CO Stripping Research Articles

Design the PdCu/Ni2P electrocatalyst with high efficiency for ethanol oxidation reaction in alkaline media

To facilitate the electrocatalytic behavior of Direct Ethanol Fuel Cells (DEFCs), a sequence of bimetallic PdxCuy/Ni2P-C catalysts are synthesized via the microwave-assisted ethylene glycol reduction method. The results indicate that our designed Pd2Cu/Ni2P-C(1:1) catalyst owns high activity (3974.08 mA mg−1Pd), 8.3 times higher than the commercial Pd/C. The durability and the CO tolerance of the corresponding catalysts are also investigated by chronoamperometry (CA) and CO stripping measurements, implying Pd2Cu/Ni2P-C(1:1) shows good durability and the anti-CO poisoning ability for EOR in alkaline media. The electrochemical impedance spectra (EIS) analysis reveals lower charge transfer resistance for Pd2Cu/Ni2P-C(1:1). Combined with the results of XRD, HRTEM, XPS and electrochemical measurements, we found that the good electrocatalytic activity, CO tolerance and long-term durability of Pd2Cu/Ni2P-C(1:1) may be provided by the electronic and strain effect among Pd, Cu and Ni2P, which will bring the downshift in the d-band center of catalysts and the weakened adsorption of intermediates.

Electrochemical Investigation of Pt/Fe-N-C Catalysts as a Promising Low Pt-Loading Catalyst for Polymer Electrolyte Membrane Fuel Cells

Nowadays polymer electrolyte membrane fuel cells (PEMFC) become commercially established systems used in automobile, stationary and portable power generation industry which provide an improved kinetic, high efficiency, zero emission and high tolerance to the impurities. A standard catalyst used in PEMFC is based on Pt nanoparticles on carbon support materials. However, high cost of this critical raw material stimulates interest in the development of platinum group metals-free electrocatalysts (1). Fe-N-C materials are one of the promising non-precious metal electrocatalysts for these fuel cells which have recently reached outstanding performance in terms of oxygen reduction reaction (ORR) activity (2). However, the volumetric activity and durability of Fe-N-C catalyst is still significantly lower compared to Pt/C in PEMFC mainly due to their lower turnover frequency and active sites density (3).Therefore, in this work, a Black Pearl (BP) based Fe-N-C catalyst is used as ORR active support material for Pt-nanoparticles to investigate synergetic effects of active Pt and Fe-N sites towards stability and volumetric activity. We present a comparative electrochemical characterization of Pt/Fe-N-C and Pt/BP electrocatalysts with loadings of 0.01, 0.1 and 1 wt.% Pt. The measurements were done in terms of rotating ring-disk electrode experiments in 0.1 M HClO4 electrolyte under accelerated stress test (AST) during 5000 cycles in potential range 0.6 – 1.5 V vs RHE in N2-saturated electrolyte, in order to provoke Pt as well as carbon support degradation. The electrochemical surface area (ECSA) of the Pt was determined by hydrogen underpotential deposition (HUPD) and CO stripping methods before and after AST for Pt/BP catalysts. However, for Pt/Fe-N-C the ECSA determination was impossible because for metal-oxide supported Pt catalysts, a straightforward analysis of HUPD and CO methods fails to give meaningful values. The RRDE experiments revealed that ORR mass and specific activities were significantly decreased after AST especially for Pt/BP (Fig. 1) which was attributed to the platinum dissolution as well as carbon support degradation during cycling.1. E. Eren, N. Özkan, Y. Devrim, Int J Hydrogen 2020, 45 (58), 33957-33967.2. Y. He, S. Liu, C. Priest, Q.Shi , G. Wu, Chem. Soc. Rev., 2020, 11, 3484–3524.3. T. Reshetenko, A. Serov, M. Odgaard, G. Randolf, L. Osmieri, A. Kulikovsky, Electrochem Commun 2020, 118, 106795.Figure 1. Cathodic scan of ORR curves of 0.1 wt.% Pt/BP catalyst with 40 µg cm-2 Pt loading in O2-saturated 0.1 M HClO4 at scan rate 5 mV s-1 by 1600 rpm before and after AST. Figure 1

