AbstractCapacitive carbons are attractive for energy storage on account of their superior rate and cycling performance over traditional battery materials, but they usually suffer from a far lower volumetric energy density. Starting with expanded graphene, a simple, multifunctional molten sodium amide treatment for the preparation of high-density graphene with high capacitive performance in both aqueous and lithium battery electrolytes is reported. The molten sodium amide can condense the expanded graphene, lead to nitrogen doping and, what is more important, create moderate in-plane nanopores on graphene to serve as ion access shortcuts in dense graphene stacks. The resulting high-density graphene electrode can deliver a volumetric capacitance of 522 F cm−3 in a potassium hydroxide electrolyte; and in a lithium-ion battery electrolyte, it exhibits a gravimetric and volumetric energy density of 618 W h kg−1 and 740 W h L−1, respectively, and even outperforms commercial LiFePO4.

There is considerable interest in the synthesis of activated carbons from biomass through hydrothermal carbonization (HTC) followed by activation. Here we report our findings that using NH4Cl additive for HTC of glucose changes the product from nanosphere carbon to N-doped microsphere carbon with a much lower surface area, but unexpectedly, the following KOH-activated N-doped microsphere carbon shows a significantly higher specific surface area (exceeding 3000 m2 g−1) than that (2385 m2 g−1) of activated conventional HTC carbon. Under similar conditions, other HTC additives, such as NaCl and HCl, can also lead to the formation of microsphere carbons with decreased surface area, but the specific surface area of the corresponding activated carbons decreased accordingly. These comparisons together with XPS and FTIR analyses suggest that the doped N in the HTC carbon play an important role on the formation of extra pores during the activation. Furthermore, the activated N-doped microsphere carbon delivers the highest specific capacity (349 F g−1) at a current density of 1 A g−1 in 6 mol L−1 KOH. Our findings promise an efficient route to the preparation of N-doped highly porous carbon with high capacitive performance.

Electrolysis of solid chalcopyrite (CuFeS2) against a graphite inert anode has been studied in equimolar NaCl-KCl melt at 700 °C. During electrolysis, S2− ions are released from the solid CuFeS2 cathode, transfer to the graphite anode and discharge to S2 gas. The reduction mechanism of CuFeS2 was investigated by cyclic voltammetry, potentiostatic and constant voltage electrolysis together with spectroscopic and scanning electron microscopic analyses. The reduction contains mainly three stages: the insertion of Na+ or K+ into CuFeS2, forming LxCuFeS2 (L = Na or K, x ≤ 1); the partial reduction of LxCuFeS2 to Lx-wCuFe1-yS2-z and Fe; the complete reduction to a mixture of Cu and Fe, which can be magnetically separated. After the separation, pure Cu can be obtained by leaching out the residual Fe with acid. Electrolysis at a cell voltage of 2.4 V has led to a rapid reduction of CuFeS2. The current efficiency and energy consumption were 85% and 1.68 kWh/kg-CuFeS2, respectively.

Graphene and graphene/metal oxide composite materials have attracted considerable interest for use as energy materials due to their excellent electrochemical performances. Here, we propose using melamine as a template for the synthesis of cambered nano-walls of SnO2/rGO materials. Melamine powder can effectively absorb SnO2/GO from the solution to form a core–shell structure of melamine@SnO2/GO. After thermal reduction of GO at 200 °C to form the melamine@SnO2/rGO, melamine was dissolved in hot water at 80 °C, leaving behind the cambered SnO2/rGO nano-walls. Melamine is recyclable since it precipitates when its solution cools to room temperature. The thickness of the SnO2/rGO nano-walls can be easily controlled by adjusting the mass ratio of melamine to SnO2/GO. When the mass ratio was set to ten, cambered walls of SnO2/rGO with a thickness of about 100–200 nm were achieved. The resulting SnO2/rGO delivered an initial reversible capacity of 998 mA h g−1 at a current density of 100 mA g−1 and a capacity of 855 mA h g−1 after 100 discharge–charge cycles in a potential range between 0.02 and 3.0 V vs. Li/Li+. It also showed good rate performance with a reversible capacity of 460 mA h g−1 at 1 A g−1. These high capacities can be linked to the special cambered nano-walls which ensure fast solid diffusion in addition to providing an effective liquid-channel and buffer-volume in the electrode. The proposed synthesis method is easily scalable and should be applicable to many other graphene based energy materials.

