CO Electrochemistry Research Articles

Surface Electrochemistry Decorated By Andrzej Wieckowski

Surface electrochemistry aimed towards materials with advanced catalytic processes will be thoroughly discussed in a view of footprint, contributions and approach that was pursued by Prof. Andrzej Wieckowski. A number of well-defined single crystalline surfaces will be brought to the audience to recall past time when many groups were focused on CO electrochemistry and design of new real world catalysts that would meet strict CO-tolerant requirements.Andrzej was pioneering a difficult pathway in executing the most complex in-situ measurements and he was always challenging others in order to push the field towards new frontiers. That time created a number of highly visible articles and research groups were competing who would be the first in proposing the new reaction mechanism, novel material or to dispute work of others who made questionable claims. That time is known as golden era of electrochemistry, where scientific knowledge did not have alternative in front highly critical audience. This field is missing those times, and for that reason, this symposium is an outstanding opportunity to recall past, and to remember Prof. Wieckowski as a pillar who was setting the new limits.

Isolating the impact of CO concentration in syngas mixtures on SOFC performance via internal reforming and direct oxidation

One of the distinct advantages of solid oxide fuel cells (SOFCs) is their ability to directly oxidize CO in addition to H2, which allows them to be run on syngas mixtures. However, membrane-electrode-assembly (MEA) models typically neglect CO electrochemistry in the presence of H2 and H2O, assuming that the water–gas-shift reaction proceeds faster than sluggish CO electro-oxidation. In this paper, however, we demonstrate with a comprehensive 1D-MEA model that CO electro-oxidation cannot be neglected in syngas mixtures, particularly at high current densities for high CO-content syngas. We first demonstrate that incoming CO is not all shifted to form H2 before reaching the triple-phase boundary, as previously assumed, due to the equilibrium limitation of the water–gas-shift reaction at 800 °C. Furthermore, we confirm that direct oxidation of CO contributes non-negligible current relative to H2 at high anode overpotentials in syngas mixtures. Together these results show that CO electro-oxidation plays an important role in SOFC performance not only via water–gas-shift reforming, but also via direct oxidation even when H2 is present. This work suggests that accurate models for both surface reforming and direct electro-oxidation of CO in SOFC anodes must be included in order to capture performance when using syngas mixtures.

Mass-Action Kinetics Approach to Concurrent H2 and CO Electrochemistry in a Patterned SOFC Anode

SOFC is a fuel flexible technology suitable for producing clean energy. Understanding of the multi-reaction mechanism that a complex H2-CO fuel presents to the electrochemical kinetics is numerically approached in this work. By using existing fundamental reaction mechanisms and kinetic parameters, elementary reactions involved in an SOFC anode have been assembled, modeled and analyzed. This involves both homogeneous and heterogeneous chemistry, electrochemistry and surface diffusion. The use of the patterned anode approach removes the mass transport complications and allows comparison with pre-existing experimental data. The model provides both polarization curves and surface coverage distribution, among other results, providing a high level of detail and understanding of the physical phenomena involved. In particular, analysis is focused upon understanding how the competitive H2 and CO reactions behave.The presence of CO was found to stabilize OCV response to temperature and while it occupied most of the active sites it did not penalize overall performance except when significant product species were present.

Detailed H2 and CO Electrochemistry for a MEA Model Fueled by Syngas

SOFCs are capable of directly oxidizing CO in addition to H2, which allows them to be coupled to a gasifier that converts coal, biomass or other carbonaceous fuels to synthesis gas. Most membrane-electrode-assembly (MEA) models neglect CO electrochemistry because H2 oxidizes more readily and most CO presumably shifts to H2 by the water-gas-shift reaction in the presence of H2O. In this paper we examine this assumption, especially at conditions of high current density and high CO-content syngas. The comprehensive 1D-MEA model presented here incorporates detailed mechanisms for both H2 and CO oxidation. The rate parameters and rate-limiting steps for the H2 mechanism are selected by fitting the model to experimental porous anode data for H2/H2O mixtures. The kinetic parameters of the CO mechanism were determined by matching the data to EIS measurements. The anode’s physical parameters, triple-phase-boundary length and nickel pattern width, are then determined by fitting the CO mechanism to porous anode data for CO/CO2 mixtures. The resulting H2 and CO mechanisms are then combined, along with surface reforming kinetics, into a single model that iterates through anode activation overpotential to resolve the individual current contributions of each fuel. We find that the model under-predicts experimental data for H2/CO mixtures when CO electrochemistry is neglected, but fits the data well at high current densities when CO oxidation is included. Furthermore, the combined model fits H2/CO data best when a single charge-transfer step in the H2 mechanism is assumed to be rate-limiting over the full range of current densities. This single rate-limiting step assumption for H2/CO mixtures differs from a previous finding that the H2 adsorption step becomes rate-limiting at high current densities for H2/H2O mixtures. This implies that adding CO to the fuel stream can fundamentally alter the H2 oxidation process. The individual H2 and CO current contributions for CO-rich syngas confirm that the addition of CO oxidation can delay H2 current saturation at high anode activation overpotentials, hence improving cell performance. These results indicate that CO oxidation cannot be neglected in MEA models running on CO-rich synthesis gas, and further studies are needed on the combined CO and H2 mechanism on Ni-YSZ.

