Ultrahigh Proton Conductivity Research Articles

Tuning Proton Conduction by Staggered Arrays of Polar Preyssler-Type Oxoclusters.

Polyoxometalates (POMs), anionic nanosized oxoclusters that can be considered as fragments of metal oxides, have been extensively studied for their diverse composition and structure, showing promise in various fields such as catalysis and electronics. Proton conduction, relevant to catalysis and electronics, has attracted interest in materials chemistry, and POM anions are advantageous in terms of their proton carrier density and mobility. Recently, polar POMs have attracted attention for their unique ferroelectric behaviors, yet they have been little studied with regard to proton conduction, as their polarity has generally been believed to have a negative impact. Here, we propose that polar POMs can be used to align polar proton carriers, such as H2O and polymers, to construct efficient proton-conducting pathways. In this study, we present ionic crystals composed of polar Preyssler-type POMs ([Xn+(H2O)P5W30O110](15-n)-, Xn+ = Ca2+, Eu3+) and K+ exhibiting ultrahigh proton conductivity surpassing 10-2 S cm-1, which is required for practical applications. In contrast, ionic crystals with nonpolar Preyssler-type POMs show an order of magnitude lower proton conductivity. Structural and spectroscopic studies combined with theoretical calculations reveal that proton carriers align with the aid of staggered arrays of polar POMs, forming a hydrogen-bonding network favorable for proton conduction. This study integrates molecular chemistry by the design of POMs and solid-state chemistry by exploring long-range proton conduction mechanisms, offering novel insights for future materials design.

Ultrahigh proton conductivity of four ionic hydrogen-bonded organic frameworks based on functionalized terephthalates

Recently, the utilization of hydrogen-bonded organic frameworks (HOFs) with high crystallinity and inherent well-defined H-bonding networks in the field of proton conduction has received increasing attention, but obtaining HOFs with excellent water stability and prominent proton conductivity (σ) remains challenging. Herein, by employing functionalized terephthalic acids, 2,5-dihydroxyterephthalic acid, 2-hydroxyterephthalic acid, 2-nitro terephthalic acid, and terephthalic acid, respectively, four highly water-stable ionic HOFs (iHOFs), [(C8H5O6)(Me2NH2)]∙2H2O (iHOF 1), [(C8H5O5)(Me2NH2)] (iHOF 2), [(C8H4NO6)(Me2NH2)] (iHOF 3) and [(C8H5O4)(Me2NH2)] (iHOF 4) were efficiently prepared by a straightforward synthesis approach in DMF and H2O solutions. The alternating-current (AC) impedance testing in humid conditions revealed that all four iHOFs were temperature- and humidity-dependent σ, with the greatest value reaching 10-2 S·cm−1. As expected, the high density of free carboxylic acid groups, crystallization water, and protonated [Me2NH2]+ units offer adequate protons and hydrophilic environments for effective proton transport. Furthermore, the σ values of these iHOFs with different functional groups were compared. It was discovered that it dropped in the following order under 100 °C and 98 % relative humidity (RH): σ iHOF 1 (1.72 × 10−2 S·cm−1) > σ iHOF 2 (4.03 × 10−3 S·cm−1) > σ iHOF 3 (1.46 × 10−3 S·cm−1) > σ iHOF 4 (4.86 × 10−4 S·cm−1). Finally, we investigated the causes of the above differences and the proton transport mechanism inside the framework using crystal structure data, water contact angle tests, and activation energy values. This study provides new motivation to develop highly proton-conductive materials.

Square-planar Tetranuclear Cluster-based Alkaline Earth Metal-organic Frameworks with Enhanced Proton Conductivity.

