Solid-state Proton Conductors Research Articles

Room-Temperature Superprotonic Conductivity beyond 10-1 S cm-1 in a Co(II) Coordination Polymer.

An efficient design of crystalline solid-state proton conductors (SSPCs) is crucial for the progress of clean energy applications. Developing such materials to make them work at room temperature with a conductivity of ≥10-1 S cm-1 is of significant interest in terms of technical and commercial aspects. Utilizing the recently highlighted "coordinated-water-driven proton conduction" approach, herein, we have rationally synthesized two highly stable and scalable 1D Co(II) coordination polymers (CPs) as SSPCs, PCM-2 {[Co(bpy)(H2O)2(NO3)2]·H2O}n and PCM-3 {[Co2(bpy)2(SO4)2(H2O)6].4H2O}n, with distinct alignments in coordinated water and coordinated oxo-anions (nitrate and sulfate, respectively). The acidity of the metal-bound water molecules in PCM-2 is further enhanced through cooperative long-range continuous H bonds with coordinated Brønsted basic nitrates (proton acceptors), leading to ultrahigh superprotonic conductivities even at 25 °C (1.03 × 10-1 S cm-1 under 95% RH), and reached a maximum of 2.99 × 10-1 S cm-1 at 85 °C (95% RH). The conductivity at 25 °C is even higher than that of commercial Nafion 117 (6.74 × 10-2 S cm-1 at 100% RH). The absence of such an H-bonding interaction in PCM-3 (closed loops) resulted in a lesser conductivity of 5.87 × 10-5 S cm-1 (95% RH, 85 °C). PCM-2 represents the first example of SSPC exhibiting conductivity in the order 10-1 S cm-1 at ambient temperature (25 °C) with excellent recyclability.

Efficient Proton Conductor Based on Bismuth Oxide Clusters and Polyoxometalates.

Developing new solid-state electrolyte materials for improving the proton conductivity remains an important challenge. Herein, a novel two-dimensional layered solid-state proton conductor Bi2O2-SiW12 nanocomposite, based on silicotungstic acid (H4SiW12O40) and Bi(NO3)3·5H2O, was synthesized and characterized. The composite consists of a layered cation framework [Bi2O2]2+ and interlayer-embedded counteranionic [SiW12O40]4-, which forms continuous hydrogen bond (O-H···O) networks through the interaction of adjacent oxygen atoms on the surface of the [Bi2O2]2+ and oxygen atoms of the H4SiW12O40. Facile proton transfer along these pathways endows the Bi2O2-SiW12 (30:1) nanocomposite with an excellent proton conductivity of 3.61 mS cm-1 at 90 °C and 95% relative humidity, indicating that the nanocomposite has good prospects as a highly efficient proton conductor.

MOF-Based Solid-State Proton Conductors Obtained by Intertwining Protic Ionic Liquid Polymers with MIL-101.

Solid-state proton conductors based on the use of metal-organic framework (MOF) materials as proton exchange membranes are being investigated as alternatives to the current state of the art. This study reports a new family of proton conductors based on MIL-101 and protic ionic liquid polymers (PILPs) containing different anions. By first installing protic ionic liquid (PIL) monomers inside the hierarchical pores of a highly stable MOF, MIL-101, then carrying out polymerization in situ, a series of PILP@MIL-101 composites was synthesized. The resulting PILP@MIL-101 composites not only maintain the nanoporous cavities and water stability of MIL-101, but the intertwined PILPs provide a number of opportunities for much-improved proton transport compared to MIL-101. The PILP@MIL-101 composite with HSO4 - anions shows superprotonic conductivity (6.3×10-2 Scm-1 ) at 85°C and 98% relative humidity. The mechanism of proton conduction is proposed. In addition, the structures of the PIL monomers were determined by single crystal X-ray analysis, which reveals many strong hydrogen bonding interactions with O/NH···O distances below 2.6Å.

