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In this work, a novel aqueous battery consisting of manganese in (Mn Sn) redox chemistries is proposed, where Mn redox reactions occur in the positive electrode and Sn deposition/stripping reactions occur in the negative electrode. The battery exhibits an open circuit voltage of 1.7 V, while a coulombic efficiency of higher than 98.8% is obtained at an operating current density of 30 mA cm−2. Cyclic tests show that the energy efficiency can be kept above 91.5% with no observable decay at a current density of 10 mA cm−2. In addition, due to the high hydrogen overpotential of Sn metal, the parasitic hydrogen evolution reaction can be largely avoided. With decent battery performance, the Mn Sn system offers a promising solution for future energy storage applications.

Conventional zinc bromide electrolytes offer low ionic conductivity and often trigger severe zinc dendrite growth in zinc-bromine flow batteries. Here we report an improved electrolyte modified with methanesulfonic acid, which not only improves the electrolyte conductivity but also ameliorates zinc dendrite. Experimental results also reveal that the kinetics and reversibility of Zn2+/Zn and Br2/Br− are improved in this modified electrolyte. Moreover, the battery's internal resistance is significantly reduced from 4.9 to 2.0 Ω cm2 after adding 1 M methanesulfonic acid, thus leading to an improved energy efficiency from 64% to 75% at a current density of 40 mA cm−2. More impressively, the battery is capable of delivering an energy efficiency of about 78% at a current density of as high as 80 mA cm−2 when the electrode is replaced by a thermally treated one. Additionally, zinc dendrite growth is found to be effectively suppressed in methanesulfonic acid supported media, which, as a result, enables the battery to be operated for 50 cycles without degradation, whereas the one without methanesulfonic acid suffers from significant decay after only 40 cycles, primarily due to severe zinc dendrite growth. These superior results indicate methanesulfonic acid is a promising supporting electrolyte for zinc-bromine flow batteries.

The zinc-bromine flow battery (ZBFB) is regarded as one of the most promising candidates for large-scale energy storage owing to its high energy density and low cost. However, because of the large internal resistance and poor electrocatalytic activity of graphite- or carbon-felt electrodes, conventional ZBFBs usually can only be operated at a relatively low current density, which limits their widespread application. Herein, we propose an asymmetrical cell by replacing the conventional thick felt electrode with a thin and electrocatalytically active carbon-paper (CP) electrode interfacing with a flow-field structure. With the enhanced electrocatalytic activity of CP for the Br-2/Br- redox reaction and the reduced internal resistance of the thinner electrode, the ZBFB with this newly proposed structure exhibits an energy efficiency of up to 83.5% at a current density of 40 mA cm(-2), much higher than that with a graphite-felt electrode of only 73.0%. Remarkably, the battery can even be operated at a high current density up to 100 mA cm(-2) while maintaining an energy efficiency of more than 70%, demonstrating an excellent rate capability.

Mn3O4 nanoparticle-decorated carbon cloths are prepared as the electrodes for vanadium redox flow batteries. The Mn3O4 nanoparticles are prepared using a two-step process, in which MnOOH nanoflakes are electrodeposited on the surface of the carbon cloth and subsequently converted to Mn3O4 nanoparticles by a thermal treatment. The prepared electrode exhibits a superior electrocatalytic activity toward the VII/VIII redox reaction. It is demonstrated that the peak potential separation of the VII/VIII redox reaction on the as-prepared electrode is reduced from 402.0 mV to 82.4 mV at a scan rate of 5 mV s−1 when the pristine carbon cloth is deposited with Mn3O4 nanoparticles. The flow battery with carbon cloths decorated with 0.31 mg cm−2 of Mn3O4 nanoparticles as the electrode exhibits an energy efficiency of 88 %, which is 17 % higher than that of the battery assembled with the pristine carbon cloth at a current density of 100 mA cm−2. Therefore, the superior electrocatalytic activity of Mn3O4 nanoparticles reported offers a promising approach to improve the performance of carbon-based electrodes, which can reduce the capital cost of vanadium redox flow batteries. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Electrospinning has been employed to fabricate carbonaceous materials with larger surface areas for vanadium redox flow batteries. However, the woven carbon nanofibers prepared with conventional electrospinning methods are plagued by the low porosity and poor permeability, thereby causing a significant mass-transport resistance during the operation of batteries. To tackle this problem, we report a novel method by self-assembling porous carbon fibers into large bundles to form electrodes. This electrode is fabricated by electrospinning polyacrylonitrile and polystyrene binary solutions. Instead of forming single fibers, the individual fibers are self-assembled into fiber bundles by properly managing the viscosity of the precursor solution. The formation of large fiber bundles significantly enlarges the pore size while retaining large specific surface areas. The single cell with the as-prepared electrodes achieves an energy efficiency of 87.7% at a current density of 100 mA cm−2, which is 15.2% higher than that of the single cell with conventional electrospinning electrodes. The energy efficiency still maintains over 80% at 200 mA cm−2. More importantly, the discharge capacity and electrolyte utilization are nearly doubled. All these results demonstrate that this electrode preparation method is effective to improve the mass transport properties of traditional electrospun electrodes in vanadium redox flow batteries. © 2018 Elsevier B.V.

