Gilberto Maia

A remarkably simple characterization of glassy carbon-supported films of graphite, graphene oxide, and chemically converted graphene using Fe(CN)3−6/Fe(CN)4−6 and O2 as redox probes

This study investigated the behavior of thin films of graphene oxide (GO) and chemically converted graphene (CCG) on a glassy carbon surface in the presence of two redox probes (Fe(CN)3−6/Fe(CN)4−6 and O2), employing cyclic voltammetry, electrochemical impedance spectroscopy, and hydrodynamic voltammetry as a simple procedure for characterizing these films. The feasibility of using these electrochemical techniques for this purpose opens up the possibility of applying them to biosensors and electrocatalysts using surface-supported GO and CCG materials. Raman spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy were employed to characterize GO and CCG.

Blocking properties of gold electrodes modified with 4-nitrophenyl and 4-decylphenyl groups

The electrochemical properties of Au electrodes grafted with 4-nitrophenyl and 4-decylphenyl groups have been studied. The electrografting of gold electrode surface with aryl groups was carried out by electroreduction of the corresponding diazonium salts in acetonitrile. The nitrophenyl film growth on gold was examined by atomic force microscopy, electrochemical quartz crystal microbalance and X-ray photoelectron spectroscopy. These measurements showed that a multilayer film of nitrophenyl groups was formed. Cyclic voltammetry was used to study the blocking properties of aryl-modified gold electrodes towards the Fe(CN)63−/4− redox system. The reduction of oxygen was strongly suppressed on these electrodes as evidenced by the rotating disc electrode results.

Electrochemical Modification of Gold Electrodes with Azobenzene Derivatives by Diazonium Reduction

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X-ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi-reversible voltammogram of the Fe(CN)6 (3-/4-) redox couple on bare Au and a sigmoidal shape for the GBC- and FBK-modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)6 (3-/4-) response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)6 (3-/4-) redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge-transfer rates to the Fe(CN)6 (3-) probe for the GBC- and FBK-modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.

Use of electrochemical techniques to characterize methamidophos and humic acid specifically adsorbed onto Pt and PtO films

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to study methamidophos (MAP) and humic acid (HM) specifically adsorbed onto Pt and PtO films in pH-7.0 universal buffer. The approach was found to be sufficiently selective for use in studies involving adsorption of species in environmental systems (e.g., soil minerals), typically evaluated by batch experiments and high performance liquid chromatography (HPLC) or gas chromatography (GC). The proposed method allowed quantification of active hydrogen adsorption sites blocked by HM, both when this compound is adsorbed alone or co-adsorbed with MAP. At higher amounts of MAP in the adsorption solution, the compound was co-adsorbed more effectively than HM (kept at constant concentration). In the case of sequential specific adsorption, the first compound adsorbed typically predominates over the second. EIS was more effective for determining the number of blocked active sites on Pt than CV, which was superior for PtO films.

Direct electron transfer from alcohol dehydrogenase

This study employed hydrodynamic cyclic voltammetry (HCV) with a glassy carbon (GC) rotating disk electrode (RDE), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) to investigate the direct electron transfer (DET) behavior of alcohol dehydrogenase (ADH, EC 1.1.1.1) embedded or otherwise in polymyxin (PM) and adsorbed concomitantly with cofactor NAD+ or NADH, or none on GC and covered with Nafion®. The hybrid GC/PM-ADH-NAD+/Nafion electrode thus constructed (and visualized by scanning electron microscopy (SEM)) persistently bioelectrocatalyzed ethanol oxidation and performed DET involving bakers yeast ADH, GC, and ethanol—a key finding in the present study—making this system a promising anode for use in biofuel cells. A rate constant (ks) of 0.82 s−1 was obtained for this electrode at a potential scan rate (ν) of 120 mV s−1. EIS experiments, particularly those conducted after higher potential HCV scans, allowed resistance to electron hopping (Reh) between redox centers in ADH and between these centers and ethanol molecules to be estimated as 84 kΩ lower for the GC/PM-ADH-NAD+/Nafion electrode during ethanol oxidation than for bare GC in the presence of ethanol. Nuclear magnetic resonance (NMR) unequivocally confirmed acetaldehyde production from ethanol oxidation by ADH DET.