Synthesis and In Situ Monitoring of Mechanochemical Preparation of Highly Proton Conductive Hydrogen-Bonded Metal Phosphonates.

Crystalline porous materials are recognized as promising proton conductors for the proton exchange membrane (PEM) in fuel cell technology owing to their tunable framework structure. However, it is still a challenging bulk synthesis for real-world applications of these materials. Herein, we report the mechanochemical gram-scale synthesis of two isostructural metal hydrogen-bonded organic frameworks (MHOFs) of Co(II) and Ni(II) based on 1-hydroxyethylidenediphosphonic acid (HEDPH4) with 2,2'-bipyridine (2,2'-bipy): Co(HEDPH3)2(2,2'-bipy)·H2O (1) and Ni(HEDPH3)2(2,2'-bipy)·H2O (2). In situ monitoring of the mechanochemical synthesis using different synchrotron-based techniques revealed a one-step mechanism - the starting materials are directly converted to the product. With the existence of extensive hydrogen bonds with amphiprotic uncoordinated phosphonate hydroxyl and oxygen atoms, both frameworks exhibited proton conduction in the range of 10-4 S cm-1 at room temperature under humid conditions. This study demonstrates the potential of green mechanosynthesis for bulk material preparation of framework-based solid-state proton conductors.

Exploring Proton Pair Motion Away from the Global Proton–Tuple Energy Minimum in Yttrium-Doped Barium Zirconate

Yttrium-doped barium zirconate is one of the fastest solid-state proton conductors. While previous studies suggest that proton–tuples move as pairs in yttrium-doped barium zirconate, a systematic catalog of possible close proton–tuple moves is missing. Such a catalog is essential to simulating dual proton conduction effects. Density functional theory with the Perdew–Burke–Ernzerhof functional is utilized to obtain the total electronic energy for each proton–tuple. The conjugate gradient and nudged elastic band methods are used to find the minima and transition states for proton–tuple motion. In the lowest-energy configuration, protons are in close proximity to each other and the dopant, significantly affecting the backbone structure. The map of moves away from the global minimum proton–tuple shows that the most critical move for long-range proton conduction is a rotation with a barrier range of 0.31–0.41 eV when the two protons are in close proximity.

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

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

BaCe0.6Zr0.3Y0.1O3-α electrochemical hydrogen sensor for fusion applications

Potentiometric sensors are broadening their application scope because they are affordable and scalable devices capable of being used in-field and online. In that frame, the use of proton-conducting solid-state electrolytes in these sensors architecture offers the possibility to operate at high temperatures, with high physical and chemical stabilities. A potential application for this kind of sensor is tritium monitoring in future fusion reactors. The first steps toward the development of this analytical tool will be devices capable of measuring hydrogen in high temperature applicationsIn the present work, a potentiometric sensor based on BaCe0.6Zr0.3Y0.1O3-α (BCZY) solid-state proton-conductor was tested for hydrogen monitoring at 325 °C, 350 °C and 375 °C. BCZY was synthesized by the solid-state method and characterized using XRD. The sensor's linear range, accuracy and limit of detection were evaluated at the selected temperatures (325 °C, 350 °C and 375 °C). Moreover, sensors presented a very short response and recovery times (<100 s).

Proton-conducting oxides based on LaScO3: structure, properties and electrochemical applications. A focus review

Solid state proton conductors are promising materials for various electrochemical applications. LaScO3 - based oxides are representatives of the proton-conducting oxides with perovskite structure, alternative to conventional cerates and zirconates of alkaline-earth elements. These oxides exhibit a sufficient level of proton conductivity with a combination of good chemical stability in the H2O and CO2 - containing atmospheres. The current review is focused on summarizing and analyzing of the currently available data on LaScO3 - based oxides. The peculiarities of crystal structure and proton defect formation, aspects of synthesis and obtaining dense ceramics, and electrical properties are provided. Additionally, the current state of applications in electrochemical devices of LaScO3 - based oxides is briefly discussed.

