Proton Conduction in Metal–Organic Frameworks and Related Modularly Built Porous Solids

Proton‐conducting materials are an important component of fuel cells. Development of new types of proton‐conducting materials is one of the most important issues in fuel‐cell technology. Herein, we present newly developed proton‐conducting materials, modularly built porous solids, including coordination polymers (CPs) or metal–organic frameworks (MOFs). The designable and tunable nature of the porous materials allows for fast development in this research field. Design and synthesis of the new types of proton‐conducting materials and their unique proton‐conduction properties are discussed.     

Novel proton-conductive nanochannel membranes with modified SiO2 nanospheres for direct methanol fuel cells

A novel approach is proposed to prepare a proton-conductive nanochannel membrane based on polyvinylidene difluoride (PVDF) porous membrane with modified SiO₂ nanospheres. The hydrophilic PVDF porous membrane with a 450-nm inner pore size was chosen as the supporting structure. Pristine SiO₂ with a uniform particle size of 95–110 nm was synthesized and functionalized with –NH₂ and –COOH, respectively. Through-plane channels of porous membrane and arranged functional nanoparticles in pores could contribute to constituting efficient proton transfer channels. The characteristics such as morphology, thermal stability, water uptake, dimensional swelling, proton conductivity and methanol permeability as proton exchange membranes, of the SiO₂ nanospheres, and the composite membrane were investigated. The formation of ionic channels in membrane enhanced the water uptakes and proton conduction abilities of the composite membranes. PVDF/Nafion/SiO₂–NH₂ exhibited superior proton conductivities (0.21 S cm^?1) over other samples due to several proton sites and the acid–base pairs formed between –NH₂ and –SO₃H. Furthermore, all the composite membranes exhibited improved methanol resistance compared with Nafion. Therefore, such a design based on porous membrane provided feasibility for high-performance proton exchange membrane in fuel cell applications.     

Hydrogen‐Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton‐Conducting Materials

Two porous hydrogen‐bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra‐high proton conduction values (σ) 0.75× 10^?2 S cm^?1 and 1.8×10^?2 S cm^?1 under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic‐based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen‐bonded porous organic frameworks as solid‐state proton conducting materials.     

Water‐Mediated Proton Conduction in a Robust Triazolyl Phosphonate Metal–Organic Framework with Hydrophilic Nanochannels

The development of water‐mediated proton‐conducting materials operating above 100 °C remains challenging because the extended structures of existing materials usually deteriorate at high temperatures. A new triazolyl phosphonate metal–organic framework (MOF) [La₃ L ₄(H₂O)₆]Cl ? x H₂O ( 1 , L ^2?=4‐(4H‐1,2,4‐triazol‐4‐yl)phenyl phosphonate) with highly hydrophilic 1D channels was synthesized hydrothermally. Compound 1 is an example of a phosphonate MOF with large regular pores with 1.9 nm in diameter. It forms a water‐stable, porous structure that can be reversibly hydrated and dehydrated. The proton‐conducting properties of 1 were investigated by impedance spectroscopy. Magic‐angle spinning (MAS) and pulse field gradient (PFG) NMR spectroscopies confirm the dynamic nature of the incorporated water molecules. The diffusivities, determined by PFG NMR and IR microscopy, were found to be close to that of liquid water. This porous framework accomplishes the challenges of water stability and proton conduction even at 110 °C. The conductivity in 1 is proposed to occur by the vehicle mechanism.     

Enhanced local density of states at the Fermi level of the surface platinum in carbon-supported platinum particles by Nafion ionomer

¹⁹⁵Pt nuclear magnetic resonance (NMR) was employed to study the influence of Nafion ionomers (NIs), a common ingredient used as a proton conducting mediator and binder in a membrane-electrode-assembly (MEA) for proton exchange membrane fuel cells, on the local density of states at the Fermi surface (E_F-LDOS) of Pt/C catalysts. NIs were observed to increase the surface E_F-LDOS of Pt particles, thereby altering of the catalytic properties of metal particles. This correlates well with the reduction of fuel oxidation rates. Thus, ionomers decreasing the surface E_F-LDOS of Pt particles would be desirable. In addition, a new concept to interpret the surface E_F-LDOS was introduced and validated.     