Precise Electrochemical Testing in Half-Cells: Controlling Catalyst Layer Formation and the Three-Phase Boundary in Gas Diffusion Electrodes

Novel and tailored electrocatalysts need solid benchmarking for evaluation of e.g. the oxygen reduction reaction activity (ORR), and the performance in a model approach should strictly reflect their eventual performance in the fuel cell devices. It is strongly believed that the routinely used testing methods, like rotating disk electrode (RDE) method, provide erroneous results, as they do not mimic the original fuel cell conditions in terms of water management, presence of ionomer, relevant current densities, etc. Half-cell tests of gas diffusion electrodes (GDEs) have been shown to be a good compromise as these tests exhibit comparable oxygen mass transport to the electrode as a fuel cell device, in contrast to the limited mass transport in RDEs. Realistic operational conditions, such as potential range and higher current densities in the GDE set-ups, are more akin to membrane electrolyte assemblies (MEA) testing than RDEs without the hassle and cost of full-cell tests [1-4]. In this presentation, half-cell test results will be reported for gas diffusion electrodes prepared using various methods, namely: 1) Drop-casting; 2) Spray coating with sonication; 3) Air brushing with spray gun; and 4) Hand painting. Commercially available Pt/C HiSpec 4000 was used as electrocatalyst and results of parallel RDE and MEA tests with similar Pt loadings will be discussed. Detailed electrochemically active surface area (ECSA) analysis using Hupd and CO stripping studies of various loadings from 5μg cm-2 Pt loading to 50 μg cm-2 will be reported. Drop-casting method on GDE was found to be the most efficient process to study the ECSA using Hupd (38 m2 gPt -1) and CO-stripping (61 m2 gPt -1) which were achieved at a loading of 20 μgPt cm- 2. Spray-coating with ultrasonication with a diluted catalyst ink was the ideal procedure to fabricate a uniform thin layer of catalyst with multiple deposition cycles. The current densities achieved during ORR evaluation in half-cell set-up were almost 2 orders of magnitude higher than those achieved using the RDE method. Different strategies of implementing the Nafion ionomer were utilized and compared, like: 1) Nafion 117 membrane hot-pressed on the GDE; 2) Nafion solution sprayed on the surface of the GDE reaching a loading of 2 mg cm-2 and; 3) GDE without a membrane. This report will also aim to assess how the presence of phosphoric acid in the perchloric acid electrolyte and its purity affects the catalyst’s performance for the ORR reaction. Scanning electron microscopy (SEM) was used to study the catalyst layer formation on the gas diffusion layers (GDLs). The GDEs were characterized using X-Ray Absorption Spectroscopy (XAS) [5, 6], ex-situ and in-situ, allowing us to extract structural information through the extended X-ray absorption fine structure analysis (EXAFS) (catalyst particle size, composition, morphology, and oxidation state); furthermore, by using a subtractive-normalization technique in the near-edge region (Δμ XANES) it is possible to obtain adsorbate coverage (H, O, PO4 species adsorbed on Pt surface) and binding-site information for a detailed comparison between the above samples.

High oxygen reduction reaction activity and durability of Pt catalyst photo-deposited on SnO2-coated and uncoated multi-walled carbon nanotubes