A focused review on structures and ionic conduction mechanisms in inorganic solid-state proton and hydride anion conductors

This review provides a focused discussion on the structures and ionic conduction mechanisms of inorganic solid-state proton and hydride anion conductors.

Phosphate induced formation of core-shell tin pyrophosphate via an acid method for proton conduction at elevated temperatures

Tin pyrophosphate is a feasible solid-state proton conductor as electrolyte and ionomers for applications in fuel cells and sensors, etc. Nevertheless, the morphology and proton conductivity of the SnP2O7 varies vigorously due to different fabrication methods, molar ratios of phosphorous to tin and sintering temperature. In this work, a core-shell SnP2O7 (c-SnP2O7) has been fabricated via a conventional acid method between SnO2 nanoparticles and various phosphoric acids including H3PO4, H4P2O7, (HPO3)x and P2O5 under temperature as low as 200 °C. The transformation of SnO2 to c-SnP2O7 in these phosphoric acids follows dissolving and reprecipitation mechanism where Sn(HPO4)2 is the intermediate species. In addition, the dominated factor for the transformation is the presence of P2O74− due to the condensation or hydration of various phosphoric acids. The as-synthesized c-SnP2O7 contains crystalline SnP2O7 inner core, an amorphous phosphate intermediate layer and an outer-layer with gel-like phosphorous-rich species. Furthermore, the phosphorous-rich gel layer contributes to outstanding proton conductivity of c-SnP2O7 up to 8.0 × 10−2 S cm−1 at 260 °C and excellent durability of 115 h at 240 °C and anhydrous conditions. Overall, the core-shell SnP2O7 is a promising proton conductor to be used in elevated temperature fuel cells.

Solid-State Proton Conduction Driven by Coordinated Water Molecules in Metal–Organic Frameworks and Coordination Polymers

As efficient alternative renewable energy resources, proton-exchange membrane fuel cells (PEMFCs) have received immense attention where proton-conducting materials act as PEMs for efficient proton migration. The recent past has witnessed considerable progress in the use of metal–organic frameworks (MOFs) and coordination polymers (CPs) as solid-state proton conductors (SSPCs) due to their structural superiority and architectural diversity. In order to impart proton conductivity, a variety of intrinsic and extrinsic proton sources have been installed onto these frameworks; however, reports on proton conduction where metal-coordinated water molecules act as solely intrinsic proton sources by virtue of their enhanced acidity are largely unexplored. The key examples of proton conduction driven by coordinated water molecules discussed in this Perspective present some unique features of MOFs and CPs as SSPCs, displaying a wide range of proton conductivity. The scope of works with plausible design principles that are urgently required to be investigated is also highlighted.

(Invited) Mechanism of Cathodic Reaction in Proton-Conducting Ceramic Fuel Cells Investigated By Patterned Model Electrodes

