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Evaluation of reduced-graphene-oxide-supported gold nanoparticles as catalytic system for electroreduction of oxygen in alkaline electrolyte

Chemically-reduced graphene-oxide-supported gold nanoparticles are considered here as catalytic materials for the reduction of oxygen in alkaline medium. Gold nanoparticles are prepared by the chemical reduction method, in which the NaBH4-prereduced Keggin-type phosphomolybdate heteropolyblue acts as the reducing agent for the precursor (HAuCl4). Polyoxmetallate (PMo12O403-) capping ligands stabilize gold nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm-3 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of both gold and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of electrochemical diagnostic experiments (performed in 0.1 mol dm-3 KOH): (i) the phosphomolybdate adsorbates are removed from the interface as they undergo dissolution in alkaline medium; and (ii) the Au nanoparticles (Au loading, 30 {\mu}g cm-2) remain well-dispersed on the carbon as evident from transmission electron microscopy. High electrocatalytic activity of the reduced-graphene oxide-supported Au nanoparticles toward reduction of oxygen in alkaline medium is demonstrated using cyclic and rotating ring-disk voltammetric experiments. Among important issues are possible activating interactions between gold and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy).


Introduction
A tremendous amount of research has been carried out in the field of oxygen electroreduction, particularly with respect to potential applications in the fuel cell research [1][2][3][4][5][6][7][8][9][10][11]. Obviously, many efforts have been made to develop suitable alternative electrocatalysts efficient enough to replace electrocatalysts based on scarce strategic elements such as platinum-group metals (PGMs) [5,6,8,9,12,13]. Despite intensive research in the area of lowtemperature fuel cells with proton-conducting electrolytes, the practical oxygen reduction catalysts still utilize systems based on PGMs.
Because they operate in alkaline environment, other elements beyond PGMs can act as reasonable candidates for implementation as the oxygen reduction catalytic sites [16].
Representative examples include gold, silver and nickel-based catalytic materials that are obviously much cheaper and more abundant, and are promising for future commercialization.
In this respect, the systems based on gold nanoparticles capable of oxygen reduction in alkaline medium [17][18][19][20] should be mentioned. The enhanced catalytic activity of gold nanostructures is often attributed to existence of the high fraction of low-coordinated surface atoms at the corners and edges of Au nanoparticles, that play the roles of active sites [21,22].
The kinetics and mechanism of the oxygen electroreduction at Au(hkl) catalysts in alkaline medium is critically dependent on the interfacial gold structure as well as on the potential applied to the electrode [23]. It has been postulated that, in alkaline solutions, the fourelectron reduction of oxygen on Au(100) involves the disproportionation mechanism [19] which seems to be enhanced on Au(100) in comparison to the performance of other two low-Miller index faces of Au. For example, on Au(111), the reduction of oxygen proceeds according to the 2-electron reaction mechanism with hydrogen peroxide as the final reduction product [24]. The role of the superoxide (O 2 -) adsorptive intermediate has been postulated to be more significant during the O 2 -electroreduction in alkaline media relative to that in acidic 4 media [19,23]. When it comes to application of carbon-supported gold nanoparticles (with a diameter of 7-8 nm) for oxygen reduction, their electrocatalytic activity seems to vary inversely with the size of the particles [17]. In this respect, the decreased hydrophilicity and coordination of the smaller clusters of gold is believed to increase to the energy of binding of between O and Au and to decrease the activation energy during the dioxygen dissociative chemisorption.
The O 2 -reduction electrocatalysts are typically nanocomposite materials utilizing metal nanoparticles bearing the active sites dispersed on suitable supports. While exhibiting long-term stability, a useful support should facilitate dispersion, provide easy access of reactants, and assure good electrical contact with active sites. In spite of limitations related to the durability, carbon nanoparticles of approximately 20-50 nm diameters (e.g. Vulcan XC-72R) are commonly utilized as supporting materials. Because of the high specific surface area and excellent thermal, mechanical and electrical properties, graphene and graphene-based materials [25][26][27] have recently been considered as supports for catalysts [28][29][30][31][32]. Under such conditions, the parasite effects related to agglomeration and thus degradation of catalytic nanoparticles are likely to be largely prevented.
In the present work, we consider the chemically reduced graphene-oxide-supported gold nanoparticles as catalytic systems for the electroreduction of oxygen in basic medium.
The electrocatalytic diagnostic experiments involve comparative measurements utilizing commonly-used Vulcan (carbon) supports as carriers for the analogous gold nanoparticles.
Here application of inorganic Keggin-type heteropolymolybdates (PMo 12 O 40 3-) as capping ligands (capable of chemisorbing on both gold and carbon substrates [33][34][35][36][37][38][39][40] facilitates deposition, nucleation, stabilization and thus controlled growth of gold nanoparticles on surfaces of both Vulcan and graphene nanostructures. In the latter case, we have utilized the so-called reduced graphene oxide which, contrary to conventional graphene, still contains oxygen functional groups regardless of subjecting it to the chemical reduction step [25][26][27]. By analogy to graphene oxide, the existence of oxygen groups in the plane of carbon atoms of 5 reduced graphene oxide not only tends to increase the interlayer distance but also makes the layers somewhat hydrophilic. Furthermore, during fabrication of the catalytic systems (in acid medium), the adsorbed polymolybdates [9,34] are likely to bind gold nanoparticles via the oxygen or hydroxyl groups on graphene and Vulcan surfaces. During operation in alkaline medium, polyoxometallates decompose and disappear; but the catalytically active gold centers remain stable on carbon surfaces. It is apparent from the diagnostic cyclic voltammetric and rotating ring-disk measurements in the oxygen-saturated 0.1 mol dm -3 KOH that the system utilizing Au catalyst deposited on the chemically reduced graphene oxide has exhibited higher electrocatalytic currents (and produced lower amounts of the undesirable hydrogen peroxide intermediate) during oxygen reduction (relative to the performance of gold nanoparticles deposited on conventional Vulcan-carbon support or simple bare Au nanoparicles). The enhancement effect is particularly sound in the high potential range (0.8-1.0 V vs. RHE). On the whole, the combined effect of the high surface area and electrical conductivity of reduced graphene oxide should contribute to the overall enhancement effect and activity of Au nanoparticles toward the oxygen reduction.

