Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl

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Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl Carboxylate Frameworks Yin Qin, Tianli Gao, Wenping Xie, Zifeng Li, and Gang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11056 • Publication Date (Web): 05 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Applied Materials & Interfaces

Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl Carboxylate Frameworks Yin Qin, Tian-li Gao, Wen-Ping Xie, Zifeng Li, and Gang Li* College of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, P. R. China ABSTRACT: Nowadays, although the research of proton conductive materials has been extended from traditional sulfonated polymers to novel crystalline solid materials such as MOFs, COFs, and HOFs, the research on crystalline ferrocene-based carboxylate materials is very limited. Herein, we selected two hydrogen-bonded and - interactions supported ferrocenyl carboxylate frameworks (FCFs), [FcCO(CH2)2COOH] (FCF 1) and [FcCOOH] (FCF 2) (Fc = (5-C5H5)Fe(5-C5H4)) to fully investigate their water-mediated proton conduction. Their excellent thermal, water and chemical stabilities were confirmed by the means of thermogravimetric analyses, PXRD and SEM determinations. The two FCFs indicate temperature- and humidity-dependent proton conductive features. Intriguingly, their ultrahigh proton conductivities are 1.17 × 10-1 and 1.01 × 10-2 S/cm, respectively, under 100 °C and 98% RH, which are not only comparable to the commercial Nafion membranes, but also rank among the highest performing MOFs, HOFs and COFs ever described. Based on the structural analysis, calculated Ea value, H2O vapor adsorption, PXRD and SEM measurements, reasonable conduction mechanisms are highlighted. Our research provides a novel inspiration for finding new high proton conducting crystalline solid materials. Importantly, the outstanding conducting performances of 1 and 2 suggest their hopefully potentials in fuel cells and related electrochemical fields. KEYWORDS: Proton conduction, Ferrocenyl carboxylate frameworks, crystal structure, high stability,

mechanism

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▇ INTRODUCTION Proton exchange membrane fuel cells have received extensive attention due to their prominent advantages of high power density, lofty energy conversion efficiency and environmentally friendly features.1 Proton exchange membrane (PEM) directly determines their performance and service life. Therefore, the design and development of PEM with high proton transfer capability and excellent water, chemical and electrochemical stabilities are the frontier and hot field.2 Currently, to overcome the rebarbative disadvantages (complicated manufacturing process, high cost, limited working conditions and fuzzy structures) of current commercial perfluorosulfonic acid membranes, Nafion and Nafion-like membranes,3,4 people have explored several types of rapidly developing crystalline solid materials, such as metal-organic frameworks/coordination polymers (MOFs/CPs),5-10 covalent organic frameworks (COFs),11-14 and hydrogen-bonded organic frameworks (HOFs),15-17 and

metalo-hydrogen-bonded organic frameworks (MHOFs)18 as proton conductive

candidates. Related investigations have confirmed that the good crystallinity of these materials is indeed good for the precise investigation for the proton transferring pathway and mechanism. Consequently, a few of high proton conductive materials based on the above crystalline solid materials have been exhibited by careful molecular design and tactful preparation strategies.10-13,16 Nevertheless, the reported proton conductive


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materials with ultra-high conductivities (10-1-10-2 S/cm) are very limited.16,19-26 Therefore, it is particularly important to find more crystalline materials to expand the research scope of high proton conductive materials. Meanwhile, the pivotal role of hydrophilic carboxyl group (-COOH) in proton conduction process has been strongly concerned by us. It is because the hydrophilic carboxylic acid groups may not only donate protons but also construct the hydrogenbonding networks with guest H2O molecules for proton transfer.27-30 In the field of proton conducting MOFs, people have tried to adopt organic ligands bearing -COOH groups to build stable frameworks containing uncoordinated -COOH groups, serving as a high efficient proton conduction pathway. Simultaneously, the carboxylic acid units can also enhance the poor performance and microstructure of the electrolyte materials.29 It is to be pointed that, due to the strong coordination interactions between carboxylate units and metal ions, it is not easy to control and obtain a substantial number of proton conductive MOFs with uncoordinated -COOH groups. Nevertheless, the limited examples of conductive MOFs also showed the pivotal role of uncoordinated -COOH units in proton conduction process.29-36 For instance, Li J. R. and co-authors designed and prepared one new 3D porphyrinic MOF, [Co(DCDPP)] (DCDPP = 5,15-di(4carboxylphenyl)-10,20-di(4-pyridyl)porphyrin),

carrying

high-density

uncoordinated-

COOH units and exhibiting a high proton conductivity () of 3.9 × 10-2 S/cm under 80 °C and 97% relative humidity (RH).29 Higuchi M. and coworkers compared the proton conductivities of three Eu(III)-based metalo-supramolecular polymers containing