The molten eutectic mixture of magnesium, sodium and potassium chlorides (MgCl2–NaCl–KCl) has inappreciable solubility for oxide ions, and can help disengage a carbon anode from the oxide ions generated at a metal oxide cathode, and effectively avoid carbon dioxide formation. This “disengaging strategy” was successfully demonstrated in electro-reduction of solid oxides of zirconium and tantalum. It has led to significantly higher current efficiency (93%), and lower energy consumption (1.4 kW h kg−1) in electrolysis of tantalum oxide to tantalum metal compared to the conventional electrolysis in molten calcium chloride (e.g. 78% and 2.4 kW h/kg-Ta).

Electrochemical reduction of solid GeO2 has been investigated in the mixed CaCl2-NaCl melt at 1023 K for developing a more efficient process for preparation of Ge. Cyclic voltammetry and potentiostatic electrolysis were applied to study the GeO2-loaded metallic cavity electrode. In addition, porous GeO2 pellets were reduced by potentiostatic and constant cell voltage electrolysis with a graphite anode, and the electrolysis products were analyzed by powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry, focusing on understanding the reduction mechanism and the impact of electrode potential on the product purity. It was found that the reduction of GeO2 to Ge occurred at a potential of about -0.50 V (vs. Ag/Ag+), but generating various calcium germanates simultaneously, whose reduction was a little more difficult and needed a potential more negative than -1.00 V. However, if the cathode potential exceeded -1.60 V, Ca (or Na) - Ge intermetallic compounds might form. These results gave an appropriate potential range between -1.10 and -1.40 V for the production of pure germanium. Rapid electrolysis of GeO2 to pure Ge has been realized at a cell voltage of 2.5 V with a current efficiency of about 92%.

•Porous metal electrodes prepared by direct electro-reduction of oxide precursors.•High capacity porous Sn/SnSb negative electrodes for lithium ion batteries.•Pore-forming of Sn/SnSb anode by NH4HCO3 for enhanced electrochemical performances.Porous Sn/SnSb negative electrodes for lithium ion batteries were directly prepared by electro-reduction of the SnO2–Sb2O3 (molar ratio = 4:1) composite electrodes in 1 mol/L H2SO4. After the reduction, the original dense SnO2–Sb2O3 composite electrode changed into a porous structure with the oxides almost completely reduced to nanoparticles of Sn and SnSb alloy. As the precursor electrode showed very poor electrochemical performances in lithium ion batteries, the resultant metallic porous Sn/SnSb electrode exhibited high charge capacity (800 mAh/g) and good cycling stability (70% of capacity retention at the 40th cycle) between 0.02 and 1.5 V (vs. Li/Li+) at a current density of 100 mA/g. More porous Sn/SnSb electrode was derived from the SnO2–Sb2O3 composite precursor using pore-forming by NH4HCO3 (15 vol.%), showing enhanced electrochemical performances with an initial capacity of 900 mAh/g at 100 mA/g, and 520 mAh/g at 1 A/g at the 40th charging–discharging cycle.

Electrolysis of MoS2 to produce Mo nanopowders and elemental sulfur has been studied in an equimolar mixture of NaCl and KCl at 700 °C. The reduction mechanism was investigated by cyclic voltammetry (CV), potentiostatic and constant voltage electrolysis together with spectroscopic and scanning electron microscopic analyses. The reduction pathway was identified to be MoS2 → LxMoS2 (x ≤ 1, L = Na or K) → L3Mo6S8 and LMo3S3 → Mo, and the last step to format metallic Mo was found to be relatively slow in kinetics. Electrolysis at a cell voltage of 2.7 V has led to a rapid reduction of MoS2 to nodular Mo nanoparticles (50–100 nm), with the current efficiency and energy consumption being about 92% and 2.07 kW h kg−1-Mo, respectively.

