Xueyi He

Bioinspired Ultrastrong Solid Electrolytes with Fast Proton Conduction along 2D Channels

Solid electrolytes have attracted much attention due to their great prospects in a number of energy- and environment-related applications including fuel cells. Fast ion transport and superior mechanical properties of solid electrolytes are both of critical significance for these devices to operate with high efficiency and long-term stability. To address a common tradeoff relationship between ionic conductivity and mechanical properties, electrolyte membranes with proton-conducting 2D channels and nacre-inspired architecture are reported. An unprecedented combination of high proton conductivity (326 mS cm-1 at 80 °C) and superior mechanical properties (tensile strength of 250 MPa) are achieved due to the integration of exceptionally continuous 2D channels and nacre-inspired brick-and-mortar architecture into one materials system. Moreover, the membrane exhibits higher power density than Nafion 212 membrane, but with a comparative weight of only ≈0.1, indicating potential savings in system weight and cost. Considering the extraordinary properties and independent tunability of ion conduction and mechanical properties, this bioinspired approach may pave the way for the design of next-generation high-performance solid electrolytes with nacre-like architecture.

Facilitating Proton Transport in Nafion-Based Membranes at Low Humidity by Incorporating Multifunctional Graphene Oxide Nanosheets

Nafion, as a state-of-the-art solid electrolyte for proton exchange membrane fuel cells (PEMFCs), suffers from drastic decline in proton conductivity with decreasing humidity, which significantly restricts the efficient and stable operation of the fuel cell system. In this study, the proton conductivity of Nafion at low relative humidity (RH) was remarkably enhanced by incorporating multifunctional graphene oxide (GO) nanosheets as multifunctional fillers. Through surface-initiated atom transfer radical polymerization of sulfopropyl methacrylate (SPM) and poly(ethylene glycol) methyl ether methacrylate, the copolymer-grafted GO was synthesized and incorporated into the Nafion matrix, generating efficient paths at the Nafion-GO interface for proton conduction. The Lewis basic oxygen atoms of ethylene oxide (EO) units and sulfonated acid groups of SPM monomers served as additional proton binding and release sites to facilitate the proton hopping through the membrane. Meanwhile, the hygroscopic EO units enhanced the water retention property of the composite membrane, conferring a dramatic increase in proton conductivity under low humidity. With 1 wt % filler loading, the composite membrane displayed the highest proton conductivity of 2.98 × 10-2 S cm-1 at 80 °C and 40% RH, which was 10 times higher than that of recast Nafion. Meanwhile, the Nafion composite exhibited a 135.5% increase in peak power density at 60 °C and 50% RH, indicating its great application potential in PEMFCs.

Phosphorylated graphene monoliths with high mixed proton/electron conductivity

A mixed ionic–electronic conductor (MIEC) plays a crucial role in electrochemical technologies relevant to energy conversion and storage. The existing composite materials often suffer from poor mixed conducting performance due to the distinct phase boundaries and random distributions of the transport channels. Herein, we propose the concept for the fabrication of a single-phase MIEC using two-dimensional (2D) building blocks-phosphorylated graphene nanosheets, which are assembled into three-dimensional (3D) interconnected networks with long-range ordered nanochannels. Attributed to the sufficient proton carriers (phosphate groups) confined in electron-conductive nanochannels and the integration of conductive pathways, simultaneously enhanced proton conductivity (0.13 S cm−1) and electron conductivity (0.265 S cm−1) are achieved at 98% RH and 35 °C, surpassing the current performance of all graphene-based MIECs. This approach may pave the way for designing MIECs with high mixed conduction by utilizing the unique properties of 2D materials, beyond the limitations of pure proton-conducting/electron-conducting materials.

One-pot synthesis of silica–titania binary nanoparticles with acid–base pairs via biomimetic mineralization to fabricate highly proton-conductive membranes

Simultaneous manipulation of vehicle-type and Grotthuss-type proton conduction within proton exchange membranes (PEMs) to induce satisfactory proton conductivity is crucial and challenging for promising environmentally friendly devices such as PEM fuel cells. In this study, a facile one-pot biomimetic mineralization approach is proposed for the construction of binary SiO2–TiO2 nanoparticles with a tunable ratio of silica to titania. Then the nanoparticles are functionalized by acid–base pairs and introduced into a Nafion matrix to fabricate novel hybrid membranes. The interaction between functional groups on SiO2–TiO2 binary nanoparticles and the polymer endows the hybrid membrane with good interfacial compatibility and enhanced dimensional stability. The incorporation of acid–base pairs reduces the activation energy for proton transfer; as a result, the hybrid membrane exhibits the highest proton conductivity of 1.37 × 10−2 S cm−1 at 26.1% RH and 80 °C, which is two orders of magnitude higher than that of recast Nafion. Compared with recast Nafion, a 51.3% increase in maximum power density is achieved for the Nafion/Si1–Ti2-160 hybrid membrane at 60 °C.

A highly conductive and robust anion conductor obtained via synergistic manipulation in intra- and inter-laminate of layered double hydroxide nanosheets

Layered double hydroxides (LDH), bearing trivalent cationic charge centers and hydroxyl slabs, hold great promise for use in efficient solid-state ion conductors. In this study, a composite membrane with ion-conducting 2D channels was prepared based on exfoliated LDH nanosheets and quaternized polyvinyl alcohol via a filtration process. The LDH laminates, which were formed by the stacking of exfoliated LDH nanosheets, primarily afforded the conductive performance of the membranes. Within the intra-laminate galleries of LDH, charge-balancing anions determined the channel size, water absorption capacity and electrostatic interactions inside the 2D channels. As a result, hydroxide ion transport was greatly affected by the anionic species present. The organic moieties inside the inter-laminate galleries of LDH conferred hydrogen bonds and covalent linkages at the organic–inorganic interfaces, resulting in a nacre-mimetic structure, leading to the improved mechanical properties of the composite membranes. The synergistic manipulation of composition and interactions within the intra- and inter-laminate galleries endowed the novel anion conductors with both high conductivity (156.3 mS cm−1 at 80 °C) and good mechanical performance (a tensile strength of 48.4 MPa and toughness of 2.09 MJ m−3).