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uncoordinated –COOH groups (PolyEu-H), coordinated –COOH groups (PolyEu) and without -COOH groups (PolyEu-2), respectively, and found that PolyEu-H has the  being 5.6 × 10-2 S/cm under 75 °C and 95% RH that is 10000 times larger than that of PolyEu and about 400 times higher that of PolyEu-2.31 Our group reported a 2D imidazole dicarboxylate MOF, [Cu(p-IiPhHIDC)]n (p-IiPhH3IDC = 2-(p-N-imidazol-1-yl)phenyl-1H-imidazole-4,5-dicarboxylic acid) bearing uncoordinated -COOH units between the sheets and presenting the optimal  of 1.51 × 10-3 S/cm under 100 °C and 98% RH.32 Also, Huang Y. M. and et al. pointed out that the construction of the H-bonds between -COOH and –SO3H units are extremely helpful for the proton transfer.25 On the basis of the above considerations, the crystalline ferrocene-based carboxylate frameworks (FCFs), as a class of undiscovered proton conductive materials, have drawn our considerable attention. Due to the introduction of the free carboxylate groups and a suitable stable organometallic ferrocene unit with special physical and chemical properties,37 such materials will meet the two crucial elements as proton conductors: high proton conduction ability and high structural stability, such as water, thermal and chemical stabilities. Additionally, like other explored conducting solid crystalline materials, the good crystallinity of FCFs will be advantageous to the exploration for proton conduction pathway and mechanism. Herein, two FCFs, [FcCO(CH2)2COOH] (FCF 1) and [FcCOOH] (FCF 2) (Fc = (5C5H5)Fe(5-C5H4)) were selected and prepared. We discovered that the threedimensional frameworks of FCFs 1 and 2 were fabricated with intermolecular H-bonds


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and π−π interactions. Both the FCFs denote the temperature- and humidity-dependent proton conductive features. Excitingly, we found that both FCFs 1 and 2 possess not only ultra-high proton conductivities being 1.17 × 10-1 and 1.01 × 10-2 S/cm under 100 °C and 98% RH, respectively, but also outstanding thermal stability, and high stability towards water and acid solutions. Based on the structural analyses, calculated

Ea value, H2O adsorptions, powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) determinations, we will highlight the conduction mechanism here. Moreover, the advantages of such materials, such as simple preparation, high yield, low cost and electrochemical stability, offer more opportunities as real-world conductive materials in fuel cells and relative fields.

▇ EXPERIMENTAL SECTION

Reagents and Apparatus The reagents were analytically pure and were used as commercially purchased. Ferrocenyl derivatives, FcCOCH338 and FcCOOCH339 were synthesized in the light of the literature. The FLASH SMART analyzer was adopted to conduct elemental analyses. The infrared spectrum was performed on KBr pellet with a Bruker Tensor II FTIR spectrophotometer. Thermogravimetric (TG) analyses were determined on a Netzsch STA 449F3 differential thermal analyzer. PXRD pattern was collected on a Panalytical X’pert PRO X-ray diffractometer. Nitrogen (under -196C) and H2O vapor (under 25 C) adsorption−desorption isotherms were carried out on a ASAP 2420 adsorptometer, and a 3H-2000PW multistation weight method analyzer (BeiShiDe Instrument Technology (Beijing) Co. Ltd.), respectively. SEM image was collected by a Zeiss SIGMA 500 scanning electron microscope. The photographs of samples of FCFs 1 and 2 were performed on a LEXT 3D measuring laser microscope ols4100 instrument.