Graphene-based composite materials have attracted considerable interest due to their exceptional performance in various applications. However, the present synthesis processes, usually via graphene oxide (GO), are still very expensive. Here we propose an easy and affordable strategy based on sulfuric-acid-intercalated GO (SIGO) for the preparation of graphene-clamped nano-SnO2 (GCSnO2) with high performance for lithium-ion batteries. SIGO is the direct and readily available intermediate product during the oxidation of graphite in sulfuric acid, but has been overlooked for nearly a century. In the past, SIGO was washed to produce clean GO with great difficulties. An interesting characteristic of SIGO that we have found is its easy expansion and exfoliation to high-quality graphene at very low temperatures (just above 100 °C). In this work, GCSnO2 containing 55 wt% SnO2 nanoparticles (5–10 nm in diameter) has been prepared by the expansion and exfoliation of nano-SnO2 coated SIGO at 300 °C in air. The samples have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA). The initial reversible charge–discharge capacity of GCSnO2 was 858 mA h g−1 at a current density of 200 mA h g−1 in the potential range between 0.02 and 2.00 V. The capacity decayed to about 600 mA h g−1 after 10 cycles and then remained almost unchanged; 572 mA h g−1 remained after the studied 270 cycles. The contribution of SnO2 was estimated to be about 800 mA h g−1 during cycling, corresponding to the full and stable utilization of the theoretical capacity of SnO2.

Laboratory studies of electrochemical reduction of refractory metal oxides, e.g. TiO2 and Ta2O5, in molten CaCl2 often involve a graphite anode and a cell voltage of 3.0 V or higher, which deviates significantly from thermodynamic predictions. The causes considered in the past have included mechanistic, kinetic and dynamic complications of cathode reactions, but little was considered on anodic processes. This paper shows that oxidation of the O2− ion on the graphite anode is also a significant contributor to the high cell voltages applied. Cyclic voltammetry in molten CaCl2 containing added CaO (up to 2.51 mol%) suggested that O2− oxidation on graphite proceeds dominantly in two steps as previously observed on glassy carbon. With increasing CaO concentration, the second step became rate-limiting over a wide range of potentials before the processes reached at diffusion controlled high current density. This understanding led to the proposal and experimental confirmation of a “low anode current density strategy” in potentiostatic reduction of thin cylindrical pellets of TiO2 and Ta2O5 in molten CaCl2 at 850 °C. It was observed that a 10-fold increase of the graphite anode area could decrease the cell voltage by about 1.0 V, which should save energy consumption by up to one third.

The electrochemical reduction of solid silica has been investigated in molten CaCl2 at 900 °C for the one-step, up-scalable, controllable and affordable production of nanostructured silicon with promising photo-responsive properties. Cyclic voltammetry of the metallic cavity electrode loaded with fine silica powder was performed to elaborate the electrochemical reduction mechanism. Potentiostatic electrolysis of porous and dense silica pellets was carried out at different potentials, focusing on the influences of the electrolysis potential and the microstructure of the precursory silica on the product purity and microstructure. The findings suggest a potential range between −0.60 and −0.95 V (vs. Ag/AgCl) for the production of nanostructured silicon with high purity (>99 wt%). According to the elucidated mechanism on the electro-growth of the silicon nanostructures, optimal process parameters for the controllable preparation of high-purity silicon nanoparticles and nanowires were identified. Scaling-up the optimal electrolysis was successful at the gram-scale for the preparation of high-purity silicon nanowires which exhibited promising photo-responsive properties.

The outward diffusion equation of the instantaneously released ion in pores of a porous layer was derived theoretically, and then applied for the study of oxygen ion diffusion in porous Fe which was generated by fast electrochemical reduction of solid Fe2O3 in molten CaCl2. The solid cathode was constructed by filling mixed Fe/Fe2O3 powders (8:1 in mass ratio) into cylindrical cavities in a Mo foil substrate. This fabricated mini porous electrode was subjected to the double-potential treatment with selected potentials according to the cyclic voltammetry (CVs) of Fe2O3. The lower potential of −0.7 V (vs. quartz sealed Ag/AgCl) enabled fast reduction of solid Fe2O3, and instantaneous release of O2− ions into the pores of the generated porous Fe. During the standing time at −0.7 V, outward O2− diffusion occurred, and the amount of O2− remaining in the porous electrode was determined by re-oxidation at a potential of 0.16 V. The experimental outward diffusion of O2− was in good accord with the theoretical equation, consequently, the diffusion coefficients of O2− in CaCl2 contained in porous Fe were evaluated as 8.2 × 10−6, 9.1 × 10−6 and 1.0 × 10−5 cm2 s−1 at 1108, 1123 and 1138 K respectively. These data followed the Arrhenius’ equation with diffusion activation energy of about 67.8 kJ mol−1. The whole work can provide a simple method for the study of diffusion in a porous electrode, and the designed ultrafast deoxidation of solid Fe2O3 may be also helpful to the present effort in developing iron metallurgy by molten salt electrolysis.

