0-Nanoporous Ni(OH)2 thin film on 3D Ultrathin-Graphite Foam for Asymmetric Supercapacitor-

exfoliated graphite oxide('a-MEGO')as the negative electrode.The highest power density of 27times higher),while the energy density was comparable to or higher,than high-supercapacitor had a capacitance retention of63.2%after10000cycles.

supercapacitor.asymmetric

shorten the electron and ion di?usion

with the aim of more e?cient

and mass exchange.Such nano-

composites have always been

metal current collector to obtain

continuous conductive structure,which

decreases the overall gravimetric

capacitance.The typical addition

(a)The XRD patterns of the Ni(OH)2/UGF

UGF alone.(b)XPS spectra acquired

/UGF composite,of the Ni2p3/2region

electrochemical cycling test.

highly random porous architecture, composed of nano?akes with thickness

(Figure2a,inset).Some Ni(OH)

observed in the solution from the same

were not attached to the UGF,and

of nano?akes,were roughly spherical

of about2μm(Figure S3,see Supporting Information).

the decomposition of urea,Ni(OH)

the UGF surface as well as in the

anisotropy of the Ni(OH)2crystals leads

'nano?akes'.13,28The pH value

growth of the nano?akes and Ostwald-ripening as larger?akes grow and consume

The large?akes were found to aggregate,

a reduction in total surface energy.

on the UGF surface leads to the

porous Ni(OH)2thin?lm,while nucleation leads to spherical particles to minimize energy.13,28,31à33The structure of the

composite was also studied by EDS

àf).The elemental maps of O and

and continuous dispersion throughout

electrochemical properties of the Ni(OH)2/UGF composite were?rst evaluated using a three-electrode in6M KOH aqueous solution.Figure3a shows

curves of Ni(OH)2/UGF composite at various rates in the potential range of0à0.5V vs Ag/AgCl.

of oxidation and reduction peaks can be seen,

are due to the reversible reaction of Ni(OH)2t

NiOOHtH2Oteà.34,35The Coulombic e?-

obtained from the CV curve at the scan rate

mV/s is99.7%.The high Coulombic e?ciency indicated fully reversible pseudocapacitive behavior of Ni(OH)2/UGF composite electrode.With increasing

rate,the current increased and the oxidation peaks shifted to a more positive position and reduction peaks to a more negative position.

due to an increase of the internal di?usion resistance within the pseudoactive material with an increase in scan rate.33,36The gravimetric capacitance

Ni(OH)2/UGF electrode at various current densities calculated from the discharge curves and is shown Figure3c.A gravimetric capacitance based on the mass of the electrode(i.e.,UGFtNi(OH)2)was

at a current density of0.5A/g(the current density normalized based on the total mass of the electrode). The gravimetric capacitance can reach(1.56(

103F/g based on the mass of Ni(OH)2alone at the

SEM images of the Ni(OH)2/UGF composite,(a)SEM image of the composite material,and the inset shows cation image of the Ni(OH)2nano?akes;(b)a cross-sectional view of the composite;(c)SEM image corresponding EDS elemental mapping images of(d)carbon,(e)oxygen,and(f)nickel.

3.Three-electrode electrochemical measurements of the Ni(OH)2/UGF composite in6M KOH aqueous solution.

at di?erent scan rates;(b)galvanostatic chargeàdischarge curves at various current densities;(c)gravimetric capacitance of Ni(OH)2/UGF composite as a function of the current densities,(d)cycling performance of Ni(OH) composite at10A/g,inset shows the CV curves before and after cycling at5mV/s,respectively;(e)CV curves(at5mV/s, magnifying CV curve of Ni(OH)2on carbon paper);(f)Nyquist plots of the as-prepared Ni(OH)2/UGF composite,and nanoparticles slurry after drop-casting on UGF or carbon paper.

highly dependent on the cell operating according to E=C single3V2/8,1.8V was used as

potential to further investigate the electrochem-performance of the asymmetric supercapacitor.

di?erent scan rates(ranging from5to

measured under1.8V in6M KOH(aq)

4b).A combination of both pseudoca-

EDL types of capacitance was clearly

scan rates.The current increased with

and the oxidation peak shifted to a more potential,similar to what was observed in three-electrode con?guration.The galvanostatic charge/discharge plots at di?erent current densities with window of1.8V are shown in Figure4c.The capacitance calculated based on the total mass of

(total mass of both positive and negative reached119F/g per electrode at a current

The capacitance retention of the asym-

10000cycles of charge/discharge at a

of5A/g is63.2%(Figure S7,see Support-

and indicates a relatively good stability asymmetric pseudocapacitor.The capacitance loss

the loss of the electrical contact between materials(Ni(OH)2)and the UGF because of

volume of the active materials during cycling.

densities and power densities of the supercapacitor(based on the total mass of

including the mass of UGF)are shown

(see Supporting Information).A maxi-gravimetric energy density and power density including the UGF current collector in

cell.The corresponding volumetric

and power density were estimated

and55.3kW/L,respectively,based on

trode density of0.65g/cm3(Table S2,

Information for details of the calculation).

the performance of the fully packaged

cell was also estimated(Table S2).With

of22%for the active electrode materials

aged device,a maximum practical gravimetric density of 6.9Wh/kg(highest power

44.0kW/kg)and volumetric energy density (highest power density of30.8kW/L)

for the packaged device.Table S3(see Information)summarizes high-end commercial capacitor devices from various companies Ragone plot is shown in Figure4d.The

of our hybrid supercapacitor is comparable

than most of the organic-based high-end capacitors,but the power density is

(2à27times higher)than all the commercially

able cells.Considering the use of environmentally benign aqueous electrolyte,our hybrid

tor shows promise for application as

energy device.

CONCLUSION

In conclusion,a3D interconnected

composite was made by growth of a

thin?lm on the lightweight and highly

curves of Ni(OH)2/UGF//a-MEGO asymmetric supercapacitor(a)measured at di?erent potential window erent scan rates in6M KOH solution and(c)galvanostatic chargeàdischarge curves at Ragone plots of the asymmetric supercapacitor based on the full cell,comparing with supercapacitors.