Insights into electrode/electrolyte interfacial processes and the effect of nanostructured cobalt oxides loading on graphene-based hybrids by scanning electrochemical microscopy Free

Increasing global demand of electric energy calls for reliable and sustainable energy conversion and storage systems, especially for portable and mobile electronics. To meet these challenges, several alternatives to diminish fossil fuel reserves are being meticulously explored.^1,2 Electrochemical energy production and storage are under serious consideration as alternative renewable energy sources including fuel cells, rechargeable secondary batteries, and electrochemical supercapacitors (and pseudocapacitors).^3–6 The realization of such devices requires the development of high-performance advanced multifunctional materials and electrode scaffolds. The persistence of nanoscale carbons for “greener” energy is due to structural polymorphism, chemical stability, and wider operational potential window in organic electrolytes, rich surface chemistry, and relatively inert electrochemistry. Likewise, transition metal oxides and hydroxides are considered promising due to facile synthesis approaches, tunable electrical conductivity, controllable surface morphologies, and relatively higher capacitance due to a variety of redox reactions during rapid charging-discharging with available multi-oxidation states (Meⁿ⁺, n = 2,3,4) and as cost-effective noble-metal-free electrocatalysts.^7,8 However, they suffer from structural degradation and mechanical stability by themselves thus leading to faster capacitance decay limiting their performance. Substantial efforts have been taken to stabilize them, for instance, by nanostructuring and combining with nanoscaled carbons to form hybrids.^9–11 

Graphene is the focus of several electrochemical technologies including energy storage systems,^12,13 chemical/biological sensing,^14–16 and electrocatalysis due to its unique two-dimensional honeycomb lattice structure, high specific surface area (∼2,630 m² g⁻¹), and exceptional physical-chemical properties.¹⁷ Moreover, atomic-scale defects in graphene alter its properties that allow optimization for targeted application.^18,19 Doping and corrugation of graphene basal plane and edge sites are also sources of enhancing its electrochemical reactivity.²⁰ Furthermore, due to easy processability for scalability, graphene derivatives (e.g., graphene oxide; GO, chemically reduced; rGO and electrochemical reduced; ErGO) are equally promising due to rich surface electrochemistry attributed to diverse functional moieties. Among these efforts, we expand the scope of graphene derivatives (GO, rGO, and ErGO) since synergistic effects result from their interactions (covalent, electrostatic, stronger chemisorption or physisorption and chemical bridged proximity)^21,22 with transition metal oxides, thus creating tailored properties and interfaces for high−performance electrochemical electrodes (see Fig. S1, supplementary material).^23,24 The electrochemical energy storage and conversion and storage rely on different mechanisms, which range from purely adsorptive (supercapacitive) to pseudocapacitive (or Faradaic) redox reactions and electrocatalytic processes at the surface of electroactive electrodes.^25–28 Considering “true” electrochemical reactions that involve charge transfer, electrochemical energy storage properties and electrocatalytical activity of low-dimensional carbon materials are intertwined,^29,30 which are effected by several electrochemical descriptors such as surface coverage, electrode potential, solvation, and electrolyte pH. Recent developments in graphene-based hybrid materials invigorated interests in creating advanced electrochemical electrode scaffolds and electrocatalytic supports. Therefore, their expansive applications in sustainable energy and noble-metal-free electrocatalysis require us to understand the fundamental underpinnings of energy storage mechanisms and kinetics, predominated by physicochemical processes occurring at electrode/electrolyte (or solid/liquid) interface, while quantifying the structure-property-electroactivity relationship establishments for enhnaced energy storage.³¹ While computational methods is proving to be powerful tools in revealing and providing valuable insights for the design of electrode materials,³² it is challenging to include all of these effects accurately into a computational model.

In this work, we report on the development of graphene/cobalt oxide hybrids as viable high-performance hybrid pseudocapacitors. They consist of electrodeposited nanostructured cobalt oxide (CoO and Co₃O₄) polymorphs on electrochemical processed GO, rGO, and ErGO establishing chemical bridged electrical contact and nanoscale manipulation (either spatially controlled with nanoscale resolution or cobalt oxide loading) and compare to those physisorbed samples.²⁶ These materials have also been the focus of electrochromic devices, gas sensing, rechargeable secondary batteries,^33,34 and electrocatalytic sensing platforms.^35,36 We employed a scanning electrochemical microscopy (SECM, hereon) to investigate the electrode kinetics and to gain insights into physicochemical interfacial processes³⁷ as well as to determine the possible redox pathways and to quantify the associated physical parameters. In general, SECM is an electrochemical surface imaging technique, where an ultramicroelectrode tip is moved from bulk solution toward the surface (substrate) under study, and the current at the tip is measured with respect to distance from the surface to obtain approach curves (see Fig. S2, supplementary material). Alternatively, keeping the tip height above the substrate fixed, the position of the tip is scanned in the xy plane to obtain plots of the tip feedback current with position above the substrate. This allows imaging (or mapping) the electrochemical activity/reactivity of the electrode surface at nanometer spatial resolution.³⁸ SECM has not been used to investigate the electron transfer kinetics of graphene-cobalt oxide hybrids, and therefore, the current study presents unprecedented fundamental insights into electrode/electrolyte interfacial processes.

