under an anhydrous N 2 atmosphere. The conductivity of PWA-[BMIM][TFSI] hybrid under anhydrous conditions increased from 10 −4 S/cm to 0.04 S/cm at 60 °C. The conductivity of the hybrids at each temperature was higher than that of pure PWA and [BMIM][TFSI] under anhydrous condition. It can be due to the protonic carriers.

New Class of Low-Humidity Fast Protonic Conductors Based on Crystalline Organic Materials with Closely Spaced Sulfonic Acid Groups

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Synthesis of Fast Proton Conductor from Waste Incineration Ash

Synthesis of Fast Proton Conductor from Waste Incineration Ash

Effect of temperature and high pressure on Mn2+ EPR spectra in K3H(SO4)2 fast-proton conductor

The linewidth ΔH pp and spin-Hamiltonian parameters under temperature and high hydrostatic pressure by X-band continuous wave electron paramagnetic resonance in the K3H(SO4)2 crystal were studied. Spin-Hamiltonian parameters, direction cosines and coordination of Mn2+ ion were determined at room temperature. The pressure at 300 MPa leads to the change of hydrogen bond potential and the transition from double well to single well potential moves about 10 K towards a higher temperature.

Pretransition Phenomena in Fast-Proton Conductors

The proton transfer along hydrogen-bonded channels has been recognized to be responsible for the charge conduction in many organic and biological systems [1–3]. Proton conducting solids are also of interest because of their potential use in fuel cells [4, 5]. Current interest to fast-proton conductors needs their synthesis, parameterization, and especially understanding of charge transport mechanism on molecular level. Since the basic work of Antonchenko, Davydov and Zolotaryuk (ADZ) [6] it has been widely believed that protons in hydrogen-bonds move in deformed double-well potential resulting from proton–lattice interactions. Combined with proton–proton interaction this characteristic provides the essential ingredients for the ionic defects to move as topological solitons. The main aim of our work is to check experimentally a validity of data description in the framework of ADZ model. To get reproducible data we have collected a number of EPR and dc current flow data for K3H(SO4)2 (KHS) fastproton conductor pure and with Mn, CrO3− 4 paramagnetic ions in the temperature region below the transition to paraelectric fast-proton conducting phase (Tg = 471 K). We were motivated by Dolinsek et al. [7] NMR work where for Rb3H(SO4)2 so-called “pretransition phenomena” were observed about 100 K below Tg. In such a way we avoided the sample decomposition and could repeat the experiments for ∗corresponding author; e-mail: Stefan.Waplak@ifmpan.poznan.pl

Brønsted acid–base and –polybase complexes as electrolytes for fuel cells under non-humidifying conditions

A novel series of Brønsted acid–base complexes were prepared by the combination of 2,5-diphenyl-1,3,4-oxadiazole (DOD) with a super strong acid bis(trifluoromethane sulfonyl)imide (HTFSI) at various molar ratios. The DOD–HTFSI complexes showed high ionic conductivities and were electroactive for H 2 oxidation and O 2 reduction at a Pt electrode. The complexes could serve as fuel cell electrolytes under entirely non-humidifying conditions and at elevated temperatures. The concept of the DOD–HTFSI complexes was applied to a polybase, poly[ p-phenylene-2,5-(1,3,4-oxadiazole)] (POD), and the ion-conduction behavior of the POD–HTFSI complexes was discussed with a view to realizing anhydrous solid-state fast proton conductors.

Characterisation of polyphosphate composites based on intermediate temperature

High temperature proton conducting solids are useful materials for many electrochemical applications such as high temperature fuel cells, hydrogen sensors and hydrogen gas separators. However, many protonic conductors decompose at temperatures above 300 ◦C. For high-temperature proton conductors, progress is a matter of combining high mobility with a high concentration of protons, but also of avoiding accompanying contributions from oxygen ions and electrons. The intermediate-temperature (200–600 ◦C) fuel cells are very attractive because they combine the advantages of highand low-temperature fuel cells. A membrane formed from an inorganic polyphosphate composite has good ionic conduction properties at temperatures between 473 and 573 K especially in humidified atmosphere [1, 2]. The number of fast protonic conductors, organic or inorganic crystalline and amorphous have been prepared during the past two decades [3], which are categorized into two types on the basis of their operating temperatures. One is hydrated crystalline compounds [4], and the other is perflurinated ionomers [5] showing high conductivities of ∼10−2 S cm−1 below 100 ◦C, but their chemical degradation limits the use in practical applications. On the other hand, fast proton conducting glasses, if developed, extend beyond the limitation of the above compounds, because of their high chemical and mechanical durability and easy formation into films and plates. Solid state proton conductors that show high conductivities in the medium temperature range (100–200 ◦C) with low humidity have been required as the electrolyte for polymer electrolyte fuel cells and direct methanol fuel cells operating in the temperature range [6]. The operation of fuel cells in the medium temperature range improves the efficiency of energy conversion in the cells and reduces the poisoning of Pt catalysts with CO in the fuel gases. In addition, working of the fuel cells under the condition of low humidity permits the reduction of the weight and volume of humidifiers, so that the fuel cells should have advantages for application in electric vehicles. These situations described above are