Among the most critical issues associated with redox flow batteries (RFBs), one is the undesirable crossover of redox-active species through the membrane, which is typically polymer-based. To address this issue, we report an inorganic mesoporous silica film based nanochannel membrane with highly ordered, vertically aligned, and subnanosized channels. Unlike conventional polymer-based membranes that have ion transport channels with fixed sizes between 2 and 4 nm in diameter, the ion transport channels of the membrane are controllable in size and uniform in size distribution, enabling the membrane to separate the ions that are larger than the critical value. Experimental results prove the expected effect that the pristine nanochannel membrane (with the channel size of ∼0.5 nm) grown on the ITO glass can completely separate protons and vanadium ions. After ITO etching, the free-standing membrane maintains extremely low vanadium-ion permeability that is four orders of magnitude lower than that of Nafion membranes, suggesting that it is a promising candidate for redox flow batteries. Moreover, the membrane fabrication is convenient, inexpensive, and does not rely on sophisticated facilities or conditions, providing potential in scalable productions. © 2018 Elsevier B.V.

Developing high-performance electrodes with high operating current densities and long-term cycling stability is crucial to the widespread application of vanadium redox flow batteries (VRFBs). In this work, boron-doped graphite felt electrodes are designed, fabricated and tested for VRFBs. The first-principles study first demonstrates that the boron-doped carbon surface possesses highly active and stabilized reaction sites. Based on this finding, boron-doped graphite felt electrodes are fabricated for VRFBs. Testing results show that the batteries with boron-doped graphite felt electrodes achieve energy efficiencies of 87.40% and 82.52% at the current densities of 160 and 240 mA cm(-2), which are 15.63% and 19.50% higher than those with the original electrodes. In addition, the batteries can also be operated at high current densities of 320 and 400 mA cm(-2) with energy efficiencies of 77.97% and 73.63%, which are among the highest performances in the open literature. More excitingly, the VRFBs with the boron-doped graphite felt electrodes exhibit excellent stability during long-term cycling tests. The batteries can be stably cycled for more than 2000 cycles at 240 mA cm(-2) with ultra-low capacity and efficiency decay rates of only 0.028% and 0.0002% per cycle. In addition, after refreshing the electrolytes, the performances of the batteries are nearly recovered regardless of the inevitable decay of the membrane. All these results suggest that highly efficient and ultra-stable boron-doped graphite felts are promising electrodes for VRFBs.

An accurate prediction of ion selective adsorption in ion exchange membranes is essential to reflect the role of fixed charges on ion transport through the membrane in vanadium redox flow batteries (VRFB). Unlike those empirical models reported in the literature, this work reports on a new ion selective adsorption model with the Donnan effect considered for movable ions distributed in the membrane pores only. This model, no longer relying on empirical coefficients, is then applied to the calculation of ion transport through membranes in VRFBs. The model shows a more accurate prediction of vanadium crossover and membrane conductivity, and enables to capture the effect of key membrane properties on battery performance. It is found that (i) an increase in H2SO4 concentration reduces the electrolyte imbalance and improves the coulombic efficiency; (ii) an increase in membrane porosity significantly improves the membrane effective conductivity; (iii) the change of fixed charges should be careful to balance all performances. Therefore, membrane properties and operating conditions need proper adjustment to improve the battery performance, and our VRFB model is a good tool to help membrane optimization. © 2018 Elsevier Ltd