We prepared the graphene-cobalt oxide hybrids via chemical and electrochemical reduction processed graphene followed by anchoring nanostructured cobalt oxides and cobalt nanoparticles by electrodeposition, where the scheme is shown in Fig. 1(a). The electrodeposition was carried out using amperometry described in detail elsewhere (supplementary material) along with the electrodeposition curves (see Figs. S3(a)–S3(c)). Figure 1(b) shows the scanning electron microscopy images revealing the surface morphology of hybrids including CoO/GO, Co₃O₄/GO, CoO/ErGO, Co₃O₄/ErGO along with GO, ErGO, CoO, Co₃O₄ constituents. We have also studied some of the samples with physical deposition (PHYS subscript with sample names)²⁷ and cobalt nanoparticles on ErGO¹¹ for comparison with those of chemical bridged interfaces obtained by electrodeposition for efficient electron transfer. The SEM images show a relatively uniform surface morphology where interconnected network and crumpled graphene nanosheets (GNS) and nanowalls are decorated with optimally loaded Co_xO_y. The presence of micro-particles with size ranging 50–100 nm is apparent that is irregularly or non-uniformly distributed. Also, the GNS is decorated with Co_xO_y preferentially at the edges or on the walls, which helps to prevent restacking of GNS. It is conceivable that below a critical concentration of nanostructured cobalt oxides, the GNS may aggregate. HRTEM images (Fig. S4, supplementary material) reveal polycrystalline microstructure and porous network consisting of nanoparticles of CoO and Co₃O₄ with grain size ranged 10–50 nm. Structural order is evident from the lattice fringe patterns, which can be associated with the crystallinity of cobalt oxides by themselves and GNS decorated particles. The cyclic voltammograms (CVs) shown in Fig. 2 are obtained from CoO/GO, CoO/ErGO, Co₃O₄/GO, Co₃O₄/ErGO, Co₃O₄/GO_PHYS, and Co₃O₄/rGO_PHYS electrodes at the scan rate of 20 mV/s between −0.2 and +0.8 V in 0.5 M KOH electrolyte. They are measured in a three-electrode electrochemical cell (macro-electrode configuration) and with SECM (disk shaped micro-electrode configuration) highlighting the unique differences between the magnitude and signatures. An illustration showing a comparison between a macroscale electrode (planar or linear diffusion) and a microscale electrode (convergent diffusion) is provided in Refs. 39 and 40.

All hybrid electrodes displayed typical pseudocapacitive behavior with characteristic broad redox peaks in stark contrast to graphene derivatives, which is nearly rectangular loops indicative of almost an ideal supercapacitor (see Ref. 42). Specifically, they displayed two sets of characteristic redox peaks centered at −0.15/+0.01 V (cathodic I/anodic II) and +0.2/+0.4 V (cathodic III/anodic IV) related to presumably redox reactions that is the conversion between different cobalt oxidation states namely, oxidation of Co(II) to Co(III) and Co(III) to Co(IV) described as⁴¹ 

corresponds to the first and second redox couple, respectively. The specific capacitance (C_s) in macroelectrode configuration is determined