Alkaline earth (AE) metal complexes have garnered significant interest in various functional fields due to their nontoxicity, low density, and low cost. However, there is a lack of systematic investigation into the structural characteristics and physical properties of AE-metal-organic frameworks (MOFs). In this research, we synthesized isostructural MOFs consisting of AE4(μ4-Cl) clusters bridged by benzo-(1,2;3,4;5,6)-tris(thiophene-2'-carboxylic acid) (BTTC3-) ligands. The resulting structure forms a truncated octahedral cage denoted as [AE4(m4-Cl)]6(BTTC)8, which further linked to a porous three-dimensional framework. Among the investigated AE ions (Ca, Sr, and Ba), the Ca4-MOF demonstrated good chemical stability in water compared to Sr4-MOF and Ba4-MOF. The N2 adsorption and solid-state UV-vis-NIR absorption behaviors were evaluated for all AE4-MOFs, showing similar trends among the different metal ions. Additionally, the proton conduction study revealed that the Ca4-MOF exhibited ultra-high proton conductivity, reaching 3.52×10-2 S cm-1 at 343 K and 98 % RH. Notably, the introduction of LiCl via guest exchange resulted in an improved proton conduction of up to 6.36×10-2 S cm-1 under similar conditions in the modified LiCl@Ca4-MOF. The findings shed light on the regulation of physical properties and proton conductivity of AE-MOFs, providing valuable insights for their potential applications in various fields.

Imparting Stable and Ultrahigh Proton Conductivity to a Layered Rare Earth Hydroxide via Ion Exchange.

Proton conductors are essential functional materials with a wide variety of potential applications in energy storage and conversion. In order to address the issues of low proton conductivity and poor stability in conventional proton conductors, a simple and valid ion-exchange method was proposed in this study for the introduction of stable and ultrahigh proton conductivity in layered rare earth hydroxides (LRHs). Test analyses by solid-state nuclear magnetic resonance, Fourier transform infrared spectroscopy, and powder X-ray diffraction revealed that the exchange of H2PO4- not only does not disrupt the layered structure of LRHs, but also creates more active proton sites and channels necessary for proton transport, thereby creating a high-performance proton conductor (LRH-H2PO4-). By utilizing this ion-exchange method, the proton conductivity of LRHs can be significantly enhanced from a low level to an ultrahigh level (>10-2 S·cm-1), while maintaining excellent long-term stability. Moreover, through methodically manipulating the guest ions and molecules housed within the interlayers of LRHs, a comprehensive explanation has been presented regarding the proficient mechanism of proton conduction in LRH-H2PO4-. As a result, this investigation presents a feasible and available approach for advancing proton conductor.

Subnanometer Nanowire‐Reinforced Construction of COF‐Based Membranes with Engineering Biomimetic Texture for Efficient and Stable Proton Conduction

AbstractAchieving rapid ion transport through nanochannels is essential for both biological and artificial membrane systems. Covalent organic frameworks (COFs) with well‐defined nanostructures hold great promise for addressing the above challenge. However, due to the limited processability and inadequate interlamellar interaction of COF materials, it is extremely difficult to integrate them to prepare high‐performance proton conductors. Herein, inspired by the ingenious bio‐adhesion strategy in nature, ultrafast proton conduction is achieved by taking advantage of COF membranes where TP‐COF nanosheets are linked by subnanometer nanowires/lignocellulosic nanofibrils composites (SNWs/LCNFs) through electrostatic and π‐π interactions to form an ordered and robust structure. Notably, the synthesized SNWs exhibited impressive proton conductivity and adhesion capacity due to their inbuilt phosphotungstic acid (HPW) molecules and multidimensional interactions. Therefore, attributed to the synergistic contribution of TP‐COFs and SNWs, the composite membrane achieves ultrahigh proton conductivity (0.395 S cm−1 at 80 °C and 100% RH), superior mechanical property (109.8 MPa), exceptional fuel cell performance (71.6 mW cm−2), and superior operational stability (OCV decay rate is about 1.5 mV h−1), demonstrating outstanding competitiveness. More importantly, the proposed design concept has the potential to be applied in membranes for various electrochemical devices and molecular separations.

