Proton Conductivity Of Membranes Research Articles

Confining Phosphoric Acid in Quaternized COF Channels for Ultra-stable and Fast Anhydrous Proton Transport.

Phosphoric acid (H3PO4) doping is a widely employed strategy to facilitate anhydrous proton transport in high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, significant H3PO4 leaching during long-term operation poses critical challenges to maintaining membrane stability and proton conductivity. Herein, H3PO4 is incorporated into positively charged nanochannels of quaternized covalent organic framework membranes (QACOFMs), leveraging strong electrostatic interactions and confinement effects to achieve exceptional H3PO4 retention under hydration conditions. Moreover, the shortened hydrogen bond length between H3PO4 (O-H…O <2.7 Å) and the highly interconnected hydrogen bond network in the H3PO4@QACOFMs facilitate ultra-fast anhydrous proton transport. As a result, the H3PO4@QACOFMs exhibit superior anhydrous proton transport in a broader temperature range (60-200 oC) and the highest proton conductivity reaches about 379.7 mS cm-1 at 200 oC.

Construction of Continuous Proton Transport Channels via Anchoring Strong Proton Conductor on Graphene Oxide in Sulfonated Poly (Ether Ether Ketone)

ABSTRACTConstructing continuous proton transport channels in proton exchange membranes (PEMs) has been considered as an efficient strategy to improve proton conductivity. In this work, a water‐insoluble hybrid graphene oxide (HDGO) is prepared via the hydrothermal reaction between polydopamine‐coated graphene oxide (DGO) and a strong proton conductor phosphotungstic acid (HPW), in which HPW is chemically bonded onto DGO. The two‐dimensional structure of DGO and the strong acidity of HPW favor the rapid proton transport in the sulfonated poly(ether ether ketone) (SPEEK) membranes. As compared to that of the SPEEK control membrane, the proton conductivity of the SPEEK composite membrane containing 6 wt% HDGO increases from 0.035 to 0.061 S cm−1 at 25°C, an increase of ~75%. The composite membrane containing 10 wt% HDGO exhibits ~49% lower area swelling ratio than the SPEEK control membrane. Meanwhile, the proton conductivity of the composite membrane remains stable during the 1440 h immersing in liquid water.

Microscopic insights into UiO-66@proton exchange composite membrane by molecular dynamics simulation

UiO-66 as one of the known metal-organic frameworks (MOFs) has been recognized as highly promising dopants for enhancing the proton conductivity of proton exchange membrane (PEM) owing to the large pore volume and structure tunability. Despite the numerous experimental reports on MOF-doped PEMs, their increased proton conductivity is commonly ascribed to enhanced water uptake. The underlying mechanisms from a microscopic perspective remain elusive. Therefore, this work explores the microstructure and water diffusion dynamics within composite membranes to decipher their mechanisms involved in proton conductivity using molecular dynamics (MD) simulations. Four types of composite membranes based on two representative MOFs i.e. UiO-66 and UiO-66-NH2 and two PEMs including Nafion and Dow, respectively, were taken into account. It is revealed that the UiO-66-NH2 doped Nafion composite membrane exhibits the highest water uptake among the four composite membranes resulting from the super hydrophilicity of UiO-66-NH2. Besides, the more concentrated distribution of sulfonic groups near the water-PEM interface and the higher interface roughness of UiO-66-NH2 doped Nafion lead to more water molecules surrounding its sulfonic groups that are favorable for the proton dissociation from sulfonic groups. Furthermore, the increased water channel connectivity of MOF-doped membranes that promotes proton transport through water via the Grotthuss mechanism demonstrates one of the mechanisms for increased proton conductivity. On the other hand, although the reduced lifetime of the hydrogen bond network and the enhanced water diffusion coefficient within MOF-doped membranes manifest the favorable proton transfer via the Vehicle mechanism. Overall, UiO-66-NH2 doped Nafion membranes exhibiting the highest water channel connectivity and water diffusion coefficients demonstrate the greatest potential of UiO-66-NH2 doping in advancing the proton conductivity. These findings provided microscopic insights into understanding the improved proton conductivity mechanism of MOF doped PEMs, and the approaches developed in this work may be extended to other composite membranes.

Electrophoretic Deposition of Carbon-Ionomer Layers on Proton Conducting Membranes.

Synthetic and natural carbons are widely used as carrier for electrodes in electrochemical applications. They need to have a controlled morphology in order to facilitate mass and charge transport, so the process of film formation is of uttermost importance. Here we show, how carbons (after proper preconditioning) can be codeposited with an ionomer by electrophoretic deposition, a method that does allow full control of deposition conditions during the process. In view of potential applications, we focus on the direct deposition on proton-conducting membranes. Ionomers and membranes applied are based on established per-fluorinated polyethylene with SO3H-terminated side chains (PFSA). Conditions for reproducible deposition are reported in terms of optimal charge on the carbon particles, field strength in the deposition cell and necessary deposition times for a given film thickness. Additionally, a horizontal cell arrangement is suggested to avoid gravitational effects.