Spectroscopic investigation of structure and mechanism of proton conductivity CsHSO<Subscript>4</Subscript> and composites CsHSO<Subscript>4</Subscript>/SiO<Subscript>2</Subscript>

IR and RS spectra, structural, thermodynamical and transport properties of CsHSO₄ and CsHSO₄/SiO₂ composites have been investigated. It is shown that CsHSO₄ is stabilized in composites in phase II (monoclinic modification), which turns into amorphous state with increasing silicon dioxide content. A formation of disordered highly conducting states of CsHSO₄ in composites at the temperatures noticeably lower than superionic phase transition was stated. Bond length equalization in sulfate-ions, weakening of hydrogen bond system, and as a result easier proton transfer in composites in comparison with pure salt take place in these conducting states. Mechanism for formation of composites and their proton conductivity is proposed.     

Effect of hydrophilic SiO2 additive in cathode catalyst layers on proton exchange membrane fuel cells

Hydrophilic nanosized SiO₂ and sulfonated SiO₂ particles were added to the cathode catalyst layer (CL) to improve the water wettability and the performance of a proton exchange membrane fuel cell (PEMFC) at low humidity. It was found that both nanosized SiO₂ and sulfonated SiO₂ additive improved the hydrophilicity of the cathode CL by the contact angle measurement. Contrary to nanosized SiO₂, sulfonated SiO₂ improved the conductivity of the cathode CL. Increased wettability of the cathode CL from SiO₂ maintained fuel cell at hydration conditions. This phenomenon had a profound influence on electrode performance at low humidity. Since the sulfonic groups in sulfonated SiO₂ improved the proton conductivity of the cathode CL, the cell with sulfonated SiO₂ showed better performance.     

Chemically modified proton‐conducting membranes based on sulfonated polyimides: Improved water stability and fuel‐cell performance

A series of branched/crosslinked sulfonated polyimide (B/C‐SPI) membranes were prepared and evaluated as proton‐conducting ionomers based on the new concept of in situ crosslinking from sulfonated polyimide (SPI) oligomers and triamine monomers. Chemical branching and crosslinking in SPI oligomers with 1,3,5‐tris(4‐aminophenoxy)benzene as a crosslinker gave the polymer membranes very good water stability and mechanical properties under an accelerated aging treatment in water at 130 °C, despite their high ion‐exchange capacity (2.2–2.6 mequiv g^?1). The resulting polymer electrolytes displayed high proton conductivities of 0.2–0.3 S cm^?1 at 120 °C in water and reasonably high conductivities of 0.02–0.03 S cm^?1 at 50% relative humidity. In a single H₂/O₂ fuel‐cell system at 90 °C, they exhibited high fuel‐cell performances comparable to those of Nafion 112. The B/C‐SPI membranes also displayed good performances in a direct methanol fuel cell with methanol concentrations as high as 50 wt % that were superior to those of Nafion 112. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3751–3762, 2006     

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.

A proton-conducting coordination polymer glass derived from a protic ionic liquid works as a moldable solid electrolyte and the anhydrous fuel cell showed I–V performance of 0.15 W cm⁻² at 120 °C.     

pH‐Dependent Proton Conducting Behavior in a Metal–Organic Framework Material

A porous metal–organic framework (MOF), [Ni₂(dobdc)(H₂O)₂]?6 H₂O (Ni₂(dobdc) or Ni‐MOF‐74; dobdc^4?=2,5‐dioxido‐1,4‐benzenedicarboxylate) with hexagonal channels was synthesized using a microwave‐assisted solvothermal reaction. Soaking Ni₂(dobdc) in sulfuric acid solutions at different pH values afforded new proton‐conducting frameworks, H⁺@Ni₂(dobdc). At pH 1.8, the acidified MOF shows proton conductivity of 2.2×10^?2 S cm^?1 at 80 °C and 95 % relative humidity (RH), approaching the highest values reported for MOFs. Proton conduction occurs via the Grotthuss mechanism with a significantly low activation energy as compared to other proton‐conducting MOFs. Protonated water clusters within the pores of H⁺@Ni₂(dobdc) play an important role in the conduction process.     