Icing on the Cake: Imidazole-Anchored Strategy To Enhance the Proton Conductivity of Two Isostructural Ce(IV)/Hf(IV) Metal-Organic Frameworks.

In the field of proton conduction, the acquisition of crystalline metal-organic frameworks (MOFs) with high stability and ultrahigh proton conductivity has been of great research value and is worth continuous exploration. Here, we greenly synthesized a three-dimensional porous MOF (MOF-801-Ce) by using [(NH4)2Ce(NO3)6 and fumaric acid as starting materials and solvothermally synthesized Hf-UiO-66-NO2 by using HfCl4 and 2-nitroterephthalic acid as starting materials. A series of measurements have shown that both MOFs exhibit good water stability, acid-base stability, and thermal stability and demonstrate outstanding proton conductivity. At 100 °C and 98% relative humidity (RH), the proton conductivities (σ) could be 2.59 × 10-3 S·cm-1 for MOF-801-Ce and 0.89 × 10-3 S·cm-1 for Hf-UiO-66-NO2. To pursue higher proton conductivity, we further adopted the evaporation approach to encapsulate imidazole molecules in the pores of the two compounds, achieving the imidazole-encapsulated MOFs, Im@MOF-801-Ce and Im@Hf-UiO-66-NO2. As expected, their σ values were significantly boosted by almost an order of magnitude up to 10-2 S·cm-1. Finally, their proton-conductive mechanisms were explored in light of the structural information, gas adsorption/desorption, and other tests. The outstanding structural stability of these MOFs and their durability of the proton conduction capability manifested that they have great promise in electrochemical fields.

Incorporating graphene oxide into COF membranes enables ultrahigh proton conductivity and ultralow H2 crossover

Proton exchange membranes (PEMs) are the core components of electrochemical hydrogen energy production, storage and conversion devices. It is a great challenge to develop PEMs with a combination of high proton conductivity, minimal H2 crossover and mechanical robustness for achieving high efficiency and long-term stability in these devices. Here, a co-assembled PEM composed of ionic covalent organic framework (iCOF) nanosheets and functionalized graphene oxide (GO) nanosheets was presented. The iCOF (TpPa-SO3H) nanosheets and polydopamine-modified graphene oxide (DGO) nanosheets were co-assembled into TpPa-SO3H/DGO mixed nanosheets with ultrahigh aspect ratio, which were readily further processed into TpPa-SO3H/DGO co-assembled membranes. The abundant and uniformly distributed –SO3H groups on TpPa-SO3H nanosheets and the acid-base pairs formed between TpPa-SO3H and DGO nanosheets promoted proton conduction significantly. Additionally, the dense structured DGO nanosheets inhibited H2 crossover remarkably, and the multiple interactions between TpPa-SO3H and DGO nanosheets enhanced mechanical strength. The resultant TpPa-SO3H/DGO co-assembled membrane exhibited an ultrahigh proton conductivity of 916.4 mS cm−1 (80 °C, 100% RH), an unprecedently low H2 crossover of 9.7 barrer and a sufficient tensile stress of 87.8 MPa. The enhanced proton conduction and H2 barrier property led to enhanced performance in both electrochemical hydrogen compression and single hydrogen fuel cells.

Bioinspired Heterogeneous Construction of Lignocellulose-Reinforced COF Membranes for Efficient Proton Conduction.

The exponential interest in covalent organic frameworks (COFs) arises from the direct correlation between their diverse and intriguing properties and the modular design principle. However, the insufficient interlamellar interaction among COF nanosheets greatly hinders the formation of defect-free membranes. Therefore, developing a methodology for the facile fabrication of these materials remains an enticing and highly desirable objective. Herein, ultrahigh proton conductivity and superior stability are achieved by taking advantage of COF composite membranes where 2D TB-COF nanosheets are linked by 1D lignocellulosic nanofibrils (LCNFs) through π-π and electrostatic interactions to form a robust and ordered structure. Notably, the high concentration of -SO3 H groups within the COF pores and the abundant proton transport paths at COFs-LCNFs interfaces impart composite membranes ultrahigh proton conductivity (0.348 S cm-1 at 80 °C and 100% RH). Moreover, the directional migration of protons along the stacked nanochannels of COFs is facilitated by oxygen atoms on the keto groups, as demonstrated by density functional theory (DFT) calculations. The simple design concept and reliable operation of the demonstrated mixed-dimensional composite membrane are expected to provide an ideal platform for next-generation conductive materials.