Experimental
Chemicals were commercial materials of the highest available grade, and they were used as received. KOH, HCl, HNO 3  Graphene oxide sheets of 300-700 nm sizes (thickness, 1.1±0.2 nm) were from Megantech. Reduced graphene oxide (rGO) was obtained using sodium borohydride as reducing agent at 80˚C according to the procedure described earlier [41]. 6 Vulcan XC-72R conductive carbon black was purchased from Cabot and purified by treatment in 12 mol dm -3 HCl solution for 1h followed by washing out with distilleddeionized water several (3)(4) times to remove any possible leftovers. The resulting carbon black particles were functionalized with interfacial oxygen-containing groups by subjecting them to 3 mol dm -3 HNO 3 for 8 h under reflux conditions, then washed thoroughly with water, (collected under high-speed centrifuging conditions) and, finally, dried completely at 60°C.
Solutions (including 0.1 mol dm -3 KOH) were prepared by dissolving the respective chemical in the triply-distilled subsequently-deionized (with Milli-Q water purification system) water.   A rotating ring disk electrode (RRDE) working assembly was from Pine Instruments; it included a glassy carbon (GC) disk and a Pt ring. The radius of the GC disk electrode was 2.5 mm; and the inner and outer radii of the ring electrode were 3.25 and 3.75 mm, respectively.
As a rule, potentials presented in this study were recalculated and expressed against the reversible hydrogen electrode (RHE). The collection efficiency (N) equal to 0.39 was determined from the ratios of ring and disk currents obtained from six independent RRDE measurements performed at the rotation rate of 1600 rpm using the argon-saturated 0.005 mol 6 ] in 0.01 mol dm -3 K 2 SO 4 solution [42].
Prior to modification the glassy carbon electrodes were activated by polishing on a cloth wetted with successively finer grade aqueous alumina slurries (grain sizes from 5 to 0.5 μm). All measurements were carried out at room temperature 22±2°C. 8 Morphology of samples was assessed using Libra Transmission Electron Microscopy120 EFTEM (Carl Zeiss) operating at 120 kV.

Physicochemical Identity of Reduced Graphene Oxide
Graphene oxide, GO, and partially reduced graphene oxide, rGO contain various carbon-oxygen groups (hydroxyl, epoxy, carbonyl, carboxyl), in addition to the large population of water molecules still remaining in the reduced samples. Independent elemental analysis based on the C 1s and O 1s XPS spectra from XPS measurements [43] showed that the oxygen content in rGO was in the range from 8.6 to 12.1 at%; the C-to-O ratio was on the level of 7.1-10.3. When compared to the analogous parameters of the commercially available GO, the oxygen content and the C-to-O ratio values were more than three times lower and more than three times higher, respectively [43].
The rGO material has also been characterized (relative to GO) by Raman spectroscopy (Fig. 1). The assignment of the Raman bands has been carried out in accordance with the previous reports [44,45]. As expected, the Raman spectra of Fig. 1 show two large peaks in the range of 1300-1600 cm -1 : one peak near 1350 cm -1 , which stands for the D band undergo adsorption and to form electroactive adsorbates on rGO as well. Fig. 2A shows a cyclic voltammogram of PMo 12 -modified rGO which was spontaneously deposited on bare glassy carbon through dipping of the electrode substrate in the aqueous (unbound PMo 12 -free) 11 colloidal suspension of PMo 12 -modified rGO for 20 min. In view of our previous reports [34][35][36][37][38], the observed redox transitions (pairs of surface voltammetric peaks in Fig. 2A (1) where n is equal to 2, 4 or 6. It is noteworthy that the full-width values at half-maximum of the second oxidation or reduction peaks (appearing at about 0.2 V in Fig. 2A) are very close to the theoretical 45 mV (expected for an ideal two-electron surface-type voltammetric peak).
The peak heights of Fig. 2A have been found to be directly proportional to scan rates up to at least 500 mV s -1 . Furthermore, under such conditions, the formal potentials of three sets of the PMo 12 surface-peaks ( Fig. 2A) have been practically independent of the scan rate.