• Acid-treated CNTs were coated with SnO 2 by chemical method. • Platinum was photo-deposited on CNTs and SnO 2 -CNT supports. • Surface morphology was investigated by SEM and HAADF-STEM. • Pt/CNT and Pt/SnO 2 -CNT showed high oxygen reduction activity in acid media. Platinum nanoparticles were photo-deposited on acid-treated multi-walled carbon nanotubes (CNTs) and SnO 2 -coated CNTs. The SnO 2 -CNT nanocomposite was synthesised by a chemical method. Scanning and scanning transmission electron microscopy results indicated that the CNTs surface is partially covered by the SnO 2 layer. The surface morphology of the Pt catalyst depends on the nature of the support material. For instance, porous Pt aggregates of various sizes are formed at specific nucleation sites on the CNT surface, while a selective deposition of Pt was observed on the metal oxide part of the SnO 2 -CNT. Cyclic voltammetry and CO stripping investigations in 0.1 M HClO 4 solution confirmed different surface electrochemistry of the Pt catalysts. It is revealed that photo-deposition induces the formation of Pt-Sn nanoalloys due to increased interaction at the oxide interfaces. Both Pt/CNT and Pt/SnO 2 -CNT catalysts showed remarkable activity towards the electroreduction of oxygen in acid media. Durability measurement showed that photo-deposited Pt nanoparticles are more resistant to degradation. The corrosion-resistant nature of the SnO 2 -CNT support further improves the catalysts stability. Pt/CNT and Pt/SnO 2 -CNT catalysts retained 86% and 88% of the initial surface area respectively, after 10,000 potential cycles between 0.6 and 1.2 V RHE .

Role of dissolved CO in the solution on the origin of CO pre-oxidation on Pt(1 1 1)-Type electrodes

• CO pre-oxidation occurs on (1 1 1) terrace sites of catalyst surfaces defect-rich. • CO in solution determines the magnitude (and the shape of the curve) of CO pre-oxidation. • CO pre-oxidation involves the diffusion-limited bulk CO electro-oxidation at most active sites. • Main CO electro-oxidation peak is a typical behavior of a surface-confined process. In voltammetric CO stripping experiments in acid media, CO pre-oxidation usually takes place on catalysts with a high density of surface defects, obeyed the conditions of full CO ads layer formation at suitable low potentials into the “hydrogen region” and, importantly, CO traces in the bulk of the solution is required. This study aims to interrogate that. The electro-oxidation of CO was voltammetrically conducted applying different scan rates, employing Pt(1 1 1) terraced surfaces and full CO coverage equilibrated with different content of dissolved CO in the bulk of an acid quiescent solution; also experiments were performed in which CO ads was present only on the (1 1 0)-type steps in a CO-free solution. It was found that: (1) slopes of peak potentials of CO pre-oxidation versus the logarithm of scan rates were smaller for higher CO contents in the solution; (2) charge density of CO pre-oxidation was higher for slower scan rate; (3) CO pre-oxidation takes place at low potentials where the (1 1 0)-type step sites were inactive in the catalysis of CO electro-oxidation. We conclude that, in such a quiescent solution, the content of dissolved CO in the bulk of the solution determines the magnitude and the shape of the curve of the CO pre-oxidation. This can be understandable in terms of prompt provision of CO molecules from the solution (diffusion layer) to surface active sites, that is, the most active sites, released during the CO pre-oxidation. In the CO pre-oxidation on Pt(1 1 1) terraced surfaces, the most active sites lie at the bottom of (1 1 1) terraces adjacent to the steps. The results provide evidence that, in presence of CO traces in the bulk of the solution, on one hand, the pre-oxidation of CO involves the diffusion-limited electro-oxidation of bulk CO at most active sites released during the CO pre-oxidation . On the other hand, the main CO electro-oxidation peak is a typical behavior of a surface-confined process. Our proposition applies to Pt electrodes in acidic medium.

The CO Tolerance of Pt/C and Pt-Ru/C Electrocatalysts in a High-Temperature Electrochemical Cell Used for Hydrogen Separation.