Zinc-bromine flow batteries have been regarded as one of the most promising technologies for large-scale energy storage. However, the widespread application of this technology is still hindered by its low power density, primarily resulting from a large polarization in the Br2/Br− electrode. In this work, carbonized tubular polypyrrole with abundant nitrogen- and oxygen-containing functional groups is synthesized and, for the first time, applied to a zinc-bromine flow battery as a positive electrode material. Experimental results unravel that carbonized tubular polypyrrole exhibits a high activity toward the Br2/Br− redox reaction, and enables the zinc-bromine flow battery to be operated at a current density of 80 mA cm−2 with a high energy efficiency of 76.0%. In contrast, the battery with the pristine graphite felt electrode can only output an energy efficiency of 69.4% at the same current density. In addition, no degradation is observed for the zinc-bromine flow battery equipped with carbonized tubular polypyrrole for 100 cycles. These superior results suggest that carbonized tubular polypyrrole is a promising positive electrode material for zinc-bromine flow batteries. © 2018 Elsevier Ltd

Using the density functional theory method, we demonstrate that the recently synthesized three-dimensional carbon-honeycomb (C-honeycomb) can serve as a promising nanoporous scaffold for both lithium and sodium deposition with good electronic conductivity. Lithium/sodium can insert into the channels of C-honeycomb with low migration energy barriers along the walls of one-dimensional pores (<0.5 eV for lithium and <0.25 eV for sodium) and exist in the form of metal nanorods. A high theoretical capacity (711 mAh/g, almost twice that of graphite) can be achieved when the one-dimensional pores are filled with lithium/sodium atoms. The volume expansion of the C-honeycomb after metal insertion is less than 5 and 15% for lithium and sodium, respectively. Further introduction of defects such as pyridinic-N doping or single vacancy can provide initial nucleation sites for the metal nanorods and increase the open-circuit voltage.

We investigate thermal effects on the sedimentation behavior of elliptical particles via particle-resolved direct numerical simulation. Two scenarios of fluid-particle heat transfer in an infinitely long channel are considered: one is a cold particle settling in a hot fluid, while the other is a hot particle settling in a cold fluid. Results show that when an elliptical particle sediment in a wide channel (i.e., the block ratio is large), in addition to the two sedimentation modes reported in the literature for the particle sediments in isothermal fluids, there exist another three modes arising from thermal effects: the tumbling mode, the anomalous rolling mode and the inclined mode. Specifically, for a cold particle settling in a hot fluid, we found the tumbling mode and inclined mode, while for a hot particle settling in a cold fluid, we found the anomalous rolling mode and inclined mode. The phase diagrams of the sedimentation modes as functions of Archimedes and Grashof numbers are given. Analyses of the relationship between particle Reynolds number and Grashof number indicates that the mode presented depends on the competition between channel wall confinement, combined forced and natural convection. © 2018 Elsevier Ltd

Lacking effective anchoring materials to suppress the severe shuttle effect is a longstanding issue hindering the development of lithium-sulfur (Li-S) batteries. In this work, a first-principles study is carried out to investigate the potential of borophene and defective borophene, which have high ionic conductivity and adsorbent ability, as anchoring materials for Li-S batteries. Borophene is found to exhibit ultra-high adsorption energies towards lithium polysulfides, but the material facilitates the decomposition of Li-S clusters, leading to an undesirable sulfur loss during battery cycling. For this reason, borophene is not an ideal anchoring material for Li-S batteries. On the contrary, defective borophene is found to show moderate adsorption energies ranging from 1 to 3 eV, which not only effectively anchors lithium polysulfides to suppress the shuttle effect, but also keeps their cyclic structures undecomposed. In addition, defective borophene exhibits a metallic characteristic during the whole reaction process, ensuring the lithium polysulfides be easily charged back and not accumulate on the anchoring materials. Given these advantages, it is expected that defective borophene is a promising anchoring material, leading to a suppressed shuttle effect and enhanced capacity retention for Li-S batteries.