Proton-conducting ceramic fuel cell (PCFCs) is one type of solid oxide fuel cell using a solid-state proton-conductor, typically BaZrO3-based and BaCeO3-based ceramics, as an electrolyte. PCFC is expected to operate at intermediate temperatures with high energy conversion efficiency. One of technological challenges for realization of PCFC is the development of high performace cathode. Cathodic reaction in PCFC is electrochemical reduction of oxygen gas foriming water vapor. Since protons, electrons, and oxygen gas molcules exist in the electrolyte, electrode and gas phase, respectively, the electrode reaciton takes place at triple phase boundaries (TPBs) in principle. On the other hand, the reaction site can be expanded from TPBs to double phase boundaries (DPBs), when a mixed ionic-electronic conductor (MIEC) is used. In such an MIEC electrode, the dominant reaction sites are believed to be DPBs because of their large area. So far, some oxides are reported to exhibit mixed protonic-electronic conductor, but no clear experimental evidences have been provided to show how significantly TPB/DPB reactions contribute to the total reaction.Recently we proposed new and original model electrodes, which can be schematically illustrated in Fig. 1, to investigate a gas reaction on a solid electrolyte. These so-called "patterned thin film electrodes" are kinds of thin film electrodes. However, in contrast to a conventional thin film electrode, the electrode/electrolyte contact area was limitted by inserting a slitted insulating layer between the electrode and the electrolyte, and threfore our model electrodes can behave as a columar electrode. In the electrode of Fig. 1(a), the electrode reaction proceeds only through DPBs, and the reaction current gradually decreased with increasing the distance from the electrode/electrolyte interface depending on proton diffusion in the electrode layer. On the other hand, in the electrode of Fig. 1(b), a part of the electrode film was removed from the electrode of Fig.1(a) to introduce TPBs. In this case, the electrode reaction takes place not only through DPBs but also through TPBs. Then, by comparing the electrode performances between these novel model electrodes, the contribution of TPB/DPB reactions can be separately evaluated.We applied these model electrodes to investigate the mechanism of the PCFC cathodic reactions. (La,Sr)CoO3-d was chosen as a cathode material, and the model electrodes were fabricated on a Ba(Zr,Yb)O3-d electrolyte. On the lateral and opposite sides of the electrolyte, porous Pt and Pd electrode was set as a reference and a counter electrodes. DC polarization and electrochemical impedance spectroscopy measurements were performed with the model electrodes at 773-973 K. As results, it was found TPBs are the dominant reaction site for (La,Sr)CoO3-d PCFC cathode, although DPB reaction slightly contributes to the total reaction. More detailed disucssion will be given in the presentation based on the results by operando X-ray absorption measurements. Figure 1

Hydrogen Sensor with 3D Printed BaCe0.6Zr0.3Y0.1O3-α electrolyte for High-Temperature Applications

Fusion energy is considered a promising source of energy for the near future. The most efficient reaction for that purpose is the fusion of deuterium and tritium, two hydrogen isotopes, to release helium, neutrons and energy. To assure the correct operation of this technology, new online devices able to monitor hydrogen isotopes will be required.Electrochemical sensors based on solid-state proton conductor ceramics can be used for that purpose. These materials have attracted significant interest because of their chemical and physical durability, especially at elevated temperatures. These electrolytes are perovskite-type materials with electrical carriers, positive holes, excess electrons and oxide ion vacancies. In previous work [1], amperometric hydrogen sensors based on BaCe0.6Zr0.3Y0.1O3-α (BCZY) electrolyte have been developed and tested in hydrogen atmospheres obtaining good results. These prototypes were constructed using 13 mm BCZY disks obtained by uniaxial pressure.In the present work, hydrogen sensors based on BCZY were constructed and evaluated in amperometric mode. The BCZY powder was disk-shaped using uniaxial pressure and 3D printing. Sensors were constructed as follows: first, sintered disks were platinized using platinum ink. Then, disks were sealed with a glass binder to alumina tubes. This way, the external face of the disk acted as a working electrode (WE) and the inner part of the sensor as a counter electrode (CE). 3D printed disks were fabricated using a Delta WASP 2040 ceramic 3D printer. For that, the ceramic powder was mixed with PEG400 and water until a viscous paste was obtained. Once green bodies were dried, they were heated for debinding and sintering using an optimized thermal program to reduce porosity. XRD (for crystallographic phases) and SEM (for microstructural defects and sintering) were used for the 3D printed and uniaxial pressed sintered disks characterization. The amperometric response of sensors constructed with both electrolytes (the uniaxially pressed and the 3D printed disks) was compared.[1] A. Hinojo, I. Soriano, J. Abellà, S. Colominas, Evaluation of High-Temperature Hydrogen Sensors Based on BaCe0.6Zr0.3Y0.1O3-α and Sr(Ce0.9Zr0.1)0.95Yb0.05O3-α Perovskites for Industrial Applications, Sensors. 20 (2020) 7258. https://doi.org/10.3390/s20247258.