This paper describes an experimental evaluation and comparison of Pt/C and Pt-Ru/C electrocatalysts for high-temperature (100–160 °C) electrochemical hydrogen separators, for the purpose of mitigating CO poisoning. The performances of both Pt/C and Pt-Ru/C (Pt:Ru atomic ratio 1:1) were investigated and compared under pure hydrogen and a H2/CO gas mixture at various temperatures. The electrochemically active surface area (ECSA), determined from cyclic voltammetry, was used as the basis for a method to evaluate the performances of the two catalysts. Both CO stripping and the underpotential deposition of hydrogen were used to evaluate the electrochemical surface area. When the H2/CO gas mixture was used, there was a complex overlap of mechanisms, and therefore CO peak could not be used to evaluate the ECSA. Hence, the hydrogen peaks that resulted after the CO was removed from the Pt surface were used to evaluate the active surface area instead of the CO peaks. Results revealed that Pt-Ru/C was more tolerant to CO, since the overlapping reaction mechanism between H2 and CO was suppressed when Ru was introduced to the catalyst. SEM images of the catalysts before and after heat treatment indicated that particle agglomeration occurs upon exposure to high temperatures (>100 °C)

Sub-Monolayer YOx /MoOx on Ultrathin Pt Nanowires Boosts Alcohol Oxidation Electrocatalysis.

A crucial issue restricting the application of direct alcohol fuel cells (DAFCs) is the low activity of Pt-based electrocatalysts for alcohol oxidation reaction caused by the reaction intermediate (CO*) poisoning. Herein, a new strategy is demonstrated for making a class of sub-monolayer YOx /MoOx -surface co-decorated ultrathin platinum nanowires (YOx /MoOx -Pt NWs) to effectively eliminate the CO poisoning for enhancing methanol oxidation electrocatalysis. By adjusting the amounts of YOx and MoOx decorated on the surface of ultrathin Pt NWs, the optimized 22% YOx /MoOx -Pt NWs achieve a high specific activity of 3.35mA cm-2 and a mass activity of 2.10 A mgPt -1 , as well as the enhanced stability. In situ Fourier transform infrared (FTIR) spectroscopy and CO stripping studies confirm the contribution of YOx and MoOx to anti-CO poisoning ability of the NWs. Density functional theory (DFT) calculations further reveal that the surface Y and Mo atoms with oxidation states allow COOH* to bind the surface through both the carbon and oxygen atoms, which can lower the free energy barriers for the oxidation of CO* into COOH*. The optimal NWs also show the superior activities toward the electro-oxidation of ethanol, ethylene glycol, and glycerol.

Effect of Sn content on Pt/CNT electrocatalysts for direct ethanol fuel cell application

AbstractSeveral bimetallic PtSn catalysts supported on multiwalled carbon nanotubes for ethanol electro‐oxidation were prepared by conventional impregnation‐reduction method. The Pt loading was kept at 20 wt.% and Pt:Sn atomic ratios of 5:1, 3:1, and 1:1 were selected. The catalysts were structurally characterized by temperature programmed reduction, X‐ray diffraction, X‐ray photoelectron spectroscopy, H2 chemisorption, cyclohexane dehydrogenation reaction, and transmission electron microscopy. The electrochemical characterization of the electrocatalytic materials was carried out in acid medium by cyclic voltammetry, linear sweep voltammetry, chronoamperometry, and CO stripping techniques. Among the different stoichiometries tested, the Pt(20)Sn(12.17)/CNT catalyst exhibited the highest electrocatalytic activity for ethanol oxidation reaction, with mass current density of 189.5 mA mgPt−1 at 796 mV (vs. Ag/AgCl).

Surface Basicity of Metal@TiO2 to Enhance Photocatalytic Efficiency for CO2 Reduction.

