Mohamed S. El-Deab

An extraordinary electrocatalytic reduction of oxygen on gold nanoparticles-electrodeposited gold electrodes ☆

The cathodic reduction of oxygen has been investigated at a gold nanoparticles-electrodeposited gold electrode in 0.5 M H2SO4 solution. Two well-defined reduction peaks were observed at +50 and −250 mV vs. Ag/AgCl/KCl (sat.). Those two peaks indicated a 2-step 4-electron reduction pathway of O2 in this strong acidic medium. The former peak was ascribable to the 2-electron reduction of O2 to H2O2, while the latter was assigned to the reduction of H2O2 to H2O. The observed electrocatalysis for the reduction of O2 is attributable to the extraordinary catalytic activity of the gold nanoparticles over the bulk gold electrode, at which the 2-electron reduction peak of O2 to H2O2 was observed at −200 mV.

Hydrodynamic voltammetric studies of the oxygen reduction at gold nanoparticles-electrodeposited gold electrodes

The electrocatalytic reduction of oxygen at Au nanoparticles-electrodeposited Au electrodes has been studied using rotating disk electrode (RDE) voltammetry in 0.5 M H2SO4. Upon analyzing and comparison of the limiting currents data obtained at various rotation speeds of this RDE with those obtained at the bulk Au electrode, an effective value of the number of electrons, n, involved in the electrochemical reduction of O2 was estimated to be ca. 4 for the former electrode and ca. 3 for the bulk Au electrode at the same potential of −350 mV versus Ag/AgCl/KCl(sat.). This indicates the higher possibility of further reduction and decomposition of H2O2 at Au nanoparticles-electrodeposited Au electrode in this acidic medium. The reductive desorption of the self-assembled monolayer of cysteine, which was formed on the Au nanoparticles-electrodeposited Au electrode, was used to monitor the change of the specific activity of the bulk Au electrode upon the electrodeposition of the Au nanoparticles.

Electrochemical Reduction of Oxygen on Gold Nanoparticle-Electrodeposited Glassy Carbon Electrodes

The electrochemical reduction of oxygen (O 2 ) on Au nanoparticle-electrodeposited glassy carbon electrodes (GCEs) has been performed in 0.1 M phosphate buffer solution (pH 7.2). Two well-separated electrochemical reduction peaks for O 2 on GCE were observed at about -750 and -2000 mV vs. Ag/AgCl/KCl (sat.) i.e., a peak separation of ca. 1250 mV. Those two peaks were attributed to the two-step four-electron reduction of O 2 to H 2 O through H 2 O 2 . A remarkable decrease of the separation of the two peaks (down to 550 mV) along with a significant positive shift of the two reduction peaks of O 2 to -350 and -9880 mV, respectively, were observed upon loading of a very minute amount of Au nanoparticles onto the GCE (typically 2.78 X 10 - 7 g cm - 2 ). Further positive potential shift of the two peaks along with a concurrent decrease of the peaks separation could be achieved by controlling the extent of Au loading on the GCE. Au-electrodeposited GCE with an equivalent Au film thickness of 10 nm showed almost the same behavior toward the O 2 reduction as the bulk Au electrode. These observations were interpreted in view of the increase of the effective (real) surface area of the Au film by the increase of its thickness, as indicated by scanning electron micrographs in addition to the characteristic cyclic voltammogram for Au nanoparticle-electrodeposited GCEs in N 2 -saturated 0.05 M H 2 SO 4 .

Size and Crystallographic Orientation Controls of Gold Nanoparticles Electrodeposited on GC Electrodes

Gold nanoparticles were electrodeposited onto glassy carbon-electrodes (nano-Au/GC) in the presence of two different additives, namely, cysteine and iodide ions. The electrochemical characterization of the electrodeposited nano-Au/GC electrodes was performed via the measurements of the reductive desorption patterns of a thiol (e.g., cysteine) self-assembled monolayer as well as the cyclic voltammetric response toward the oxygen reduction reaction in alkaline medium. The structural characterization of the electrodeposited Au nanoparticles was performed by the scanning electron microscope. The nano-Au/GC electrodes prepared in the presence of 100 μM cysteine were surprisingly found to be enriched in the Au(100) and Au(110) facets and are characterized by a relatively big particle size up to 300 nm as well as low particle density (number of particles per unit area). The Au nanoparticles prepared in the presence of 100 μM I - ions were found to be much enriched in the Au(111) facets and are characterized by a relatively narrow particle size distribution range (10-40 nm) as well as a high particle density. Analysis of the X-ray diffraction data revealed a significantly decreased ratio of Au(111) domains of the Au nanoparticles electrochemically deposited in the presence of cysteine. These preliminary results suggest a simple way to control the size as well as the preferential crystallographic orientations of gold nanoparticles.

