Origin of Multiple Peaks in the Potentiodynamic Oxidation of CO Adlayers on Pt and Ru-Modified Pt Electrodes

The study of the electrooxidation mechanism of CO_ad on Pt based catalysts is very important for designing more effective CO-tolerant electrocatalysts for fuel cells. We have studied the origin of multiple peaks in the cyclic voltammograms of CO stripping from polycrystalline Pt and Ru modified polycrystalline Pt (Pt/Ru) surfaces in both acidic and alkaline media by differential electrochemical mass spectrometry (DEMS), DFT calculations, and kinetic Monte Carlo (KMC) simulations. A new CO_ad electrooxidation kinetic model on heterogeneous Pt and Pt/Ru catalysts is proposed to account for the multiple peaks experimentally observed. In this model, OH species prefer to adsorb at low-coordination sites or Ru sites and, thus, suppress CO repopulation from high-coordination sites onto these sites. Therefore, CO_ad oxidation occurs on different facets or regions, leading to multiplicity of CO stripping peaks. This work provides a new insight into the CO electrooxidation mechanism and kinetics on heterogeneous catalysts

New Insights into the Mechanism and Kinetics of Adsorbed CO Electrooxidation on Platinum: Online Mass Spectrometry and Kinetic Monte Carlo Simulation Studies

The electrooxidation of saturated CO adlayers on Pt/Vulcan and polycrystalline Pt has been studied by potential step techniques combined with differential electrochemical mass spectrometry (DEMS) and kinetic Monte Carlo (KMC) simulations. DEMS was used to selectively monitor the CO_ad electrooxidation, via the CO₂ formation rate, without interference from the pseudocapacitive double-layer charging and electrode surface oxidation, while the KMC simulations were employed to understand the mechanism and kinetics of CO_ad electrooxidation at the molecular level. Our DEMS data show that the current transients of CO_ad electrooxidation on polycrystalline Pt and Pt/Vulcan exhibit an initial spike immediately after the potential step, followed by a slow current decay and finally a broad main peak. The temporal evolution of the transients depends strongly on the oxidation potential applied, resulting in the overlap of the initial spike and the main peak for high potentials. A model is proposed to account for the observed phenomena. On the basis of this model, we developed a kinetic Monte Carlo simulation code specific to the electrooxidation of adsorbed CO on Pt. The simulations reproduce the experimental data very well, confirming the robustness of our model

CO₂ and O₂ Evolution at High Voltage Cathode Materials of Li-Ion Batteries: A Differential Electrochemical Mass Spectrometry Study

A three-electrode differential electrochemical mass spectrometry (DEMS) cell has been developed to study the oxidative decomposition of electrolytes at high voltage cathode materials of Li-ion batteries. In this DEMS cell, the working electrode used was the same as the cathode electrode in real Li-ion batteries, i.e., a lithium metal oxide deposited on a porous aluminum foil current collector. A charged LiCoO₂ or LiMn₂O₄ was used as the reference electrode, because of their insensitivity to air, when compared to lithium. A lithium sheet was used as the counter electrode. This DEMS cell closely approaches real Li-ion battery conditions, and thus the results obtained can be readily correlated with reactions occurring in real Li-ion batteries. Using DEMS, the oxidative stability of three electrolytes (1 M LiPF₆ in EC/DEC, EC/DMC, and PC) at three cathode materials including LiCoO₂, LiMn₂O₄, and LiNi_0.5Mn_1.5O₄ were studied. We found that 1 M LiPF₆ + EC/DMC electrolyte is quite stable up to 5.0 V, when LiNi_0.5Mn_1.5O₄ is used as the cathode material. The EC/DMC solvent mixture was found to be the most stable for the three cathode materials, while EC/DEC was the least stable. The oxidative decomposition of the EC/DEC mixture solvent could be readily observed under operating conditions in our cell even at potentials as low as 4.4 V in 1 M LiPF₆ + EC/DEC electrolyte on a LiCoO₂ cathode, as indicated by CO₂ and O₂ evolution. The features of this DEMS cell to unveil solvent and electrolyte decomposition pathways are also described