Construction of hydrogel composites with superior proton conduction and flexibility using a new POM-based inorganic–organic hybrid

Polyoxometalates (POMs) have attracted extensive interests as solid-state proton conductors due to their high conductivity and good thermal stability. However, the low humidity stability and poor mechanical property of crystalline POMs greatly hinder their practical application. Herein, on the basis of the remarkable improvement of the humidity stability that can be achieved by hybridizing POMs with organic components, we prepared a new POM-based inorganic–organic hybrid, H[Zn(Hapca)₃][PW₁₂O₄₀]·7H₂O <strong>1</strong> (Hapca = 3-aminopyrazine-2-carboxylic acid), composed of Keggin-type PW₁₂O₄₀³⁻ anions and Zn(Hapca)₃²⁺ cations. Compound <strong>1</strong> exhibits a proton conductivity of 1.16 × 10⁻⁴ S·cm⁻¹ at 55 °C, which remains stable at a relative humidity of 95%. To construct POM-based conductive materials with good mechanical property, we introduced <strong>1</strong> into a poly(vinyl alcohol) (PVA)/glycerol (Gly) hydrogel and obtained the hydrogel composites <strong>1@PVA/Gly-</strong><i><strong>x</strong></i> (<i>x</i> = 20%, 30%, and 40%, where <i>x</i> is the percentage of the mass of <strong>1</strong> to the total mass of <strong>1</strong> and PVA<strong></strong>). Remarkably, the hydrogel composites <strong>1@PVA/Gly-</strong><i><strong>x</strong></i> combine the merits of their components. The proton conductivity of <strong>1@PVA/Gly-40%</strong> reaches 1.11 × 10⁻² S·cm⁻¹ at ambient humidity and 75 °C, outperforming most POM-based composite materials reported to date. Moreover, <strong>1@PVA/Gly-40%</strong> displays good elasticity and toughness and high strain sensitivity. Additionally, we assembled <strong>1@PVA/Gly-40%</strong> into a strain sensor for the effective monitoring of various human activities. This work provides a new platform for the transformation of crystalline POM-based inorganic–organic hybrids into materials with good mechanical properties.

Intermediate-Temperature Proton Exchange Membranes Based on Cerium Ultraphosphate Composited with Polybenzimidazole

This paper reports the rational fabrication and structural, thermal, mechanical and electrochemical characterization of a new type of intermediate-temperature (IT) polymer-inorganic composite (PIC) proton exchange membranes (PEMs) that are made of cerium ultraphosphate (CeP5O14—CUP) as the solid-state proton conductor composited with a high-temperature (HT) polybenzimidazole (PBI) as the polymeric binder. Flexible PBI-CUP PIC membranes with the thickness of ∼135 μm and CUP mass fraction of up to 75% were prepared by solution-casting without additional acid-doping (e.g., phosphoric acid). The proton conductivity of the fabricated IT-PIC-PEMs was up to 5.80 × 10−2 S cm−1 as measured from a prototype IT PEM fuel cell (PEMFC) operated at 200 °C in the humidified hydrogen and air environment. This type of IT-PIC-PEMs also demonstrated sufficient mechanical strength and flexibility, excellent thermal stability (up to 350 °C), and very good durability of the proton conductivity (within the test duration of 500 h). The present experimental study shows the promising future of the IT-PIC-PEMs for applications in various IT electrochemical processes including IT-PEMFCs, IT-electrolyzers, etc.