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The DC conductivity was calculated by determining voltage and current for the pelletized samples of 1 and 2 on a Keithley 4200 semiconductor testing system under H2 atmosphere and 98% RH. Preparation of FCF (1) 1 was synthesized according to the previous literature.40 Good quality crystal of FCF 1 were obtained as following procedure: 0.15 g of FcCO(CH2)2COOH (0.52 mmol) was firstly dissolved in 200 mL diethyl ether, and then 200 mL of petroleum ether was added. Then, the resulting yellow solution was placed at 25 C in the dark to avoid the photolysis.41 After 2-3 days, red good quality crystals of FCF 1 appeared. Preparation of FCF (2) 2 was prepared according to the previous literature.42 Red good quality crystal of FCF 2 were obtained in a manner analogous to that used to 1, only 80 mL diethyl ether instead of 200 mL diethyl ether. Crystal Structure Determinations Suitable crystals of FCFs 1 and 2 were chose and put on a Xcalibur, Eos, Gemini diffractometer. The crystals were maintained under 293(2) K during data collection. Adopting Olex2 software,43 their structures were solved by the ShelXS44 program (Direct Methods) and refined by the ShelXL45 refinement package employing Least Squares minimisation. The crystallographic data, selected bond parameters, and H-bonding parameters are listed in Tables S1-S4, respectively. The CCDC numbers are 1886016 for 1 and 1886017 for 2. Stability Experiment The as-synthesized crystals of 1 and 2 were dipped in H2O for 30 d at room temperature, separated and air-dried to get solids after water treatment for PXRD and SEM measurements. Crystals of 1 or 2 (15 mg) were immersed in 25 mL of aqueous solutions with various pH values (pH = 1, 2, ..., 6) adjusted by HCl for 1 d. Subsequently, the crystals were filtered and airdried for PXRD and SEM measurements.


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Proton Conductivity Measurement Alternating-current (AC) impedance measurement was conducted on a PARSTAT 2273 impedance analyzer (voltage of 100 mV, frequency of 1-106 Hz) bearing a quasi-four-probe cell (The opposite electrode and the reference electrode are connected to a Pt electrode, and the first reference electrode and the working electrode are connected to another Pt electrode). The cylindrical pellet was obtained by pressing about 30-35 mg of microcrystalline powder at 2.5-3 MPa for 4 min, whose thickness (about 1 mm) was determined by a Vernier caliper. Prior to the AC measurement, the wafer was equilibrated at a fixed RH for 1 d to insure optimum electrochemical data. All impedance data were obtained through PowerSuite software. The  was calculated by the equation: σ = L/(RS). The general resistance, R (Ω), was collected from arc extrapolation to the Z′axis on the Nyquist plot (the low frequency end), and the semicircle exists in the high frequency of the Nyquist plot. Symbols L and S present the thickness and flat surface area of the cylindrical wafer, respectively. The Arrhenius plots of log(σT) versus 1000/T display a good linear relationship and can be fitted to the Arrhenius equation: Tσ = σ0 exp(−Ea/kT), where Ea is the apparent activation energy for conduction, σ0, k, and T are the pre-exponential factor, Boltzmann’s constant, and the absolute temperature, respectively. Analyses on the Impedance Plots The Nyquist plots of FCFs 1 and 2 under 30 or 100 C and 98% RH are well fitted by ZSimpWin software. The equivalent circuit LR(C(R(Q(R(LR)(CR))))) was used to fit the Nyquist plots. The fitting curves were demonstrated in Figures S1 and S2. ▇ RESULTS AND DISCUSSION Structural Analysis. Good quality crystals of FCFs 1 and 2 were grown from the mixed solvents of ether and petroleum ether (v:v = 1:1) (see Supporting Information). The two FCFs crystallize in the monoclinic system with P21/n space group (see Table S1). The


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bond distances and angles within the ferrocenyl groups are unexceptional (see Tables S2 and S3). Also, the bond parameters of the substituted carboxylate groups are close to the related compounds in the literature.46,47 As shown in Figure 1, in the frameworks of FCFs 1 and 2, two adjacent symmetric units are connected with classical strong O– H⋯O hydrogen bonds formed by the -COOH group (see Figure S3 and Table S4), and then joined another asymmetric neighboring part through weak non-classical intermolecular C–H⋯O H-bonds involved in CH units of the cyclopentadienyl rings and O atoms of the C=O or COOH units. Moreover, the π⋯π interactions between the neighboring cyclopentadienyl rings contribute to consolidating the crystal structures. Apparently, by means of above intermolecular forces, the two FCFs form the ordered solid-state frameworks containing different channels (Figure 1). Importantly, the arrangement of carboxylate groups and carbonyl oxygen atoms in the channels is extremely neat, which is vitally beneficial to the construction of proton conduction pathways and the exploration of proton conduction.


