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

Abstract 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.

water and methanol for five times, and finally dried at room temperature.

Preparation of 2D-Co-NS-PBS.
The 2D-Co-Bulk powder was immersed in 0.2 M PBS solution (pH = 7.0). The pink powder turned to purple after 30 minutes. The precipitation was filtered after 12 hours, washed with distilled water for five times, and finally dried at room temperature.
Scanning electron microscopy (SEM) measurement. 2D-Co-NS and 2D-Co-NS-PBS were dispersed in methanol and aqueous solution (v/v = 1:1) by ultrasound for 30 minutes, respectively, and then stood for 5 min. Suck the supernatant and dropped it on the SiO 2 /Si slices by capillary, dried at room temperature and characterized by SEM. Similarly, 2D-Co-Bulk and 2D-Co-Bulk were soaked in K 2 SO 4 , Na 2 SO 4 and K 2 Cr 2 O 7 solutions obtained samples that were similar to the preparation method of 2D-Co-NS except that ultrasound was not required.

Transmission electron microscopy (TEM) measurement. 2D-Co-NS and 2D-
Co-NS-PBS were ultrasonically dispersed in mixture of H 2 O/MeOH (v/v = 1:1) for 30 min, respectively, then let it stand for 5 minutes. suck the supernatant with a pipette (10 L) and dropped the double copper mesh carbon film, which was dried at room temperature and characterized by TEM. Similarly, preparation method of 2D-Co-Bulk was a similar way to that described for 2D-Co-NS except that ultrasound was not required.

Atomic Force Microscope (AFM) measurement. 2D-Co-Bulk, 2D-Co-NS and
2D-Co-NS-PBS were dispersed by ultrasound in mixture of H 2 O/MeOH (v/v = 1:1) for 30 min, respectively. The supernatant droplets were absorbed with a pipetting gun (10 mL) for spin coating on a SiO 2 /Si slice. AFM characterization was performed after drying at room temperature.
Calibration of selected area electron diffraction (SAED). The diffracted electron beam from the sample comes from a two-dimensional network of atoms. It has translational symmetry in two directions, denoted by unit vectors a and b, respectively. These two vectors define the plane of the two-dimensional network, the two-dimensional plane has the thickness of a single layer, and the diffracted electron beam is also irradiated vertically on this plane. According to the crystal plane required for structural analysis, the crystal data (CCDC: 268305) of the sample was imported into the Single Crystal software, exported the electron diffraction points of the desired crystal plane with Crystal Maker, and then use Photoshop software to mark the crystal plane.
The inductively coupled plasma atomic emission spectrometry (ICP-AES) measurement. Considering the metal element content of 2D-Co-NS-PBS, 2D-Co-Bulk (10 mg) and 2D-Co-NS-PBS (10 mg) were dissolved in 5% HNO 3 (5 mL) matrix and H 2 O 2 (10 mL) respectively, then diluted to 1000 mL and analyzed with a Co internal standard against a 6-point standard curve over the range from 0.1 ppb to 500 ppb. The correlation coefficients of all correlation analyses were >0.9997, and each sample was tested three times in parallel, and each sample was repeated several times. Similarly, the ICP-AES tests for samples obtained by immersing 2D-Co-Bulk in solutions K 2 SO 4 , Na 2 SO 4 and K 2 Cr 2 O 7 were performed in the same way as 2D-Co-Bulk and 2D-Co-NS-PBS methods.
EXAFS curve fitting details. Data reduction, data analysis, and EXAFS fitting were performed and analyzed with the Athena and Artemis programs of the Demeter data analysis packages [J. Synchrotron Rad. 2005, 12, 537] that utilizes the FEFF6 program [Phys. Rev. B 1995, 52, 2995 to fit the EXAFS data. The energy calibration of the sample was conducted through a standard Co foil, which as a reference was simultaneously measured. A linear function was subtracted from the pre-edge region, then the edge jump was normalized using Athena software. The χ(k) data were isolated by subtracting a smooth, three-stage polynomial approximating the absorption background of an isolated atom. The k3-weighted χ(k) data were Fourier transformed after applying a Hanning window function (Δk = 1.0). For EXAFS modeling, The global amplitude EXAFS (CN, R, σ 2 and ΔE 0 ) were obtained by nonlinear fitting, with least-squares refinement, of the EXAFS equation to the Fourier-transformed data in R-space, using Artemis software, EXAFS of the Co foil is fitted and the obtained amplitude reduction factor S 0 2 value (0.792) was set in the EXAFS analysis to determine the coordination numbers (CNs) in the Co-O scattering path in sample. For Wavelet Transform analysis, the χ(k) exported from Athena was imported into the Hama Fortran code. [Phys. Rev. B 2005, 71, 094110] The parameters were listed as follow: R range, 1-3.5 Å, k range, 0-13.0 Å -1 for sample (0-13.0 Å -1 for Co foil and CoO); k weight, 2; and Morlet function with κ=10, σ=1 was used as the mother wavelet to provide the overall distribution. The diameter of the washer is the same as the inner diameter of the stainless steel coils (1.58 cm in diameter). In addition, the shell of self-made coin cell has holes drilled into it to allow humidity control. We put the dense wafer into the self-made coin cell and placed in an oven with adjustable humidity and temperature. Both ends of the Coin Cell respectively were respectively connected with silver glue and gold wire to concatenate the electrochemical apparatus for proton conductivity testing.
Subsequently, the electrochemical workstation was replaced with Keithley 6517B instrument to test the electronic conductivity of self-made Coin Cell at 99% RH.
Electrochemical impedance spectroscopy (EIS) under different conditions (temperature range 298K~373k, relative humidity 99%). The charge conducted by proton was a sinusoidal AC signal input (frequency 10 -1 -10 6 Hz and amplitude 50 mV) in the electrochemical workstation (CHI760E) when the DC static voltage is 0, then using the ZView2 program to output the arc region and the linear state composition, namely the Nquist plot. Finally, the radius of the arc part, that was, the R value of the ion transfer resistance, was obtained by the equivalent circuit fitting. The proton conductivity of the sample is calculated as follows [Chem. Rev. 2020, 120, 8416] : where σ i is the proton conductivity, R is ion transfer resistance, L is the length and A is cross-sectional area through the sample to be tested.
The activation energy (E a ) of the sample was calculated through the Arrhenius equation: where B is pre-exponential factor, E a is the Arrhenius activation energy, k is Boltzmann constant, and T is the thermodynamic temperature.
Electrical conductivity, σ e , measures the ability of a material to conduct electrical current. Measuring σ e usually involves connecting the material under test to an electronic apparatus, typically a resistor. and measuring the electrical conductance (G), length (L), and cross-sectional area (S) of the channel. The electrical conductance is generally obtained by fitting the linear region of the current voltage (I-V) curve using Ohm's law [J. Am. Chem. Soc. 2016, 138, 14772] . (hv) 1/n = W(hv-E g ) Among them,  is the absorption index, h is Planck's constant, v is the frequency, W is the absorbance, E g is the forbidden band width of the semiconductor, n is related to the type of semiconductor, n = 1/2 for direct bandgap semiconductors; indirect bandgap semiconductors n = 2. Take hv as the abscissa and (hv) 1/n as the ordinate to draw a graph, make a tangent, and the value that intersects with the abscissa is E g .

Calculation of electronic band structure and density of states. First-principles
calculations for structural optimization and analysis of the electronic properties were performed with the Vienna ab initio simulation package using the projectoraugmented wave method. [Phys. Rev. B 1996, 54, 11169] The cutoff points for the kinetic energy and k-point spacing were set to 500 eV and approximately 0.1 Å −1 , respectively.