Ag:BiVO4 dendritic Hybrid-architecture for High Energy Density Symmetric Supercapacitors

We demonstrate the fabrication of Ag:BiVO4 with a dendritic architecture by a template free hydrothermal method. Then, symmetric cells based on Ag:BiVO4 electrodes were assembled which exhibit an extended voltage window of up to 1.6 V with an excellent energy density of 2.63 mW h cm−3 (38.43 W h kg−1) and a power density of 558 mW cm−3 (8.1 kW kg−1).

Germany) were used as starting materials.All the chemicals were of AR grade and were used without any further purification.BiVO 4 , Ag:BiVO 4 nanostructures were synthesized by facile solvothermal method.In a typical procedure, initially a mixed solvent was prepared using 5 ml HNO 3 , 5 ml C 2 H 5 OH and 65 ml H 2 O. Subsequently, 2.5 mmol of Bi (NO 3 ) 3 .5H 2 O and 2.5 mmol of NH 4 VO 4 were dissolved separately in each of 35 ml of as prepared solvent.The two solutions were mixed at room temperature under vigorous magnetic stirring and maintained at pH=7 by addition of NH 3 .
Furthermore, the solution of Ag (1 wt.%) was added dropwise and stirred for 10 min.After being cooled to room temperature naturally, the precipitate was collected and washed with distilled water and ethanol thoroughly, dried at 60 o C for 4 h and used for further characterization.For comparison pure BiVO 4 was also prepared were prepared by same method except use of C 2 H 5 OH in solvent system.

Materials Characterization:
The phase analysis of the samples were performed by X-ray diffraction (XRD) on a Rigaku-Ultima III with CuKα radiation (λ = 1.5418Å).The surface morphology of as-prepared samples were investigated using the field-emission scanning electron microscopy (FEI Quanta 650F Environmental SEM) attached with an energy-dispersive X-ray spectroscopy (EDS) analyzer to measure the sample composition.Microstructure investigation was conducted using field emission transmission electron microscopy with a JEOL JSM 2200 FS microscope operating at 200 kV.

Electrochemical measurements
In order to check the electrochemical performances, the working electrodeswere prepared by using Doctor Blade method.For this, 85 % of active material (Ag:BiVO 4 ) was mixed with 10 % PVDF as binder and 5 % acetylene black.A few drops of N-Methyl-2-pyrrolidone (NMP, solvent) were added and the mixture was homogenized using a mortar to get a uniform paste.Finally, the paste was applied on commercial flexible carbon cloth which was further used as SCs electrodes.The resultant thin films were then annealed at 180 o C for two hours in order to remove the binder.The typical mass loading of the electrode material was around 0.5-0.9mg/cm2.The electrochemical properties were measured using standard three electrode system which contain working electrodes (BiVO 4 and Ag:BiVO 4 ), counter electrode (platinum) and reference electrode (Ag/AgCl) in 6 M KOH electrolyte.Symmetric cell was constructed in a 3-way Teflon Swagelok cell using two identical electrodes of Ag:BiVO 4 with polypropylene separator sandwiched between them and few drops of 6 M KOH electrolyte.Two channels from potentiostat were connected together in such a way that one channel records voltage between two electrodes (positive and negative) and other channel records potential contributed from positive and negative electrodes with respect to reference electrode.All electrochemical measurements (cyclic voltammetry (CV) and galvanostatic charge-discharge techniques) were carried out using a Biologic VMP3 potentiostat.