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(a)

(b) Figure 1. The solid-state 3D frameworks of FCFs 1 (a) and 2 (b) supported by intermolecular Hbonds and - interactions.

Thermal, Water and Chemical Stabilities. The thermal, water, chemical stabilities are crucial factors in ensuring that proton conductivity measurement can be carried out efficiently. Therefore, the thermal decomposition behaviors of FCFs 1 and 2 were determined by Thermogravimetric (TG) analyses to confirm their thermal stability. It can be observed from TG curves (see Figure S4) that the two FCFs could maintain their


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structural robustness up to 175.4 °C for 1 and 183.4 °C for 2 in air. Above these temperatures,

both

compounds

exhibit

a

strong

thermogravimetric

process

corresponding to the thermal decomposition of organic components. Finally, a plateau region from 420 to 800 °C for 1 (from 460 to 800 °C for 2) can be found in the TG curves. The final black residues of FCFs 1 and 2 were confirmed to be -Fe2O3 through powder X-ray diffraction (PXRD) analyses (see Figure S5). It is conspicuous the introduction of the special organometallic unit, ferrocene, offers a special role in the stability of the two simple carboxylate compounds. To measure the stability of the two FCFs towards H2O and acidic solutions, the assynthesized sample was immersed in water for 30 d or refluxed in boiling water for 1 d, or soaked in HCl aqueous solutions (pH = 1, 3, 5 and 6) for 24 h, and subsequently the PXRD and SEM determinations were performed. As indicated in Figures 2 and S6, FCFs 1 and 2 can survive in water and acidic solution, as evidenced by the almost unchanged PXRD patterns. Moreover, in comparison with the SEM images (see Figure 3), it was found that crystals of 1 and 2 still retain their original crystallinity and the morphologies after treatment in water or HCl (aqueous). As discussed above, the introduction of the bulk and hydrophobic ferrocene unit leads to the extremely high water stability of FCFs 1 and 2. Reasonably, the two carboxylate compounds, FCFs 1 and 2, must exhibit excellent stabilities towards acidic solutions.


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Figure 2. PXRD patterns of 1 and 2: simulated from the single-crystal data, as-synthesized, after H2O treatment of crystals and after soaking in different acidic solutions of crystals.


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Figure 3. SEM images of FCFs 1 and 2: (a) as-synthesized crystals of 1; (b) water-treated crystals of 1; (c) crystals of 1 treated with HCl (aqueous, pH = 1); (d) as-synthesized crystals of 2; (e) watertreated crystals of 2; (f) crystals of 2 treated with HCl (aqueous, pH = 1).

Proton Conduction. Based on the excellent water, chemical and thermal stabilities of 1 and 2, we conducted the proton conduction exploration of the pelletized samples for the two FCFs. Note that FCFs 1 and 2 did not display proton conductive behavior at anhydrous condition and 30 C. Below 53% RH for 1 and below 68% RH for 2, the impedance patterns of 1 and 2 are irregular; it is impossible to collect the relevant electrochemical data. Probably because that under low RH, the frameworks absorbed fewer water molecules, which resulted in insufficient to form adequate H-bonding networks as good


