Electroreduction of dioxygen (ORR) in alkaline medium on Ag/C and Pt/C nanostructured catalysts—effect of the presence of methanol

Ag/C catalysts with different loading were prepared using a colloidal route to obtain well dispersed catalysts on carbon, with a particle size close to 15 nm. An amount of 20 wt.% Ag on carbon was found to be the best loading in terms of current density and mass activity. The 20 wt.% Ag/C catalyst was then studied and the kinetics towards ORR was determined and compared with that of a 20 wt.% Pt/C catalyst. The number of exchanged electrons for the ORR was found to be close to four with the rotating disk electrode (RDE) as well as with the rotating ring disc electrode (RRDE) techniques. From the RDE results, the Tafel slopes b, the diffusion limiting current density inside the catalytic film (j_l^film) and the exchange current density (j₀) were evaluated. The Tafel slopes b and diffusion limiting current densities inside the catalytic film (j_l^film) were found to be in the same order for both catalysts, whereas the exchange current density (j₀), which is a suitable estimation of the activity of the catalyst, was at least 10 times higher at the Pt/C catalyst than at the Ag/C catalyst. The behavior of both catalysts in methanol containing electrolyte was investigated and it was found that at a low methanol concentration, the Pt/C catalyst was quasi-tolerant to methanol. But, at a high methanol concentration, the ORR at a Pt/C was affected. However, the Pt/C catalyst showed in each case better activity towards ORR than the Ag/C catalyst, even if the latter one was less affected by the presence of methanol than the former one.     

Influence of surfactants on the electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes

The effect of surfactants on the electroreduction of O₂ to H₂O₂ was investigated by cyclic voltammetry and batch electrolysis on vitreous carbon electrodes. The electrolytes were either 0.1 M Na₂CO₃ or 0.1 M H₂SO₄ at 295 K, under 0.1 MPa O₂. Electrode kinetics and mass transport parameters showed the influence of surfactants on the O₂ electroreduction mechanism. The cationic surfactant (Aliquat 336^®, tricaprylmethylammonium chloride), at mM levels, increased the standard rate constant of O₂ electroreduction to H₂O₂ 15 times in Na₂CO₃ and 1900 times in H₂SO₄, to 1.8 × 10^–6 m s^–1 and 9.9 × 10^–10 m s^–1, respectively. This effect on the reaction rate might be due to an increase of the surface pH, induced by the Aliquat 336^® surface film. The nonionic (Triton X-100) and anionic (sodium dodecyl sulfate) surfactants retarded the O₂ electroreduction, presumably by forming surface structures, which blocked the access of O₂ to the electrode. Ten hour batch electrosynthesis experiments performed at 300 A m^–2 superficial current density, 0.1 MPa O₂, 300 K, on reticulated vitreous carbon (30 ppi), showed that compared to the values obtained in the absence of surfactant, mM concentrations of Aliquat 336^® increased the current efficiency for peroxide from 12% to 61% (0.31 M H₂O₂) in 0.1 M Na₂CO₃ and from 14% to 55% (0.26 M H₂O₂) in 0.1 M H₂SO₄, respectively.     

Development of platinum-free catalyst and catalyst with low platinum content for cathodic oxygen reduction in acidic electrolytes

Studies are presented of the kinetics and mechanism of oxygen electroreduction on CoPd catalysts synthesized on XC 72 carbon black. As shown both in model conditions and in tests with the cathodes of hydrogen–oxygen fuel cells with proton conducting electrolyte, the CoPd/C system features higher activity as compared to Co/C. It is found by means of structural analysis that CoPd alloy is formed in the course of the catalyst synthesis. This provides the higher catalytic activity of the binary systems. CoPd/C catalyst is also more stable in respect to corrosion than Pd on carbon black. Measurements on a rotating ring–disc electrode show that the CoPd/C system provides preferential oxygen reduction to water in the practically important range of potentials (E > 0.7 V). The similarity of the kinetic parameters of the oxygen reduction reaction on CoPd/C and Pt/C catalysts points to a similar reaction mechanism. The slow step of the reaction is the addition of the first electron to the adsorbed and previously protonated O₂ molecule. Studies of the most active catalyst in the fuel cell cathodes are performed. Binary PtCo catalysts (metal atomic ratio of 1 : 1) with low platinum content (7.3 wt.%) modified by phosphorus or sulfur are developed and studied. It is demonstrated that the specific activity of the PtCoS/C (Pt : S = 1 : 1) catalytic system for the O₂ reduction reaction exceeds that of a commercial Pt/C catalyst (E-TEK). The tolerance of the catalyst modified with sulfur is at least six times higher than that of Pt/C (E-TEK).     