A theoretical model correlating the precursor porosity, P (in volume percentage), the metal-to-oxide molar volume ratio, R, and the cathode volume shrinkage S   (in volume fraction, experimentally determined) (the PRS model) with the deoxidation speed of the solid metal oxide cathode in molten chlorides has been developed, allowing accurate prediction of the optimal cathode porosity by a simple equation, Popt=3R+S−13R×100%. Tests of this equation in practices suggest Popt to be an intrinsic parameter to the oxide. For example, the predicted values of Popt for the electrolysis of TiO2 and Ta2O5 cathodes are about 67% and 25% respectively, and a cathode porosity far away from Popt would suppress the deoxidation seriously. The established model has been well verified by the electrolysis of the oxides of Si, Ti and Ta.Highlights► Theoretical model for electro-reduction of solid oxide cathode in molten chlorides ► Accurate prediction of the optimal cathode structure for the fastest deoxidation ► The optimal cathode porosity is an intrinsic parameter of an oxide. ► The precursor porosity has great impact on the deoxidation speed. ► Verification of the model by electrolyses of the oxides of Si, Ti and Ta

Hydrogen bonding between protonated monoethanolamine and chloride ion can benefit the capture and thermal stabilisation of carbon dioxide in hydroxyl imidazolium based ionic liquids for potential reclamation of the captured carbon by, for example, electrolysis and catalytic synthesis.

Electrochemical desulfidation of solid sulfide has been studied for the first time in the mixed NaCl–KCl melt. At 700 °C, the equimolar NaCl–KCl enabled highly efficient transport of the S2− ion and high speed electro-reduction of porous WS2 pellets to W nanonodules (50–100 nm). The graphite anode was experimentally confirmed to be an ideal non-consumable anode, on which the S2− ion was oxidised to gaseous sulfur. The current efficiency of the electrolysis of WS2 was higher than 90% and the energy consumption could be lower than 1.23 kWh/kg-W. These findings promise new green methods for the preparation of various transition metals and alloys by electro-desufidation of their solid sulfides.

The cyclic voltammograms of a silica sheathed tungsten disc (W-SiO2) electrode in molten CaCl2 at 900 °C exhibited an unusually increasing reduction current with decreasing the potential scan rate. When the cathodic limit was less negative than −1.00 V (vs. a quartz sealed Ag/AgCl reference electrode), the reduction current was also smaller in the forward (negative) potential scan than that in the reversed (positive) scan. However, at a given reduction charge, the reduction current increased with the scan rate, following approximately a logarithm law. These unique features have been elaborated according to the dynamic model of the conductor (silicon)/insulator (silica)/electrolyte (molten salt) three-phase interlines (3PIs). Combining the voltammetric observations with the composition analysis of the products from potentiostatic electrolysis of porous silica pellets, the optimal potential window was identified to be from −0.65 V to −0.95 V. In this potential range, silica was converted to pure silicon with the oxygen content being less than 0.5 wt.%. At potentials more negative than −0.95 V, the reduction of Ca2+ ions in the reduction-generated porous silicon layer led to the formation of various calcium silicides. These findings can help the development of an electrolytic process for clean, efficient and inexpensive production of high purity silicon.

Electrochemical reduction of a solid TiO2−ZrO2 mixture (molar ratio = 1:1) to the TiZr alloys in a consolidated porous structure was studied by constant cell voltage electrolysis in molten CaCl2 at 900 °C. For the first time, and surprisingly, it was found that tuning the α- and β-phases in the TiZr alloys could be easily realized by controlling the electrolysis time or, specifically, the oxygen content in the alloys. Oxygen acted as a phase-transition inhibitor from the high-temperature β-TiZr to the low-temperature α-TiZr, leading to metastable β-TiZr or (α+β)-TiZr as the product at room temperature. However, if the oxygen content decreased to <1100 ppm, only α-TiZr was collected, even when the electrolytic alloy was directly quenched in water. The consolidated porous (α+β)-TiZr alloys possessed elastic moduli in the range of 20−40 GPa, matching closely to that of natural bone (10−30 GPa). They also exhibited excellent corrosion resistance in the Ringer solution. These novel findings promise a simple, one-step, low-energy, and environmentally friendly electrochemical process for the production of β-phase containing TiZr alloys as medical implant materials without any added expensive and/or bio-incompatible elements.

