Formation of nano-scaled crevices and spacers in NiO-attached graphene oxidenanosheets for supercapacitors

Abstract

Graphene oxide nanosheets were deposited directly onto the stainless steel substrate using electrophoretic deposition (EPD). The nickel ions adsorbed at the graphene oxide surface were reduced electrochemically to form nickel nanoparticles during the EPD process in the presence of a nickel nitrate additive. The attached Ni nanoparticles converted to NiO nanoparticles after annealing at 300 C. The NiO nanoparticles functioned as nanospacers to improve the face-to-face aggregation of the graphene oxide sheets. The nano-scaled crevices could be formed in the NiO-attached graphene oxide electrode after heat treatment due to different shrinkage or expansion rates between the graphene oxide and NiO nanoparticles. The wettability of electrode might be improved by the deposition of hydrophilic NiO nanoparticles at the surface of graphene sheets. Therefore, the specific capacitance of the NiO-attached graphene oxide reached as high as 569 F g À1 , which is 40 times higher than that of the bare graphene oxide electrode (13 F g À1 ) at a discharge current density of 5 A g À1 .

Electrophoretic deposition of graphene-related materials: A review of the fundamentals

“…The charge neutralisation mechanism has also been reported to govern the deposit formation for GRM sheets decorated with other charged species, similar to the case of GO, including PIHA (polymeric isocyanate crosslinked with hydroxy functional acrylic adhesive) [127], hyaluronic acid [133], methyl violet dye [112], aluminon [134], safranin [113] and Ni ions [96]. In one instance, however, chemical bonding was shown to occur between the deposit, rGO, and the working electrode, TiO 2 nanotube arrays, [71] during anodic EPD from rGO aqueous suspensions.…”

“…However, these hydroxides may also improve the deposits' adhesion onto the substrates [142]. Metallic nanoparticles may form within graphene/GO layers either by reduction of the metal ions when they come into contact with the cathode's surface during EPD [63,96] or during post-EPD annealing or reduction steps [99]. While this phenomenon can be beneficial for production of composite films, care is needed to avoid the unwanted contamination of the deposits.…”

“…However, high concentrations of these ions can destabilize the suspensions, whilst, their electrophoresis and accumulation on the electrode's surface can have negative effects on the deposition of the desired GRM [90] as described in Section 5. iv. Typical metal salts that are used for charging GRM for EPD are nitrates of aluminium [91][92][93][94], nickel [95][96][97], zinc [98], cobalt [99], manganese [63,100], magnesium [17,20,31,39,68,85,86,[101][102][103][104][105][106][107][108] and magnesium chloride [109], although, copper sulfate has been used recently [110]. The performance of these ions, however, may be pH dependent.…”

“…For example, electrodes for electrochemical double layer supercapacitors rely on high structural porosity as well as a large interlayer separation of GRM sheets to allow effective electrolyte access and to maximise the active surface area for charge storage [15,63,96,124,143]. On the other hand, field emission of electrons at low voltages requires a high surface roughness and a predominance of well distributed, individualised GRM edges [39,85,91,92,101,[144][145][146].…”

Graphene based metal and metal oxide nanocomposites: synthesis, properties and their applications

“…In particular, their oxides, in particular HRG/NiO and HRG/Ni(OH) 2 have been applied as electrode materials in supercapacitors. 346,347 Owing to their high theoretical capacity (2584 F g -1 within 0.5 V for NiO), their low price and excellent pseudocapacitive behavior, both NiO and Ni(OH) 2 have been considered as suitable replacement for the more expensive amorphous RuO 2 . 348,349 However, these materials largely suffer from poor electrical conductivity, low accessible surface areas and volume changes during the cycling process.…”