(Invited) One-Dimensional Ammonium Nickel Phosphate Nanorods for the Fabrication of High-Performance and Flexible Symmetric Pseudocapacitors
Authors: Ozoemena Kenneth I. , Raju Kumar 
Published: February 2020

There has been a considerable research interest in the use of electrochemical capacitors (ECs), supercapacitors (i.e., all-carbon based energy storage systems) and pseudocapacitors (i.e., redox-based energy storage systems) for the development of next generation energy storage systems for portable, flexible and wearable electronics. Supercapacitors are known for their high power density, while the pseudocapacitors are known for their high energy density [1-3]. The most attractive flexible electronics are those that employ high-performance ECs, i.e., with high energy and power densities.
The performance of any EC-driven technology, including flexible electronics, is solely dependent on the physicochemical properties of their electrode materials. Electrode materials with 1D and 2D nanostructures (such as the nanorods and nanosheets) maximize the super-/pseudo-capactive properties by their unique ability to permit ion propagations [4,5]. In addition, 1D nanostructures greatly address the space-confined transport phenomena thus enhance the charge accumulation and Faradaic redox reactions [4,6].
Phosphate-rich mesoporous materials (PRMM) such as NH4CoPO4.H2O and NH4NiPO4.H2O have been reported to exhibit attractive properties for pseudocapacitors. Hitherto, however, there is no report on PRMM nanowires/nanorods for pseudocapacitors. Motivated by the attractive properties of PRMM and 1D nanomaterials in supercapacitor applications, and paucity of literature on 1D PRMM nanostructures, we have explored the possibility of tuning the structure of porous ammonium nickel phosphate hydrate (NH4NiPO4.H2O) (ANP) nanorods for high-performance all-solid-state, flexible pseudocapacitors.
Here, we introduce a facile one-pot strategy to synthesize various morphologies of ANP by hydro/solvothermal route in ethylene glycol (EG), water and mixed solvents of EG/water using nickel acetate and ammonium phosphate in template-free and additive-free protocols. Three different morphologies (nanorods, nanodendrites, and nanoplatelets) with different sizes were obtained based on the reaction times and types of solvents employed.
In this presentation, we will prove that ANP nanorods (Figure 1a), in contrast to their nanoplatelets and nanodentdrites counterparts, exhibit excellent improvement in pseudocapacitance. As exemplified in Figure 1b, the energy and power densities of the nanorods of ANP
Figure 1: (a) FESEM image of ANP nanorods and (b) Ragone plots of different nanostructures of ANP-based all-solid-state flexible symmetric pseudocapacitors obtained at different times (24, 36 and 48 h) in the presence of EG, water (W) and mixed solvents. The electrolyte used was PVA/KOH.

References

1) A.T. Chidembo, K.I. Ozoemena, B. O. Agboola, V. Gupta, G.G. Wildgoose, R.G. Compton, Energy Environ. Sci.3 (2010) 228–236.
2) J. N. Lekitima, K.I. Ozoemena, C.J. Jafta, N. Kobayashi, Y. Song, D. Tong, S.W. Chen, M. Oyama, J. Mater. Chem. A, 1 (2013), 2821 – 2826.

3) C.J. Jafta, F. Nkosi, L. le Roux, M.K. Mathe, M. Kebede, K. Makgopa, Y. Song, D. Tong, M. Oyama, N. Manyala, S.W. Chen, K.I. Ozoemena, Electrochim. Acta
2013, 110, 228 – 233
4) X. Zhao, B.M Sánchez, P.J. Dobson, P.S. Grant, Nanoscale,
3,839-855 (2011).
5) X Peng, L. Peng, C. Wu, Yi Xie. Chem. Soc. Rev.
43,3303-3323 (2014).
6) Y.R. Ahna, C.R. Park, S.M. Jo, D.Y. Kim,Appl. Phys. Lett. 90, 122106 (2007).

Figure 1