Fe2O3 and TiO2 powders were compounded in different proportions at elevated temperatures. Porous thin pellets were made from the compounded oxides and then electro-reduced to the respective ferrotitanium alloys and/or intermetallic compounds in solid state in molten CaCl2. Typical electrolysis conditions were 800–1000 °C, 2.8–3.2 V and 4–15 h. X-ray diffraction, scanning electron and optical microscopy, and potentiodynamic polarisation were used to characterise the oxide precursors and/or the products. The results showed that the obtained Fe–Ti alloys achieved the designated elemental compositions. When the Fe content in the oxide precursor was less than 50 wt.%, the products were mainly mixed Ti and Fe–Ti alloys. At higher Fe contents, the products changed to a mixture of Fe2Ti and Fe. Between 8 and 15 wt.% Fe, the products sintered most severely. The Fe-rich Fe–Ti alloys had better corrosion resistance than a common ship hull steel (E36) in simulated sea water, i.e. the aqueous solution of 3 wt.% NaCl. The Ti-rich Fe–Ti alloys (8 wt.% Fe) had good corrosion resistance to the 1.0 mol/L HCl solution. The addition of Nb in the alloys improved the corrosion resistance, but the addition of Al caused the opposite effect.

Graphene and graphene/metal oxide composite materials have attracted considerable interest for use as energy materials due to their excellent electrochemical performances. Here, we propose using melamine as a template for the synthesis of cambered nano-walls of SnO2/rGO materials. Melamine powder can effectively absorb SnO2/GO from the solution to form a core–shell structure of melamine@SnO2/GO. After thermal reduction of GO at 200 °C to form the melamine@SnO2/rGO, melamine was dissolved in hot water at 80 °C, leaving behind the cambered SnO2/rGO nano-walls. Melamine is recyclable since it precipitates when its solution cools to room temperature. The thickness of the SnO2/rGO nano-walls can be easily controlled by adjusting the mass ratio of melamine to SnO2/GO. When the mass ratio was set to ten, cambered walls of SnO2/rGO with a thickness of about 100–200 nm were achieved. The resulting SnO2/rGO delivered an initial reversible capacity of 998 mA h g−1 at a current density of 100 mA g−1 and a capacity of 855 mA h g−1 after 100 discharge–charge cycles in a potential range between 0.02 and 3.0 V vs. Li/Li+. It also showed good rate performance with a reversible capacity of 460 mA h g−1 at 1 A g−1. These high capacities can be linked to the special cambered nano-walls which ensure fast solid diffusion in addition to providing an effective liquid-channel and buffer-volume in the electrode. The proposed synthesis method is easily scalable and should be applicable to many other graphene based energy materials.

Electrolysis of MoS2 to produce Mo nanopowders and elemental sulfur has been studied in an equimolar mixture of NaCl and KCl at 700 °C. The reduction mechanism was investigated by cyclic voltammetry (CV), potentiostatic and constant voltage electrolysis together with spectroscopic and scanning electron microscopic analyses. The reduction pathway was identified to be MoS2 → LxMoS2 (x ≤ 1, L = Na or K) → L3Mo6S8 and LMo3S3 → Mo, and the last step to format metallic Mo was found to be relatively slow in kinetics. Electrolysis at a cell voltage of 2.7 V has led to a rapid reduction of MoS2 to nodular Mo nanoparticles (50–100 nm), with the current efficiency and energy consumption being about 92% and 2.07 kW h kg−1-Mo, respectively.

The electrochemical reduction of solid silica has been investigated in molten CaCl2 at 900 °C for the one-step, up-scalable, controllable and affordable production of nanostructured silicon with promising photo-responsive properties. Cyclic voltammetry of the metallic cavity electrode loaded with fine silica powder was performed to elaborate the electrochemical reduction mechanism. Potentiostatic electrolysis of porous and dense silica pellets was carried out at different potentials, focusing on the influences of the electrolysis potential and the microstructure of the precursory silica on the product purity and microstructure. The findings suggest a potential range between −0.60 and −0.95 V (vs. Ag/AgCl) for the production of nanostructured silicon with high purity (>99 wt%). According to the elucidated mechanism on the electro-growth of the silicon nanostructures, optimal process parameters for the controllable preparation of high-purity silicon nanoparticles and nanowires were identified. Scaling-up the optimal electrolysis was successful at the gram-scale for the preparation of high-purity silicon nanowires which exhibited promising photo-responsive properties.