Current status of ab initio quantum chemistry study for oxygen electroreduction on fuel cell catalysts

Recent progress in the ab initio quantum chemistry study of cathode oxygen reduction on fuel cell catalysts is reviewed with emphasis on density functional theory and ab initio molecular dynamics methods. The capabilities of these methods are illustrated using examples of oxygen adsorption on transition metals and alloys, and the reduction mechanism. Ab initio studies can calculate adsorption geometry, energy, the dissociation energy barrier, reversible potential, activation energy, and potential dependant properties for elementary electron transfer steps. Even though ab initio study in this field is still at an early stage, it has already demonstrated its predictive ability in the trend of adsorption energy on transition metals and alloys, and illustrated its potential in identifying better electrocatalysts.     

Study of the electroreduction of nitrate on copper in alkaline solution

The electrocatalytic activity of a Cu electrode for the electroreduction of nitrate in alkaline medium was investigated by linear sweep voltammetry at stationary and rotating disc electrodes. Nitrate-reduction products generated upon prolonged electrolyses at different potentials were quantified. In addition, adsorption phenomena associated with the nitrate electroreduction process were characterized by electrochemical quartz crystal microbalance (EQCM) experiments. This data revealed that nitrate electroreduction process strongly depends on the applied potential. Firstly, at ca. −0.9 V vs. Hg/HgO, the electroreduction of adsorbed nitrate anions to nitrite anions was identified as the rate-determining step of the nitrate electroreduction process. Between −0.9 and −1.1 V, nitrite is reduced to hydroxylamine. However, during long-term electrolyses, hydroxylamine is not detected and presumably because it is rapidly reduced to ammonia. At potential more negative than −1.1 V, nitrite is reduced to ammonia. At ca. −1.45 V, i.e. just before the hydrogen evolution reaction, the abrupt decrease of the cathodic current is due to the electrode poisoning by adsorbed hydrogen. In addition, during the first minutes of nitrate electrolysis, a decrease of the copper electrode activity was observed at the three investigated potentials (−0.9, −1.1 and −1.4 V). From polarization and EQCM measurements, this deactivation was attributed to the adsorption of nitrate-reduction products, blocking the electrode surface and slowing down the nitrate electroreduction rate. However, it was demonstrated that the Cu electrode can be reactivated by the periodic application of a square wave potential pulse at −0.5 V, which causes the desorption of poisoning species.     

Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition

The kinetics of oxygen reduction in methanol-containing acid electrolyte was investigated at platinum-based electrodes using the porous rotating disk electrodes (RDE) technique. Utilization of commercial-grade (E-TEK) carbon-supported Pt particles with narrow size distribution provided evidences for a particle size effect on the tolerance of oxygen reduction electrocatalysts to methanol competition. In methanol-containing perchloric acid electrolyte, the mass activity (MA, A g⁻¹ Pt) for oxygen reduction increases continuously with a decrease in particle size from d=4.6 to 2.3 nm, whereas in methanol-free electrolyte MA is roughly independent of the size, when d≤3.5 nm. Effects of addition of a second metal to Pt were also investigated. Based on particle size considerations Pt:Cr-C appears to be a more active catalyst than Pt-C for oxygen reduction in methanol-containing electrolyte.     

Performance and durability of PtCo alloy catalysts for oxygen electroreduction in acidic environments

The performance and durability of PtCo alloy catalysts for oxygen electroreduction in acidic environments were investigated by the electrochemical rotating disk electrode (RDE) technique, in situ electrochemical scanning tunneling microscopy (STM), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). In comparison with benchmark Pt catalysts, PtCo alloy catalysts offer both enhanced catalytic activity towards molecular oxygen electroreduction and improved durability under electrochemical forces in acidic environments for short-term operation; however, PtCo alloy catalysts do not provide long-term improvement in catalytic activity and electrochemical durability due to the leaching of elemental cobalt from the alloys under intensive potential cycling. Therefore, similar to Pt, PtCo alloys are not viable cathode catalysts for low temperature fuel cells for transportation applications.     