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proton transfers. Therefore, the proton conductivity measurements were recorded on 1 under 53%, 68%, 75%, 85%, 93% and 98% RHs (see Figures S7-S12, and 4a) and on 2 under 68%, 75%, 85%, 93% and 98% RHs from 30 to 100 °C (see Figures S13-S17, and 4b). As displayed in these impedance plots, under relative low temperatures and low RHs, the Nyquist spectra exhibit a semicircle at the high frequency and a characteristic spur at low frequency. As temperature and humidity increase, the semicircle will gradually become smaller, and the spur will become more and more apparent. Such AC impedance spectra are characteristic of proton conduction.6-8, 27 Through the comprehensive tests of the proton conductivities of both FCFs 1 and 2 at constant temperature and varying RHs, and at fixed RH and varying temperatures, we discovered that the proton conductivities of the FCFs are temperature and humidity dependent. In other words, the  values of the two FCFs magnify gradually with the increasing of temperature and humidity (Table S5 and S6). Furthermore, by carefully analyzing the proton conductivity behaviors of the two compounds, the slight difference is observed in the proton conductivity with increasing temperature and humidity (Figures 4c-4f). For 1, under 30 C, the calculated  is rapidly elevated from 3.19 × 10−7 S/cm at 53% RH to 1.95 × 10-3 S/cm at 98% RH, which is increased by four orders of magnitude (Figure 4c) and can be comparable with that of Nafion 117 (1.40 × 10-3 S/cm under 40% RH and 31 C)48 and of other conductive materials, such as MOF-801 (1.88 × 10-3 S/cm under 98% RH and 25 C),49 JLU-Liu44 (8.4 × 10-3 S/cm under 98% RH and 27 C),47 [Fe(oxa)(H2O)2] (oxa = oxalate ion; 1.3 × 10-3 S/cm under 98% RH and 25 C),51 and


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(NH4)2(adpi)[Zn2(oxa)3]·3H2O (adpi = adipic acid; 1.3 × 10-3 S/cm under 98% RH and 25 C).36 At fixed 40, , and 100 C, the  enhances by 2-3 orders of magnitude with increasing humidity, respectively (Figures 4c and 4e, S18 and S19). At different fixed RHs, the  of 1 was magnified by 2 to 4 orders of magnitude with increasing temperature (see Figure 4d and Table S5). A case in point is that, under 98% RH, the  for 1 changes from 1.95 × 10-3 S/cm at 30 °C to the highest value of 1.17 × 10-1 S/cm under 100 °C (Figure 4a).

(a)

(b)


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(c)

(e)

(d)

(f)

Figure 4. Nyquist plots of 1 (a) and 2 (b) under 98% RH and 100 °C; (c) Humidity dependence of the proton conductivities for 1; (d) Temperature dependence of the proton conductivities for 1; (e) Humidity dependence of the proton conductivities for 2; (f) Temperature dependence of the proton conductivities for 2.

To verify the conductivities of 1 and 2 under high humidity and temperature, we performed the DC conductivity determinations. According to the results of both DC and AC tests, the conductivities are at the same order of magnitude for 1 (10-1 S/cm) and 2 (10-2 S/cm) (Figure S20). For FCF 2, similar to FCF 1, proton conductivity is prone to increase with temperature and humidity by 3 orders of magnitude (Table S6, Figures 4e and 4f). Although the calculated conductivity value merely is 3.90 × 10-5 S/cm at 30 C and 98% RH, the


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highest value is 1.01 × 10-2 S/cm at 100 C and 98% RH (see Table S6, and Figure S17) displaying a dramatic increase. By analyzing the structural features of the two FCFs, it can be perceived that 1 has – COOH and –C=O units, both which can bond with adsorbed water molecules. In 2, there exists only –COOH group. In addition, in 1, the flexible linker, -C(O)-CH2-CH2-, between the ferrocene unit and –COOH group, will not only reduce the steric resistance of bulk ferrocene unit, but also meet the structural needs of formation hydrogen bonds via its own torsion. In contrast, in 2, -COOH group directly connects with the huge ferrocene group. The above structural differences result in the different hydrogen bonding networks inside the frameworks of the two FCFs, which further explains the difference in proton conduction behaviors between the two compounds, for example, the optimal  of 1 is an order of magnitude larger than that of 2. Noteworthy, the two FCFs exhibit remarkable ultra-high  values of 1.17 × 10-1 and 1.01 × 10-2 S/cm under 100 °C and 98% RH, respectively. As demonstrated in Table 1, the highest  of 1 is comparable to that of Nafion52 and several conductive materials of COF(TfBD) (3.0 × 10-1 S/cm, 80°C and 95% RH),12 PSM 1 (1.67 × 10-1 S/cm, 80°C and 95% RH),53 MOF(Co) (1.49 × 10-1 S/cm, 80 °C and 98% RH),22 H3PO4@S1-15 (1. 37 × 10-1 S/cm, 30 °C and 100% RH),54 and BUT-8(Cr)A (1.27 × 10-1 S/cm, 80 °C and 100% RH),52 and higher than that of a few of efficient proton conducting materials with high conductivities around 10-2 S/cm.18,19, 22,31, 29,56-63