Cross-layer alternating paired charge distribution to boost proton conductivity of COF lamellar membrane

Covalent organic frameworks (COFs) have attracted great interest for the development of proton exchange membranes (PEMs) in the field of hydrogen fuel cells. However, the Grotthuss mechanism of proton transfer fails to discriminate the preferential pathways on the complicated hydrogen-bond network, thus limiting cell performance. Herein, a heterocharged COF lamellar membrane with cross-layer alternating paired charge distribution was fabricated by alternately assembling the positively charged TpEB and negatively charged TpPa-SO3H nanosheets. We demonstrated that the TpEB nanosheet drives the proton to directional transfer along the hydrogen-bond network on the sulfonic acid periphery of TpPa-SO3H by significantly reducing ineffective motions in branched hydrogen-bond network for both through-plane and in-plane directions. Meanwhile, it provides a low absorption energy for proton transfer from the sulfonic acid shell. This assembly effect occurs within the three nanosheet layers during each alternate film formation (A3B3) and contributes to the long-term stability of the proton conductivity for TpEB@TpPa-SO3H. Notably, it achieves a maximum power density of 223 mW cm−2 at 60 °C and 100 % RH, which is superior to those of 83 mW cm−2 and 22 mW cm−2 for the homocharged TpPa-SO3H and TpEB membranes, respectively. This work provides new insights into the design of high-conductivity PEMs from engineered COF membranes.

Preparation of Proton-Conducting Solid Electrolyte Membrane Based on Carboxymethyl Chitosan Complexed with Ammonium Acetate

This research aims to prepare the proton-conducting solid electrolyte membrane based on carboxymethyl chitosan (CMCh) complexed with ammonium acetate (CH3COONH4). The membranes were prepared by using casting solution technique where the various weight percentages of ammonium acetate mixed to CMCh for obtaining optimum condition based on ionic conductivity analysis. Some characterizations were conducted to analysis the functional groups (involving complexation studies), ionic conductivities, mechanical properties, crystallinities, and thermal analysis by using FTIR, EIS, tensile tester, XRD, and TGA. The results showed that the optimum proton conductivity was obtained at the addition of 40% (w/w) CH3COONH4 salt as high as 1.39×10-4 S/cm and a tensile strength of 9.06 MPa. Based on the results, it can be concluded that the optimum condition of the membrane shows good characteristics to be applied as a proton conducting solid electrolyte. Keywords: Ammonium acetate, carboxymethyl chitosan, polymer electrolyte, proton conductivity, proton-conducting membrane

Molecular Vibration and Physicochemical Performance of Proton-Conducting Solid Polymer Electrolyte Membrane based on CMC/PVA/CH3COONH4

This work studied examined the influence of ammonium acetate (CH3COONH4) on CMC/PVA-based solid polymer electrolyte (SPE) membranes, focusing on molecular vibration, proton conductivity, and physicochemical properties. SPE membranes were prepared via the casting solution method with varying CH3COONH4 concentrations to determine the optimal proton conductivity. Various characterizations, including FTIR, EIS, XRD, and TGA, were performed. The optimal membrane condition was achieved with 10 wt-% CH3COONH4 in the CMC/PVA (80/20) blend, yielding proton conductivity of 3.93×10⁻⁴ S/cm and favorable mechanical, thermal, and crystallinity properties, making it suitable for proton-conducting polymer applications. Keywords: ammonium acetate, carboxymethyl cellulose, ionic conductivity, poly(vinyl alcohol), proton battery, solid electrolyte membrane

Development, Evaluation and Characterization of Hydrophilic Oxide/Polymer Composite Membranes for Polymer Electrolyte Fuel Cells Operating at Temperatures over 80oc

