Oxygen Plasma-Mediated Microstructured Hydrocarbon Membrane for Improving Interface Adhesion and Mass Transport in Polymer Electrolyte Fuel Cells.

Developing a method for fabricating high-efficient and low-cost fuel cells is imperative for commercializing polymer electrolyte membrane (PEM) fuel cells (FCs). This study introduces a mechanical and chemical modification technique using the oxygen plasma irradiation process for hydrocarbon-based (HC) PEM. The oxygen functional groups were introduced on the HC-PEM surface through the plasma process in the controlled area, and microsized structures were formed. The modified membrane was incorporated with plasma-treated electrodes, improving the adhesive force between the HC-PEM and the electrode. The decal transfer was enabled at low temperatures and pressures, and the interfacial resistance in the membrane-electrode assembly (MEA) was reduced. Furthermore, the micropillar structured electrode configuration significantly reduced the oxygen transport resistance in the MEA. Various diagnostic techniques were conducted to find out the effects of the membrane surface modification, interface adhesion, and mass transport, such as physical characterizations, mechanical stress tests, and diverse electrochemical measurements.

Nafion Composite Membranes Impregnated with Polydopamine and Poly(Sulfonated Dopamine) for High-Performance Proton Exchange Membranes.

We prepared Nafion composite membranes by impregnating Nafion-212 with polydopamine, poly(sulfonated dopamine), and poly(dopamine-co-sulfonated dopamine) using the swelling-filling method to generate nanopores in the Nafion framework that were filled with these polymers. Compared to the pristine Nafion-212 membrane, these composite membranes showed improved thermal and mechanical stabilities due to the strong interactions between the catecholamine of the polydopamine derivatives and the Nafion matrix. For the composite membrane filled with poly(sulfonated dopamine) (N-PSDA), further interactions were induced between the Nafion and the sulfonic acid side chain, resulting in enhanced water uptake and ion conductivity. In addition, filling the nanopores in the Nafion matrix with polymer fillers containing aromatic hydrocarbon-based dopamine units led to an increase in the degree of crystallinity and resulted in a significant decrease in the hydrogen permeability of the composite membranes compared to Nafion-212. Hydrogen crossovers 26.8% lower than Nafion-212 at 95% relative humidity (RH) (fuel cell operating conditions) and 27.3% lower at 100% RH (water electrolysis operating conditions) were obtained. When applied to proton exchange membrane-based fuel cells, N-PSDA exhibited a peak power density of 966 mW cm-2, whereas N-PSDA showed a current density of 4785 mA cm-2, which is 12.4% higher than Nafion-212 at 2.0 V and 80 °C.

Multiscale simulation approach to investigate the binder distribution in catalyst layers of high-temperature polymer electrolyte membrane fuel cells.

A multiscale approach involving both density functional theory (DFT) and molecular dynamics (MD) simulations was used to deduce an appropriate binder for Pt/C in the catalyst layers of high-temperature polymer electrolyte membrane fuel cells. The DFT calculations showed that the sulfonic acid (SO3-) group has higher adsorption energy than the other functional groups of the binders, as indicated by its normalized adsorption area on Pt (- 0.1078 eV/Å2) and carbon (- 0.0608 eV/Å2) surfaces. Consequently, MD simulations were performed with Nafion binders as well as polytetrafluoroethylene (PTFE) binders at binder contents ranging from 14.2 to 25.0 wt% on a Pt/C model with H3PO4 at room temperature (298.15 K) and operating temperature (433.15 K). The pair correlation function analysis showed that the intensity of phosphorus atoms in phosphoric acid around Pt ([Formula: see text]) increased with increasing temperature because of the greater mobility and miscibility of H3PO4 at 433.15 K than at 298.15 K. The coordination numbers (CNs) of Pt-P(H3PO4) gradually decreased with increasing ratio of the Nafion binders until the Nafion binder ratio reached 50%, indicating that the adsorption of H3PO4 onto the Pt surface decreased because of the high adsorption energy of SO3- groups with Pt. However, the CNs of Pt-P(H3PO4) gradually increased when the Nafion binder ratio was greater than 50% because excess Nafion binder agglomerated with itself via its SO3- groups. Surface coverage analysis showed that the carbon surface coverage by H3PO4 decreased as the overall binder content was increased to 20.0 wt% at both 298.15 and 433.15 K. The Pt surface coverage by H3PO4 at 433.15 K reached its lowest value when the PTFE and Nafion binders were present in equal ratios and at an overall binder content of 25.0 wt%. At the Pt (lower part) surface covered by H3PO4 at 433.15 K, an overall binder content of at least 20.0 wt% and equal proportions of PTFE and Nafion binder are needed to minimize H3PO4 contact with the Pt.