Proton Conduction Properties of Intrinsically Sulfonated Covalent Organic Framework Composites

The long–term stability of proton conductors is one of the most important factors in evaluating materials. Guest molecules can act as “bridges” for proton conduction channels and reside in the channels of covalent organic frameworks, but they are prone to leakage. Therefore, it is important to develop proton conductors with intrinsic proton conductivity. In this paper, we synthesized an intrinsically sulfonated covalent organic framework, TpPa–SO3H, which has a more stable proton conducting performance than that of TpPa@H2SO4 by loading guest molecules. Meanwhile, the proton conductivity of TpPa–SO3H was further improved by coating a superabsorbent polymer through an in situ reaction to obtain PANa@TpPa–SO3H (PANa: sodium polyacrylate). As a result, the modified composite exhibits an ultrahigh proton conductivity of 2.33 × 10−1 S cm−1 at 80 °C under 95% relative humidity (RH). The stability of PANa@TpPa–SO3H makes it an efficient proton transport platform with excellent proton conductivity and long–term durability.

High Proton Conductivity of a Bismuth Phosphonate Metal–Organic Framework with Unusual Topology

Despite the interest in proton exchange membrane (PEM) technologies (fuel cells and electrolyzers) for energy applications, the low stability of the electrolyte materials under working conditions (i.e., humidity and temperature) is one of their major limitations. Metal–organic frameworks (MOFs) have recently emerged as promising electrolytes due to their higher stability compared with the currently applied organic polymers, proton conductivity, and outstanding porosity. Here, a novel robust Bi phosphonate MOF (branded as IEF-7) was successfully synthesized and fully characterized, exhibiting an unusual topology due to the irregular coordination geometry of the bismuth cations. Furthermore, IEF-7 exhibited potential porosity, very high chemical and thermal stability, and free −PO3H groups involved in its ultrahigh proton conductivity, reaching 1.39 × 10–2 S cm–1 at 90 °C and 90% relative humidity for, at least, 3 cycles. In order to improve the consolidation and shaping of the powder for testing its ion conductivity properties, a highly MOF-loaded composite (90 wt %) was prepared by adding a proton conductive sulfonated polysulfone binder. The proton conductivity of the resulting composite was in the same order of magnitude as the compacted MOF powder, making this polymeric composite electrolyte very promising for PEM technologies.

Ultra-High Proton Conductivity iHOF Based on Guanidinium Arylphosphonate for Proton Exchange Membrane Fuel Cells

Ultra-High Proton Conductivity iHOF Based on Guanidinium Arylphosphonate for Proton Exchange Membrane Fuel Cells

Proton transfer in layered hydrogen-bonded system γ-MOOH (M = Al, Sc): Robust bi-mode ferroelectricity and 1D superionic conductivity

Layered γ-MOOH, such as synthetic boehmite γ-AlOOH and γ-ScOOH, has been explored for various applications since 1950s. In this paper, based on first-principles calculations, we show the evidence of two proton transfer modes in their hydrogen-bonded network that give rise to extraordinary properties: (1) they, respectively, result in two distinct types of ferroelectricity with different switching mechanisms and polarizations, while the exhibiting mode under an electric field depends on various factors, including the field intensity and direction, the existence of vacancies, and temperature; and (2) the combination of two modes can lead to ultra-high proton conductivity along 1D channels. Their proton migration barriers ensure high ferroelectric Curie temperature, while still much lower compared with current proton conductors, giving rise to 1D superionicity with unprecedented protonic conductivity over 24 mS/cm. Those light weight nontoxic layered materials with high polarizations, Curie temperature, and ultra-high protonic conductivity should provide vast opportunities for various applications.