Also, the highest proton conductivity of 2 can be comparable with above high

proton conducting materials under similar testing conditions.18,19, 22,31, 29,56-63


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The elevated moisture promotes the conductivity of FCFs 1 and 2 revealing that the proton transferring process is controlled by water-mediated proton conduction and the H2O units have a significant function on improving the proton conduction. When RH increased, the H2O units enter into the frameworks to interact with the -COOH or carbonyl (-C=O) groups establishing H-bonding networks or else serve as proton carriers for proton transfer, which would lead to a high efficiency enhancement of proton transportation.29,64 To prove this point, the H2O adsorption/desorption experiments for 1 and 2 have been performed on and the curve of water adsorption/desorption isotherms (Figure S21a). The water vapor sorption isotherms of evacuated samples for 1 and 2 at 298 K indicate that the maximum uptakes are 13.2 and 9.0 mg/g, respectively (P/P0 being 0.96). Obviously, the strong hydrophobicity of ferrocene group leads to the low adsorption values. However, the water adsorption capacity of 1 is larger than that of 2, which is consistent with the difference in proton conductivity. At higher RH, both the FCFs can accommodate more water molecules inside the frameworks due to the hydrophilicity of –COOH units, which can be confirmed by the fact that the FCFs exhibit higher conductivity at higher RH. Additionally, N2 adsorption at 77 K was measured by ASAP 2420 to evaluate their inherent porosity, revealing typical type-III isotherms (Figure S21b). The small BET surface areas 1.21 for m²/g 1 and 1.55 m²/g for 2 can be observed. Apparently, the two compounds have no obvious pores. Moreover, to confirm the effect of water molecules on proton conductivities under 100 C and 98% RH, PXRD as well as a TG curve of the pellet of 1 or 2 after AC determinations under this condition was carried out to detect any structural change and the absolute amount of absorbed water. As displayed in Figure S22, the two compounds maintain structural stability after AC measurement. However, thermal analysis tests showed that both


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compounds indicated high water absorption, and each structural unit of 1 and 2 adsorbed 3 and 2.5 water molecules, respectively (Figure S23). It is to be noticed that although the two compounds have no pores, the pellet samples have higher water storage capacity under high humidity and temperature. This may be due to the existence of abundant small interstices within the pellets, or the flexibility of the frameworks for 1 and 2. The effect of temperature on the proton conduction of the two FCFs is mainly due to that H2O molecules act as proton carriers to transmit protons by the H-bonding networks as form of H+(H2O)n, and the mobility of H+(H2O)n increases with the rising of temperature, therefore, the higher acid character (pKa) of H2O molecules at higher temperatures is liable for the increase of proton conductivity along with rising in temperature.12 To acquire the insights into the proton conductive mechanisms, Ea values of FCFs 1 and 2 were calculated by least-squares fits of the slopes (see Figure 5). At 98% RH, Ea values for 1 are 0.39 eV from 30-70 °C and 0.60 eV from 80-100 °C. For 2, the Ea values are calculated to be 0.39 (30-60 °C) and 0.23 eV (70-100 C). Obviously, in the light of Grotthuss (Ea values being 0.1-0.4 eV) and Vehicle (Ea values being 0.5-0.9 eV) mechanisms adopted to state proton conduction,65,66 there exist different proton conducting mechanisms in the two FCFs. At low temperature ranges, the Ea value (0.39 eV) of 1 and 2 reveals that proton conduction of the two FCFs corresponds to the Grotthuss (hopping) mechanism indicating that the main H+ ions, dissociated from – COOH or H+(H2O)n were transporting along the H-bonding networks established by – COOH, C=O units and the adsorbed water units in 1 or by –COOH units and the