Single-Step Fabrication of Polymeric Composite Membrane via Centrifugal Colloidal Casting for Fuel Cell Applications.

Recent interest in polymer electrolyte membranes (PEMs) for fuel cell systems has spurred the development of infiltration technology by which to insert ionomers into mechanically robust reinforcement structures by solution casting in order to produce a cost effective and highly efficient electrolyte. However, the results of the fabrication process often continue to present challenges related to the structural complexity and self-assembly dynamics between the hydrophobic and hydrophilic parts of the constituents which in turn, necessitates additional processing steps and increases production costs. Here, a single-step process is reported for highly compact polymeric composite membranes (PCMs), fabricated using a centrifugal colloidal casting (C3) method. Combined structural analyses as well as coarse-grained molecular dynamics simulations are employed to determine the micro-/macroscopic structural characteristics of the fabricated PCMs. These findings indicate that the C3 method is capable of forming highly dense ionomer matrix-reinforcement composites consisting of microphase-separated ionomer structures with tailored crystallinity and ionic cluster sizes. An outcome that is very unlikely with the single-step coating steps in conventional methods. These structural attributes ensure PCMs with better proton conductivity, greater strain stability, and lower gas crossover properties compared to commercial pristine membranes, expanding their possible range of applicability to PEMs.

Nafion-Based Proton-Exchange Membranes Built on Cross-Linked Semi-Interpenetrating Polymer Networks between Poly(acrylic acid) and Poly(vinyl alcohol).

We report semi-interpenetrating polymer network (semi-IPN) membranes prepared easily from a cross-linked network using poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) with interpenetrated Nafion for both proton-exchange membrane fuel cell (PEMFC) and proton-exchange membrane water electrolyzer (PEMWE) applications. Thermal esterification between PAA and PVA induced three-dimensional cross-linking to improve mechanical toughness and reduce hydrogen crossover, while the hydrophilic nature of the PAA-PVA-based cross-linked matrix still enhanced the water uptake (WU) and hence conductivity of the Nafion penetrant. The semi-IPN membrane (NPP-95) composed of Nafion, PAA, and PVA with a ratio of 95:2.5:2.5 showed a hexagonal cylindrical morphology and improved thermal, mechanical, and dimensional stability compared to a recast Nafion membrane (re-Nafion). The membrane was also highly effective at managing water due to its low WU and high conductivity. Furthermore, its hydrogen permeability was 49.6% lower than that of re-Nafion under the actual fuel cell operating conditions (at 100% RH and 80 °C). NPP-95 exhibited significantly improved conductivity and PEMFC performance compared to re-Nafion with a current density of 1561 mA/cm2 at a potential of 0.6 V and a peak power density of 1179 mW/cm2. Furthermore, in the PEMWE performances, NPP-95 displayed about a 1.5-fold higher current density of 4310 mA/cm2 at 2.0 V and much lower ohmic resistance than re-Nafion between 60 and 80 °C.

Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency.

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoOx@CoNy) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoOx@CoNy nanorods, composed of metallic Co4N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm-2) in an alkaline medium. Although pristine NCNF or CoOx@CoNy alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoOx@CoNy nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoOx@CoNy nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoOx@CoNy/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.

Synthesis of Sulfonated Poly(Arylene Ether Sulfone)s Containing Aliphatic Moieties for Effective Membrane Electrode Assembly Fabrication by Low-Temperature Decal Transfer Methods.