Improvement of proton conductivity and efficiency of SPEEK-based composite membrane influenced by dual-sulfonated flexible comb-like polymers for vanadium flow battery

A sulfonated poly(ether ether ketone)-based composite membrane (S/SMA-SN) is developed by introducing dual-sulfonated flexible comb-like polymer (SMA-SN), varying from 0.1 to 1.5 wt%, which was prepared from the poly(styrene-co-maleic anhydride)-ethylenediamine through a main-/side-chain two-step sulfonation procedure. The S/SMA-SN composite membranes demonstrate ultrahigh proton conductivity and excellent cell performance at ultralow contents of SMA-SN polymers. Synergistic main-/side-chain proton channels formed by SPEEK and SMA-SN offer the highest proton conductivity (49.2 mS cm-1) and the lowest vanadium ion permeability (19.6 × 10-7 cm2 min-1) for S/SMA-SN-0.5, which is superior to SPEEK and Nafion membranes. Moreover, S/SMA-SN-0.5 displays its remarkable energy efficiency (72–81.3%) at 100–200 mA cm-2 current density, further illustrating the tradeoff effect of conductivity and resistance. Even at 150 mA cm-2 current density, stable coulombic efficiency (97%) and energy efficiency (80%) maintained after 500 charge-discharge cycles confirms the stable membrane structure and increased chemical stability, which originated from the main-/side-chain synergistic structure. This opens a promising method to explore the high-performance proton exchange membrane for vanadium flow battery application by the side-chain structuralized comb-like polymer at ultralow concentrations.

Enhanced stability and ultrahigh proton conductivity of hydrogen-bonded organic frameworks

A highly stable HOF-based composite (HDSD-1@Tp) was obtained by in situ reaction with HDSD-1. The introduction of NH4+ optimizes its proton conduction pathways, making HDSD-1@Tp-NH3 possess a high proton conductivity of 3.8 × 10−1 S cm−1.

Giant Water Uptake Enabled Ultrahigh Proton Conductivity of Graphdiyne Oxide

AbstractProton conductors have attracted great attention in various fields, especially in energy production. Here, we find that graphdiyne oxide (GDYO), derived from graphdiyne (GDY), features the highest proton conductivity of 0.54 S cm−1 (100 % RH, 348 K) among the oxidized carbon allotropes reported so far. The sp‐ and sp2‐co‐hybridized carbon skeleton of GDY enables GDYO with the giant water uptake, which is 2.4 times larger than that of graphene oxide (GO), resulting in ultrahigh proton conductivity by increasing the proton concentration and proton conduction pathways. This ultrahigh proton conductivity of GDYO is further proved in a methanol fuel cell by using GDYO membrane as proton exchange membrane. The GDYO membrane enables the cell with higher open circuit voltage, larger power density and lower methanol permeability, compared with commercial Nafion 117. Moreover, the GDYO membrane bears high ion exchange capacity, good acidic stability and low swelling ratio.

Giant Water Uptake Enabled Ultrahigh Proton Conductivity of Graphdiyne Oxide.

Proton conductors have attracted great attention in various fields, especially in energy production. Here, we find that graphdiyne oxide (GDYO), derived from graphdiyne (GDY), features the highest proton conductivity of 0.54 S cm-1 (100 % RH, 348 K) among the oxidized carbon allotropes reported so far. The sp- and sp2 -co-hybridized carbon skeleton of GDY enables GDYO with the giant water uptake, which is 2.4 times larger than that of graphene oxide (GO), resulting in ultrahigh proton conductivity by increasing the proton concentration and proton conduction pathways. This ultrahigh proton conductivity of GDYO is further proved in a methanol fuel cell by using GDYO membrane as proton exchange membrane. The GDYO membrane enables the cell with higher open circuit voltage, larger power density and lower methanol permeability, compared with commercial Nafion 117. Moreover, the GDYO membrane bears high ion exchange capacity, good acidic stability and low swelling ratio.

