Advanced charged sponge-like porous membranes with ultrahigh stability and selectivity are designed and fabricated for vanadium flow battery (VFB) applications. The designed porous membranes are fabricated via constructing positively charged cross-linked networks on the pore walls of polysulfone membranes. The charge density of the pore walls can be tuned by changing the crosslinking time. The positively charged pore walls can effectively retain vanadium ions via Donnan exclusion, hence keeping extremely high selectivity, while the crosslinked network effectively increases the membrane stability. As a result, the designed membranes exhibit an outstanding performance, combining extremely high selectivity and stability. The single cell assembled with the prepared porous membrane shows a columbic efficiency of 99% and an energy efficiency of 86% at a current density of 80 mA cm⁻², which is much higher than Nafion 115 (93.5%; 82.3%). A battery assembled with the prepared membrane shows a stable battery performance over more than 6000 cycles, which is by far the longest record for porous membranes ever reported. These results indicate that advanced, charged, sponge-like, porous membranes with a crosslinked pore-wall structure are highly promising for VFB applications.

Anion exchange membranes (AEMs) with excellent stability and high ion conductivity are fabricated via the formation of internal cross linking networks. The internal crosslinking networks are constructed by reacting 4,4′-bipyridine with chloromethylated polysulfone. The bipyridine group simultaneously functions as ionic conductor and cross linker in this system. The performance of the membrane is tuned via controlling the 4,4′-bipyridine content in the casting solution. The prepared membranes demonstrate excellent chemical stability and high ion conductivity under acidic conditions. As a consequence, the membranes show very promising performance for vanadium flow battery application, exhibiting a Coulombic efficiency of 99.2% and an energy efficiency of 81.8% at a current density of 140 mA cm⁻². The battery that is assembled with the prepared membrane shows a stable battery performance over more than 1600 cycles, which is by far the longest cycle life reported. These results indicate that the AEMs with internal crosslinking structures are promising candidates for battery systems and even for fuel cells.

Sluggish kinetic characteristics of the electrode reactions and low electrode space utilization for solid Li₂O₂ deposition are limiting factors for Li–O₂ batteries. In this work, iron-/cobalt-doped micron-sized honeycomb-like carbon (Fe-MHC, Co-MHC) with a hierarchical pore structure is prepared by using nano-CaCO₃ as a template. The effect of the transition-metal nanoparticles as a second template on further pore construction and optimization is presented. As graphitization catalysts, the added transition metals also affect the formation of the graphene-rich structure in the carbon framework. As a result, the obtained hierarchical carbon materials doped with trace metals exhibit higher electrochemical catalytic activity and stability than non-doped samples. In particular, an enhanced discharge capacity as high as 9260 mAh g⁻¹ is achieved for the Fe-MHC cathode, which is attributed to its enlarged pore volume and high utilization of the electrode space.

The electrolyte in the vanadium flow battery (VFB) is one of the key materials in determining the energy density, which further limits its commercialization. A novel strategy of varying the proton concentration to improve electrolyte utilization is proposed to enhance the energy density of the VFB. The effects of proton concentration on the equilibrium potential and polarization have been investigated systematically. Superior electrolyte utilization at a proton concentration of 5 mol L⁻¹ is achieved; this corresponds to a high volumetric energy density (22.5 Wh L⁻¹) and volumetric capacity density (14.8 Ah L⁻¹), which is favorable toward the commercial application of VFBs. This general strategy also inspires research on improving electrolyte utilization in other flow battery systems.

A sulfur/carbon composite with a hollow cagelike configuration is synthesized and employed as a lithium/sulfur battery. The material provides a hierarchical porous structure with small pores in local regions and large pores connected to each other throughout the electrode. It exhibits a good polysulfide retention ability, promotes the efficient transfer of electrons and ions, and suppresses the redistribution of active material during cycles. Thus, the battery shows an improved rate performance, as well as a superior cycling stability of 94 % capacity retention, after 100 cycles at 0.5 C.

