Combined Intrinsic and Extrinsic Proton Conduction in Robust Covalent Organic Frameworks for Hydrogen Fuel Cell Applications

Abstract

Developing new materials for the fabrication of proton exchange membranes (PEMs) for fuel cells is of great significance. Herein, a series of highly crystalline, porous, and stable new covalent organic frameworks (COFs) have been developed by a stepwise synthesis strategy. The synthesized COFs exhibit high hydrophilicity and excellent stability in strong acid or base (e.g., 12 m NaOH or HCl) and boiling water. These features make them ideal platforms for proton conduction applications. Upon loading with H3PO4, the COFs (H3PO4@COFs) realize an ultrahigh proton conductivity of 1.13×10−1 S cm−1, the highest among all COF materials, and maintain high proton conductivity across a wide relative humidity (40–100 %) and temperature range (20–80 °C). Furthermore, membrane electrode assemblies were fabricated using H3PO4@COFs as the solid electrolyte membrane for proton exchange resulting in a maximum power density of 81 mW cm−2 and a maximum current density of 456 mA cm−2, which exceeds all previously reported COF materials.

Confining H3PO4 network in covalent organic frameworks enables proton super flow

“…H 3 PO 4 @TPB-DMeTP-COF enables a proton flow that is even twofold compared to molten neat H 3 PO 4 (~1 × 10 -1 S cm -1 ) 25 . Note that the proton flow is significantly increased by 2-8 orders of magnitude higher than those of other analogues under anhydrous condition (Table 1) 3,5,6,10,21,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] . Meta-organic frameworks (MOFs) are hardly efficient to construct anhydrous proton-conducting systems by loading neat H 3 PO 4 into their pores.…”

“…These data confirmed that H 3 PO 4 @TPB-DMeTP-COF with dense and large pores enables super proton flow over a wide range of temperature. Note that a recent example of NKCOF-1 with H 3 PO 4 shows no proton conductivity under anhydrous conditions 27 . TPB-DMeTP-COF retains its crystallinity and porosity after AC impedance for 20 h ( Supplementary Fig.…”

Weakly Humidity‐Dependent Proton‐Conducting COF Membranes

“…Covalent organic frameworks (COFs), a class of crystalline porous polymers with pre‐designable pore structures, hold grand promise. [ 21–23 ] Unlike amorphous, flexible crosslinked networks, the crystalline, rigid organic frameworks of COFs are expected to afford well‐defined and stable proton‐conducting nanochannels, [ 24–30 ] as well as good water retention capability. [ 18 ] However, leveraging COF materials in the field of COF‐based PEMs for practical application remains a great challenge arising from the poor processing ability of COF materials and the poor structural integrity of COF membranes.…”

“…a) Temperature‐dependent proton conductivity of IPC‐COF membrane and Nafion 212 at 98% RH. b) Comparison of proton conductivity data for IPC‐COF membrane (solid stars), COF materials (open diamonds) [ 26–29,31 ] and state‐of‐the‐art PEMs (open triangles). [ 2,9,10,37–39 ] c) Proton conductivity of IPC‐COF membrane and Nafion 212 versus RH at 40 °C.…”

“…Interestingly, the MEA‐IPC‐COF can maintain a peak power density of 0.93 W cm −2 (threefold higher than Nafion 212) even at low RH of 35%, demonstrating a weakly humidity‐dependent feature. As compared in Figure 5c and Table S2 (Supporting Information), the as‐prepared IPC‐COF membrane achieves the highest power density among the current COF materials (0.08 W cm −2 ) [ 27 ] and even prevails over those of state‐of‐the‐art PEMs at low RH. We envisage that the fuel cell performance at low RH could be further improved by optimizing the catalyst layer composition (e.g., using weakly humidity‐dependent proton‐conducting materials as binder) and improving the compatibility between IPC‐COF membrane and catalyst layers in MEAs.…”