Shanfu Lu

Layer-by-layer self-assembly in the development of electrochemical energy conversion and storage devices from fuel cells to supercapacitors

As one of the most effective synthesis tools, layer-by-layer (LbL) self-assembly technology can provide a strong non-covalent integration and accurate assembly between homo- or hetero-phase compounds or oppositely charged polyelectrolytes, resulting in highly-ordered nanoscale structures or patterns with excellent functionalities and activities. It has been widely used in the developments of novel materials and nanostructures or patterns from nanotechnologies to medical fields. However, the application of LbL self-assembly in the development of highly efficient electrocatalysts, specific functionalized membranes for proton exchange membrane fuel cells (PEMFCs) and electrode materials for supercapacitors is a relatively new phenomenon. In this review, the application of LbL self-assembly in the development and synthesis of key materials of PEMFCs including polyelectrolyte multilayered proton-exchange membranes, methanol-blocking Nafion membranes, highly uniform and efficient Pt-based electrocatalysts, self-assembled polyelectrolyte functionalized carbon nanotubes (CNTs) and graphenes will be reviewed. The application of LbL self-assembly for the development of multilayer nanostructured materials for use in electrochemical supercapacitors will also be reviewed and discussed (250 references).

HPW/MCM‐41 Phosphotungstic Acid/Mesoporous Silica Composites as Novel Proton‐Exchange Membranes for Elevated‐Temperature Fuel Cells

Proton-exchange membrane and direct methanol fuel cells (PEMFCs and DMFCs) have attracted much attention as clean energy sources for various applications, such as electric vehicles, portable electronics, and domestic power generation, because of their high power density, high efficiency, and low greenhouse gas emission. Especially, the operation of PEMFCs and DMFCs at temperatures above 100 8C is considered to have many advantages, such as the elimination of CO poisoning of the platinum electrocatalyst, faster electrode reaction kinetics, simplified water and heat management, higher energy efficiency, and reduced usage of precious Pt and Pt alloy catalyst. However, the state-of-the-art proton-exchange membranes (PEMs) based on perfluorosulfonic acid (PFSA), such as Nafion, are unstable at elevated temperatures ( 100 8C) and proton conductivity decreases significantly due to the loss of water from the membrane under conditions of high temperatures or low humidities. Therefore, development of PEMs with high proton conductivity and stability at elevated temperatures is a major challenge. Great efforts have been dedicated to developing PEMs for operation at elevated temperatures based on mesoporous or nanoporous inorganic materials. Mesoporous inorganic materials have a pore size range of 2–50 nm and are characterized by high specific surface area, nanometer-sized channels or frameworks with an ordered or disordered interconnected internal structure, and high structural stability, which make feasible potential applications as proton-exchange membranes operating at elevated temperatures. Lu and co-workers reported sol–gel-derived mesostructured zirconium phosphates with proton conductivities of about 10 –10 6 S cm . Colomer et al. synthesized nanoporous anatase thin films with conductivity values from 10 5 to 10 3 S cm 1 in the range of 33%–81% relative humidity (RH) at room temperature. Li and Nogami prepared proton-conducting mesoporous silica films with conductivity ranging from 10 6 to 10 4 S cm 1 under 40%–90% humidity. However, the proton conductivity of the pure mesoporous materials depends significantly on their textural characteristics. For instance, Colomer et al. also reported a proton conductivity of 2.0 10 2 S cm 1 on mesoporous acid-free silica xerogels and 3.78 10 2 S cm 1 on a nanoporous anatase thin film at 80 8C and 81% RH. Yamada et al. reported a TiO2-P2O5 mesoporous nanocomposite with a proton conductivity value of 2 10 2 S cm 1 at 160 8C. Halla et al. synthesized meso-SiO2-C12EO10OH-CF3SO3H as a new protonconducting electrolyte and reported a conductivity of 1 10 3 S cm 1 at room temperature and 90% RH. Although the conductivity values of mesoporous acid-free silica xerogels, meso-SiO2-C12EO10OHCF3SO3H or anatase thin films are adequate for fuel cell applications, their performance as an electrolyte in a PEMFC has not been evaluated yet. Yamada and Honma synthesized a H3PW12O40 (abbreviated as HPW) and polystyrene sulfonic acid (PSS) composite by self-assembly of –SO3H onto the HPW surface, achieving a proton conductivity of 1 10 2 S cm 1 at 180 8C. Nevertheless, the power density of the cell based on a PSS with 10wt% HPW composite membrane is very low, ca. 3mW cm 2 at 160 8C in H2/O2 with no external humidity. Uma and Nogami synthesized an inorganic glass composite membrane consisting of a mixture of phosphotungstic acid (HPW) and phosphomolybdic acid (HPM), and reported very high conductivity values, 1.014 S cm 1 at 30 8C and 85% RH for a mesoporous-structured HPW/HPM-P2O5-SiO2 glass, [24] and 1.01 10 1 S cm 1 at 85 8C under 85% RH for a mesoporousstructured HPW-P2O5-SiO2 glass. [17] The cell performance based on these inorganic PEMs was 35–42mW cm 2 in H2/O2 at ca. 30 8C under 30% RH. However, there is little information on the performance of HPWand HPM-incorporated P2O5-SiO2 glass electrolyte cells at elevated high temperatures or in methanol fuels. Here, we present a novel inorganic PEM based on highly ordered mesoporous MCM-41 silica with assembled HPW nanoparticles by the vacuum-assisted impregnation method (VIM). The proton conductivity of the HPW/MCM-41 mesoporous silica inorganic PEM is 0.018 and 0.045 S cm 1 at 25 and 150 8C, respectively. Most significantly, the PEMFCs based on the HPW/MCM-41 mesoporous-silica membrane showed a very impressive performance, achieving a maximum power density of 95mWcm 2 in H2/O2 at 100 8C and 100% RH, and 90mWcm 2 in methanol/O2 at 150 8C and 0.67% RH of the cathode. Highly orderedmesoporous silica MCM-41 can be synthesized according to the procedure given in the literature. Thus, the key issue is to anchor and assemble HPW into the mesopores or channels of MCM-41 host. We have derived a VIM to assemble HPW molecules into the mesoporous silica. In this process, the impurities or trapped air inside the mesopores are removed under vacuum, and the vacuum-treated mesoporous silica