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adsorbed water units in 2. As discussed in the previous section, although the Ea values of 1 and 2 are the same at low temperatures, the more complex H-bond networks result in 1 having the higher proton conductivities than those in 2 (see Tables S5 and S6). With the increase of temperature, the Ea value (0.23 eV) of 2 becomes much smaller than 0.4 eV, and the proton conduction behavior is a more typical hopping mechanism. In contrast, the Ea value (0.60 eV) of 1 increases sharply showing that the proton conduction in the high-temperature region may belong to the mainly Vehicle mechanism and partially Grotthuss mechanism. We try to explain that on the basis of structural analysis, H2O vapor adsorption and the acid strength of ferrocene carboxylic acid derivatives. The Ea value (0.60 eV) of 1 is higher than that (0.23 eV) of 2, which is in accord with that 1 has stronger affinity for protons than 2. In general, for proton conductive materials, low activation energy leads to high proton conductivity.22 However, as illustrated in Tables S5 and S6, the  values of 1 are 10-20 times larger than those of 2. This can be revealed by the fact that more complex hydrogen bonding networks in 1 can be formed and more hydrated protons will be dissociated, which is more conducive to the transmission of protons. In order to prove the assistance of water to proton conduction, we also conducted a control experiment on the two dehydrated FCFs at D2O atmosphere. Under 98% RH and 100 C, the  values of 1 and 2 in the presence of D2O became 4.11 × 10−3 (Figure S24a) and 1.90 × 10−5 S/cm (Figure S24b), respectively, which are much smaller than


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those would be under the same conditions in water vapor. This is clearly due to the heavier isotope substitution effect.

Figure 5. Arrhenius plots of the proton conductivities of 1 and 2 under 98% RH.

In addition, to illustrate the crucial role of free carboxylate groups in the proton conduction process of FCFs 1 and 2, the proton conductivities of two ferrocenyl derivatives congaing C=O groups and without –COOH groups, FcC(O)CH3 (AF)38 and FcC(O)OCH3 (AFM)39 under 98% RH and 30-50 C have been investigated below their melting points (AF: m. p. 85-86C; AFM: m. p. 69.6-70.5C). As listed in Table S7 and shown in Figure S25, both two derivatives show certain proton conduction abilities, which confirms the role of carbonyl oxygen atom in proton conduction. Under similar testing conditions, the proton conductivities of the two ferrocenyl derivatives without – COOH groups are lower than those of 1 and 2. For example, at 98% RH and 50C, the

 values are for AF and AFM are 3.29  10-6 and 3.29  10-6 S/cm, respectively. Contrastively, the values for 1 and 2 are 4.26 × 10-3 and 5.07 × 10-5 S/cm, respectively.


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These results thus demonstrate again the key role of free carboxylate groups in proton conduction for FCFs 1 and 2. In addition, as discussed in the early literature,67,68 the significant enhancement of water-mediated proton conduction by hydrophobic ferrocene groups should also be noted. As displayed in Figure S26, the PXRD peaks of the pellets after AC impedance determination matched well with the simulated PXRD one explicating that the skeleton of 1 and 2 remains uniform even after proton conduction measurements. Additional evidence came from SEM images (see Figures S27 and S28) and photographs (see Figures S29 and S30) for the samples before and after AC impedance measurement. It was clearly observed from these images that the morphology and crystallinity of the sample did not alter significantly, which again proved the rigidity of the two compounds. Taking account of the practical needs in the fuel cells fields, we further determined the durability of these two proton conductive materials 1 and 2 under 100 C and 98 % RH. As revealed in Figure 6, after one month of persistent electrochemical measurement, the

 values of the samples kept mainly constant. This indicates that they can be promising candidates for proton exchange membrane.


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Figure 6. Time-dependent proton conductivities of FCFs 1 and 2 at 100 C and 98% RH.