The present transportation system based on the internal combustion engine is a major contributor to the issue of CO2 emission, and a paradigm shift is required toward to electrical motorization based on batteries and polymer electrolyte fuel cells (PEFCs). The motorization of heavy-duty vehicles (HDVs) by PEFCs is one of the essential targets. HDVs will require higher operating temperatures, at least 120oC, versus normal temperatures (60-80oC), for optimal heat management. A commercial perfluorosulfonic acid-based electrolyte (PFSA, e.g., NafionⓇ) shows better proton conductivity with chemical stability over a wide temperature range, although the conductivity decreases under low back pressure at 100~120 oC. In order to improve the proton conductivity in the higher temperature range, a composite membrane of NafionⓇ with a hydrophilic filler of Ta-doped TiO2 (Ta-TiO2) was synthesized. Ta-TiO2 nanoparticles with fused-aggregate network microstructure were synthesized by the flame oxide-synthesis method and were crushed by wet milling (Star Burst, Sugino Machine, Limited Co.). The fillers were mixed with NafionⓇsolution (20 wt.%, D2020 Chemours Co.) by use of a rotary ultrasonic dispersing processor (500 rpm, 60 min., PR-1, THINKY Co.), a planetary ball mill (500 rpm, 30 min., PULVERISETTE 6, FRITSCH Co.), and a defoaming processor (500 rpm、5 min., HM-400W, Kyoritsu Seiki Co.). The dispersion solution was coated in hot air by use of a die coating system (die coater, Taku-Dai Mini-50, Daimon Co., Ltd.), and were hot-pressed (140 ℃, 3 min., TCMD-2.5, TOHO KOGYO Co.) to form composite membranes (9 cm × 9 cm × 25 µmt). The cross-sectional backscattered electron image BSE) of the composite membrane was observed by scanning electron microscopy (SEM, acceleration voltage 2 kV, SU9000, Hitachi High-Tech Co.) and transmission electron microscopy (TEM, H-9500, Hitachi High-Tech Co.). The proton conductivity of the composite membrane was measured at 80 °C and 120 °C by the AC four-probe method. Water uptake of the composite membrane was measured by water vapor adsorption. The current-voltage (I-V) polarization curves of single cells using the prepared composite membrane (membrane thickness, 25 µm) were measured at 80 ℃, 100% RH and 120 ℃, 20% RH, back pressure 50 kPaG.The obtained composite membranes were soft and flexible. The BSE images indicated, based on their high contrast, that the Ta-TiO2 particles were uniformly dispersed in the composite membrane(Fig. 1(a)). The higher magnification of the cross-sectional TEM images also showed that the hydrophilic surface of Ta-TiO2 adhered to the Nafion® without any voids. The proton conductivity of the composite membrane at 80 °C and 80% RH increased with increasing Ta-TiO2 content and showed a maximum value of 0.13 S cm-1at 3 wt% and then decreased. The maximum proton conductivity was 1.3 times higher than that of Nafion®(Fig. 1(b)). The proton conductivity of the composite membrane (3 wt% of Ta-TiO2) at 120 °C, 20% RH reached 0.018 S cm-1, which was 1.8 times higher than that of a commercial Nafion® membrane, 0.01 S cm-1. The water uptake in each composite membrane was nearly independent of the concentration of the hydrophilic oxide at each humidity(Fig. 1(c)). Comparing with literature results, improvements of the proton conductivity by use of various types of additives have been reported, but the improvement of the proton conductivity by such a small percentage of hydrophilic oxide additive without affecting the water uptake in the membrane has not been reported, to our knowledge. We consider that the protonic hopping (proton mobility) might change as a result of adding the hydrophilic Ta-TiO2 in the membrane. The I-V performance of a single cell using a composite membrane preserved the same performance as that of a cell using an unmodified Nafion® membrane at 80 °C and 100% RH, and then improved at 120 °C and 20% RH(Fig. 1(d)).

Sulfonated Poly(phenylene sulfone)s with High Ion Exchange Capacity

Abnormal phenomena caused by climate change require the realization of carbon neutrality while ensuring a stable supply of energy (energy security). Climate change is mainly caused by environmental pollution from human society, and in recent years, research and development of environmentally friendly materials for practical use has been carried out.Energy devices such as fuel cells, water electrolysis, and RFB have polymer electrolyte components that transport ions, and up until now, fluorine-based electrolytes such as Nafion have been mainly used. However, within the framework aiming to achieve carbon neutrality, the development of materials containing fluorine is thought to be subject to restrictions. Therefore, there is a need for alternative materials to fluorine-based electrolytes.We are conducting research aimed at the practical application of sulfonated PPSU, a hydrocarbon electrolyte, as a non-fluorine electrolyte [1-6]. In this study, we report the synthesis and properties of sulfonated PPSU with high ion exchange capacity (IEC) (calculated value: 5.55meq/g), which has four sulfo groups per repeating unit of PPSU.SPPSU (2S), which has two sulfo groups per repeating unit of PPSU (IEC=3.57meq/g), was obtained by sulfonation of bis(4-fluorophenyl)sulfone using 30% Oleum. Next, polymerization was performed by condensation reaction of sulfonated bis(4-fluorophenyl)sulfone (SFPS) and 4,4'-biphenol (BP). Furthermore, by sulfonating these polymers in sulfuric acid, SPPSU (4S) having four sulfo groups per repeating unit (IEC=5.55meq/g) was synthesized. The molecular weight of SPPSU(4S) was 130,304 – 303,203. Depending on the sulfonation conditions using SPPSU(2S) polymer and sulfuric acid, the viscosity of SPPSU(4S) was 30 to 270 mP·s, which was higher than that of Nafion (D520CS), which was 10 to 40 mPa·s. In addition, the tensile strength, tensile elongation, and flexural modulus of SPPSU (4S) were 22 MPa, 251%, and 296 MPa. Compared to Nafion 212, it exhibited high tensile strength and flexural modulus while having similar tensile elongation properties. Furthermore, the conductivity of SPPSU (4S) was 11.3 mS/cm at 120°C and RH 10%. This value was approximately 4 times higher than the conductivity of Nafion212 (3.2 mS/cm) under the same conditions.References J.D. Kim, A. Donnadio, M. S. Jun, M.L. Di Vona, “Crosslinked SPES-SPPSU membranes for high temperature PEMFCs”, Int. J. Hyd. Ener., 2013, 38, 1517-1523.J.D. Kim, L.-J. Ghil, “Annealing effect of highly sulfonated polyphenylsulfone polymer”, Int. J. Hyd. Ener., 2016, 41, 11794-11800.Y. Zhang, J.D. Kim, K. Miyatake, “Effect of thermal crosslinking on the properties of sulfonated poly(phenylene sulfone)s as proton conductive membranes”, J. Appl. Poly. Sci., 2016, 133(46) 44218-44225.J.D. Kim, A. Ohira, H. Nakao, “Chemically crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes for PEM fuel cells”, membranes, 2020, 10, 31-44.J.D. Kim, A. Ohira, “Crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes for elevated-temperature PEM water electrolysis”, membranes, 2021, 11, 861-873.J.D. Kim, Y. Zhang; Japan patent no. 6548176, EU patent no. EP3340350, US patent no. US1086215.

