Coordination polymer glass from a protic ionic liquid: proton conductivity and mechanical properties as an electrolyte†

High proton conducting electrolytes with mechanical moldability are a key material for energy devices. We propose an approach for creating a coordination polymer (CP) glass from a protic ionic liquid for a solid-state anhydrous proton conductor. A protic ionic liquid (dema)(H2PO4), with components which also act as bridging ligands, was applied to construct a CP glass (dema)0.35[Zn(H2PO4)2.35(H3PO4)0.65]. The structural analysis revealed that large Zn–H2PO4−/H3PO4 coordination networks formed in the CP glass. The network formation results in enhancement of the properties of proton conductivity and viscoelasticity. High anhydrous proton conductivity (σ = 13.3 mS cm−1 at 120 °C) and a high transport number of the proton (0.94) were achieved by the coordination networks. A fuel cell with this CP glass membrane exhibits a high open-circuit voltage and power density (0.15 W cm−2) under dry conditions at 120 °C due to the conducting properties and mechanical properties of the CP glass.


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Experimental Scheme S1. Synthetic scheme of 1 and 2. Table S1. VFT fitting parameters of H + conductivity.            Table S2. Simulated RMC structure in cif format. References S3 EXPERIMENTAL Materials and methods: ZnO (99.99% trace metals basis) and diethylmethylamine were purchased from Aldrich, and phosphoric acid (85% in H2O) was purchased from Wako pure chemical industries. All reagents were used without further purification process. The reported ionic liquid (dema)(H2PO4) (2)  Thermogravimetric analysis (TGA) were recorded using Rigaku TG8120 instrument under flowing N2 with 10 K min˗1 scan rate. Differential Scanning Calorimetry (DSC) experiments were measured using Hitachi DSC 7020 instrument in an inert argon atmosphere. The sample was placed in an aluminum sample folder and a lid.
Impedance spectra from 30 °C to 120 °C under Ar atmosphere were collected using a BioLogic VSP-300 with an EC Frontier sample cell (SB1A, ϕ = 13 mm, d = 5 mm). The frequency ranges from 0.1 Hz to 1.0 MHz with a sinus voltage (10 mV) was collected at each temperature. The measurements were performed at thermal equilibrium after holding 30 min at each temperature.
Collected impedance data were treated and analyzed by a ZView software. The VFT (eq. 1) fitting (Levenberg-Marquardt Method) was performed using an Origin 2018 software.
A is proportional to the concentration of the carrier ions, B is the pseudo activation energy for the ion conduction, while T0 is the ideal glass transition temperature.

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Proton Transport number (tH+) was determined by the electromotive force (EMF) measurements using a BioLogic VSP-300. A membrane filter (Omnipore TM Merck, 10.0 uM, 2 X 2 cm) was impregnated with 1 or 2 at 120 o C or at room temperature, respectively. The membrane was then sandwiched between Pt loaded carbon papers (1.5 mg cm − ², ϕ = 5 mm, Chemix Co. Ltd.) and placed into a single cell with straight gas flow channels. On one side H2 /Ar (3.99 vol%,) was fed continuously while on the other side the H2/Ar gas was diluted with N2 (99.99995 vol%) gas to achieve variable partial pressures. The H2 concentration was precisely controlled by mass flow controllers (SEC-E40, Horiba, ltd.) while maintaining total gas flow rate of 100 mL/min on both the sides. The assembly was maintained at desired temperature using Oven and stabilized for ~10 min before measurement. The EMF generated between the sensing and counter electrodes were measured under different H2 partial pressures (P2) on equilibrating for 3 min. The proton transport number (tH+) is calculated using following equation where, E, T, R, F, P1, P2, and t denote EMF, temperature, the gas constant, Faraday constant, partial pressure of H2 gas, and transport number, respectively.
The polarization curves of a H2/O2 fuel cell were recorded using a Solartron SI 1287. Gas diffusion electrodes (0.3 mg/cm², 46.5 wt% Pt/C) for both cathode and anode were fabricated by spray coating of 46.5wt% Pt/C (TEC10E50E, Tanaka Kikinzoku, ltd.) onto the carbon paper gas diffusion layer (Sigracet ® 29 BC, SGL Carbon, GmbH.). A membrane electrode assembly (MEA) was prepared by sandwiching the electrolyte membrane between two gas diffusion electrodes, and the contact area between electrode and electrolyte were ϕ = 7 mm. The resulting MEA was installed into a single cell with straight gas flow channels, and the dry H2 (>99.999vol%) and O2 (>99.999vol%) gases were supplied to the single cell at a flow rate of both 100mL/min under anhydrous conditions. The flow rate was controlled by mass flow controllers (SEC-E40, Horiba, ltd.). The polarization curves of the single cell were measured at 120 °C.
Extended X-ray Absorption Fine-Structure Spectroscopy were collected on beamline BL14B2 at SPring-8. X-ray absorption spectra in the energy region of the Zn K-edge were measured in transmission mode. Fourier transformation was k3-weighted in the k range from 3.0 to 12.5 Å −1 .
The data processing and coordination number fitting were performed with Athena and Artemis