Liquid-electrolyte sodium-sulfur battery operated at room temperature is encountering challenges brought by the complex sulfur redox reactions, including (i) the dissolved polysulfide intermediates trigger serious side reactions on Na anode surface; (ii) the short-chain sulfide precipitation exhibits sluggish kinetics and the sulfur utilization is generally below 50% with unclear reasons. In this work, employing an ion selective polybenzimidazole-based separator we successfully suppress the polysulfide corrosion on the Na anode, which allows the investigation of the precipitation reaction. Combining DFT calculation and characterization techniques, we determine Na2S2 particles as the final discharge product and reveal that Na2S2 passivation is the predominant attributes of large polarization and capacity fading. To address these issues, we present the use of a bifunctional NaI-P2S5 based electrolyte additive, which (i) improves the Na2S2 precipitation kinetics by forming soluble Na2S2-P2S5 complex and (ii) promotes the dissolution of Na2S2 by the chemical mediation of I−/ I3 −. As a result, a much improved capacity retention (92.9% for 50 cycles at 0.2C) is attained, which sheds light on enabling the stable operation of sodium-sulfur batteries via combining advanced separator and electrolyte engineering strategies. © 2018 Elsevier B.V.

Lacking effective anchoring materials to suppress the severe shuttle effect is a longstanding issue hindering the development of lithium-sulfur (Li-S) batteries. In this work, a first-principles study is carried out to investigate the potential of borophene and defective borophene, which have high ionic conductivity and adsorbent ability, as anchoring materials for Li-S batteries. Borophene is found to exhibit ultra-high adsorption energies towards lithium polysulfides, but the material facilitates the decomposition of Li-S clusters, leading to an undesirable sulfur loss during battery cycling. For this reason, borophene is not an ideal anchoring material for Li-S batteries. On the contrary, defective borophene is found to show moderate adsorption energies ranging from 1 to 3 eV, which not only effectively anchors lithium polysulfides to suppress the shuttle effect, but also keeps their cyclic structures undecomposed. In addition, defective borophene exhibits a metallic characteristic during the whole reaction process, ensuring the lithium polysulfides be easily charged back and not accumulate on the anchoring materials. Given these advantages, it is expected that defective borophene is a promising anchoring material, leading to a suppressed shuttle effect and enhanced capacity retention for Li-S batteries.

Developing high-performance electrodes with high operating current densities and long-term cycling stability is crucial to the widespread application of vanadium redox flow batteries (VRFBs). In this work, boron-doped graphite felt electrodes are designed, fabricated and tested for VRFBs. The first-principles study first demonstrates that the boron-doped carbon surface possesses highly active and stabilized reaction sites. Based on this finding, boron-doped graphite felt electrodes are fabricated for VRFBs. Testing results show that the batteries with boron-doped graphite felt electrodes achieve energy efficiencies of 87.40% and 82.52% at the current densities of 160 and 240 mA cm(-2), which are 15.63% and 19.50% higher than those with the original electrodes. In addition, the batteries can also be operated at high current densities of 320 and 400 mA cm(-2) with energy efficiencies of 77.97% and 73.63%, which are among the highest performances in the open literature. More excitingly, the VRFBs with the boron-doped graphite felt electrodes exhibit excellent stability during long-term cycling tests. The batteries can be stably cycled for more than 2000 cycles at 240 mA cm(-2) with ultra-low capacity and efficiency decay rates of only 0.028% and 0.0002% per cycle. In addition, after refreshing the electrolytes, the performances of the batteries are nearly recovered regardless of the inevitable decay of the membrane. All these results suggest that highly efficient and ultra-stable boron-doped graphite felts are promising electrodes for VRFBs.

It is widely recognized that the oxygenated carbonaceous materials improve the performance of vanadium redox flow batteries (VRFBs). However, whether the enhanced vanadium redox reactions are due to the existence of grafted oxygen‐containing functional groups (OCFGs) or the enlarged specific surface areas (SSAs) during the course of surface treatments is still under dispute. To determine which factor is indeed responsible for the enhanced redox reactions, a set amount of OCFGs is intentionally grafted on the model carbon electrode without modifying the surface area using an alkaline‐mediated hydrothermal treatment. Experimental results, using the peak potential separation and the activation energy as electroactivity descriptors in the three‐electrode cell, demonstrate that OCFGs promote the V(II)/V(III) redox reaction, while exhibiting a minute catalytic effect on the V(IV)/V(V) redox reaction. The superior electroactivity is also testified by a flow battery test, in which the energy efficiency as well as the charge/discharge capacity of VRFBs with hydrothermally treated samples is significantly higher than those of VRFBs with the pristine one. Through these experiments, it is verified that OCFGs contribute to the electroactivity promotion for carbon electrodes rather than the enhanced SSA. The findings can guide the effort to design high‐performing carbonaceous electrodes for VRFBs.