Table 1. Comparison of the  values of 1 and 2 with other High-Performing ProtonConducting Materials. materials

conductivity (S/cm)

testing conditions

ref

COF(TfBD) PSM 1 MOF(Co) H3PO4@S1-15 BUT-8(Cr)A FCF 1 PCMOF21/2(Tz) PCMOF21/2(Pz) PolyEu-H BUT-8(Cr) MOF[Co(DCDPP)]·5H2O Im@MOF-808 Tb(III)-MOF KAUST-7′ (C6H14N2)[NiV2O6H8(P2O7)2]·2H2O Im@(NENU-3) VNU-23 (NH4)2[ZrF2(HPO4)2] Im-Fe-MOF [CuI4CuII4L4]n-NH3 FCF 2

3.0 × 10-1 1.64 × 10-1 1.49 × 10-1 1. 37×10-1 1.27 × 10-1 1.17 × 10-1 1.17 × 10-1 1.10 × 10-1 5. 6 × 10-2 4.63 × 10-2 3.9 × 10-2 3.45 × 10-2 2. 57 × 10-2 2.2 × 10-2 2.0 × 10-2 1.82 × 10-2 1.79 × 10-2 1.45 × 10-2 1.21 × 10-2 1.13 × 10-2 1.01 × 10-2

80°C and 95% RH 80°C and 95% RH 80 °C and 98% RH 30 °C and 100% RH 80 °C and 100% RH 100 °C and 98% RH 85 °C and 90% RH 85 °C and 90% RH 75 °C and 95% RH 80 °C and 100% RH 80 °C and 97% RH 65 °C and 99% RH 60 °C and 98% RH 90 °C and 95% RH 60 °C and 100% RH 70 °C and 90% RH 95 °C and 85% RH 90 °C and 95% RH 60 °C and 98% RH 100 °C and 98% RH 100 °C and 98% RH

12 53 23 54 55 This work 23 23 31 56 29 19 57 58 59 55 60 61 62 63 This work


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▇ CONCLUSION In summary, we systematically explored the water-mediated proton conductive properties of two hydrogen-bonded and - interacted ferrocenyl carboxylate frameworks (FCFs). The results revealed that the two FCFs demonstrated unexpectedly excellent proton conductivities, and their optimal proton conductivities reached 1.17 × 10-1 and 1.01 × 10-2 S/cm under 100 °C and 98% RH, respectively. In particular, the performance of 1 is more outstanding: its room temperature proton conductivity can be achieved 1.95 × 10-3 S/cm at 98 % RH, and its optimal  can be comparable to Nafion ranking among the best proton conductive materials reported at present. In addition, the existence of special organometallic ferrocene groups, as well as intermolecular interactions such as H-bonds, aromatic stacking forces insider the frameworks of the FCFs results in their excellent thermal, water and chemical stabilities, especially the acid stability. It will be very beneficial to the real-world applications in fuel cells and relevant fields. By means of structural analyses and other testing methods, we have deeply studied the pathways and mechanisms of proton conduction. Simultaneously, the prepared materials of FCFs 1 and 2 display high stability and durability for proton conduction determinations. Therefore, we believe that this class of crystalline proton conducting material with the advantages of simple preparation, low cost and stable structures has strong application prospects for proton conduction. More importantly, our


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research results will offer more afflatus for researchers to explore more similar proton conducting materials. ▇ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Details of crystal data, impedance analysis, PXRD patterns, gas adsorption/desorption. ▇ AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]. ORCID Gang Li: 0000-0001-9049-4208 Notes The authors declare no competing financial interest. ▇ ACKNOWLEDGMENT

This work was supported by the National Science Foundation of China (21571156 and J1210060). We are very grateful to Dr. Zhi-Qiang Shi for his useful discussions on crystal structural analysis ▇ REFERENCES [1]

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SYNOPSIS

The water-mediated proton conduction of two hydrogen-bonds and - interactions supported ferrocenyl carboxylate frameworks (FCFs), [FcCO(CH2)2COOH] (FCF 1) and [FcCOOH] (FCF 2) (Fc = (5-C5H5)Fe(5-C5H4)) have been investigate fully. Intriguingly, the ultrahigh proton conductivities of 1.17 × 10-1 and 1.01 × 10-2 S/cm for 1 and 2, respectively, at 100 °C and 98% RH can be observed. These  values are not only comparable to the commercial Nafion membranes, but also ranking among the highest performing MOFs, HOFs and COFs ever described.

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Ultrahigh Proton Conduction in Two Highly Stable Ferrocenyl

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