The purpose of this study was to investigate the effect of the aliphatic moiety in the sulfonated poly(arylene ether sulfone) (SPAES) backbone. A new monomer (4,4'-dihydroxy-1,6-diphenoxyhexane) was synthesized and polymerized with other monomers to obtain partially alkylated SPAESs. According to differential scanning calorimetry analysis, the glass transition temperature (Tg) of these polymers ranged from 85 to 90 °C, which is 100 °C lower than that of the fully aromatic SPAES. Due to the low Tg values obtained for the partially alkylated SPAESs, it was possible to prepare a hydrocarbon electrolyte membrane-based membrane electrode assembly (MEA) with Nafion® binder in the electrode through the use of a decal transfer method, which is the most commercially suitable system to obtain an MEA of proton exchange membrane fuel cells (PEMFCs). A single cell prepared using this partially alkylated SPAES as an electrolyte membrane exhibited a peak power density of 539 mW cm-2.

Anion Exchange Composite Membranes Composed of Quaternary Ammonium-Functionalized Poly(2,6-dimethyl-1,4-phenylene oxide) and Silica for Fuel Cell Application.

Anion exchange membranes (AEMs) with good alkaline stability and ion conductivity are fabricated by incorporating quaternary ammonium-modified silica into quaternary ammonium-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO). Quaternary ammonium with a long alkyl chain is chemically grafted to the silica in situ during synthesis. Glycidyltrimethylammoniumchloride functionalization on silica (QSiO2) is characterized by Fourier transform infrared and transmission electron microscopic techniques. The QPPO/QSiO2 membrane having an ion exchange capacity of 3.21 meq·g-1 exhibits the maximum hydration number (λ = 11.15) and highest hydroxide ion conductivity of 45.08 × 10-2 S cm-1 at 80 °C. In addition to the high ion conductivity, AEMs also exhibit good alkaline stability, and the conductivity retention of the QPPO/QSiO2-3 membrane after 1200 h of exposure in 1 M potassium hydroxide at room temperature is about 91% ascribed to the steric hindrance offered by the grafted long glycidyl trimethylammonium chain in QSiO2. The application of the QPPO/QSiO2-3 membrane to an alkaline fuel cell can yield a peak power density of 142 mW cm-2 at a current density of 323 mA cm-2 and 0.44 V, which is higher than those of commercially available FAA-3-50 Fumatech AEM (OCV: 0.91 V; maximum power density: 114 mW cm-2 at current density: 266 mA cm-2 and 0.43 V). These membranes provide valuable insights on future directions for advanced AEM development for fuel cells.

Polystyrene-Based Hydroxide-Ion-Conducting Ionomer: Binder Characteristics and Performance in Anion-Exchange Membrane Fuel Cells.

Polystyrene-based polymers with variable molecular weights are prepared by radical polymerization of styrene. Polystyrene is grafted with bromo-alkyl chains of different lengths through Friedel-Crafts acylation and quaternized to afford a series of hydroxide-ion-conducting ionomers for the catalyst binder for the membrane electrode assembly in anion-exchange membrane fuel cells (AEMFCs). Structural analyses reveal that the molecular weight of the polystyrene backbone ranges from 10,000 to 63,000 g mol-1, while the ion exchange capacity of quaternary-ammonium-group-bearing ionomers ranges from 1.44 to 1.74 mmol g-1. The performance of AEMFCs constructed using the prepared electrode ionomers is affected by several ionomer properties, and a maximal power density of 407 mW cm-2 and a durability exceeding that of a reference cell with a commercially available ionomer are achieved under optimal conditions. Thus, the developed approach is concluded to be well suited for the fabrication of next-generation electrode ionomers for high-performance AEMFCs.

Membrane/Electrode Interface Design for Effective Water Management in Alkaline Membrane Fuel Cells.

The recent development of ultrathin anion exchange membranes and optimization of their operating conditions have significantly enhanced the performance of alkaline-membrane fuel cells (AMFCs); however, the effects of the membrane/electrode interface structure on the AMFC performance have not been seriously investigated thus far. Herein, we report on a high-performance AMFC system with a membrane/electrode interface of novel design. Commercially available membranes are modified in the form of well-aligned line arrays of both the anode and cathode sides by means of a solvent-assisted molding technique and sandwich-like assembly of the membrane and polydimethylsiloxane molds. Upon incorporating the patterned membranes into a single-cell system, we observe a significantly enhanced performance of up to ∼35% compared with that of the reference membrane. The enlarged interface area and reduced membrane thickness from the line-patterned membrane/electrode interface result in improved water management, reduced ohmic resistance, and effective utilization of the catalyst. We believe that our findings can significantly contribute further advancements in AMFCs.

Synthetic approaches for advanced multi-block anion exchange membranes.