N-doped carbon catalysts have attracted great attention as potential alternatives to expensive Pt-based catalysts used in fuel cells. Herein, an ordered hierarchically porous carbon codoped with N and Fe (Fe-NOHPC) is prepared by an evaporation-induced self-assembly process followed by carbonization under ammonia. The soft template and Fe species promote the formation of the porous structure and facilitate the oxygen reduction reaction (ORR).The catalyst possesses an ordered hierarchically porous structure with a large surface area (1172.5 m² g⁻¹) and pore volume of 1.03 cm³ g⁻¹. Compared to commercial 20 % Pt/C, it exhibits better ORR catalytic activity and higher stability as well as higher methanol tolerance in an alkaline electrolyte, which demonstrates its potential use in fuel cells as a nonprecious cathode catalyst. The N configuration, Fe species, and pore structure of the catalysts are believed to correlate with its high catalytic activity.

Anion exchange membranes prepared from quaternized poly(tetramethyl diphenyl ether sulfone) (QAPES) were first investigated in the context of vanadium flow battery (VFB) applications. The membranes showed an impressive suppression effect on vanadium ions. The recorded vanadium permeability was 0.02×10⁻⁷–0.09×10⁻⁷ cm² min⁻¹, which was two orders of magnitude lower than that of Nafion 115. The self-discharge duration of a VFB single cell with a QAPES membrane is four times longer than that of Nafion 115. The morphological difference in hydrophilic domains between QAPES and Nafion was confirmed by TEM. After soaking the membranes in VO₂⁺ solution, adsorbed vanadium ions can barely be found in QAPES, whereas the hydrophilic domains of Nafion were stained. In the ex situ chemical stability test, QAPES showed a high tolerance to VO₂⁺ and remained intact after immersion in VO₂⁺ solution for over 250 h. The performance of a VFB single cell assembled with QAPES membranes is equal to or even better than that of Nafion 115 and remains stable in a long-term cycle test. These results indicate that QAPES membranes can be an ideal option in the fabrication of high-performance VFBs with low electric capacity loss.

Perfluorosulfonic acid ionomers (PFSI) with different side-chain lengths have been investigated with respect to their morphology and electrochemical properties in vanadium flow batteries (VFB). The results indicated that the membrane with the shortest side chains (SSC-M2) displayed small ion clusters and a low degree of hydrophobic–hydrophilic separation, which is favourable to reduce the cross-over of vanadium ions in the VFB. SSC-M2 shows a similar proton conductivity to Nafion, which carries longer ionic side chains but with much lower ion permeability. As a result, the VFB assembled with SSC-M2 exhibited a superior coulombic efficiency and a voltage efficiency close to that of Nafion115. In situ mass transfer revealed that SSC-M2 had a remarkably low degree of vanadium and water transfer across the membrane, which resulted in lower capacity fading than in the case of Nafion115. These results indicate that a membrane with short side chains is an ideal option in the fabrication of high-performance VFBs with low capacity loss.

We report the fabrication and properties of a high-performance, inexpensive, composite, anion-exchange membrane (AEM) for an all-vanadium flow battery (VFB) application. The AEM was fabricated by dication cross-linking without the involvement of trimethylamine, and shows well-balanced anion conductivity and robustness due to imidazolium and imidazolium–ammonium functionalities, as well as a concomitantly achieved semi-interpenetrating network structure. The VFB single cell yielded a Coulombic efficiency of 99 % and an energy efficiency of 84 % at 80 mA cm⁻², and operated for over 900 charge/discharge cycles. This work demonstrates the combined use of several favorable AEM design rationales, such as incorporating abundant and efficient anion-exchange groups, constructing a swelling- and oxidation-resistant structure, and facile fabrication; it provides an effective way of developing high-performance, low-cost AEMs for VFB applications.