Facile Synthesis of Carbon-Supported Pd–Co Core–Shell Nanoparticles as Oxygen Reduction Electrocatalysts and Their Enhanced Activity and Stability with Monolayer Pt Decoration

The rational synthesis of active, durable, and low-cost catalysts is of particular interest to fuel cell applications. Here, we describe a facile method for the preparation of Pd-rich Pd_<i>x</i>Co alloy nanoparticles supported on carbon, using an adsorbate-induced surface segregation effect. The electronic properties of Pd were modulated by alloying with different amounts of Co, which affects the oxygen reduction reaction (ORR) activity. The electrocatalytic activity of the Pd₃Co@Pd/C nanoparticles for the ORR was enhanced by spontaneously depositing a nominal monolayer of Pt. The activities of the different catalysts for the ORR could be correlated with the oxygen adsorption energy and the d-band center of the catalyst surface, as calculated using density functional theory, which is in agreement with previous theoretical studies. The materials synthesized herein are promising cathode catalysts for fuel cell applications and the facile synthesis method could be readily adapted to other catalyst systems, facilitating screening of high efficiency catalysts

An Electrochemical Quartz Crystal Microbalance Study of a Prospective Alkaline Anion Exchange Membrane Material for Fuel Cells: Anion Exchange Dynamics and Membrane Swelling

A strategy has been devised to study the incorporation and exchange of anions in a candidate alkaline anion exchange membrane (AAEM) material for alkaline fuel cells using the electrochemical quartz crystal microbalance (EQCM) technique. It involves the electro-oxidation of methanol (CH₃OH) under alkaline conditions to generate carbonate (CO₃^2–) and formate (HCOO^–) ions at the electrode of a quartz crystal resonator coated with an AAEM film, while simultaneously monitoring changes in the frequency (Δ<i>f</i>) and the motional resistance (Δ<i>R</i>_m) of the resonator. A decrease in Δ<i>f</i>, indicating an apparent mass increase in the film, and a decrease in Δ<i>R</i>_m, signifying a deswelling of the film, were observed during methanol oxidation. A series of additional QCM experiments, in which the effects of CH₃OH, CO₃^2–, and HCOO^– were individually examined by changing the solution concentration of these species, confirmed the changes to be due to the incorporation of electrogenerated CO₃^2–/HCOO^– into the film. Furthermore, the AAEM films were found to have finite anion uptake, validating the expected tolerance of the material to salt precipitation in the AAEM. The EQCM results obtained indicated that HCOO^– and CO₃^2–, in particular, interact strongly with the AAEM film and readily displace OH^– from the film. Notwithstanding, the anion exchange between CO₃^2–/HCOO^– and OH^– was found to be reversible. It is also inferred that the film exhibits increased swelling in the OH^– form versus the CO₃^2–/HCOO^– form. Acoustic impedance analysis of the AAEM-film coated quartz resonators immersed in water showed that the hydrated AAEM material exhibits significant viscoelastic effects due to solvent plasticization

Water Oxidation Catalysis by Co(II) Impurities in Co(III)₄O₄ Cubanes

The observed water oxidation activity of the compound class Co₄O₄(OAc)₄(Py–X)₄ emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co₄O₄ molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis

Water Oxidation Catalysis by Co(II) Impurities in Co(III)₄O₄ Cubanes

The observed water oxidation activity of the compound class Co₄O₄(OAc)₄(Py–X)₄ emanates from a Co­(II) impurity. This impurity is oxidized to produce the well-known Co-OEC heterogeneous cobaltate catalyst, which is an active water oxidation catalyst. We present results from electron paramagnetic resonance spectroscopy, nuclear magnetic resonance line broadening analysis, and electrochemical titrations to establish the existence of the Co­(II) impurity as the major source of water oxidation activity that has been reported for Co₄O₄ molecular cubanes. Differential electrochemical mass spectrometry is used to characterize the fate of glassy carbon at water oxidizing potentials and demonstrate that such electrode materials should be used with caution for the study of water oxidation catalysis