Electrocatalysis of oxygen reduction on glassy carbon electrodes modified with anthraquinone moieties

Anthraquinone groups were electrochemically grafted to glassy carbon (GC) electrodes via methylene linker to study the oxygen reduction reaction (ORR) in alkaline medium. Two different anthraquinone derivatives, 2-bromomethyl-anthraquinone or 2-chloromethyl-anthraquinone, were used to modify the GC electrode surface. Several modification conditions encompassing potential cycling and electrolysis at a fixed potential were employed in order to vary the surface concentration of MAQ groups (ΓMAQ) and to study the dependence of the O2 reduction behaviour on electrografting procedure. Cyclic voltammetry confirmed the presence of anthraquinone moieties attached to the GC electrode and ΓMAQ varied in the range of (0.5–2.4) × 10−10 mol cm−2. Oxygen reduction was studied on MAQ-modified GC electrodes of various surface coverage using the rotating disc electrode (RDE) and rotating ring-disc electrode (RRDE) methods. The RDE and RRDE results of O2 reduction reveal that GC/MAQ electrodes show rather similar electrocatalytic behaviour towards the ORR yielding hydrogen peroxide as the final product.

Carbon-supported metal nanodendrites as efficient, stable catalysts for the oxygen reduction reaction

The search for efficient, stable electrocatalysts for the oxygen reduction reaction (ORR) has received increased attention, given the need to speed up this reaction in fuel cells. This article reports the one-pot synthesis of novel metal nanodendrites (MNDs) of Pt or Pt–Pd alloy surface-covering patterns that, supported on Vulcan Carbon XC-72, effectively catalyzed the ORR. The surface of Vulcan Carbon XC-72 exhibits raised plains interspersed with ribbed troughs, in a pattern energetically favorable to metal precipitation (deposition) into the ribbed troughs. This produces MND/C structures that are strongly catalytic toward the ORR. Mass-specific activity (MSA) of 0.56 mA μg−1 and specific activity (SA) in the 1.17–1.35 mA cm−2 range are noteworthy findings for Pt/C, Pt@Au′/C, and Pt–Pd/C MND electrocatalysts at 0.9 ViR-free, using platinum-group metal (PGM) loadings as low as 26 μg cm−2—better values, therefore, than the United States Department of Energy (DOE) targets for MSA (0.44 A mgPt−1) and SA (0.72 mA cm−2 at 0.9 ViR-free) for electrocatalysts used in portable applications to be marketed in 2017, and for cathode areal PGM loadings (<50 μgPGM cm−2), as well as better than the commercial E-Tek Pt/C (20% Pt mass) catalyst.

The Contribution of Carbon Supports on the Activity of Pt for Glycerol Electrooxidation: the Importance of Investigating the Derivative Voltammogram and Arrhenius Plots

Glycerol electrooxidation was evaluated on Pt electrodeposited over carbon Vulcan (CV), multi-walled carbon nanotubes (MWCNTs), graphene oxide nanoribbons (GONRs), and graphene nanoribbons (GNRs). Different masses of Pt were deposited under the same conditions, producing different surface areas of Pt. The presence of GNRs slightly enhanced the specific activity of the catalyst. By investigating the derivative voltammetry of glycerol, we found that the supports did not shift the onset potential towards lower values. Moreover, we found that the apparent activation energy did not vary by changing the carbon support. In this sense, we rationalized the slight improvement in specific activity of Pt deposited on GNRs as a consequence of the frequency of collision factor due to the availability of Pt over the longitudinal flat surface of nanoribbons, as shown by the high active surface area/mass of electrodeposited Pt ratio.

Electroreduction of a CoII coordination complex producing a metal–organic film with high performance toward electrocatalytic hydrogen evolution

This paper describes the synthesis and structural characterization of a novel, cheap and simple CoII complex (CoII(L)2Cl2) based on the 1,3,5-trisubstituted-pyrazoline ligand along with the electrochemical production of metal–organic electroactive films derived from this new complex. These systems were applied as electrocatalysts for hydrogen production (ACN/ACA or ACN/TFA medium) where both materials presented high performance toward hydrogen evolution. Compared to the CoII complex, the electroactive films exhibited significant electroactivity toward hydrogen evolution, presenting a remarkable TOF for H2 production (312 900 s−1, corrected by Faraday efficiency) in the presence of TFA. In addition, the generated metal–organic film showed high stability toward the electrocatalytic hydrogen production, supporting at least 1000 cycles at 20 mV s−1 in the large potential range investigated, as well as good performance and stability in the presence of 0.5 M H2SO4. Relevant insights into the mechanistic details and the role played by the CoII complex and the films during the catalytic hydrogen production are also discussed in light of the structural features and electrochemical experiments.