Platinum-alloy nanostructured thin film catalysts for the oxygen reduction reaction

In an effort to study advanced catalytic materials for the oxygen reduction reaction (ORR), a number of metallic alloy nanostructured thin film (NSTF) catalysts have been characterized by rotating disk electrode (RDE). Optimal loadings for the ORR and activity enhancement compared to conventional carbon supported nanoparticles (Pt/C) were established. The most efficient catalyst was found to be PtNi alloy with 55 wt% of Pt. The enhancement in specific activity is more than one order of magnitude, while the improvement factor in mass activity is 2.5 compared to Pt/C. Further lowering of the platinum to nickel ratio in NSTF catalysts did not lead to increased mass activity values.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

Performance of hybrid density functional theory methods toward oxygen electroreduction over platinum

The performance of a large number of hybrid density functional theory methods is evaluated toward calculating potential-dependent activation energies for uncatalyzed and Pt-catalyzed oxygen reduction and hydroperoxyl oxidation. This reaction is the first step and the rate-determining step in the electrochemical oxygen reduction, which is the cathodic process in electrolyte-based fuel cells. Special focus is put on determining methods that allow results comparable to those previously calculated using MP2 method with the 6-31G(d,p) basis set for O and H and the LANL2DZ basis set for Pt. This level of theory was shown to reproduce well, within the model used here, key features of experimental data. It is found that hybrid density functional theory methods with small (less than 30%) Hartree-Fock exchange contributions give less accurate results mainly due to underestimated calculated activation energies while methods with higher (around 50%) Hartree-Fock exchange contributions give results closer to the target ones. New hybrid density functional theory methods with specific reaction parameters that give superior results are proposed. The best overall performance is found for the method denoted as B1B95-50 in which the Hartree-Fock exchange contribution is half. This method is computationally affordable and offers promise as a reliable method in applications to larger systems.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

环氧乙烷催化水合生产乙二醇动力学研究

在温度为70~105℃,压力为0.7~1.5 MPa,n(H2O)∶n(环氧乙烷)=6~8的条件下,以阴离子交换树脂为催化剂,研究了环氧乙烷(EO)催化水合生产乙二醇(EG)反应动力学特性,建立了动力学模型.研究结果表明,EG的生成速率对EO浓度具有一级反应特征.对试验数据进行了回归,得到了动力学模型参数,其反应活化能为42.4 kJ/mol,指前因子为5.48×106h-1。回归数据的线性相关系数均大于0.96,说明该模型能够充分体现反应特征。     

Influence of carbon support on the performance of platinum based oxygen reduction catalysts in a polymer electrolyte fuel cell

Novel carbons from the Sibunit family prepared via pyrolysis of hydrocarbons [Yermakov YI, Surovikin VF, Plaksin GV, Semikolenov VA, Likholobov VA, Chuvilin AL, Bogdanov SV (1987) React Kinet Catal Lett 33:435] possess a number of attractive properties for fuel cell applications. In this work Sibunit carbons with BET surface areas ranging from ca. 20 to 420 m² g⁻¹ were used as supports for platinum and the obtained catalysts were tested as cathodes in a polymer electrolyte fuel cell. The metal loading per unit surface area of carbon support was kept constant in order to maintain similar metal dispersions (∼0.3). Full cell tests revealed a strong influence of the carbon support texture on cell performance. The highest mass specific activities at 0.85 V were achieved for the 40 and 30 wt.% Pt catalysts prepared on the basis of Sibunit carbons with BET surface areas of 415 and 292 m² g⁻¹. These exceeded the mass specific activities of conventional 20 wt.% Pt/Vulcan XC-72 catalyst by a factor of ca. 4 in oxygen and 6 in air feed. Analysis of the I–U curves revealed that the improved cell performance was related to the improved mass transport in the cathode layers. The mass transport overvoltages were found to depend strongly on the specific surface area and the texture of the support.     

Polypyrrole and La1−xSrxMnO3 (0≤ x ≤0.4) composite electrodes for electroreduction of oxygen in alkaline medium

Composite G/PPy/PPy(La_1−xSr_xMnO₃)/PPy electrodes made of the perovskite La_1−xSr_xMnO₃ embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La_1−xSr_xMnO₃) (0≤ x ≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1 mol dm⁻³ pyrrole (Py) plus 0.05 mol dm⁻³ K₂SO₄ with and without containing a suspension of 8.33 g L⁻¹ oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5 mol dm⁻³ K₂SO₄ plus 5 mmol dm⁻³ KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.     