Phosphosilicate nano-network (PPSN)-Polybenzimidazole (PBI) composite electrolyte membrane for enhanced proton conductivity, durability and power generation of HT-PEMFC

Here we report a composite electrolyte membrane of Polybenzimidazole (PBI) with Phosphosilicate nano-network (PPSN) for enhanced proton conductivity, durability and power generation of high temperature polymer electrolyte membrane fuel cell (HT-PEMFC). Solid state proton conductor three dimensional Phosphosilicate nano-network (average particle size <10 nm) is synthesized using easy and low-cost sol gel method followed by ball milling and composited with PBI at different loading employing methane sulfonic acid (MSA) as solvent. The electrolyte membrane is characterized using FESEM, XRD, FTIR, TGA; proton conductivity, ion exchange capacity, water uptake and acid doping level, chemical stability and mechanical yield strength are measured and the membrane is tested for HT-PEMFC application. Property and performance mapping reveals that with 10% PPSN loading, composite (PPSN-PBI-10) membrane offers the maximum enhancement of all properties and power generation of HT-PEMFC, while beyond a critical loading (∼22%) properties and performance deteriorate below that of pristine PBI. Using optimum loading of PPSN, compared to pristine PBI, a remarkable rise in water uptake and acid doping level is achieved that facilitates proton conduction; also in spite of the presence of Phosphoric acid in the PPSN filler, the maximum 47.5% enhancement of ultimate strength is attained. The performance of HT-PEMFC using composite PPSN-PBI unveil that almost 2 times (100%) enhancement of peak power generation (∼0.73 W cm−2) is achieved using PPSN-PBI-10 at 170 °C operating temperature compared to pristine PBI. This may be attributed to the facilitated proton conduction through the extended tunnelling network offered by PPSN. Incorporation of PPSN improves the durability; over 48 h only 16% decay in voltage is noticed using PPSN-PBI-10 membrane which is remarkably lower than the 31% decay of pristine PBI membrane.

Local proton hopping mechanism in imidazolium-based plastic crystal: an ab initio molecular dynamics study

Abstract Protic organic ionic plastic crystals (POIPCs) are promising solid-state proton conductor materials in anhydrous proton exchange membrane fuel cells, due to their mechanical flexibility and high ionic conductivity in the plastic crystal phase. In typical POIPCs, the ions are orientationally disordered while the centers of mass are ordered (positional order) like the crystal phase. The local disorder provides more degrees of freedom for the translational and rotational diffusion of ions, thus enhancing proton conduction either via the vehicle mechanism or the Grotthuss mechanism. Yet the local dynamics and the interactions of the cations and anions during the proton transfer process are far from being fully understood. Here, we performed Car–Parrinello molecular dynamics (CPMD) simulation on the imidazolium methanesulfate ([ImH][CH3SO3]) unit cell. By artificially creating one proton hole, we found that a proton can hop directly between the cations. Though the anion is not directly involved in proton hopping, the oxygen atom in the sulfonate group interacts with the proton and has a synergetic motion along with the proton hopping process. This indicates the structural disorder of imidazolium rings and the aid of an anion can facilitate Grotthuss-type proton hopping in imidazolium-based POIPCs.

Phase transitions and ionic conductivity of ionic plastic crystals based on pyrrolidinium cations and dihydrogen phosphate anion

Ionic plastic crystals (IPCs) have been studied as solid-state proton conductors due to their properties such as non-volatility, plasticity, and high ionic conductivity. In order to further improve their proton transport property, understanding the relationship between the ion structure and the properties of IPCs is needed. For this purpose, five pyrrolidinium salts with dihydrogen phosphate anion ([DHP] − ) were synthesized. 1-Ethyl-1-methylpyrrolidinium ([C 1,2 pyr] + ) cation and 1-(2-hydroxyethyl)-1-methylpyrrolidinium ([C 1,2OH pyr] + ) cation exhibited one or more solid-solid phase transitions in DSC traces. The 1 H and 31 P solid state NMR results indicated that the component ions in [C 1,2 pyr][DHP] exhibited rotational mobility in Phase I. [C 1,2 pyr][DHP] and [C 1,2OH pyr][DHP] exhibited higher ionic conductivity values than the other pyrrolidinium salts after the addition of H 3 PO 4 , and their ionic conductivity was over 10 −3 S cm −1 at 120 °C. Thus, rotational mobility and the hydroxyl group on the pyrrolidinium cation will contribute to proton transport in the solid-state phases. • Five pyrrolidinium salts with dihydrogen phosphate anion were synthesized. • One or more solid-solid phase transitions were observed for some pyrrolidinium salts. • Ionic conductivity exhibited a large increase at the phase transition temperature. • Plastic crystalline behavior was effective in achieving high ionic conductivity.