Morphological Selection of Gold Nanoparticles Electrodeposited on Various Substrates

Gold nanopanicles with different morphologies (nanocrystallites, perfect nanospheres, plumbs, and nanoaggregates) have been electrodeposited on different substrates, namely, glassy carbon (GC), highly oriented pyrolytic graphite (HOPG), and Au(111) single-crystalline substrates. Au particles with particle size ranging from a few nanometers to a few micrometers have been prepared. The morphology of the electrodeposited Au particles was largely dependent on the nature of the substrate as well as the composition of the electrodeposition bath. For instance, the inclusion of iodide ions during electrodeposition was found to enhance two-dimensional (2D) growth of the Au nanoparticles, and particles with a relatively small particle size down to 10 nm were obtained. The inclusion of L-cysteine (as an additive) during the electrodeposition of the Au nanoparticles resulted in a significant influence on the morphology (and the particle size of the Au particles), which strongly depends on the nature of the substrate. Au nanoparticles with crystalline geometry were prepared on the Au(lll) substrates in the presence of L-cysteine, while under the same experimental conditions Au aggregates of size up to 300 nm were electrodeposited on the GC substrates. Au particles with a perfect spherical shape were electrodeposited on the HOPG electrodes. X-ray diffraction measurements of the electrodeposited Au particles revealed significantly different crystallinity of the Au particles and in turn different ratios of the single-crystalline domains constituting the Au particles. The cyclic voltammetric response toward the oxygen reduction reaction at the different Au nanoparticles showed a versatile behavior ranging from a quasi-reversible two-electron reaction to an irreversible overall four-electron reaction in O 2 -saturated 0.5 M KOH solution, demonstrating the entirely different electrocatalytic activity of the thusprepared Au nanoparticles on different substrates.

Fabrication of Au(111)-Like Polycrystalline Gold Electrodes and Their Applications to Oxygen Reduction

In the present study, we introduced a simple method for the fabrication of Au(111)-like polycrystalline Au electrodes via the formation of a submonolayer [i.e., sub-self-assembled monolayer (sub-SAM)/Au] of a thiol compound [e.g., cysteine (CYST), mercaptoacetic acid, or cystamine]. The oxygen reduction reaction (ORR) in alkaline medium (O 2 -saturated 0.5 M KOH) performed at these sub-SAM/Au electrodes proceeds via a two-electron quasi-reversible pathway irrespective of the charge of the terminal group of the thiol [with anodic-to-cathodic peak separation (ΔE p ) of about 60 mV]. This behavior is similar to that observed at the Au(111) single-crystalline electrode in the same medium. The presence of iodide ions in the alkaline medium leads to a significant negative shift of the reduction peak while the anodic peak is completely ceased. This indicates the blocking of the Au( 111) domain of the sub-SAM/Au by the iodide ions, leading to the complete inhibition of the anodic oxidation of the hydrogen peroxide formed during the cathodic scan. At the CYST sub-SAM/Au electrodes, the O 2 reduction is completely hindered in O 2 -saturated 0.1 M KI, while the quasi-reversible behavior of the sub-SAM/Au electrode toward the ORR is restored after ten successive potential cycles between +200 and -500 mV at a scan rate of 50 mV s -1 in O 2 -saturated 0.5 M KOH. This indicates the high stability of the submonolayer of CYST at the Au electrode and that the I - ions [which possess a strong adsorption tendency toward Au] cannot replace the CYST molecules.