Despite our ability to post-functionalize poly(arylene ether sulfone) multi-block copolymers by rapid chloromethylation, bromination, or acylation, with degrees of functionalization that exceeded 70% in a few hours, materials formed during attempts to prepare fully post-functionalized multi-block copolymers are poorly soluble due to undesired side reactions, such as crosslinking or di-bromination. In particular, clustered reactive sites in multi-block copolymers increase the chance of self-reactions between polymer backbones, resulting in the formation of by-products. On the other hand, the authentic multi-block copolymer with good solubility and high molecular weight was successfully synthesized using functionalized monomers. Despite its low ion-exchange capacity, the resulting multi-block copolymer outperformed the commercial FAA-3-30 membrane in terms of anion conductivity, even under low relative humidity conditions.

Poly(2,6-dimethyl-1,4-phenylene oxide)s with Various Head Groups: Effect of Head Groups on the Properties of Anion Exchange Membranes.

Poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs)-based anion exchange membranes (AEMs) with four of the most widely investigated head groups were prepared. Through a combination of experimental and simulation approaches, the effects of the different types of head groups on the properties of the AEMs, including hydroxide conductivity, water content, physicochemical stability, and fuel cell device performance were fully explored. Unlike other studies, in which the conductivity was mostly investigated in liquid water, the conductivity of the PPO-based AEMs in 95% relative humidity (RH) conditions as well as in liquid water was investigated. The conductivity trend in 95% RH condition was different from that in liquid water but corresponded well with the actual cell performance trend observed, suggesting that the AEM fuel cell performance under in situ cell conditions (95% RH, 60 °C, H2/O2) is more closely related to the conductivity measured ex situ under 95% RH conditions (60 °C) than in liquid water. On the basis of the conductivity data and molecular simulation results, it was concluded that the predominant hydroxide ion-conducting mechanism in liquid water differs from that in the operating fuel cell environment, where the ionomers become hydrated only as a result of water vapor transported into the cells.

Nanocomposite Membranes Comprising Crosslinked Polymer Blends of Poly(vinyl alcohol)/Poly(styrene sulfonic acid-co-maleic acid) and Fumed Silica Nanoparticles.

Nanocomposite polymer electrolyte membranes comprising a crosslinked polymer blend of poly(vinyl alcohol)/poly(styrene sulfonic acid-co-maleic acid) (PVA/PSSA-co-MA) and fumed silica nanoparticles were prepared for direct methanol fuel cell (DMFC) applications. Silica nanoparticles could be incorporated well uniformly in the completely miscible system, which can form a three-dimensional network structure to achieve the enhancement of mechanical properties as well as the additional reduction of methanol permeability. The optimized proton conductivities and methanol permeability of the PVA/PSSA-co-MA membrane with silica nanoparticles of 10 wt.% were 0.0482 S cm-1 at room temperature and 5.78 × 10-7 cm2 s-1 at the methanol concentration of 40% (w/w), respectively.

Transition metal alloying effect on the phosphoric acid adsorption strength of Pt nanoparticles: an experimental and density functional theory study.

The effect of alloying with transition metals (Ni, Co, Fe) on the adsorption strength of phosphoric acid on Pt alloy surfaces was investigated using electrochemical analysis and first-principles calculations. Cyclic voltammograms of carbon-supported Pt3M/C (M = Ni, Co, and Fe) electrocatalysts in 0.1 M HClO4 with and without 0.01 M H3PO4 revealed that the phosphoric acid adsorption charge density near the onset potential on the nanoparticle surfaces was decreased by alloying with transition metals in the order Co, Fe, Ni. First-principles calculations based on density functional theory confirmed that the adsorption strength of phosphoric acid was weakened by alloying with transition metals, in the same order as that observed in the electrochemical analysis. The simulation suggested that the weaker phosphoric acid adsorption can be attributed to a lowered density of states near the Fermi level due to alloying with transition metals.

Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell.

The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm(-2) and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm(-2) and an outer layer of 0.1 mg Pt cm(-2) exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm(-2).

Highly efficient and durable TiN nanofiber electrocatalyst supports.