Supporting information S. I. 2 Reaction and growth mechanism for Ag:BiVO 4 nano-architecture formation
The evolution of well-defined dendrite morphology via hydrothermal route invites the discussion for reaction mechanism and formation mechanism in detail.The plausible reactions taking place in the Ag:BiVO 4 hybrid architecture are shown below, In a present chemical reaction, the bismuth cations and vanadium anions are provided by the hydration of BiNO 3 .5H 2 O and NH 4 VO 3 , respectively.Initially, the hydrolysis of Bi(NO 3 ) 3 .5H 2 O takes place and form a soluble BiONO 3 which readily react with VO 3 -ions at Ph ~ 6-7 forming pale yellow precipitate of tetragonal BiVO 4 .During hydrothermal treatment, after getting sufficient thermal energy, the generated BiVO 4 nuclei aggregates and converted to highly crystalline form of monoclinic BiVO 4 crystals.Meanwhile, the addition of Ag into the reaction medium produces Ag (NH 3 ) 2 + complex on reaction with NH 3 . 1The formed Ag (NH 3 ) 2 + complex ions takes sufficient thermal energy to drive-reach the surface of BiVO 4 sites wherein gets reduced to Ag by ethanol under high temperature and pressure conditions. 1,2It is expected that simultaneous nucleation of BiVO 4 and Ag might occur which further grow according to self assembly and Ostwald ripening mechanism.It is known that a higher monomer (precursor) concentration in solution generally favors 1D growth whereas the lower monomer concentration favors 3D growth. 3In the present case, the hydrothermal reaction system with relatively lower reactant concentration, sufficient temperature (180 o C) and optimum pH (= 7) value provides higher chemical potential, and faster ion movement, thereby resulting in formation Ag:BiVO 4 hybrid dendritic structures.It is noteworthy that the proper growth of dendrites was observed for Ag:BiVO 4 compared to pure BiVO 4 sample which might be due to effect of addition of C 2 H 5 OH as a reducing agent in solvent system, playing a key role on facilitating crystal growth and tailoring the morphology.
From morphology evolution point of view, initially, primary particles starts to aggregate to minimize surface energy, offering driving force for self assembly.The self assembly proceeds by the rotation of particles via Brownian motion and short-range interaction between the particles, causing formation of smaller branches (rod like) that act as building blocks for dendrites.Furthermore, due to prolonged reaction time (24 h) the continuous growth of larger particles are occurred at expense of smaller particles known as Ostwald ripening to form larger subunits or branches to adjust it to achieve a minimum total surface free energy. 3It is presumed that the an isotropic growth of formed branches along [001] direction is responsible for formation of 3D hierarchical BiVO 4 dendritic structures. 4Thereby, the obtained Ag:BiVO 4 dendritic structures preserves several key features of interlinked connectivity through branched surfaces, propound effect of Ag nanocrystals, improved surface properties and stability.
Therefore it really sense to examine the performance of this kind of material for energy application.electrode.This proves that the synergetic effect between Ag and BiVO 4 hybrid plays a key role for improving the electrochemical properties by facilitating electronic conduction as well as charge discharge processes.All these results directly reveal the feasibility of using Ag:BiVO 4 as a negative electrode for high performance supercapacitor device.

Supporting information S.I. 7
The cell (device) capacitance (C) and volumetric capacitance of the symmetric devices were calculated from their CVs according to the following equation: Where I is the applied current, V is the volume (cm 3 ) of the whole device (the whole volume of our device is about 0.069 cm3), Δt is the discharging time, ΔV (V) is the voltage window.
Volumetric energy (E, Wh/cm3) and power density (P, W/cm) of the devices were obtained from the following equations: E = Cv

Fig. S. I. 5
Fig. S. I. 5 Cyclic voltammetry (CV) curves of (a) BiVO 4 and (b) Ag:BiVO 4 electrodes at different where, C A and C V are areal and volumetric capacitances, respectively.Q(C) is the average charge during the charging and discharging process, V is the volume (cm 3 ) of the whole device (The area and thickness of our symmetric cells is about 00.088 cm.Hence, the whole volume of device is about 0.069 cm3 , ΔV (V) is the voltage window.It is worth mentioning that the volumetric capacitances were calculated taking into account the volume of the device stack.This includes the active material, the flexible substrate and the separator with electrolyte.Alternatively, the cell capacitance (C cell ), areal (C A ) and volumetric (C V ) capacitance of the electrode (C V ) was estimated from the slope of the discharge curve using the following equations: /cm3 ) is the energy density, CV is the volumetric capacitance obtained from Equation (5) and ΔV (V) is the voltage window, P (W/cm3) is the power density.

Table S2 :
Comparison of Supercapacitor values of Transition Metal Oxide (TMOs) based