Direct In Situ Raman Spectroscopic Evidence of Oxygen Reduction Reaction Intermediates at High-Index Pt(hkl) Surfaces.

The study of the oxygen reduction reaction (ORR) at high-index Pt(hkl) single crystal surfaces has received considerable interest due to their well-ordered, typical atomic structures and superior catalytic activities. However, it is difficult to obtain direct spectral evidence of ORR intermediates during reaction processes, especially at high-index Pt(hkl) surfaces. Herein, in situ Raman spectroscopy has been employed to investigate ORR processes at high-index Pt(hkl) surfaces containing the [011̅] crystal zone-i.e., Pt(211) and Pt(311). Through control and isotope substitution experiments, in situ spectroscopic evidence of OH and OOH intermediates at Pt(211) and Pt(311) surfaces was successfully obtained. After detailed analysis based on the Raman spectra and theoretical simulation, it was deduced that the difference in adsorption of OOH at high-index surfaces has a significant effect on the ORR activity. This research illuminates and deepens the understanding of the ORR mechanism on high-index Pt(hkl) surfaces and provides theoretical guidance for the rational design of high activity ORR catalysts.

Toward a quantitative theoretical method for infrared and Raman spectroscopic studies on single-crystal electrode/liquid interfaces.

In situ electrochemical infrared spectroscopy and Raman spectroscopy are powerful tools for probing potential-dependent adstructures at solid/liquid electrochemical interfaces. However, it is very difficult to quantitatively interpret the observed spectral features including potential-dependent vibrational frequency and spectral intensity, even from model systems such as single-crystal electrode/liquid interfaces. The clear understanding of electrochemical vibrational spectra has remained as a fundamental issue for four decades. Here, we have developed a method to combine computational vibrational spectroscopy tools with interfacial electrochemical models to accurately calculate the infrared and Raman spectra. We found that the solvation model and high precision level in the self-consistent-field convergence are critical elements to realize quantitative spectral predictions. This method's predictive power is verified by analysis of a classic spectroelectrochemical system, saturated CO molecules electro-adsorbed on a Pt(111) electrode. We expect that this method will pave the way to precisely reveal the physicochemical mechanism in some electrochemical processes such as electrocatalytic reactions.

Role of axially coordinated surface sites for electrochemically controlled carbon monoxide adsorption on single crystal copper electrodes.

The adsorption of CO on low index copper single crystals in electrochemical environments has been investigated. The results, analysed through a combination of in situ infrared spectroscopy, DFT and cyclic voltammetry, reveal a unique adsorption behaviour when compared to previous studies on copper and the more widely studied noble metal surfaces. By employing small, weakly specifically adsorbed electrolytes, it is shown that carbon monoxide is adsorbed over a much wider electrode potential range than previously reported. The electrochemical Stark shift (δν/δE) observed is similar for the three Cu(hkl) surfaces examined despite different surface coverages. Most notably, however, is an electrochemical feature observed at ca. -1.0 V (vs. Ag/AgCl) on the (110) surface. It is proposed that this voltammetric feature arises from the reduction/oxidation of Cu(δ+) surface sites involved in the binding of carbon monoxide with the participation of the electrolyte anion. This provides additional specific sites for CO adsorption. DFT calculations support the proposed presence of low-coordination copper sites stabilised by electrolyte anions. An experimental electron transfer rate constant of 4.2 s(-1) to the Cu(δ+) surface sites formed was found. These new observations concerning the surface electrochemistry of CO on Cu indicate that the electrocatalytic behaviour of Cu electrodes in processes such as CO(2) reduction need to be re-evaluated to take account of the rich adsorption behaviour of CO, including the co-adsorption of the electrolyte anion to these sites.

The study of electrochemically active microbial biofilms on different carbon-based anode materials in microbial fuel cells.

In this communication we show that the achievable maximum current density for mature wastewater-based microbial biofilms is strongly dependent on the electrode material and the operation temperature. On graphite and polycrystalline carbon rods, the catalytic current of about 500 microA cm(-2) (projected surface area) at 30 degrees C was achieved. Carbon fiber veil or carbon-paper based materials, having a large microbially-accessible surface gave a projected current density approximately 40% higher than on graphite rod. In contrast, the biofilm cannot form well on graphite foil. Elevating the temperature from 30 to 40 degrees C increased current density by 80% on graphite rod anodes. Interestingly, the formal potential of the active site (-0.12 V (vs. standard hydrogen electrode (SHE))) is similar to all electrocatalytically active microbial biofilms and to that found for Geobacter sulfurreducens in previous studies. In addition, the real surface area values measured by BET surface area technique cannot provide a reasonable explanation for suitability of an electrode material for the formation of electrochemically active biofilm.

