Rational Construction of Molecular Electron-Conducting Nanowires Encapsulated in a Proton-Conducting Matrix in a Charge Transfer Salt

Simultaneous electron and proton conduction in a stable metal organic material with highly selective electrocatalytic oxygen reduction reaction to water

Proton coupled electron transfer (PCET) is considered as the elementary step of several chemical, electrochemical and biological processes and thus the development of dual conducting materials has recently become a major focus in Chemical Science. Herein, we report the highly selective electrocatalytic oxygen reduction to water by the stable dual conducting metal-organic material (MOM) [Cu(INA)2(H2O)4] (INA = isonicotinate). Structural analysis reveals the important role of both, hydrogen bonding and π-interactions, in the formation of a supramolecular 3D network. Theoretical calculations show that hydrogen bonding interactions among the coordinated water molecules and deprotonated carboxylate oxygen atoms induce proton transport (2.26 ± 0.10 × 10-5 S cm-1 at 98% RH) while weak intermolecular π-interactions (π-π and anion-π) provide the pathway for electron transport (1.4 ± 0.1 × 10-7 S cm-1 at 400 K). Such dual proton and electron conductivity leads to a selective oxygen reduction reaction (ORR) to water in an alkaline medium. To the best of our knowledge, this is the first report on electrocatalytic ORR by a dual-conducting metal-organic material.

Molecule-Based Proton-Electron Mixed Conductor with the Highest Ambipolar Conductivity

Proton-electron mixed conductors (PEMCs) are an essential component for potential applications in hydrogen separation and energy conversion devices. However, the exploration of PEMCs with excellent mixed conduction, which is quantified by the ambipolar conductivity, σamb = σeσH/(σe + σH) (σe: electronic conductivity; σH: proton conductivity), is still a great challenge, largely due to the lack of structural characterization of both conducting mechanisms. In this study, we prepared a molecule-based proton-electron mixed-conducting cation radical salt, (ET)4[Pt2(pop)2(Hpop)2]·PhCN (ET: bis(ethylenedithio)tetrathiafulvalene, pop2-: P2H2O52-), by electrocrystallization. The salt shows metallic electronic conduction, which arises from the (ET)2•+ layers, with a high σe value (1-2 S cm-1 at room temperature). The metallic state was corroborated by magnetic susceptibility measurement and band structure calculation. The salt also shows superprotonic conduction (σH = 2.1 × 10-2 S cm-1 at room temperature under dried conditions), which relies on the one-dimensional (1D) hydrogen-bonding network of protonated paddlewheel-type Pt-dimer complex anions, [Pt2(pop)2(Hpop)2]2-. Crystallographic and computational studies revealed the presence of infinite intra- and intermolecular O-H···O hydrogen bonds, which show a double-well-like potential energy curve with a negligible energy barrier in the 1D chain, facilitating Grotthuss-type proton hopping via Lewis basic sites. Among the structurally defined PEMCs, the present salt displays the highest room temperature σamb value of 2.1 × 10-2 S cm-1 even under dried conditions. The σamb value has the same order as those of reported perovskites-type metal oxides at high temperatures and achieves a level that is beneficial for the practical applications.

Hydrogen-Bonded Metal-Organic Framework Nanosheet as a Proton Conducting Membrane for an H2/O2 Fuel Cell

Proton-conducting metal-organic frameworks (MOFs) have attracted attention as potential electrolytes for fuel cells. However, research progress in utilizing MOFs as electrolytes for fuel cells has been limited, mainly due to challenges associated with issues such as the fabrication of MOF membranes, and hydrogen crossover through the MOF's pores. Here, proton conductivity and fuel cell performance of a self-standing membrane prepared from of a bismuth subgallate MOF nanosheets with non-porous structure are reported. The fabricated MOF nanosheet membrane with no binding agent exhibits structural anisotropy. The proton conductivity in the membrane thickness direction (4.4 × 10-3 S cm-1) at 90 °C and RH 100% is observed to be higher than that in the in-plane direction of the membrane (3.3 × 10-5 S cm-1). The open circuit voltage (OCV) of a fuel cell with ≈120 µm proton conducting membrane is 1.0 V. The non-porous nature of the MOF nanosheets contributes to the relatively high OCV. A fuel cell using ≈40 µm membrane as proton conducting electrolyte records a maximum of 25 mW cm-2 power density and a maximum of 109 mA cm-2 current density with 0.91 V OCV at 80 °C in humid conditions.

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO2 molecules as a precursor. The mass ratio of CO2 conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO2 and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO2 -derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer-Emmett-Teller surface area of 945 m2  g-1 and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10-2  S cm-1 at 85 °C, 95 % of relative humidity.

A Mixed Protonic-Electronic Conductor Base on the Host-Guest Architecture of 2D Metal-Organic Layers and Inorganic Layers

The key to designing and fabricating highly efficient mixed protonic-electronic conductors materials (MPECs) is to integrate the mixed conductive active sites into a single structure, to break through the shortcomings of traditional physical blending. Herein, based on the host-guest interaction, an MPEC is consisted of 2D metal-organic layers and hydrogen-bonded inorganic layers by the assembly methods of layered intercalation. Noticeably, the 2D intercalated materials (≈1.3 nm) exhibit the proton conductivity and electron conductivity, which are 2.02 × 10^-5 and 3.84 × 10^-4 S cm^-1 at 100 °C and 99% relative humidity, much higher than these of pure 2D metal-organic layers (>>1.0 × 10^-10 and 2.01×10^-8 S cm^-1 ), respectively. Furthermore, combining accurate structural information and theoretical calculations reveals that the inserted hydrogen-bonded inorganic layers provide the proton source and a networks of hydrogen-bonds leading to efficient proton transport, meanwhile reducing the bandgap of hybrid architecture and increasing the band electron delocalization of the metal-organic layer to greatly elevate the electron transport of intrinsic 2D metal-organic frameworks.