Electrochemical performance of pseudo-capacitor electrodes fabricated by Electrophoretic Deposition inducing Ni(OH)₂ nanoplatelets agglomeration by Layer-by-Layer

Abstract

The electrochemical behaviour of ceramic semiconductors not only depends on the characteristics of the electroactive material but also on the processing method, the nanoparticles arrangement and the consolidation degree of the formed microstructure. In this sense, the use of nanoparticles with plane morphologies (disc, platelets, etc.) results interesting due to the formation of conduction pathways produced as a consequence of their laminar structures. Electrophoretic Deposition (EPD) is a shaping methodology which allows achieving high degrees in nanoplatelets packing by controlling their alignment during the coating process specifically over 3D substrates. In this work, we have studied the effect of a moderate nanoplatelets agglomeration, by tuning their surfaces with a polyelectrolyte multilayer following a Layer-by-Layer (LbL) methodology and fixing the electric conditions of the EPD process. Overcoming the destructive effects of the full agglomeration of nanoplatelets, NiO films with a stable and extremely open macroporous structure were processed to coat Ni foams, improving the capacitive performance of pseudocapacitors leading to values of specific capacitances of 650 F/g. Results collected in this work also evidence that an efficient ordering and orientation of nanoplatelets in EPD mainly depends on tuning the suspension parameters (solid contents, conductivity, electrophoretic mobility, etc.) to avoid the massive flux and interactions among interparticles and electro-hydrodynamic forces, as well as the interference of collateral electrode phenomena.

Introduction

Among a large variety of nonspherical colloids, nanodiscs or nanoplatelets are particularly interesting as they enable the bottom-up assembly of layered nanocomposites, which enhances the ionic/electronic conduction throughout the formation of crystallographic pathways, or combines unmatched strength and toughness in laminar structures. Advances in nanoplatelets applications imply mastering their manipulation, during their assembly, orientation, packing, and the film deposition [1]. In these structures, the spontaneous organization of nanoentities is the key challenge. The nanosheets of graphite and graphene [2], [3], hydroxides and oxides [4], [5], chalcogenides [1] or metallic nanoclusters [6], have a tendency to vertically stack in suspension, forming aggregates, due to strong Van der Waals interactions between the basal planes of their lattices. Dissimilar surface charge density can be also used to force nanoplatelets arrangement by a layer-by-layer strategy (LbL, i.e. alternating negatively charged nanosheets and cationic polyelectrolytes), or their reorientation in a magnetic or electric field. Then the formulation of stable dispersions of nanoplatelets can be used as slurries or inks to prepare tailored thin films by drop casting, inkjet printing, or electrophoretic deposition (EPD) [7].

In the EPD process, charged nanoparticles move toward the electrode with the opposite charge under the influence of an electric field, and deposit forming a compact film. The EPD behaviour of a large variety of nanoplatelets in nature and dimensions was studied during the last decade for different applications (Table 1). An example are the LbL assembly of alternate modified gibbsite nanoplatelets (100–200 nm in diameter) and polyelectrolyte multilayers, where the shaped smooth surfaces play a crucial role in the mechanical properties of the final lamellar structure [8] or the nanoplatelets alignment leads to optically transparent and flexible coatings [9]. In these works, Lin et al. [8], [10] demonstrated how gibbsite can be oriented during EPD by the particular distribution of charges in their surfaces and edges, when they are electrostatically stabilised at pH 7 in a mixture of H₂O and EtOH. Further works of the same authors demonstrate that modified gibbsite-silica stabilised by the adsorption of high molecular weight Polyethylenimine (PEI) leads to crack-free coatings, but simultaneously nanoplatelets preferential alignment is slightly deteriorated. Those results suggest that the branched PEI molecule acts as flocculant disturbing the nanoplatelets ordering.

In a lower range of particle size [11], [12], [13], ZnO-based films made on aligned nanoparticles (20–35 nm in diameter) leads to high packing green density and room transparency even previously to the sintering process [12]. In those publications, the difference in charge of ZnO basal planes determines the massive adsorption of the stabilizer (branched PEI) in one side of the nanoflake, setting their hydrodynamic behaviour and then fashioning deposition. In a low concentrated suspension (1 g/L), hydrodynamic, polarization and also interparticles forces govern the arrangement of flake-like dispersed nanoparticles at the electrode. When the solid concentration increases, a massive arrival of particles prevents this ordering effect. This clearly opens the possibility of controlling the nanoplatelets orientation by EPD tuning their electrokinetics by designing the stabilization system.