To date, carbon-based materials including various carbon nanostructured materials have been extensively used as an electrocatalyst support for proton exchange membrane fuel cell (PEMFC) applications due to their practical nature. However, carbon dissolution or corrosion caused by high electrode potential in the presence of O2 and/or water has been identified as one of the main failure modes for the device operation. Here, we report the first TiN nanofiber (TNF)-based nonwoven structured materials to be constructed via electrospinning and subsequent two-step thermal treatment processes as a support for the PEMFC catalyst. Pt catalyst nanoparticles (NPs) deposited on the TNFs (Pt/TNFs) were electrochemically characterized with respect to oxygen reduction reaction (ORR) activity and durability in an acidic medium. From the electrochemical tests, the TNF-supported Pt catalyst was better and more stable in terms of its catalytic performance compared to a commercially available carbon-supported Pt catalyst. For example, the initial oxygen reduction performance was comparable for both cases, while the Pt/TNF showed much higher durability from an accelerated degradation test (ADT) configuration. It is understood that the improved catalytic roles of TNFs on the supported Pt NPs for ORR are due to the high electrical conductivity arising from the extended connectivity, high inertness to the electrochemical environment and strong catalyst-support interactions.

Green synthesis of carbon-supported nanoparticle catalysts by physical vapor deposition on soluble powder substrates.

Metal and metal oxide nanoparticles (NPs) supported on high surface area carbon (NP/Cs) were prepared by the physical vapor deposition of bulk materials on an α-D-glucose (Glu) substrate, followed by the deposition of the NPs on carbon supports. Using Glu as a carrier for the transport of NPs from the bulk materials to the carbon support surfaces, ultrafine NPs were obtained, exhibiting a stabilizing effect through OH moieties on the Glu surfaces. This stabilizing effect was strong enough to stabilize the NPs, but weak enough to not significantly block the metal surfaces. As only the target materials and Glu are required in our procedure, it can be considered environmentally friendly, with the NPs being devoid of hazardous chemicals. Furthermore, the resulting NP/Cs exhibited an improvement in activity for various electrochemical reactions, mainly attributed to their high surface area.

A highly active and durable Co-N-C electrocatalyst synthesized using exfoliated graphitic carbon nitride nanosheets.

Exfoliated graphitic carbon nitride nanosheets (g-C3N4-NS) were applied for the first time for the preparation of an electrocatalyst for the oxygen reduction reaction (ORR). A less dense structure with increased surface area was observed for g-C3N4-NS compared to bulk g-C3N4 from detailed analyses including TEM, STEM, AFM with depth profiling, XRD, and UV-Vis spectroscopy. The pyrolysis of the prepared g-C3N4-NS with Co and carbon under an inert environment provided an enhanced accessibility to the N functionalities required for efficient interaction of Co and C with N for the formation of Co-N-C networks and produced a hollow and interconnected Co-N-C-NS structure responsible for high durability. The Co-N-C-NS electrocatalyst exhibited superior catalytic activity and durability and further displayed fast and selective four electron transfer kinetics for the ORR, as evidenced by various electrochemical experiments. The hollow, interconnected structure of Co-N-C-NS with increased pyridinic and graphitic N species has been proposed to play a key role in facilitating the desired ORR reaction.

Effect of gold subsurface layer on the surface activity and segregation in Pt/Au/Pt3M (where M = 3d transition metals) alloy catalyst from first-principles.

The effect of a subsurface hetero layer (thin gold) on the activity and stability of Pt skin surface in Pt3M system (M = 3d transition metals) is investigated using the spin-polarized density functional theory calculation. First, we find that the heterometallic interaction between the Pt skin surface and the gold subsurface in Pt/Au/Pt3M system can significantly modify the electronic structure of the Pt skin surface. In particular, the local density of states projected onto the d states of Pt skin surface near the Fermi level is drastically decreased compared to the Pt/Pt/Pt3M case, leading to the reduction of the oxygen binding strength of the Pt skin surface. This modification is related to the increase of surface charge polarization of outmost Pt skin atoms by the electron transfer from the gold subsurface atoms. Furthermore, a subsurface gold layer is found to cast the energetic barrier to the segregation loss of metal atoms from the bulk (inside) region, which can enhance the durability of Pt3M based catalytic system in oxygen reduction condition at fuel cell devices. This study highlights that a gold subsurface hetero layer can provide an additional mean to tune the surface activity toward oxygen species and in turn the oxygen reduction reaction, where the utilization of geometric strain already reaches its practical limit.