where I is the response current, V_f and V_i are the integration potential limits of the voltammetric curve, v is the scan rate, and m is the mass of active electrode material. The mass was measured using a microbalance prior to and post material deposition on current carrying supports. We obtained C_s of 480–570 F.g⁻¹ for hybrids with ErGO. Likewise, we determined C_s for GO-based hybrids to be 370–410 F.g⁻¹ as well as CoO and Co₃O₄ (60–70 F.g⁻¹) comparable to those of GO and ErGO (70–90 F.g⁻¹).⁴² In traditional electrochemistry, the reactions occur across the entire electrode surface such that the ion diffusion from or to the electrode surface is planar, and the CV response current is typically described as “diffusion-limited,” giving rise to asymmetric curves (heterogeneous electrodes). However, the diffusion to or from the edge of the macroelectrode is effectively to a point (“edge effect”); therefore, the current density and the rate of mass transport are larger at the edge and diffusion becomes convergent (equivalent to microelectrode), which is the case in SECM. Alternatively, the voltammetric profiles (current magnitude and peak appearance) at disk shaped microelectrode containing a supporting electrolyte and redox agent lead to sigmoidal curves with a steady-state, diffusion-limited current, measured at electrode potentials significantly beyond the standard redox potential of the dissolved redox mediator.⁴² Since electrochemical reactions are interfacial reactions, mass transport possibly arising due to all or either of the elementary contributions (diffusion, migration, and convection) is one of the key processes to consider.⁴³ However, based on the electrodes size and time scale of the experiments, convective fluxes may still compete with diffusional fluxes in motionless solution. This may occur even in the absence of any concentration density gradients. Therefore, under given experimental conditions, it is important to consider that these differences are arising possibly due to the alteration of the diffusional steady state by natural convection close to the electrodes interface. Such a criterion then allows distinguishing properties of ultramicroelectrodes from those of other electrodes of metric sizes. Taken together, graphene-based hybrids show a significant enhancement in electrochemical performance through synergistic coupling promoted by the higher specific surface area and electrical conductivity of the GNS with redox reactions from CoO and Co₃O₄ that allow decreased ion diffusion length and faster electron transfer at the electrode/electrolyte interface in contrast to those hybrids deposited physically.

Recent electrochemical studies on graphene surface show a favorable charge transfer adsorption, i.e., at basal plane sites, which include the structural defects, imperfections, and heterogeneity disrupting sp² C conjugation and nanoscale corrugations. Also speculation is that sites with larger defect density, interfaces, and rare surface sites (edge sites) are more electroactive due to higher surface energy than those of the basal and clean surfaces. Therefore, maximizing these electroactive sites is an active area of research, though challenging. Interestingly, when these active graphene domain sites are combined with other nanomaterials including basal plane sites, they are expected to show a higher diffusion (D) and effective kinetic rate (k) transfer distribution depending upon the interaction sites (see Fig. S5, supplementary material).^41,44 Therefore, it is pertinent that we investigate the complex electrochemical processes to probe the local electron transfer/charge transport and using SECM and hopefully correlating with the structure as opposed to other scanning probe microscopy.⁴⁵ By detecting redox reactions occurring in close proximity to the electrode surface (probe approach mode), SECM is used to obtain the quantitative information of local reaction rates. Figures 3(a)–3(d) provide probe approach curves for electrodeposited and physically deposited hybrids along with constituents with the normalized distance (L = d/a), where d is the substrate (electrode)-tip distance and a is the radius of the tip. The tip electrode current (i_T) reaches plateau behavior with the steady-state current following:

where n is the number of electrons transferred at the electrode tip (O + ne^-→R), F is the Faraday constant, C is the concentration of oxidized species, and D is the diffusion coefficient limited by the hemispherical region. With tip approaching the surface of the heterogeneous electrode surface, the reduced species formed at the tip is oxidized at the conductive surface, yielding an increase in the tip current following (i_T > i_T,∞):

that creates a regenerative “positive” feedback loop. The opposite effect is observed when probing insulating region creating a “negative” feedback loop that decreases the tip current (i_T < i_T,∞)

where k₁, k₂, k₃, and k₄ are rate transfer and the related coefficients depend on the RG (ratio of insulating sheath radius to a, which is equal to 5) and normalized distance L = d_o-d_exp/a (see Table T1, supplementary material, for feasible results).³⁸ The total tip current is given by

where the symbols have usual meaning. The diffusion (D) of redox species at the hybrids ranged from 3 × 10⁻¹¹ to 7 × 10⁻⁸ m² s⁻¹ in the increasing order CoO < Co₃O₄ < CoO/GO_PHYS < Co₃O₄/GO_PHYS < Co₃O₄/rGO_PHYS < CoNP/ ErGO < Co₃O₄/GO < CoO/ErGO < Co₃O₄/ErGO complying with those determined from traditional CV curves.^7,11,42,46 A quasi-linear behavior, while plotting the current versus square root scan rate, is reminiscence of diffusion-limited (mass transport) phenomenon, and a plateaued behavior especially at higher scan rates is reflective of heterogeneous diffusion and electron transport attributed to the composite nature of hybrid electrodes. The magnitude of the ion current observed is typically governed by the Randles-Ševćik equation.¹¹ The second term in Eq. (8) is reminiscent of the extent of concavity (or convexity) of probe approach curves plotted in Fig. 3 as dashed curves. Table T1 (supplementary material) enlists the parameter values with an accuracy of <1%, which is usually smaller than typical experimental uncertainties. They are determined by fitting d_o in L and by taking different RG values for all of the heterogeneous kinetics at the tip and diffusion-controlled mediator regeneration at the substrate samples studied. The feedback approach curve then represents a measure for the absolute distance once the electrode diameter and the RG ratio are known. Once again, the electrodeposited hybrids show higher diffusion coefficients as compared to physically deposited hybrids and much smaller for constituents signifying the importance of hybrids.