Transport Properties and Morphology of Cerium Oxide Composite Sulfonated(Poly Arylene Ether Sulfone) Membranes for Application in Heavy-Duty Fuel Cell Vehicles

Fuel cells for Heavy Duty Vehicles (HDVs) offer easy scalability and the potential for critical reduction in CO2 emissions. During the drive cycles of HDVs, the fuel cells will operate over a range of temperatures, with increases while climbing up steep gradients. Operating temperatures above 90°C will be necessary, along with low relative humidity (RH) due to the need to avoid pressurization of gases. Therefore, Proton Exchange Membranes (PEMs) for HDV applications need to be suitable over this range of conditions, including functioning at high temperatures and low relative humidity. Moreover, the membranes should not swell extensively when more water is present.In this study, sulfonated poly (arylene ether sulfone) multiblock copolymers were modified with the incorporation of clusters of modified cerium oxide nanoparticles to form composites. These synthesized novel membranes approach operational proton conductivity requirements for HDV vehicles, with conductivity of 15mS/cm at 120°C and 25% relative humidity, remarkably avoiding the typical large decrease in conductivity observed below 70% RH in non-fluorinated PEMs. One purpose of this study is to understand the water, proton, and polymer interactions within these membranes. The cerium nanoparticles and sulfonamide clusters play a critical role in the membrane, affecting conductivity, and water transport, especially at high temperatures and low relative humidity, while also suppressing swelling of the membrane. Membrane proton conductivity is also strongly influenced by physical properties such as water uptake and dimensional swelling behavior, which is further impacted by polymer morphology. To probe the dynamics of the components, the diffusion coefficient and relaxation time of water in the membrane and the protonic conductivity of the membrane as functions of membrane water content are analyzed using nuclear magnetic resonance (NMR) measurements and water uptake measurements. Film property and morphology changes are evaluated using Transmission Electron Microscopy (TEM) and Small-Angle X-ray Scattering (SAXS). This study demonstrates the advantageous and non-linear benefits of closely packed acid clusters with cerium oxide composites on membrane functionality under HDV operating conditions.

Molecular Engineering of Ion-Conducting Polymer Membranes: Synthesis, Properties, and Applications

Ion-conducting polymers (e.g., proton and hydroxide) are used as polymer electrolyte membranes, and they are a key component of electrochemical energy conversion and storage technologies such as fuel cells, electrolyzers, and flow batteries. For example, hydrogen fuel cell and water electrolyzer industry also heavily relies on Nafion for a proton-conducting membrane for decades, though it is not ideal proton-conducting membrane material for those applications.Perfluoroalkyl substrates (PFASs), which include Nafion, have recently received significant pressure from government and environmental activist groups due to their environmental and health hazard of PFAS. Thus, the development of alternative ion-conducting polymers based on hydrocarbon polymers (mostly made of C and H) has attracted new attention within polymer society. Compared to perfluorosulfonated Nafion, hydrocarbon ion-conducting polymers can offer advantages of flexible synthetic tunability, lower gas permeability, and importantly lower production cost. However, their systematic polymer chemistry–morphology structure–property relationships are poorly understood until now.In this presentation, we describe an overview of recent progress in the development of advanced ion-conducting (H+ and OH–) polymers, key membrane properties, and their state-of-the-art performance in in real world applications of hydrogen fuel cells and water electrolyzers.[1,2] References [1] S. Adhikari, M. K. Pagels, J. Y. Jeon, C. Bae* “Ionomers for Electrochemical Energy Conversion & Storage Technologies”, Polymer (Perspective Special Issue celebrating the 100 years "From ‘Makromolekel' to POLYMER”), Polymer, 2020, 211, 123080.[2] S. Noh, J. Y. Jeon, S. Adhikari, Y. S. Kim, C. Bae* “Molecular Engineering of Hydroxide Ion Conducting Anion Exchange Polymers for Electrochemical Energy Conversion Technology” Acc. Chem. Res. 2019, 52, 2745–2755.

Nanoscale Proton-Conducting Oxide Membranes for Low Temperature Water Electrolysis

Operating water electrolyzers at higher current densities is an attractive approach to improving the economics of hydrogen production from water electrolysis. Conventional proton exchange membrane (PEM) electrolyzers based on Nafion membranes already operate at relatively high current densities (1.5-2.5 A cm-2), but increasing the current density to 5 A cm-2 or higher would improve the economics even further. Currently, a major limitation of operating low-temperature PEM electrolyzers at such high current densities is the large ohmic drop across the Nafion membrane, which can dramatically lower electrolyzer efficiency. To reduce the membrane resistance and enable efficient operation at high current densities, our team has been exploring the use of nanoscopic proton-conducting silicon oxide (SiOx) membranes. Although the proton conductivity of these oxide membranes is lower than Nafion, we show that their total resistance can be made much lower than conventional Nafion-117 membranes by decreasing their thickness to the nanoscale. The use of sub-micron thick membranes is made possible by the high density of SiOx compared to Nafion, which makes SiOx membranes excellent hydrogen (H2) diffusion barriers for preventing H2 crossover. In this work, we show that the area specific membrane resistance of nanoscale SiOx membranes can be reduced to less than 20% that of Nafion-117 membranes while still maintaining desirable H2 blocking capabilities and avoiding problematic electronic leakage current.