Formation and morphology of dispersed gold nanoparticles
The PMo 12 -modified Au nanoparticles (supported and unsupported) were characterized using Transmission Electron Microscopy (TEM). It is apparent from Fig. 3 Fig. 3a).

Electrochemical identity of Au nanoparticles
To comment on the influence of chemically-reduced graphene-oxide (rGO), relative to  14 It is apparent from the diagnostic linear scan voltammetric experiments that oxygen reduction proceeds at gold nanoparticles at fairly positive peak potentials, namely at ca. 0.8 V.

Electroreduction of oxygen
( Fig. 6). But the electrooxidation currents (net or background subtracted) have been found to be dependent on the application and choice of carbon support though sizes and distribution of Au nanoparticles are, in all cases (Fig. 3), comparable. For comparison, the activities of carbon (rGO, Vulcan) supports themselves toward reduction of oxygen under analogous voltammetric conditions are illustrated in a form of dotted lines (in Figs 6B and 6C).
The voltammetric data of Fig. 6 is consistent with the relatively highest catalytic activity of the rGO-supported Au-nanoparticles during electroreduction of oxygen in 0.1 mol dm -3 KOH. Here it should be noted that the rGO-support itself exhibits appreciable activity toward O 2 -reduction but at more negative potentials ( Fig. 6B; dotted line). Indeed the respective voltammetric peak is shifted more than 250 mV toward more negative values. In other words, at potentials where the main voltammetric peak (characteristic of rGO-supported Au) is developed (at ca. 0.8 V in Fig. 6B), the contribution from the rGO-support is negligible (compare dotted and solid lines in Fig. 6). In this respect, rGO seems to act as highly conductive robust support exhibiting both hydrophobic and hydrophilic domains that facilitate stability and mobility of charge (e.g. OHions), respectively, at the electrocatalytic interface.
Roles of possible metal (Au)support (rGO) interactions, as postulated earlier [56], in addition to interfacial structural defects on rGO, as apparent from Raman measurements ( Fig.   1), would require additional future investigations.
Similar observations concern electroreduction of oxygen at Vulcan-supported Aunanoparticles (Fig. 6C). As in a case of rGO-supported Au (Fig. 6B), application of highly conductive carbon currier has led to the enhancement of the electrocatalytic performance of the Au nanoparticles, as demonstrated by the positive shift of the peak potential from 0.77 to 0.8 V accompanied by an enlargement of the peak current (Fig. 6C), when compared to the performance of unsupported Au bare gold nanoparticles (Fig. 6A). Explanation of these phenomena should take into account possibility of better distribution of gold active sites upon 15 deposition onto carbon support and improved charge distribution at the interface.
Nevertheless, the application of rGO has resulted in the highest catalytic activity toward electroreduction of oxygen (Fig. 6B). When compared to the Fig. 6C data (Vulcan support), more than 20% higher voltammetric current density (net O 2 -reduction) has been observed upon application of rGO. It can be concluded that, in comparison to the commonly utilized Vulcan carriers, rGO seems to be a promising nanostructured material for supporting catalytic metal nanoparticles.

Diagnostic RRDE experiments
To get insight into the mechanism and dynamics of the electroreduction of oxygen at rGO-supported Au-nanoparticles, relative to the system's behavior at unsupported and Vulcan-supported Au, the rotating ring-disk electrode (RRDE) voltammetric experiments have been performed. between Au and rGO [56]. Further research is needed in this respect. where I ring and I disk are the ring and disk currents, respectively, and N is the collection efficiency. The results clearly show that the production of H 2 O 2 is the lowest for system utilizing gold nanoparticles supported onto chemically-reduced graphene-oxide (Fig. 9A). 17 The overall number of electrons exchanged per O 2 molecule (n) was calculated as a function of the potential using the RRDE voltammetric data of Fig. 10 and using equation [58,59]

Conclusions
This study clearly demonstrates that the chemically-reduced graphene-oxide acts as a robust and activating support for dispersed gold nanoparticles during electrocatalytic reduction of oxygen in alkaline medium (0. Moreover the onset potential for the oxygen reduction has been the most positive (0.9 V) in a case of the system utilizing reduced graphene oxide. It should be mentioned here that the synergistic effects and activating interactions between catalytic metal nanoparticles and nanostructured graphene supports were postulated before [56,57,59]