## 5 Conclusions  
In summary, hollow Co\({}_{3}\)O\({}_{4}\) nanoboxes were synthesized successfully by using a facile method, in which the overall sizes of the Co\({}_{3}\)O\({}_{4}\) were effectively controlled and as a consequence resulted in different supercapacitor performances. An overwhelming specific capacitance of 1832.7 F/g at 1 A/g and a comparable cycling stability of 85.9% after 5000 cycles are achieved in optimal Co\({}_{3}\)O\({}_{4}\) electrode. In particular, the Co\({}_{3}\)O\({}_{4}\) electrode still retains 72.3% of the specific capacitance, even at 20 A/g. The outstanding performances qualify the hollow Co\({}_{3}\)O\({}_{4}\) nanoboxes an attractive electrode material in research field and commercial application of supercapacitors. Furthermore, the morphologic features  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Material** & \
\begin{tabular}{c} **Specific Capacitance (F/g)/** \\ **Low Current Density (A/g)** \\ \end{tabular} &
\begin{tabular}{c} **Specific Capacitance (F/g)/** \\ **High Current Density (A/g)** \\ \end{tabular} & **Ref.** \\ \hline hollow Co\({}_{3}\)O\({}_{4}\) 3D-nanonet & 739/1 & 533/15 & [35] \\ hollow Co\({}_{3}\)O\({}_{4}\) spheres & 474.8/1 & 377.4/10 & [37] \\ hollow Co\({}_{3}\)O\({}_{4}\) spheres & 460/4 & 401/20 & [41] \\ hollow Co\({}_{3}\)O\({}_{4}\) corals & 527/1 & 412/10 & [42] \\ hollow Co\({}_{3}\)O\({}_{4}\) dodecahedron & 1100/1.25 & 437/12.5 & [43] \\ hollow Co\({}_{3}\)O\({}_{4}\) nanowires & 599/2 & 439 /40 & [44] \\ Co\({}_{3}\)O\({}_{4}\) nanowires & 977/2 & 484/10 & [45] \\ Co\({}_{3}\)O\({}_{4}\) nanosheets & 1121/1 & 873/25 & [46] \\ Co\({}_{3}\)O\({}_{4}\) nanoflaxes & 448/0.5 & 421/10 & [47] \\ Co\({}_{3}\)O\({}_{4}\) nanoboxols & 590/0.5 & 421/8 & [48] \\ Co\({}_{3}\)O\({}_{4}\) nanoplates & 355.6/0.4 & 230/4 & [49] \\ Co\({}_{3}\)O\({}_{4}\) nanofibers & 340/1 & 296/10 & [50] \\ carbon incorporated Co\({}_{3}\)O\({}_{4}\) & 978.9/0.5 & 303.3/15 & [51] \\ Mn doped Co\({}_{3}\)O\({}_{4}\) & 773/1 & 485/16 & [52] \\ Co\({}_{3}\)O\({}_{4}\)/CoMoO\({}_{4}\) & 1902/1 & 1200/10 & [53] \\ Co\({}_{3}\)O\({}_{4}\)/NiO & 1236.7/1 & 836.7/20 & [54] \\ Co\({}_{3}\)O\({}_{4}\)/NiO(OH)\({}_{2}\) & 1306.3/1.2 & 600 /12.1 & [55] \\ graphene/Co\({}_{3}\)O\({}_{4}\) & 1765/1 & 1266 /20 & [56] \\ FGO/Co\({}_{3}\)O\({}_{4}\) & 546/0.5 & 496/5 & [57] \\ NiO/Co\({}_{3}\)O\({}_{4}\)/MnO\({}_{2}\) & 1055.3/0.2 & 727.4/4 & [58] \\ hollow Co\({}_{3}\)O\({}_{4}\) nanoboxes & 1832.7/1 & 1324.5/20 & Ours \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specific capacitances at low current density and high current density (rate performance) of Co\({}_{3}\)O\({}_{4}\) based electrode materials.  
and tunable sizes make the applications of as-prepared Co\({}_{3}\)O\({}_{4}\) promisingly extend to other aspects, especially in highly accessible surface area required fields, such as catalysts and gas sensors.  
Conceptualization, X.C.; Methodology, P.O. and X.C.; Investigation, X.F., P.O. and X.C.; Data Curation, X.F.; Formal Analysis, X.F.; Validation, X.F., P.O. and X.C.; Writing--Original Draft Preparation, X.F.; Writing--Review & Editing, P.O. and X.C.; Supervision, P.O. and X.C.; Project Administration, P.O. and X.C. All authors have read and agreed to the published version of the manuscript.  
This research was mainly funded by Research Council of Norway (RCN), grant number 221860/F60.  
X.F. is funded by China Scholarship Council (CSC), grant number 201506930018. The authors also thank the Norwegian Micro- and Nano-Fabrication Facility, NorFab, project number 245963 and [PERSON] for SEM characterization. Critical comments by [PERSON] and [PERSON] are acknowledged.  
The authors declare no conflict of interest.