Detailed H2 and CO Electrochemistry for a MEA Model Fueled by Syngas

SOFCs can directly oxidize CO in addition to H2, which allows them to be coupled to a gasifier. Many membrane-electrode-assembly (MEA) models neglect CO electrochemistry due to sluggish kinetics and the water-gas-shift reaction, but CO oxidation may be important for high CO-content syngas. The 1D-MEA model presented here incorporates detailed mechanisms for both H2 and CO oxidation, individually fitted to experimental data. These mechanisms are then combined into a single model, which provides a good fit to experimental data for H2/CO mixtures. Furthermore, the model fits H2/CO data best when a single charge-transfer step in the H2 mechanism is assumed to be rate-limiting for all current densities. This differs from the result for H2/H2O mixtures, where H2 adsorption becomes rate-limiting at high current densities. These results indicate that CO oxidation cannot be neglected in MEA models running on CO-rich syngas, and that CO oxidation can alter the H2 oxidation mechanism.

Surface electrochemistry of CO2 reduction and CO oxidation on Sm-doped CeO(2-x): coupling between Ce(3+) and carbonate adsorbates.

The efficient electro-reduction of CO2 to chemical fuels and the electro-oxidation of hydrocarbons for generating electricity are critical toward a carbon-neutral energy cycle. The simplest reactions involving carbon species in solid-oxide fuel cells and electrolyzer cells are CO oxidation and CO2 reduction, respectively. In catalyzing these reactions, doped ceria exhibits a mixed valence of Ce(3+) and Ce(4+), and has been employed as a highly active and coking-resistant electrode. Here we report an operando investigation of the surface reaction mechanism on a ceria-based electrochemical cell using ambient pressure X-ray photoelectron spectroscopy. We show that the reaction proceeds via a stable carbonate intermediate, the coverage of which is coupled to the surface Ce(3+) concentration. Under CO oxidation polarization, both the carbonate and surface Ce(3+) concentration decrease with overpotential. Under CO2 reduction polarization, on the other hand, the carbonate coverage saturates whereas the surface Ce(3+) concentration increases with overpotential. The evolution of these reaction intermediates was analyzed using a simplified two-electron reaction scheme. We propose that the strong adsorbate-adsorbate interaction explains the coverage-dependent reaction mechanism. These new insights into the surface electrochemistry of ceria shed light on the optimization strategies for better fuel cell electrocatalysts.

Using potassium catalytic gasification to improve the performance of solid oxide direct carbon fuel cells: Experimental characterization and elementary reaction modeling

The performance of a solid oxide electrolyte direct carbon fuel cell (SO-DCFC) is limited by the slow carbon gasification kinetics at the typical operating temperatures of cell: 650–850 °C. To overcome such limitation, potassium salt is used as a catalyst to speed up the dry carbon gasification reactions, increasing the power density by five-fold at 700–850 °C. The cell performance is shown to be sensitive to the bed temperature, emphasizing the role of gasification rates and that of CO production. Given the finite bed size, the cell performance is time-dependent as the amount of CO available changes. A reduced elementary reaction mechanism for potassium-catalyzed carbon gasification was proposed using kinetic data obtained from the experimental measurements. A comprehensive model including the catalytic gasification reactions and CO electrochemistry is used to examine the impact of the catalytic carbon gasification process on the device performance. The power density is maximum around 50% of the OCV, where carbon utilization is also near maximum. Results show that bed height and porosity impact the power density; a thicker bed maintains the power almost constant for longer times while lower porosity delivers higher power density in the early stages.

Catalysis of the electrochemical reduction of carbon dioxide

The direct and catalyzed electrochemistry of CO(2) partake in the contemporary attempts to reduce this inert molecule to fuels by means of solar energy, either directly, after conversion of light to electricity, or indirectly in that all elements of comprehension derived from electrochemical experiments can be used in the design and interpretation of photochemical experiments. Following reviews of the activity in the field until 2007-2008, the present review reports more recent findings even if their interpretation remains uncertain. It also develops useful notions that allow analyzing and comparing more rigorously the performances of existing catalysts when the necessary data are available. Among the general trends that transpire presently and are likely to be the object of active future work emphasis is put on the favorable role of acid addition in homogeneous catalytic systems and on the crucial chemical role of the electrode material in heterogeneous catalysis.