Synthesis, characterization and electrocatalytic behaviour of non-alloyed PtCr methanol tolerant nanoelectrocatalysts for the oxygen reduction reaction (ORR)

In order to point out the effect of the second metal in platinum-based catalysts, a synthesis method by colloidal route derived from that of Bönnemann was used to prepare non-alloyed Pt_1−xCr_x/C electrocatalysts active towards the oxygen reduction reaction (ORR). The non-alloyed character of the catalysts was showed by XRD analysis. The Pt/Cr electrocatalyst having an nominal atomic ratio, as determined by EDX then corresponding to bulk composition and not surface composition, close to (0.8:0.2) showed higher activity for ORR in methanol-free oxygen saturated electrolyte, whereas the catalyst having an atomic ratio of (0.7:0.3) displayed higher activity for ORR at low overpotentials in saturated oxygen electrolyte containing 0.1 M methanol.Correlation of XRD and electrochemical results allows us to point out the effect of electronic interactions in catalyst activity towards ORR. It was also shown that adding chromium to platinum does not alter the reaction mechanism of oxygen reduction, and that in presence of low methanol concentration, the ORR occurs via the four-electron process according to the same mechanism as in methanol-free solution.     

Electrochemical oxidation of ethylene glycol on Pt-based catalysts in alkaline solutions and quantitative analysis of intermediate products

Electrocatalytic activities of Pt/C, Pt-Ru/C, and Pt-Ni/C for the oxidation of ethylene glycol in a basic solution are evaluated by cyclic voltammetry and quasi-steady state polarization. Based on the results of Tafel slopes from quasi-steady state polarization, the catalytic activities for ethylene glycol oxidation are in the order of Pt-Ru/C > Pt-Ni/C > Pt/C. The analysis of intermediate products for ethylene glycol oxidation by higher performance liquid chromatograph (HPLC) demonstrates that the degree of ethylene glycol oxidation is dependent on catalysts. Pt-Ru/C shows the highest current densities for ethylene glycol oxidation, but shows lower fuel utilization. On the other hand, Pt-Ni/C shows higher ability to cleavage C–C bonds, but is suffered from catalyst poisoning. To improve the tolerance for catalyst poisoning, we construct a novel Pt-Ni-SnO₂/C catalyst, compare its catalytic activities, and evaluate the intermediates. Pt-Ni-SnO₂/C shows superior catalytic activities for ethylene glycol oxidation, resulting in the highest degree of complete electro-oxidation of ethylene glycol to CO₂.     

A comparative study of the electrocatalytic oxidation of ethylene glycol on PtAu nanocomposite catalysts in alkaline, neutral and acidic media

Electrocatalytic oxidation of ethylene glycol on platinum-gold nanocomposite catalysts is investigated by cyclic voltammetry. Platinum-gold nanoparticles are prepared by chemical reduction, and cyclic voltammograms of carbon-supported platinum-gold nanocomposite catalysts show significant differences in alkaline, neutral and acidic solutions. The catalysts exhibit high electrocatalytic activity and stability in alkaline solution, showing oxidation peaks at low potentials with high current densities. Oxidation peaks at higher potentials with significant current declines are observed in neutral solution, and further positive shifts in peak potential are observed in acidic solution. The concentrations of ethylene glycol and the supporting electrolytes also affect the reaction. A higher alkaline concentration is favorable for oxidation at low potential with high current density. Increased phosphate concentration in neutral buffer solution yields a negative shift in peak potential and minor enhancement in peak current density. Changes in the sulfuric acid concentration mainly affect peak current density. Factors such as the synergic catalysis effect and increased active surface area are thought to be responsible for the reactivity of the platinum-gold nanocomposite catalysts.     

肉桂醛电解还原制备肉桂醇的研究

研究了电化学法还原肉桂醛制备肉桂醇的反应条件,分析了电解电压、三维电极因素对电还原反应的影响,产物通过红外光谱、元素分析进行定性。反应的电解电压为4 V,活性炭作为三维电极。提出了制备天然肉桂醇的新方法。     

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.     

Size-dependent catalytic activity of copper nanoclusters for oxygen electroreduction in alkaline solution

By using 2-mercapto-5-n-propylpyrimidine (MPP) as capping ligands, copper nanoclusters with different core sizes were prepared using a chemical reduction method. The as-prepared copper nanoclusters were loaded onto a glassy carbon electrode and their size effect on the electrocatalytic activity towards the oxygen reduction reaction (ORR) was investigated with electrochemical techniques in alkaline electrolyte. Cyclic voltammetric (CV) studies showed that the onset potential of ORR on smaller copper nanoclusters is more positive than that on larger copper nanoclusters. Compared to the larger clusters, higher current density of ORR was obtained using the smaller copper nanoclusters. These CV results indicate that the smaller Cu nanoclusters exhibit higher catalytic performance for ORR. In rotating-disk voltammetric studies, ORR on the synthesized MPP monolayer-protected copper nanoclusters is mainly dominated by a two-electron transfer pathway to produce H₂O₂.