Preparation and evaluation of composite membranes using cellulose nanofibers (CNF)

Polymer Electrolyte Fuel Cells (PEFCs) have several advantages, such as high efficiency and compactness, and are suitable for residential and mobile applications. The fluorinated polymers (e.g., Nafion) are typically proton-conducting membranes, which are used as the electrolyte of PEFCs under a wide range of humidity conditions, although the operation under low-humidity, high-temperature conditions decrease the proton conductivity due to water evaporation. Nafion membranes that have been prepared by mixing with transition metal oxide nanoparticles have shown higher proton conductivities, reaching values as high as 0.12 S cm-1 (80 ℃, 80% RH)1, 2. The improvement is based upon the increased hydrophilicity of the surface, which helps to maintain the membrane water content even at elevated temperatures. In the present study, we focus on the highly hydrophilic surface of carboxymethyl cellulose nanofibers (CMCNFs) and have investigated the proton conductivity of the composite membranes. The CMCNFs were mixed with Nafion (20 wt.%, DE2020 CS, Chemours) neutralized with potassium hydroxide, and formed into films by use of a die-coater (Taku-dai Mini-50 Daimon Co., Ltd.), with a final hot-pressing (160 ℃, 3 min. TCMD-2.5: TOHO KOGYO Co.). The proton conductivities of the composite membranes obtained were measured by the AC four-probe method. The types of water (chemically bonded, intermediate, and free water) present in the composite membrane were determined by the use of differential scanning calorimetry (DSC).The composite membranes obtained were flexible and translucent. The proton conductivity of the composite membrane was 1.4 times higher (0.15 S cm-1) than that of a commercial Nafion membrane (NRE211) at 80℃, 80% RH, and was 1.3 times even at 80 ℃, 20% RH. The improved water retention properties achieved by mixing with CMCNF is considered to increase the proton concentration and proton conductivity. DSC measurements showed that bonded water (water that does not freeze down to -55 ℃) remained constant even with variations in the CMCMF content. The intermediate water, with a higher melting point range (melting between -55 ℃ and 0 ℃) increased with increasing CMCMF content. The increase of intermediate water in the composite membrane is considered to contribute to the proton conductivity.

Development of a Composite Electrolyte Membrane with Chain-like Microstructure CeO2-Based Oxide for Achieving High Durability and Performance in Polymer Electrolyte Fuel Cells

Polymer electrolyte fuel cells (PEFCs) are expected to be applied in heavy-duty vehicles (HDVs), in which the operating temperature approaches around 120 ℃. The higher operation temperature, in comparison with that of passenger cars, leads to a significant decrease of the proton conductivity of perfluorosulfonic acid-based electrolyte membranes (PFSA, such as Nafion) under typical atmospheric conditions. Metal oxide surfaces, on which adsorbed water is stable even at temperatures exceeding 100 ℃, have the potential to maintain the water uptake at elevated temperatures, if the oxide is mixed with the PFSA membrane. In our Center, the improvement of proton conductivity over a wide temperature range has been confirmed in composite membranes prepared from Ta-TiO2 and Nafion.1) In the present study, Nafion / CeO2-based oxide composite membranes were fabricated to investigate their proton conductivity and microstructure, as well as to examine the durability of the electrolyte membrane.CeO2-based nanoparticles (CeO2, Nb-CeO2, Zr-CeO2) with chain-like microstructure were prepared by the flame spray pyrolysis method. Each type of nanoparticle was mixed with Nafion dispersion (20 wt.%, DE2020 CS, Chemours) at various ratios, coated and dried by use of a die coater apparatus, and then hot-pressed (140 ℃, 3 min.) to form a membrane. The microstructure was observed by scanning electron microscopy (SEM), small angle X-ray scattering (SAXS), and transmission microscopy (TEM). The proton conductivity was measured by the four-probe alternating current method, and water uptake was measured by a water vapor adsorption method. Additionally, the open circuit voltage of a single cell with the fabricated composite membranes (OCV test) was characterized to confirm the durability of the membrane.The proton conductivity of a composite membrane with 3 wt.% Nb-CeO2 was 1.3 times higher than that of commercial Nafion (NRE211, Chemours) at 80 ℃ and 80% RH, and 1.6 times higher at 80 ℃ and 20% RH. The water content of the composite membranes was independent of the Nb-CeO2 content, and the intensity of a SAXS peak for water clusters (ca. 3 nm) was comparable to that of Nafion under all relative humidity conditions. OCV durability tests using composite membranes with 3 wt.% Zr-CeO2 maintained high OCV values up to 1500 hours of operation, even though unmodified Nafion membranes were unable to maintain high OCV values for more than 250 hours. These results indicate that even small amounts of CeO2-based oxide can improve the proton conductivity and the chemical stability. Acknowledgement This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO). The results of the OCV test were evaluated by the Fuel Cell Cutting-Edge Research Center Technology Research Association (FC-Cubic). Reference (1) Ohno, K. Shudo, T. Tano, and K. Kakinuma, ACS Appl. Energy Mater., 6, 10098-10104 (2023).