Synthesis, structure and electrochemistry of Co(III) complexes with an unsymmetrical Schiff base ligand derived from 2-aminobenzylamine and pyrrole-2-carboxaldehyde

A series of cobalt(III) complexes with a new unsymmetrical tetradentate Schiff base ligand N,N’-bis(pyrrol-2-ylmethylene)-2-aminobenzylamine, H 2pyrabza, formed by the condensation of 2-pyrrole carboxaldehyde and 2-aminobenzylamine, has been synthesized and characterized by elemental analyses, IR, UV–Vis and 1H NMR spectroscopy. These complexes have the general formula trans-[Co III(L)(amine) 2]X {L 2− = pyrabza, and amine = N-methylimidazole (N-MeIm) ( 1), 3-methylpyridine (3-Mepy) ( 2), 4-methylpyridine (4-Mepy) ( 3), pyridine (py) ( 4), X = BPh 4 - }. The structure of compound 1 has been determined by X-ray crystallography. The geometry around the central cobalt ion is distorted octahedral. The electrochemical behavior of these complexes was also investigated. The reduction potential of Co(III), ranging from −0.79 V for 1 to −0.44 V for 4, shows a relatively good correlation with the σ-donor ability of the axial ligands.

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.

Comparative Surface Electrochemistry of Co and Co3O4 Nanoparticles: Nitrite as an Analytical Probe

Comparative Surface Electrochemistry of Co and Co3O4 Nanoparticles: Nitrite as an Analytical Probe

Surface electrochemistry of CO as a probe molecule on carbon-supported Se-surface modified Ru nanoparticles via infrared reflection absorption spectroscopy

The carbon monoxide molecule is used to probe the electronic properties of Ru/C and Se surface-modified Ru/C nanocatalysts. Coordination of a Se to a Ru surface strongly affects the vibrational properties and reactivity of the CO adsorbed. Marked alteration of the CO stretching frequency; increase of the Stark tuning rate for C-O vibration; and a positive shift of the onset of CO oxidation are observed for Se-modified Ru particles and attributed to the electronic effect of Se.

Synthesis and electrochemistry of Co(III) and Co(I) complexes having C 5Me 5 auxiliary

The synthesis and electrochemistry of half-sandwich type of Co(III) complexes [(C 5Me 5)Co(bidentate)(CH 3CN)](BF 4) 2 {bidentate = dppe (1,2-bis(diphenylphosphino)ethane), dppp (1,3-bis(diphenylphosphino)propane), bpy (2,2′-bipyridine), en (ethylenediamine)) are reported. Cyclic voltammograms of [(C 5Me 5)Co(bidentate)(CH 3CN)](BF 4) 2 in CH 3CN s showed two redox couples assignable to Co(II)/Co(III) and Co(I)/Co(II). The Co(I) complex having C 5Me 5 and dppe was also prepared. Two redox couples of this Co(I) complex, (C 5Me 5)Co(dppe), in CH 3CN coincided with those of [(C 5Me 5)Co(dppe)(CH 3CN)](BF 4) 2 in spite of the structural change around the metal center.

Synthesis, structure and electrochemistry of CO incorporated diruthenium metallacyclic compounds [Ru2(CO)6{μ-η1:η1:η2:η2-1,4-Fc2C5H2O}] and [Ru2(CO)6{μ-η1:η1:η2:η2-1,5-Fc2C5H2O}

Ferrocenyl substituted ruthenium metallacyclic compounds, [Ru 2 (CO) 6 {μ-η 1 :η 1 :η 2 :η 2 -1,4-Fc 2 C 5 H 2 O}] ( 1 ) and [Ru 2 (CO) 6 {μ-η 1 :η 1 :η 2 :η 2 -1,5-Fc 2 C 5 H 2 O}] ( 2 ) have been synthesized and structurally characterized. Electrochemical studies for 1 and 2 and the respective quinones, 3 and 4 show electrochemical coupling between the two redox active ferrocenyl units via the bridging ligand. The electrochemical coupling has been observed to depend upon the orientation and position of the ferrocenyl units. Ferrocenyl substituted ruthenium metallacyclic compounds, [Ru 2 (CO) 6 {μ-η 1 :η 1 :η 2 :η 2 -1,4-Fc 2 C 5 H 2 O}] ( 1 ) and [Ru 2 (CO) 6 {μ-η 1 :η 1 :η 2 :η 2 -1,5-Fc 2 C 5 H 2 O}] ( 2 ) have been synthesized and structurally characterized. Electrochemical studies for 1 and 2 and the respective quinone derivatives 3 and 4 show weak to no electrochemical coupling at the mixed-valent intermediate state which is dependent on the complex frameworks.

Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode.

The electrooxidation of CO has been studied on reconstructed gold single-crystal surfaces by a combination of electrochemical (EC) and infrared reflection absorption spectroscopy (IRAS) measurements. Emphasis is placed on relating the vibrational properties of the CO adlayer to the voltammetric and other macroscopic electrochemical responses, including rotating disk electrode measurements of the catalytic activity. The IRAS data show that the C-O stretching frequencies are strongly dependent on the surface orientation and can be observed in the range 1940-1990 cm(-1) for the 3-fold bridging, 2005-2070 cm(-1) for the 2-fold bridging, and 2115-2140 for the terminal position. The most complex CO spectra are found for the Au(110)-(1 x 2) surface, i.e., a band near 1965 cm(-1), with the second, weaker band shifted positively by about 45 cm(-1) and, finally, a weak band near 2115 cm(-1). While the C-O stretching frequencies for a CO adlayer adsorbed on Au(111)-(1 x 23) show nu(CO) bands at 2029-2069 cm(-1) and at 1944-1986 cm(-1), on the Au(100)-"hex" surface a single CO band is observed at 2004-2029 cm(-1). In the "argon-purged" solution, the terminal nu(CO) band on Au(110)-(1 x 2) and the 3-fold bridging band on the Au(111)-(1 x 23) disappear entirely. The IRAS/EC data show that the kinetics of CO oxidation are structure sensitive; i.e., the onset of CO oxidation increases in the order Au(110)-(1 x 2) > or = Au(100)-"hex" > Au(111)-(1 x 23). Possible explanations for the structure sensitivity are discussed.

Novel Impact of Short Term Aging on the Electrochemistry of CO 2 Treated Synthetic Graphite

A study was conducted of graphite prepared from heat-treatment of synthetic graphite LK-702 in flowing Samples were characterized by percent mass loss, X-ray powder diffraction, scanning electron microscopy imaging, thermogravimetric analysis, Brunauer-Emmett-Teller surface area, and methylene blue adsorption surface area measurements. Treated graphites were studied as possible electrode materials for Li-ion cells. Cycle tests demonstrated that the reversible capacity and percent capacity retained were relatively unaffected by treatment, while the irreversible capacity was significantly influenced by treatment. The change in irreversible capacity was strongly dependent upon the age of the treated samples, where samples freshly treated in showed a decrease in irreversible capacity with treatment time, and treated samples aged for at least fourteen days (in a dry environment) showed an increase in irreversible capacity with treatment time. To our knowledge, this is the first report of the impact of short term aging on the electrochemistry of treated graphite. © 2004 The Electrochemical Society. All rights reserved.

CO adsorption and kinetics on well-characterized Pd films on Pt( [formula omitted]) in alkaline solutions

The electrochemistry of CO on a bare Pt(1 1 1) electrode as well as a Pt(1 1 1) electrode modified with pseudomorphic thin palladium films has been studied in alkaline solution by means of Fourier transform infrared (FTIR) spectroscopy. First Pd films were prepared and well characterized in ultra high vacuum and subsequently transferred into the electrochemical cell for the registration of the voltammetric profiles. The charge corresponding to the formation of underpotentially deposited hydrogen (H upd) on these Pt(1 1 1)– xPd surfaces was established in sulfuric acid solution as a function of x (0⩽ x⩽1 Pd monolayer (ML)). All subsequent measurements were then performed on electrochemically deposited palladium films using the above H upd charge vs. Pd coverage relationship to evaluate the amount of electrochemically deposited palladium. FTIR spectra for CO adsorbed on 1 ML and a submonolayer coverage are compared to those of the unmodified Pt(1 1 1) surface, all surfaces having identical 2D lattice structures. Infrared absorption bands of CO bound on either Pt(1 1 1) or Pt(1 1 1)–1 ML Pd are clearly distinguished. Spectra of CO adsorbed on Pd submonolayers show characteristic features of both CO bound to Pt and to Pd, indicating that on Pt(1 1 1)– xPd surfaces there is no coupling between Pt–CO ad and Pd–CO ad molecules. The kinetics of CO oxidation on these surfaces is determined either by rotating disk electrode measurements or by FTIR spectroscopy, monitoring the CO 3 2− production. The oxidation of CO ad on Pt(1 1 1) and on Pd modified platinum surfaces starts at the same potential, at ≈0.2 V. The oxidation rate is, however, considerably lower on the Pt(1 1 1)– xPd surfaces than on the Pt(1 1 1) surface. The kinetics of CO oxidation appears to be determined by the nature of adsorbed hydroxyl anions (OH ad), which are more strongly (less active) adsorbed on the highly oxophilic Pd atoms.