ATR-SEIRAs characterization of surface redox processes in G. sulfurreducens.

In this work we report on the occurrence of at least two different redox pairs on the cell surface of the electrogenic bacteria Geobacter sulfurreducens adsorbed on gold that are expressed in response to the polarization potential. As previously reported on graphite (Environ. Sci. Technol. 42 (2008) 2445) a typical low potential redox pair is found centered at around -0.06 V when cells are polarized for a few hours at 0.2 V, while a new pair centered at around 0.38 V is expressed upon polarization at 0.6 V. Reversible changes in the IR band pattern of whole cells where obtained by Attenuated Total Reflection-Surface Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) upon potential cycling around both redox pairs. Changes clearly resemble the electrochemical turnover of oxidized/reduced states in c-type cytochromes, thus evidencing the nature of the involved molecules. The expression of external cytochromes in response to the potential of the electron acceptor suggests the existence of alternative pathways of electron transport with different energy yield, though it remains to be demonstrated.

Whole cell electrochemistry of electricity-producing microorganisms evidence an adaptation for optimal exocellular electron transport.

The mechanism(s) by which electricity-producing microorganisms interact with an electrode is poorly understood. Outer membrane cytochromes and conductive pili are being considered as possible players, but the available information does not concur to a consensus mechanism yet. In this work we demonstrate that Geobacter sulfurreducens cells are able to change the way in which they exchange electrons with an electrode as a response to changes in the applied electrode potential. After several hours of polarization at 0.1 V Ag/AgCl-KCl (saturated), the voltammetric signature of the attached cells showed a single redox pair with a formal redox potential of about -0.08 V as calculated from chronopotentiometric analysis. A similar signal was obtained from cells adapted to 0.4 V. However, new redox couples were detected after conditioning at 0.6 V. A large oxidation process beyond 0.5 V transferring a higher current than that obtained at 0.1 V was found to be associated with two reduction waves at 0.23 and 0.50 V. The apparent equilibrium potential of these new processes was estimated to be at about 0.48 V from programmed current potentiometric results. Importantly, when polarization was lowered again to 0.1 V for 18 additional hours, the signals obtained at 0.6 V were found to greatly diminish in amplitude, whereas those previously found at the lower conditioning potential were recovered. Results clearly show the reversibility of cell adaptation to the electrode potential and pointto the polarization potential as a key variable to optimize energy production from an electricity producing population.

Spectroelectrochemical examination of the interaction between bacterial cells and gold electrodes.

The interaction between bacterial cells of Pseudomonas fluorescens (ATCC 17552) and gold electrodes was analyzed by cyclic voltammetry (CV) and attenuated total reflection-surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The voltammetric evaluation of cell adsorption showed a decrease in the double-layer capacitance of polyoriented single-crystal gold electrodes with cell adhesion. As followed by IR spectroscopy in the ATR configuration, the adsorption of bacterial cells onto thin-film gold electrodes was mainly indicated by the increase in intensity with time of amide I and amide II protein-related bands at 1664 and 1549 cm(-1), respectively. Bands at 1448 and 2900 cm(-1) corresponding to the scissoring and the stretching bands of CH2 were also detected, together with a minor peak at 1407 cm(-1) due to the vs COO- stretching. Weak signals at 1237 cm(-1) were due to amide III, and a broad band between 1100 and 1200 cm(-1) indicated the presence of alcohol groups. Bacteria were found to displace water molecules and anions coadsorbed on the surface in order to interact with the electrode intimately. This fact was evidenced in the SEIRAS spectra by the negative features appearing at 3450 and 3575 cm-1, corresponding to interfacial water directly interacting with the electrode and water associated with chloride ions adsorbed on the electrode, respectively. Experiments in deuterated water confirmed these assignments and allowed a better estimation of amide absorption bands. In CV experiments, an oxidation process was observed at potentials higher than 0.4 V that was dependent on the exposure time of electrodes in concentrated bacterial suspensions. Adsorbed bacterial cells were found to get closer to the gold surface during oxidation, as indicated by the concomitant increment in the main IR bacterial signals including amide I, a sharp band at 1240 cm(-1), and a broad one at 1120 cm(-1) related to phosphate groups in the bacterial membranes. It is proposed to be due to the oxidation of lipopolysaccharides on the outermost bacterial surface.

In-situ infrared study of the adsorption and oxidation of oxalic acid at single-crystal and thin-film gold electrodes: a combined external reflection infrared and ATR-SEIRAS approach.

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR-SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm(-)(1) that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the nu(s)(O-C-O) and nu(C-OH) + delta(C-O-H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK(a) for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.