The Performance of PEFC with Marimo-like Carbon Considering Relative Humidity

1.IntroductionThe Pt catalyst supported on carbon black (Pt/CB) is widely used as a catalyst for the polymer electrolyte fuel cell (PEFC). As an alternative carbon support to the CB, which is a type of particulate carbon, we have used Marimo-like carbon (MC) which is a type of fibrous carbon. MC has a spherical structure comprised of carbon nanofilaments (CNFs) radially grown from an oxidized diamond core 1). The CNFs in the MC have a cup-stack primary structure of graphene sheets; thus, the sheet edge can effectively function as a supporting site for the Pt catalyst. There are voids of several hundred nanometers between the CNFs of the MC. These voids are expected to improve the diffusion of hydrogen gas and air and the removal of produced water. The diffusivity of hydrogen gas and air and the removal of produced water are different between the MC and CB due to their structural differences. These differences in mass diffusivity changed the state of water management and the PEFC performance. In this study, the performance of a PEFC with Pt/MC while considering the relative humidity (RH) of the hydrogen gas and air were investigated.2.ExperimentalThe catalyst supports were CB and MC. MC was synthesized by thermal chemical vapor deposition (CVD). For the synthesis, methane (CH4) gas was used as the carbon source of the CNFs and Ni supported diamond oxide (Ni/O-dia.) was used as a growth catalyst. The Ni/O-dia. was contacted with the CH4 gas in a reactor rotating at 5 rpm. The CH4 gas was supplied at the flow rate of 1000 CCM for 3 hours. During this time, the rotary reactor was heated to 550°C. Pt/MC was synthesized by reducing Pt ions in a suspension with MC. Pt/CB was prepared as a commercial Pt nanoparticle supported on the CB. Nafion was used as the ionomer in this study. The catalyst ink was prepared by adding a Nafion dispersion to the Pt/MC and Pt/CB. The ionomer/carbon (I/C) ratio was 0.1 and 0.7 for the MC and CB, respectively 3). The catalyst layer (CL) was prepared by spraying the catalyst ink onto a 50 mm × 50 mm PTFE sheet. The membrane electrode assembly (MEA) was fabricated by transferring CL to the Nafion membrane by hot pressing. The PEFC performance was evaluated at the cell temperature of 80.0°C. Hydrogen gas and air were saturated at this temperature using a humidifier. Humidified hydrogen gas was supplied to the anode and humidified air was supplied to the cathode. The relative humidity of the hydrogen gas and air was controlled by the humidifier temperature.3.Results and discussionThe structure of the MC was observed by a scanning electron microscope (SEM). SEM images of the MC showed numerous CNFs. Voids of several hundred nanometers were observed between the CNFs. The Pt supporting state of the Pt/MC was observed by a transmission electron microscope (TEM). TEM images of the Pt/MC showed Pt supported on the surface of the CNFs.The PEFC performances of the Pt/MC and the Pt/CB were compared. At a high current density, the PEFC performance of the Pt/MC was higher than that of the Pt/CB. The PEFC performance in the high current density region consumed a large amount of hydrogen gas and air. This result was due to the high hydrogen gas and air diffusivity and the high removal of produced water from the MC.The relative humidity of the air was controlled from 20 to 100% RH by supplying it to a humidifier at a temperature of 42.7 to 80.0°C. The humidification conditions of the air affected the PEFC performance of both the Pt/MC and the Pt/CB. Under the low humidification condition, the PEFC performance was reduced due to the decrease in proton conductivity of the Nafion membrane. Under the high humidification conditions, the PEFC performance decreased due to flooding. The change in the PEFC performance under these humidifying conditions was particularly high by the MC.1) N. Hirata, et.al., RSC Adv., 11, 39216-39222 (2021)2) M. Eguchi, et.al., Jpn. J. Appl. Phys., 52, 06GD06 (2013)3) M. Eguchi, et.al., polymers, 4, 1645-1656 (2012)

All-Solid-State Rechargeable Air Batteries Using Redox Active Naphthoquinone in the Negative Electrode