Alkyl and aryl substituted corroles. 2. Synthesis and characterization of linked "face-to-face" biscorroles. X-ray structure of (BCA)Co(2)(py)(3), where BCA represents a biscorrole with an anthracenyl bridge.

The synthesis, spectroscopic properties, and electrochemistry of (BCA)Co(2) and (BCB)Co(2) are described where BCA and BCB represent biscorroles linked by an anthracenyl (A) or a biphenylenyl (B) bridge. The pyridine and CO binding properties of (BCA)Co(2) and (BCB)Co(2) are also presented, and one of the compounds in its pyridine-ligated form, (BCA)Co(2)(py)(3), is structurally characterized. The data on the biscorroles are compared on one hand to the monocorrole having the same substitution pattern and on the other hand to bisporphyrins having two Co(II) ions and the same anthracenyl or biphenylenyl linkers in order to better understand the interaction which occurs between the two corrole macrocycles. A parallel study on five different Co(III) phenyl-substituted corroles showed that bis-pyridine and mono-CO adducts are readily formed from the complexes in CH(2)Cl(2). This present paper examines how the ligand binding properties and electrochemistry of these Co(III) corroles are modified by the anthracenyl or biphenylenyl bridge which links the two macrocycles in a face to face orientation. An X-ray crystal structure was obtained for the tris-pyridine adduct of the anthracenyl bridged derivative, (BCA)Co(2)(py)(3), and gives the following results: C(127)H(99)Co(2)N(11).2CHCl(3), M = 2135.90, triclinic, space group P&onemacr;, a = 13.2555(5) A, b = 18.6406(8) A, c = 22.2140(9) A, alpha = 94.186(9) degrees, beta = 102.273(9) degrees, gamma = 94.205(9) degrees, V = 5326.8(4) A(3), 9293 independent reflections collected, R(F) = 0.066.

Electrochemical properties of octakis(hydroxyethylthio)-substituted phthalocyanines

Cyclic voltammetry of octakis(hydroxyethylthio)-substituted phthalocyanines with 2 H ( H 2 OHETPc ), Zn (II) ( Zn II OHETPc (−2)) and Co (II) ( Co II( OHETPc (−2)) in the phthalocyanine core has been investigated in dimethylsulfoxide ( DMSO ), and some of the results were evaluated together with their absorption spectra in the visible range. The redox processes due to both aggregated and non-aggregated species were observed on the voltammograms of phthalocyanines with Zn (II). The effect of chloride ion on the cyclic voltammetry of Co II OHETPc (−2) has been investigated in the DMSO / ClO 4− couple. The comparison of cyclic voltammetry for Co II OHETPc in the absence and presence of chloride ion showed that chloride ion does not bind to either Co (II) or Co (I) but does bind very strongly to Co (III). The detailed electrochemistry of Co II OHETPc (−2) was investigated in terms of the Co (I)/ Co (II) and Co (II)/ Co (III) redox couples, and various equilibria related to the species coordinated to Co (II) and Co (III) in the phthalocyanine core.

Surface Electrochemistry of CO and Methanol at 25−75 °C Probed in Situ by Infrared Spectroscopy

Effects of temperature and CO partial pressure on the adsorption of carbon monoxide at a polycrystalline platinum electrode in 0.1 M HClO4 were studied with cyclic voltammetry and infrared spectroscopy. Appreciable CO coverages were detected with infrared spectroscopy from ambient up to the high-temperature limit of the experiments (75 °C). In CO saturated solution, voltammograms were dominated by the diffusion-controlled current arising from the oxidation of bulk CO. However, under these conditions the adsorbed CO layer was readily detected with infrared spectroscopy. At 75 °C, a strong atop CO band (2075−2080 cm-1) was observed at double layer and hydrogen adsorption potentials. Loss of the adlayer due to oxidation occurred at potentials positive of 0.3 V (vs KCl saturated Ag/AgCl). In CO-free electrolyte solution, atop CO band intensities measured at 75 °C were lower than those recorded in CO saturated solution by about 50%, and the adlayer was more easily oxidized. CO formed from solutions of 0.3 M me...