Layer-by-layer self-assembly of PDDA/PWA-Nafion composite membranes for direct methanol fuel cells

A novel PDDA/PWA-Nafion composite electrolyte membrane with enhanced proton conductivity (sigma) to methanol permeability (P) ratio, sigma/P, was fabricated by layer-by-layer self-assembly of negatively charged water soluble PWA and positively charged polyelectrolyte PDDA.

A novel inorganic proton exchange membrane based on self-assembled HPW-meso-silica for direct methanol fuel cells

Direct methanol fuel cells (DMFCs) based on high-temperature (100–300 °C) proton exchange membranes (HT-PEMs) offer significant advantages over the current low-temperature DMFCs based on perfluorosulfonic acid (e.g., Nafion™), such as reduction in CO poisoning via faster reaction kinetics, thus increasing the energy efficiency and reducing precious metal loading. This paper reports a novel inorganic proton exchange membrane based on 12-tungstophosphoric acid mesoporous silica (HPW-meso-silica) nanocomposites. The HPW-meso-silica was synthesized via a one-step self-assembly route assisted by a triblock copolymer, Pluronic P123, as the structure-directing surfactant. The threshold of the HPW content in the nanocomposites for the conductivity of mesoporous silica is 5 wt%. The best results were obtained at 25 wt% HPW-meso-silica, delivering a high proton conductivity of 0.091 S cm−1 at 100 °C under 100% relative humidity (RH) and 0.034 S cm−1 at 200 °C under 3% RH and a low activation energy of 14.0 kJ mol−1. The maximum power density of a cell with a 25 wt% HPW-meso-silica membrane is 19 mW cm−2 at 25 °C and increased to 235 mW cm−2 at 150 °C in methanol fuel.

Enhanced oxygen reduction at Pd catalytic nanoparticles dispersed onto heteropolytungstate-assembled poly(diallyldimethylammonium)-functionalized carbon nanotubes

Both Keggin-type phosphotungstic acid (HPW) and Pd are not prominent catalysts towards the oxygen reduction (ORR), but their composite Pd-HPW catalyst produces a significantly higher electrochemical activity for the ORR in acidic media. The novel composite catalyst was synthesized by self-assembly of HPW on multi-walled carbon nanotubes (MWCNTs) via the electrostatic attraction between negatively charged HPW and positively charged poly(diallyldimethylammonium (PDDA)-wrapped MWCNTs, followed by dispersion of Pd nanoparticles onto the HPW-PDDA-MWCNT assembly. The as-prepared catalyst was characterized by transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). TEM images show that Pd nanoparticles were uniformly dispersed on the surface of MWCNTs even when the Pd loading was increased to 60 wt%. Electrochemical activity of the catalysts for the ORR was evaluated by steady state polarization measurements using a rotating disk electrode. Compared with the acid treated MWCNTs, Pd nanoparticles supported on the HPW-assembled MWCNTs show a much higher ORR activity that is comparable to conventional Pt/C catalysts. The high electrocatalytic activities could be related to high dispersion of Pd nanoparticles as well as synergistic effects originating from the high proton conductivity of HPW. The Pd/HPW-PDDA-MWCNTs system as the cathode catalyst in proton exchange membrane fuel cells is demonstrated.