LaNi0.6Fe0.4O3 as a cathode contact material for solid oxide fuel cells

In solid oxide fuel cells (SOFCs) the interconnects electrically link air and fuel electrodes on either side to produce a practical electrical power output. The long-term stability of intermediate temperature (650–800 °C) SOFC operation strongly depends on the composition of the ferritic steel interconnection material and the steel/ceramic interface. During high-temperature operation the Cr-containing ferritic steel forms an oxide scale at its surface, thereby causing high ohmic electrical contact resistance when connected to the surface of an electronically conducting ceramic cathode material. In the long run, the vaporization of Cr species from these oxide scales also affects the cathode activity, eventually leading to cell deterioration. One way of overcoming the problem is to incorporate another electronically conducting ceramic compliant layer, commonly known as the contact layer, between the cathode and metallic interconnect. In this contribution, LaNi_0.6Fe_0.4O₃ was tested as a cathode contact material. Its performance at 800 °C in the form of a ~50 μm thick film applied on two ferritic steel compositions was examined. After 600 h of testing, contact resistances of 60 and 160 mΩ cm² were obtained. The different values are explained by the variation in steel composition.     

Composite electrode materials for solid oxide fuel cells with the protonic electrolyte of La<Subscript>1 – <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>ScO<Subscript>3 – δ</Subscript>

Heterogeneous systems based on the proton–conducting oxide of La_0.95Sr_0.05ScO_{3 – δ} with Cu, Fe, Ni, Pd, La_0.9Sr_0.1MnO_{3 – δ} considered as potential materials of solid oxide fuel cell (SOFC) electrodes are synthesized. Chemical interaction between individual components of composite materials is studied, dependences of thermal and chemical expansion of the electrolyte and composites are obtained, conductivity of electrodes is measured under the conditions of SOFC operation.     

Chemically stable anode-supported solid oxide fuel cells based on Y-doped barium zirconate thin films having improved performance

Anode-supported solid oxide fuel cells (SOFCs) based on thin BaZr_0.8Y_0.2O_3 ? δ (BZY) electrolyte films were fabricated by pulsed laser deposition (PLD) on sintered NiO–BZY composite anodes. After in situ reduction of NiO to Ni, the anode substrates became porous, while retaining good adhesion with the electrolyte. A slurry-coated composite cathode made of La_0.6Sr_0.4Co_0.2Fe_0.8O_3 ? δ (LSCF) and BaCe_0.9Yb_0.1O_3 ? δ (BCYb), specifically developed for proton conducting electrolytes, was used to assemble fuel cell prototypes. Depositing by PLD 100 nm thick LSCF porous films onto the BZY thin films was essential to improve the cathode/electrolyte adhesion. A power density output of 110 mW/cm² at 600 °C, the largest reported value for an anode-supported fuel cell based on BZY at this temperature, was achieved. Electrochemical impedance spectroscopy (EIS) measurements were used to investigate the different contributions to the total polarization losses.     

A completely spray-coated membrane electrode assembly

We present a proton exchange membrane fuel cell (PEMFC) manufacturing route, in which a thin layer of polymer electrolyte solution is spray-coated on top of gas diffusion electrodes (GDEs) to work as a proton exchange membrane. Without the need for a pre-made membrane foil, this allows inexpensive, fast, large-scale fabrication of membrane-electrode assemblies (MEAs), with a spray-coater comprising the sole manufacturing device. In this work, a catalyst layer and a membrane layer are consecutively sprayed onto a fibrous gas diffusion layer with applied microporous layer as substrate. A fuel cell is then assembled by stacking anode and cathode half-cells with the membrane layers facing each other. The resultant fuel cell with a low catalyst loading of 0.1 mg Pt/cm² on each anode and cathode side is tested with pure H₂ and O₂ supply at 80 °C cell temperature and 92% relative humidity at atmospheric pressure. The obtained peak power density is 1.29 W/cm² at a current density of 3.25 A/cm². By comparison, a lower peak power density of 0.93 W/cm² at 2.2 A/cm² is found for a Nafion NR211 catalyst coated membrane (CCM) reference, although equally thick membrane layers (approx. 25 μm), and identical catalyst layers and gas diffusion media were used. The superior performance of the fuel cell with spray-coated membrane can be explained by a decreased low frequency (mass transport) resistance, especially at high current densities, as determined by electrochemical impedance spectroscopy.     

