2120
J.B. Wu et al. / Electrochimica Acta 56 (2011) 2116–2121
Table 1
Pseudocapacitances at different discharge current densities (corresponding to Fig. 6b and c).
Discharge current densities (A g−1
)
2
4
8
10
20
40
Pseudocapacitance for NiO/Ag composite film (F g−1
Pseudocapacitance for NiO film (F g−1
)
330
261
328
247
322
225
308
221
287
203
281
191
)
we can see that when the discharge current density increases to
40 A g−1, the pseudocapacitance for the porous NiO film decreases
to 191 F g−1, while that for the porous NiO/Ag film maintains a
high level of 281 F g−1. The enhancement of pseudocapacitance is
due to the conductive network formed by the highly dispersed Ag
the charge efficiency and diminish the polarization with lower
charge voltage plateau and higher discharge voltage. Thus, the
reaction NiOOH → NiO can proceed to a higher extent with higher
specific capacitances.
OER and ease the oxygen bubbles striking, leading to better cycling
stability.
For pseudocapacitor application, the reaction kinetics is an
important factor. The reaction kinetics has been evaluated by
EIS (Fig. 8). The impedances of both film electrodes consist of a
depressed arc in high frequency regions and a straight line in low
frequency regions. Generally, the semicircle reflects the electro-
chemical reaction impedance of the film electrode and the straight
line represents the diffusion of electroactive species [18,19]. Obvi-
ously, the NiO/Ag film exhibits a much smaller semicircle and
slower slope. It is well accepted that bigger semicircle means larger
charge transfer resistance and higher slope signifies lower ions dif-
fusion rate. It is concluded that the NiO/Ag composite film has much
lower charge transfer resistance and ion diffusion resistance than
the NiO film, indicating the composite film is favorable for charge
transfer and ion diffusion. The difference of reaction kinetics can be
attributed to the introduction of Ag nanoparticles. The presence of
Ag nanoparticles can keep the NiO flakes electrically connected, and
offer conductive pathways among the NiO nanoflake, the substrate,
and the electrolyte [15], leading to lower charge transfer resistance
and ion diffusion resistance with fast reaction kinetics.
Fig. 7 shows the capacity retention properties of the porous
NiO and NiO/Ag composite films at 2 A g−1. Similar cycling char-
acteristics are observed for both films. Upon cycling, the porous
NiO/Ag composite film exhibits higher specific capacitance and bet-
ter cycling stability than the unmodified NiO film. The capacity
deterioration of the porous NiO/Ag composite film is restricted to
a very lower level, even after long-term cycling, while that of the
NiO film diminishes quite quickly. The porous NiO/Ag composite
film delivers a specific capacity of 312 F g−1 after 3000 cycles, higher
than that of the NiO film (206 F g−1 after 3000 cycles). It is indicated
that the introduction of Ag nanoparticles in NiO/Ag film is beneficial
to the enhancement of cycling stability. The degradation mecha-
nism of NiO film is a complex process mainly associated with two
factors: a self-discharge phenomenon associated with a partial dis-
solution of NiO and oxygen bubbles striking [17]. It is well accepted
that the pseudocapacitive process of NiO is a reversible reaction
of NiO/NiOOH, which is accompanied by a spontaneous chemical
conversion of NiO into Ni(OH)2 at the electrode/electrolyte inter-
face. This side process leads to a progressive degradation of the NiO
film. Besides, the oxygen evolution reaction (OER) is a competitive
reaction with the electrochemical process of Ni(II)/Ni(III) couple
during the cycle. The oxygen bubbles strikes the film accelerat-
ing the degradation process. As shown in CV result above (Fig. 5),
the porous NiO/Ag composite film shows a potential of OER with
0.576 V. The potential difference between oxidation peak potential
and OER potential is 0.105 V, while that for the porous NiO film is
0.084 V, meaning that the Ag in the composite film can suppress
4. Conclusion
A porous net-like NiO/Ag composite film is successfully pre-
pared by combining chemical bath deposition and silver mirror
reaction, successively. The as-prepared NiO/Ag composite film
exhibits a highly porous cross-linked structure with diameter of
pores ranging from 30 to 250 nm. Its high porosity and large
surface area can facilitate the contact between electrolyte and
the oxide surface and allow easy diffusion of ions among them.
Compared to the unmodified porous NiO film, the NiO/Ag com-
posite film exhibits better pseudocapacitive performance with
weaker polarization, higher specific capacity, better reversibility
and preferable cycling performance. The NiO/Ag composite film
shows pseudocapacitances of 330 F g−1 at 2 A g−1 and 281 F g−1 at
40 A g−1, respectively. The improved electrochemical performances
are attributed to the introduction of Ag nanoparticles in the com-
posite film, which improves the electric conductivity of the film
electrode.
9
8
Porous NiO/Ag film
Porous NiO film
7
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0
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Zre / Ω
Fig. 8. EIS plots of both film electrodes with 100% depth of discharge.