A Gemini Quaternary Ammonium Poly (ether ether ketone) Anion‐Exchange Membrane for Alkaline Fuel Cell: Design, Synthesis, and Properties

To reconcile the tradeoff between conductivity and dimensional stability in AEMs, a novel Gemini quaternary ammonium poly (ether ether ketone) (GQ-PEEK) membrane was designed and successfully synthesized by a green three-step procedure that included polycondensation, bromination, and quaternization. Gemini quaternary ammonium cation groups attached to the anti-swelling PEEK backbone improved the ionic conductivity of the membranes while undergoing only moderate swelling. The grafting degree (GD) of the GQ-PEEK significantly affected the properties of the membranes, including their ion-exchange capacity, water uptake, swelling, and ionic conductivity. Our GQ-PEEK membranes exhibited less swelling (≤ 40 % at 25-70 °C, GD 67 %) and greater ionic conductivity (44.8 mS cm(-1) at 75 °C, GD 67 %) compared with single quaternary ammonium poly (ether ether ketone). Enhanced fuel cell performance was achieved when the GQ-PEEK membranes were incorporated into H2 /O2 single cells.

Ultra-low loading Pt decorated coral-like Pd nanochain networks with enhanced activity and stability towards formic acid electrooxidation

Novel Pd based coral-like nanochain networks decorated with ultra-low loading (0.66 at%) Pt (Pt-on-PdCNNs) were successfully synthesized through a facile wet chemical method. The Pt-on-PdCNNs exhibited significantly enhanced activity and stability towards formic acid electrooxidation, which was ascribed to their unique properties such as the highly interconnected networks, the more exposed Pd(111) planes and the reduced CO formation and adsorption on the Pt–Pd surface.

Pt-based nanoparticles on non-covalent functionalized carbon nanotubes as effective electrocatalysts for proton exchange membrane fuel cells

Due to their unique electronic and mechanical properties, carbon nanotubes (CNTs) have been attracting much attention as favourite catalyst supports for energy conversion and storage applications. However, CNTs require molecular engineering such as solubilization and surface modification or functionalization to tailor their surface properties for the catalyst support applications. Among various functionalization methods, non-covalent functionalization is preferred, because it enables the attachment of molecules, solvents or polyelectrolytes through π–π, CH–π or hydrophobic interactions, forming effective active sites for the uniform assembly and dispersion of Pt-based precursor and/or nanoparticles (NPs) and at the same time preserving the intrinsic electronic and structural integrity of CNTs. Non-covalent functionalization is also effective to incorporate a multi-component electrocatalyst system with significantly enhanced synergistic effects. Here, the progress in the synthesis and development of highly dispersed Pt, and Pt-based NPs such as PtRu, PtSn and PtPd on non-covalent functionalized CNTs will be presented. The significant effect of interactions between CNTs, Pt-based NPs and functionalization agents on the electrocatalytic activity of Pt-based NPs on non-covalent functionalized CNTs is discussed.

Enhancement of hydrogen production in a single chamber microbial electrolysis cell through anode arrangement optimization

Reducing the inner resistances is crucial for the enhancement of hydrogen generation in microbial electrolysis cells (MECs). This study demonstrates that the optimization of the anode arrangement is an effective strategy to reduce the system resistances. By changing the normal MEC configuration into a stacking mode, namely separately placing the contacted anodes from one side to both sides of cathode in parallel, the solution, biofilm and polarization resistances of MECs were greatly reduced, which was also confirmed with electrochemical impedance spectroscopy analysis. After the anode arrangement optimization, the current and hydrogen production rate (HPR) of MEC could be enhanced by 72% and 118%, reaching 621.3±20.6 A/m3 and 5.56 m3/m3 d respectively, under 0.8 V applied voltage. A maximum current density of 1355 A/m3 with a HPR of 10.88 m3/m3 d can be achieved with 1.5 V applied voltage.

New anhydrous proton exchange membranes for high-temperature fuel cells based on PVDF–PVP blended polymers

A novel high-temperature proton exchange membrane (PEM) consisting of polyvinylpyrrolidone (PVP) and polyvinylidene fluoride (PVDF) has been successfully prepared by a simple and scalable polymer blending method. PVP is miscible with PVDF, forming a single-phased PVDF–PVP polymer with excellent flowability and uniform microstructure when the PVP content is equal to or higher than 40 wt% due to the effective hydrogen bonding between the functional groups of PVP and PVDF. A proton conductivity of 0.093 S cm−1 was obtained for a 20 wt% PVDF–80 wt% PVP membrane with a H3PO4 doping level of 2.7 at 200 °C under anhydrous conditions, compatible with the state-of-the-art PBI/PA membranes. PEM fuel cells with PA/PVDF–PVP membranes showed a high power density of 530 mW cm−2 at 180 °C in H2/O2 and excellent stability without external humidification. The results indicate the high structural and chemical stability and high retention capability of the blended membranes for doped PA at elevated operation temperatures. The PVDF–PVP hybrids show promising potential as alternative PEMs for fuel cells at elevated high temperatures and under anhydrous conditions.