Electrochemical Synthesis of a Microporous Conductive Polymer Based on a Metal–Organic Framework Thin Film

A new approach to preparing 3D microporous conductive polymer has been demonstrated in the electrochemical synthesis of a porous polyaniline network with the utilization of a MOF thin film supported on a conducting substrate. The prepared porous polyaniline with well‐defined uniform micropores of 0.84 nm exhibits a high BET surface area of 986 m² g^?1 and a high electric conductivity of 0.125 S cm^?1 when doped with I₂, which is superior to existing porous conducting materials of porous MOFs, CMPs, and COFs.     

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

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.     

Proton-conducting polymer membranes for direct methanol fuel cells

Three methods to block the methanol transport through proton-conducting polymer membranes while maintaining the proton conductivity unchanged have been conducted; 1) selective layer formation on the surface of the membrane, 2) prearation of nanoclay composite membrane providing tortuous pathway of methanol, 3) control and fixation of the proton transport channels. The methanol permeability through the membranes decreased significantly at the expense of the small decrease in the proton conductivity. It is thus concluded that both the structure and the fixation of the proton transport channels are crucial in optimizinging proton conducting membranes for direct methanol fuel cells.     

Enhancing proton conductivity of proton exchange membrane with SPES nanofibers containing porous organic cage

Effective proton conducting sites and establishing proton channels are two critical factors in developing high‐performance proton exchange membranes. This study first establishes a strategy in designing effective proton conducting channels for Nafion by using solution blowing of sulfonated polyethersulfone (SPES) nanofibers containing CC3, which is an emerging porous organic cage that possesses the advantages of dissolvable organic solvents and high proton conduction from its interconnected three‐dimensional pore structure. Our strategy results in SPES nanofiber networks with CC3 uniformly involved in and composite membranes with Nafion‐filled interfiber voids. Benefiting from such structural features, the composite membrane exhibits high proton conductivity (0.315 S cm^?1 at 80°C and 100% RH), low methanol permeability (0.69 × 10^?7 cm² S^?1), excellent water absorption, thermal and dimensional stability, and single‐cell performance. This study provides not only a valuable reference for the application of CC3 but also a new idea for establishment of proton transfer channels.     

NaIO4/KI/NaCl: a new reagent system for iodination of activated aromatics through in situ generation of iodine monochloride

A new reagent system consisting of NaIO₄/KI/NaCl in aq AcOH has been found to be effective in iodinating a variety of activated aromatic substrates via in situ-generated iodine monochloride, to furnish iodoaromatics in excellent yields. This iodination procedure has been applied successfully for a cost-effective synthesis of 3,3′-diaminobenzidine, a key intermediate for preparing proton conducting membranes for fuel cell applications, with high yield and a purity of 99.7%.     

Micro-tubular solid oxide fuel cells fabricated by phase-inversion method

A novel structured micro-tubular solid oxide fuel cell (MT-SOFC) has been fabricated by combining a phase-inversion, dip-coating and high temperature co-sintering process with impregnation of the electrode catalyst into a porous electrode matrix. The asymmetric porous anode made by phase-inversion is divided into two different layers, a thick fuel delivery layer with large finger-like pores and a thin function layer with small finger-like pores. The MT-SOFC demonstrates maximum power densities of 0.44, 0.54, 0.65 and 0.78 W/cm² at 650, 700, 750 and 800 °C, respectively with H₂–15%H₂O as fuel and ambient air as oxidant. Combining the power output with the quick start-up behavior, novel structured MT-SOFC offers a potential solution for rapid start-up high performance power devices.