The mechanisms of the deposit formation of graphene sheets have been also recently described in the literature. Most of the studies describe the stabilization of graphene by an electrostatic mechanism, and argue that the charge neutralisation is the main cause of the deposit formation [14]. But in the EPD of graphene sheets, side reactions, such as GO reduction, aqueous electrolysis, composite clusters (ceramic or metallic nanoparticles/graphene) have to be considered. Although low solid loadings deserve the graphene sheets alignment [15], even moderately for the deposition of composite clusters (TiO₂ nanoparticles and graphene nanoplatelets) [3], side reactions can disturb the nanoplatelets arrangement.

Finally, most recent studies of the EPD of flakes-like clusters of Au and Cu [6], in the micronic range, also suggest that the key for governing the deposition efficiency is the lateral size of the sheets (the shape factor), inferring that the sheets with a large lateral size possess high electrophoretic mobility and strong face-to-face Van del Waals interactions, thus leading to high deposition efficiencies.

The experimental work described in those manuscripts (Table 1) suggests that there are several reasons that can explain the nanoplatelets alignment during EPD. Depending on the nanoflakes crystallography the charge distribution in basal planes is different than in the flake edges, or even different between both basal planes. That promotes strong Van der Walls interactions among the nanoplatelets and the electrode surface, but also among particles, during deposit growth. From the point of view of dispersion, the mechanism of stabilization (electrostatic/electrosteric) should promote the individual movement of dispersed nanoentities. However, it is necessary to consider that the yield rate prevail over other parameters, and the solid content, the electrophoretic mobility (zeta potential) and the conductivity should be adjusted to avoid the massive arrival of particles to the electrode, and also the premature agglomeration of nanoplatelets that can disturbs the contribution of the osmotic flow of the solvent to the nanoplatelets ordering, predicted by models [16], [17], [18]. Those premises should be considered in the fabrication by EPD of electrodes and structures based on nanoplatelets stacking.

On the other hand, in energy storage applications, electrodes based on NiO and Ni(OH)₂ nanostructures (3D and 2D nanoparticles) have demonstrated a high performance, especially when metallic foams were used as substrates of the active material or collectors [19], [20]. It is well known the viability of EPD to cover 3D substrates or complex shapes, so the number of publications dealing with the preparation of films by EPD for energy storage and generation devices, and the functionalization of biostructures, is increasing. EPD is also recognised as the available technique to achieve the highest degrees of nanoparticles packing, even using diluted suspensions or sols, where rheological requirements are lowered [21]. Consequently, there are a strong research interest to transfer packing and ordering of the coatings achieved by EPD in flat substrates, to cover the skeleton of 3D structures, especially scaffolds or metal foams [22], [23], [24].

In this line, Wu et al. [25] described the EPD of synthetic Ni(OH)₂ nanoplatelets, with a specific surface area of 170 m²/g and a mayor diameters of 50–150 nm, in which nanoplatelets are stabilized by means an electrostatic mechanisms, adding water, acetone and iodine to Isopropyl alcohol. These suspensions had an elevated conductivity (provided by the iodine addition) and zeta potential. Nanoflakes coagulate at the electrode due to the increases of solid content and the neutralisation (reduction) of the ionic double layer, since the pH of the cathode is mainly basic. But significant hydrogen bubbling had also observed. These EPD conditions and an elevated solid loading of the suspension (10 g/L) result on a random deposition of Ni(OH)₂ nanoplatelets, which promotes the formation of the mesoporous microstructures after the coating aneling (300 °C-1 h), achieving capacities of 1409 F/g at 1.0 A/g with 92% of capacity retention after 2500 cycles.

Ni(OH)₂ electrodes based on aligned nanoplatelets, and also built by a full yield EPD process on 3D substrates, have been recently reported in the literature for both pseudocapacitors and Li-Batteries (LIBS) [23], [26]. Described results demonstrate the viability of the EPD process for replicate the nanoplatelets alignment in a dense and connected microstructure either in flat substrates or in 3D metallic foams. However, in those systems the high packing density limits the capacity performance of Ni(OH)₂ nanoplatelets. Consequently, in order to increases the open meso- and macroporosity in the semiconductor electrodes, evading the loss of connectivity, the surface modification of nanoplatelets by the Layer-by-Layer (LbL) approach has been proposed [22]. Recent results shown that in a porous microstructure shaped by EPD of Ni(OH)₂ modified by LbL adsorption of polyeletrolites, the direct contact between the electrolyte ions and the semiconductor material is favoured leading to a specific capacitance of 400 F/g at 2 A/g with 100% retention. The use of LbL approach as porous template seemed to be limited to the 3 layers assembly of polyanions and polycations, due to side effect of agglomeration induced by the LbL modification of nanoplatelets, which impedes the homogeneous deposition by EPD.

This work is aimed to overcome this agglomeration effect, by tuning nanoplatelets surface modification and EPD conditions, to coat 3D electrodes with a stable and extremely open mesoporous structure, which will increase the capacitive performance of the pseudocapacitor electrode.