Molecular aggregation in composite electrolytes based on phosphotungstic acid and calixarene

Solid-state proton conductors are the most promising candidates for use as solid electrolytes for gas sensors and fuel cells. The transport of protons in most of these materials depends on the humidity of the environment; therefore, it is important to search for materials that stabilize conductivity at low humidity. Such materials can be hydrophilic heteropoly acids and calixarene sulfonic acids, which conductivity is well even in a dry atmosphere. Therefore, the proton conductivity of a mixture of these components was investigated in the work. The effect of the formation of the “molecular composite” with new properties was found during cryogenic drying of the initial solution. This material can retain significant amounts of water, more than the sum of the original components, even at low ambient humidity. As a result, the material with an optimal composition retains a high conductivity at low humidity. The mechanism of calixarene aggregation into more complex hierarchical structures due to the binding of several of its micellar aggregates by heteropoly acid molecules has been proposed as an explanation for the discovered effects. Condensation of such aggregates during cryogenic drying leads to the formation of a composite material with a non-linear composition-property relationship.

New Applications of Zirconium Phosphate Nanomaterials

ConspectusThe 2-D layers of the inorganic ion exchanger α-zirconium phosphate (Zr(HPO4)2·H2O, α-ZrP) make this compound particularly stable to low pH, high temperature, and ionizing radiation. Initially studied for its ion exchange properties, once the conditions for its synthesis in crystalline form was accomplished by James Stynes and Abraham Clearfield in 1964, numerous other types of studies and applications followed. Extensive studies in the 1960s and 1970s on the thermodynamics of ion exchange led to insights into the intercalation mechanism of this material. The Clearfield group solved the crystal structure in 1968 and refined it in 1977. Powder methods were pioneered by the Clearfield group to solve the structure of this type of materials. In 1968 Giulio Alberti reported means to prepare zirconium phosphonates expanding the chemistry of these layered compounds. New phases of ZrP were also discovered (e.g., γ, θ, λ, τ) and the applications ranged from heterogeneous catalysis to intercalation chemistry and solid-state proton conductors. Methods to exfoliate the layers of ZrP were developed in the 1990s as interest grew in new applications of these types of materials. For example, protein and enzyme intercalation was accomplished starting in the 1990s by the McLendon, Mallouk, and Kumar groups. In the early 2000s, the Colón group pioneered the use of the θ phase of ZrP for the direct intercalation of large inorganic metal complexes that could not be directly intercalated into the α phase. Initial studies in the Colón group ranged from applications of these directly intercalated ZrP derivatives in photophysics and photochemistry, amperometric biosensors, vapochromism, and vapoluminescence. Over the past decade, new applications of these materials have been developed in anticancer drug delivery and electrocatalysis of the oxygen evolution reaction (OER). ZrP has now proven to be a promising drug nanocarrier and its unique chemical microenvironment provided by the α-type layers and the interlayer space enhances catalytic activity for numerous types of reactions. Further elucidation of the catalytic active species under operando conditions as well as the chemical structure of drug-intercalated derivatives should provide new insights that will advance the design and development of new compounds with desired properties. The initial pioneering efforts of Clearfield and Alberti are being continued by numerous research groups providing new exciting areas of development on the chemistry of layered M(IV) phosphate and phosphonate compounds. In this Account we present the efforts of the Colón group during the past decade on studies of the applications of ZrP for anticancer drug delivery and electrocatalysis of the OER.