Typical air batteries are composed of metallic negative electrode, liquid electrolyte, and gas (oxygen) diffusion positive electrode. The use of metals often causes dendrite growth, which deteriorates the durability of the air batteries similar to the secondary batteries. The liquid electrolyte poses the risk of leakage, limiting application of air batteries to small devices. Recently, air batteries that use organic redox molecules as the negative electrode have been explored. This type of emerging air batteries can be charged/discharged by the redox reactions of the organic molecules associated with protons. We have recently succeeded in developing an all-solid-state rechargeable air batteries (SSAB) combining a redox-active organic molecule as negative electrodes and a proton-conductive polymer membrane as solid electrolyte. We initially focused on 2,5-dihydroxy-1,4-benzoquinone (DHBQ) as the redox-active molecule and commercial Nafion as the solid electrolyte1 (Fig. 1a and b) to fabricate an SSAB. For improving the capacity and cyclability, polymeric DHBQ was also used. In the present study, the principle of SSAB was further verified with other redox-active organic molecules. 1,4-Naphthoquinone (NQ) was used for the negative electrode, which is known to undergo reversible redox reaction via two electrons / two protons transfer process (Fig. 2a). Nafion or a hydrocarbon-based polymer electrolyte membrane (SPP-QP)2 developed at the University of Yamanashi was used as the solid electrolyte (Fig. 2b). Pt/CB (TEC10E50E, Tanaka Kikinzoku) or Pt/GCB catalyst (TEC10EA50E, Tanaka Kikinzoku) was used for the oxygen electrode. NQ, carbon powder (Ketjen black, Tanaka Kikinzoku) and Nafion were used for the inks to be used for the negative electrode. The ink was sprayed onto one side of the electrolyte membrane. The ink for the oxygen electrode composed of Pt/CB or Pt/GCB and Nafion. was similarly sprayed onto the other side of the membrame to obtain catalyst-coated membrane (CCM). SSAB was tested at a cell temperature of 40 °C and 100% RH. Cyclic voltammograms (CV) were obtained at a scan rate of 20 mV s-1 and all charge-discharge cycling tests were performed at 15 C rate. The redox potential of the negative electrode observed in the CV measurements was 0.47 V (vs RHE) for DHBQ and 0.34 V (vs RHE) for NQ, indicating that NQ with a lower redox potential could provide higher electromotive force. Fig. 3 shows the charge-discharge curves of SSABs, in which DHBQ or NQ was used as the negative electrode. The SSAB-DHBQ had an open circuit voltage of 0.80 V, a discharge capacity of 73 mAh g-1, and a Coulomb efficiency of 58%, while the SSAB-NQ had an open circuit voltage of 0.88 V, a discharge capacity of 78 mAh g-1, and a Coulomb efficiency of 85%. Replacing Nafion with the SPP-QP membrane resulted in improved Coulomb efficiency (98%) since SPP-QP was much less permeable than Nafion and thus suppressed oxygen permeation to the negative electrode. Acknowledgement This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, through Grants-in-Aid for Scientific Research (KAKENHI 23H02058). References 1) M. Yonenaga, Y. Kaiwa, K. Oka, K. Oyaizu, K. Miyatake, Angew. Chem. Int. Ed., 62, e202304366(2023).2) J. Miyake, R. Taki, T. Mochizuki, R. Shimizu, R. Akiyama, M. Uchida, K. Miyatake, Sci. Adv., 3, eaao0476 (2017). Figure 1

Novel Mesoporous Carbon Supporting PGM As an Effective ORR Catalyst in PEM Fuel Cells

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen into electrical energy through a proton-conducting polymer membrane. These cells are highly valued in compact applications such as electric vehicles and portable power systems for their high power density and low operating temperatures. The catalyst support plays a crucial role, not only anchoring the catalyst nanoparticles but also enhancing Oxygen Reduction Reaction (ORR) activity through heteroatom doping, facilitating mass transfer via microchannels, and improving conductivity.1 Mesoporous carbon, merging electrochemical and high surface area properties, serves as an ideal catalyst support. Its mesoscopic structures enable high dispersion of electrocatalysts, and although mesopores contribute relatively less to surface area compared to micropores, they are more effective for mass transport. 2-3 We employ Metal Organic Frameworks (MOFs)-derived porous carbon, synthesized through pyrolysis, to provide structurally optimized mesoporous support that enhances surface area and electrochemical stability. MIL-101 is a highly porous, crystalline MOF known for its exceptional surface area and potential in gas storage, separation, and catalysis.4 Our research details the production of a series of mesoporous carbon C-X-MIL-101 (X = NH2, CH3, NO2, OMe) through carbonization of X-MIL-101 MOFs at 1100 °C followed by acid etching to remove residual aluminum oxides, resulting in high-surface-area, controlled-mesoporosity carbon materials. Platinum nanoparticles are subsequently embedded using a solvent impregnation method to produce Pt@C-X-MIL-101, optimizing platinum use and preventing surface deposition. The resulting Pt@C-X-MIL-101 catalysts have been tested in fuel cell and comprehensive findings will be presented at the conference. Zaman, S., Wang, M., Liu, H., Sun, F., Yu, Y., Shui, J., ... & Wang, H. (2022). Trends in Chemistry, 4(10), 886-906.Chang, H., Joo, S. H., & Pak, C. (2007). Journal of Materials Chemistry, 17(30), 3078-3088.Xu, J. B., & Zhao, T. S. (2013). RSC Advances, 3(1), 16-24.Zorainy, M. Y., Alalm, M. G., Kaliaguine, S., & Boffito, D. C. (2021). Journal of Materials Chemistry A, 9(39), 22159-22217.