The visualization of electrochemical (re)activity and mapping adsorption sites density is performed in the constant height feedback imaging mode taking advantage of an amperometric tip current that originates from the redox mediator, modulated by variations in the tip-to-sample distance and by the local electrochemical activity of the electrode surface (see Fig. S2, supplementary material). For instance, on insulating homogeneously, active surface variations in the tip current reflect variations in the sample topography. On the other hand, variations in the tip current on reasonably flat conductive surfaces are indicative of variations in the local electrochemical (re)activity of the sample. Thus, changes in the electrochemical activity (electron rate transfer) give rise to changes in the feedback (tip) current. Figures 4 and S6 (supplementary material) combined display SECM area (250 × 250 μm² or (400 × 400 μm²) scans in two-dimension contour “heat maps” and three-dimension current distribution maps using Origin software (ver. 16.0), for constituents: GO, rGO, ErGO, CoO, and Co₃O₄ and hybrids: Co₃O₄/ErGO, CoO/ErGO, CoNP/ErGO, Co₃O₄/rGO_PHYS, and Co₃O₄/GO. The tip was polarized at potential sufficient to cause an electrochemical redox reaction (generator), and the current was recorded (collected) over the polarized electrode surfaces. They show occasional higher (peak)/lower (valley) tip current characteristics of conductive/semiconductive or insulating behavior at the cobalt oxides/graphene-solution interfaces. It is apparent from the probe current distribution that the samples Co₃O₄/ErGO, CoO/ErGO, and Co₃O₄/rGO_PHYS yielded several regions of highly electroactive sites “hot spots” with areal density determined from full-width at half maximum of peaks ∼40 to 70 μm corroborated with surface morphology (Fig. 1(b)), the proposed model (Fig. S5, supplementary material) as well as theoretical calculations of electron density of states that give rise to finite density of states at or near Fermi level attributed to localized re-hybridized pockets at graphene-metal oxide interfaces.¹¹ These findings reinforce the multiple roles played by ErGO and rGO in providing a robust interconnected topological conductive network, edge plane sites (the position at which the current peaks), surface functionalities associated with π bonding and defects sites serving as anchoring sites for nanoparticles of cobalt and cobalt oxides, which not only enabled tailored and chemically bridged interfaces but also sufficient enhancement in electroactivity in electrodeposited hybrids than those of the constituents.

In summary, we have investigated hybrids consisting of graphene/cobalt oxides prepared as pseudocapacitive electrodes via electrodeposition. The SECM measurements were performed and compared with physically deposited and constituent samples. The noteworthy findings are: (1) SECM provided quantitative information about electrode kinetics and a local charge transport behavior, (2) they exhibited the synergistic coupling effects due to tailored properties, chemical bridged interfaces, and optimized Co_xO_y loading on graphene supports, and (3) SECM mapped electrochemical redox activity, i.e., higher/lower tip current and electroactive sites distribution at Co_xO_y/graphene-solution interfaces. They indicate a favorable ion adsorption and electron transfer in the order: Co₃O₄/ErGO ≥ CoO/ErGO > Co₃O₄/GO > Co₃O₄/rGO_PHYS > Co₃O₄/GO_PHYS > CoO/GO_PHYS > Co₃O₄ > CoO. These studies provided insights into physicochemical processes and nanoscale surface morphology promoted interphases/interfaces that govern their operation.

See supplementary material for brief introduction to graphene-metal oxide interfaces, the working principle of advanced electrochemical technique, scanning electrochemical microscopy (SECM), electrochemical processing of graphene (GO, rGO, ErGO), and electrodeposition of nanostructured cobalt oxides (CoO and Co₃O₄) on graphene supports to form graphene-based hybrids using the chronopotentiometry mode. The high-resolution transmission electron microscopy exhibits the polycrystalline nature of hybrid nanomaterials, scanning electrochemical microscopy images/maps of constituents and hybrids, and tabulates the parameter values derived from the Probe approach curve fit.

This work was financially supported in parts by the NSF-MRI Grant (No. 1429563), NSF KY EPSCoR RSP Grant, and WKU Research Foundation RCAP internal Award.