Electrochemical Preparation of a Au Crystal with Peculiar Morphology and Unique Growth Orientation and Its Catalysis for Oxygen Reduction

Gold nanocrystals with a peculiar pin-like morphology have been obtained on glassy carbon (GC) electrodes by electrochemical deposition from 0.5 M H 2 SO 4 solution containing 1.0 mM Na[AuCl 4 ] in the presence of cysteine as an additive. The amount of such electrodeposited Au particles as well as the particle size increased with the increase of deposition time (t d ). Crystalline facets of Au(100) and Au(110) were significantly enriched in the presence of cysteine during the electrodeposition, and the Au crystals grew along a unique crystalline orientation ofAu(lll). Finally, Au crystals (with the average diameter of ca. 100 nm and length of ca. 1660 nm at t d = 2000 s) with a peculiar pin-like morphology were obtained, in contrast to the plump Au particles prepared in the absence of cysteine. The cathodic peak current density for the O 2 reduction in acidic electrolyte (0.5 M H 2 SO 4 ) at the Au crystals-electrodeposited GC electrode prepared in the presence of cysteine was larger than that at the electrode prepared in its absence. This behavior could be reasonably explained in view of the higher specific surface area of the Au crystals prepared in the presence of cysteine than those prepared in the absence of cysteine.

Electrocatalytic Reduction of Oxygen at Au Nanoparticles–Manganese Oxide Nanoparticle Binary Catalysts

The electrocatalytic reduction of molecular oxygen (O 2 ) has been performed in O 2 -saturated 0.5 M KOH solution at binary catalysts of Au nanoparticles and manganese oxide (MnOx) nanoparticles electrodeposited onto glassy carbon (GC) electrodes. Cyclic voltammetric (CV) and rotating ring-disk electrode (RRDE) techniques have been utilized in this study. CV measurements showed that the electrochemically deposited MnOx nanoparticles have a good catalytic activity toward the disproportionation of hydrogen peroxide (HO 2 - in alkaline media) to water and O 2 . A significant positive shift of the peak potential of the oxygen reduction reaction (ORR) was observed upon loading Au nanoparticles onto the GC substrate. The combined use of MnOx and Au nanoparticles resulted in the occurrence of the ORR at a potential close to that obtained at Pt electrodes, supporting an apparent four-electron reduction pathway as revealed from the RRDE voltammetric results. The current study proposed a novel binary catalyst system (composed of metal/metal oxide nanoparticles) as a very promising candidate to replace the costly Pt-based electrocatalysts in alkaline air electrodes.

Tailor-Designed Platinum Nanoparticles Electrodeposited onto Gold Electrode Catalytic Activity for Oxygen Reduction

This work examines the electrocatalytic reduction of molecular oxygen at Pt nanoparticles (nano-Pt) electrodeposited onto polycrystalline gold (poly-Au) electrode in O 2 -saturated 0.5 M KOH. Cyclic and hydrodynamic voltammetry techniques were used for the evaluation of the electrocatalytic activity of the nano-Pt modified poly-Au electrodes. The nano-Pt was electrodeposited onto unmodified poly-Au electrode (nano-Pt/Au) and poly-Au electrode modified with a submonolayer (Au sm ) of a thiol compound (typically cysteine) (nano-Pt/Au sm ). In the latter case, Pt nanoparticles were electrodeposited, preferentially, at the bare fraction of the poly-Au electrode free from cysteine [i.e., the Au(111) domains of the poly-Au] while the other facets (i.e., Au(100) and Au(110)) were under the protection of cysteine. The nano-Pt electrodeposited onto the Au sm is characterized by small average particle size (ca. 10 nm) compared to that electrodeposited onto the unmodified Au electrode (ca. 50-100 nm). The nano-Pt/Au sm electrode offers a high electrocatalytic activity toward the oxygen reduction reaction (ORR) as compared with the nano-Pt/Au and bare polycrystalline Pt electrodes. A 60 mV positive shift of the onset potential of the ORR was obtained at the nano-Pt/Au sm electrode as compared to the bare Pt electrode. A proper design of the nano-Pt/Au binary catalyst is the key point which controls the catalytic activity of the proposed electrocatalyst.

Electrocatalytic production of hydrogen on reticulated vitreous carbon

Reticulated vitreous carbon (RVC) was used as a porous cathode for the production of hydrogen gas from flowing alkaline solution. Polarization curves were measured to evaluate the overall performance of the RVC electrode. By an aid of a mathematical model, the kinetic parameters for HER at different conditions were estimated by fitting the experimental data with the model predictions. Black nickel coatings onto the RVC porous matrix reduced the potential and consequently the electric power needed for water electrolysis. The results showed also that operating the cell at high electrolyte concentrations and/or temperatures decreases the potential required to obtain a certain rate of hydrogen production. Stable current-transient and SEM micrographs were obtained after operation of the coated electrodes for a relatively long time at high rates of hydrogen evolution. This indicated that black Ni coatings did not flack off the surface after this period.