Development of High Proton Conductive Membrane Using the Nano Cellulose Fibers for Polymer Electrolyte Fuel Cells

Recently, woody biomass has been used in multiple ways as a carbon-neutral and renewable resource. Among them, cellulose nanofibers (CNFs) are attracting attention as a new nanomaterial derived from cellulose, which is one of the major components of trees. We established a unique CNF production method by introducing phosphate groups to the hydroxyl groups in the wood pulp. The obtained phosphorylated CNFs were completely nanofibrillated (approximately 3 nm in width) with high yield, and their aqueous dispersion was highly transparent, viscous, and stable at pH 3−11. An aqueous dispersion of phosphorylated CNFs can be dehydrated and dried to form transparent CNFs sheet with densely-packed CNFs. This sheet has high transparency, strength, and thermal dimensional stability, and at the same time, it has paper-like flexibility. To promote the practical use of this technology, we have currently been operating a demonstration plant for the production of CNFs aqueous dispersion and CNFs sheet.By using CNF, which have excellent water retention and film-forming properties, we developed a nanocellulose PEM that is weakly acidic, has low environmental impact, and has high proton conductivity. The PEM using cellulose nanofibers showed a high proton conductivity of >7×10-2Scm-1 under 95% RH at 80 ℃. By combining a PVPA filler in a CNF suspension, it has become possible to easily create a self-supporting membrane using CNF as a binder. This composite membrane showed an especially high proton conductivity of 9×10-2 Scm-1 under 95% RH at 80 ℃. These results suggest that CNF presents a high potential substitute for perfluorosulfonic acid polymers and serves as a pivotal component of environmentally sustainable PEMs.

Nature Derived Gas Diffusion Layers for Proton Exchange Membrane Fuel Cells

The adverse effects of global warming have accelerated the push for sustainable energy sources to meet the global energy demand. Renewable energy can be stored in the form of hydrogen to continuously produce clean power using a proton exchange membrane fuel cell (PEMFC). A typical PEMFC consists of a proton conductive membrane coated with platinum-based catalyst layers, which promote the hydrogen oxidation (anode) and oxygen reduction (cathode) reactions. Moreover, these catalyst layers are supported by gas diffusion layers, which supply reactants and facilitate the removal of water generated from the reduction reaction. While membrane hydration is necessary for maintaining protonic conductivity, excess water in the gas diffusion layer can severely inhibit the chemical reaction. Thus, efficient water management not only enhances reactant delivery but also substantially reduces the required platinum catalyst content in PEMFCs to ensure cost-effective power generation.Interestingly, the simultaneous transport of water and oxygen is precisely optimized in trees, which are all around us. Thus, the hypothesis is that exceptional mass transport characteristics can be obtained using the organized microchannels of a novel nature-based basswood gas diffusion layer. First, basswood was carbonized to establish high electrical conductivities, and this was achieved while preserving their microstructure. Next, the microstructure of the carbonized basswood blocks was characterized using scanning electron microscopy. Finally,electrochemical testing of the carbonized basswood gas diffusion layer was performed to compare its performance with a conventional carbon paper gas diffusion layer. The mass transport behavior was characterized using electrochemical impedance spectroscopy. Through the application of our novel basswood gas diffusion layer, we provide new insights into the potential for high performance wood-based materials for clean energy, guiding the design of next generation sustainable fuel cell materials.

Nanoscale in-Situ Effects of Humidity on Proton Exchange Membrane Fuel Cell Catalyst Layers Using Scanning Transmission X-Ray Microscopy

Polymer electrolyte membrane (PEM) fuel cells produce clean energy using hydrogen and oxygen with water and heat as the only byproducts. However, their widespread adoption is still hindered by high cost and low efficiency. PEM fuel cells have a proton exchange membrane (ionomer) supported by platinum (Pt) catalyst layers to facilitate the hydrogen oxidation (anode) and the oxygen reduction (cathode) reactions. While sufficient membrane hydration is necessary to maintain proton conductivity, excess water in the catalyst layer can severely inhibit electrochemical reactions, limiting efficient fuel cell operation.To design high performance catalyst layer nanostructures, the formation of liquid water and the ionomer swelling with humidity in PEM fuel cell catalyst layers need to be investigated. In-situ hard X-ray techniques have been used to study the incipience of water; however, the high energy hard X-rays prevent spectroscopy of the low-Z membrane elements like carbon, fluorine, oxygen, and sulfur. Soft X-ray scanning transmission X-ray microscopy (STXM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy can uniquely perform high resolution (30 nm) imaging of the chemical constituents of catalyst layers with minimal radiation damage. In this work, we report a novel in-situ characterization of ionomer swelling with humidity in fuel cell catalyst layers using STXM and NEXAFS spectroscopy to reveal the nanoscale reactant and proton transport pathways in PEM fuel cell catalyst layers. Pristine catalyst coated membrane samples were cut using an ultramicrotome to obtain 300 nm thick sections and mounted in a custom cell to observe the in-situ effects of humidity. The thickness of liquid water in the membrane and catalyst layer as a function of relative humidity was obtained by performing in-situ imaging at the oxygen K-edge. We observed that at 55% relative humidity (RH), a through-plane water accumulation of 10-20 nm on the silica nanoparticles maintained the membrane proton conductivity and at 90% relative humidity, the hydration of the sulfonic acid sidechains of Nafion promoted nanoscale proton transport in the catalyst layers. Moreover, in-situ characterization of ionomer swelling with humidity was performed for the first time using NEXAFS at the fluorine K-edge. The ionomer volume fraction increased from 28% to 36% from dry (~6% RH) to humidified conditions (~90% RH) and resulted in higher nanoscale oxygen and water diffusion resistance. From these analyses, the effects of hydration on nanoscale mass transport and proton conductivity were quantified to inform the design of next-generation proton exchange membrane fuel cells. Figure 1