Short hydrogen-bond network confined on COF surfaces enables ultrahigh proton conductivity

The idea of spatial confinement has gained widespread interest in myriad applications. Especially, the confined short hydrogen-bond (SHB) network could afford an attractive opportunity to enable proton transfer in a nearly barrierless manner, but its practical implementation has been challenging. Herein, we report a SHB network confined on the surface of ionic covalent organic framework (COF) membranes decorated by densely and uniformly distributed hydrophilic ligands. Combined experimental and theoretical evidences have pointed to the confinement of water molecules allocated to each ligand, achieving the local enrichment of hydronium ions and the concomitant formation of SHBs in water-hydronium domains. These overlapped water-hydronium domains create an interconnected SHB network, which yields an unprecedented ultrahigh proton conductivity of 1389 mS cm−1 at 90 °C, 100% relative humidity.

Two Novel Three-Dimensional Tetraphenylethylene-Based Rare Earth MOFs with Ultra-High Proton Conductivity and Performance Stability.

Invited for the cover of this issue are the groups of Lin Du and Qi-Hua Zhao at Yunnan University. The image depicts astronauts as protons moving along the hydrogen-bond network in the channel of Eu-ETTB/Gd-ETTB. Read the full text of the article at 10.1002/chem.202202154.

Two Novel Three-Dimensional Tetraphenylethylene-Based Rare Earth MOFs with Ultra-High Proton Conductivity and Performance Stability.

In this work, the two example rare earth-based metal-organic frameworks (LaIII -based MOFs), Eu-ETTB and Gd-ETTB, were obtained by self-assembly. Both materials showed extremely high proton conductivity, with the proton conductivity of Eu-ETTB being 1.53×10-2 S cm-1 at 98 % relative humidity (RH) and 85 °C and that of Gd-ETTB being 2.63×10-2 S cm-1 at 98 % RH and 75 °C. This was almost the best performance observed for three-dimensional porous MOFs without post-synthetic modification and was based on milder conditions than for most materials. Furthermore, cycle test experiments and continuous work tests showed that both materials had excellent performance both in terms of stability and durability. Water vapor adsorption experiments showed that a large number of water molecules are adsorbed the hydrogen-bond network's being rebuilt by the adsorbed water molecules in the pore channel and thus optimizing the channels for proton transfer explained the MOF's high performance.

Simultaneously anchoring free carboxyl and sulfonate groups into a metal-organic framework for high proton conductivity

Although the acid groups are regarded as one of the best species for proton conduction, introducing them into MOFs to enhance the proton conductivity still faces challenges due to their high coordination ability. Herein, we develop a coordination strategy to simultaneously introduce the free carboxyl and sulfonate groups into MOF-808 pores by grafting the monosodium 2-Sulfoterephthalate (SBDC) which contains one sulfonate group and two carboxyl groups onto the metal nodes. DFT calculations reveal the carboxyl group on the ortho position of the sulfonate groups shows a stronger coordination ability and binds onto the Zr6 cluster of MOF-808 in priority, leading to one free carboxyl group and one sulfonate group being preserved and dangling in the pores. The resultant MOF-808-XSBDC (X is the molar ratio of SBDC to MOF-808) series materials with free acid groups show an enhanced water adsorption behavior, thus significantly improving the proton conductivity. Particularly, MOF-808-5SBDC exhibits an ultrahigh proton conductivity, up to 4.11 × 10−2 S cm−1 at 80 °C and 100% RH, not only over two orders of magnitude higher than MOF-808, but also reaching the highest proton conductivity benchmark (>10−2 S cm−1) of MOFs. This work offers an effective way to simultaneously introduce the carboxyl and sulfonate groups into MOFs, providing vast opportunities to construct various proton conducting MOFs with the much better proton conducting performance.