Photocatalytic reduction of CO2 to valuable chemical fuels is of broad interest, given its potential to activate stable greenhouse CO2 using renewable energy input. We report how to choose the right metal cocatalysts in combination with the surface basicity of TiO2 to enhance their photocatalytic efficiency for CO2 photoreduction. Uniform ligand-free metal nanoparticles (NPs) of Ag, Cu, Au, Pd, and Pt, supported on TiO2, are active for CO2 photoreduction using water as an electron donor. The group XI metals show a high selectivity to CO and Ag/TiO2 is most active to produce CO at a rate of 5.2 μmol g-1 h-1. The group X metals, e.g., Pd and Pt, mainly generate hydrocarbons including methane and ethane, and Pd/TiO2 is slightly more active in methane production at a rate of 2.4 μmol g-1 h-1. The activity of these photocatalysts can be enhanced by varying the surface basicity of TiO2 with primary amines. However, proton reduction selectivity is greatly enhanced in the presence of amine except amine-modified Ag/TiO2, which shows an activity enhancement by 2.4 times solely for CO2 photoreduction as compared to that without amines without switching its selectivity to proton reduction. Using in situ infrared spectroscopy and CO stripping voltammetry, we demonstrate that the improvement of electron density and the low proton affinity of metal cocatalysts are of key importance in CO2 photoreduction. As a systematic study, our results provide a guideline on the right choice of metals in combination of the surface functionality to tune the photocatalytic efficiency of supported metal NPs on TiO2 for selective CO2 photoreduction.

Electrocatalytic activity of bimetallic PtPd on cerium oxide-modified carbon nanotube for oxidation of alcohol and formic acid

• Bimetallic PtPd nanoparticles on CeO 2 modified CNT were prepared by reduction . • Effective catalysts provide better electrocatalytic performance towards oxidations. • High current density and durability compared to commercial Pt/C were found. This paper proposes an approach for the fabrication of an anode electrocatalyst for application as a fuel cell catalyst. It comprises three components, multiwall carbon nanotube (CNT), ceria oxide (CeO 2 ), and metal (i.e., Pt and/or Pd), using the reduction method for preparation. The characterization of the prepared catalysts was determined by transmission electron microscopy (TEM), X-ray diffraction, and Raman spectroscopy. The electrocatalytic performance of the prepared catalysts was examined by electrochemical measurements, e.g., cyclic voltammetry, chronoamperometry and CO stripping voltammetry. Among the catalysts, the obtained 1Pt1Pd − CeO 2 /CNT electrocatalyst provides a high electrochemical surface area, as well as high oxidation activity and durability for the oxidation of methanol, ethanol, and formic acid. The enhancement of the catalytic activity is attributed to changes in the surface electronic structures of Pt, Pd, and CeO 2 on the CNT surface that incrementally effect the active sites for oxidation. A required catalytic performance for these oxidations were observed with small-size and high-dispersion of the metal i.e.1Pt1Pd (3.34 nm) on the CeO 2 /CNT support nanocomposite. The results also show substantial improvement in the kinetics for oxidations and mass transfer efficiency owing to the catalyst structure. Therefore, the prepared catalysts have promising potential for application in low-temperature fuel cells.

Bimetallic Pd-Co Nanoparticles Supported on Nitrogen-Doped Reduced Graphene Oxide as Efficient Electrocatalysts for Formic Acid Electrooxidation

In this work, bimetallic PdxCoy nanoparticles supported on nitrogen-doped reduced graphene oxide catalysts were synthesized and tested for formic acid oxidation as potentially efficient and durable electrocatalysts. Graphene oxide was nitrogen doped through hydrothermal chemical reduction with urea as a nitrogen source. The PdxCoy nanoparticles were deposited on the nitrogen-doped graphene oxide support using the impregnation-reduction method with sodium borohydride as a reducing agent and sodium citrate dihydrate as a stabilizing agent. The structural features, such as phases, composition, oxidation states, and particle sizes, of the nanoparticles were characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy–energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The Pd nanoparticle sizes in Pd1Co1/N-rGO, Pd/N-rGO, and Pd1Co1/CNT were 3.5, 12.51, and 4.62 nm, respectively. The electrochemical performance of the catalysts was determined by CO stripping, cyclic voltammetry, and chronoamperometry. Pd1Co1/N-rGO showed the highest mass activity of 4833.12 mA–1 mg Pd, which was twice that of Pd1Co1/CNT. Moreover, Pd1Co1/N-rGO showed a steady-state current density of 700 mA–1 mg Pd after 5000 s in chronoamperometry carried out at +0.35 V. Apart from the well-known bifunctional effect of Co, nitrogen-doped graphene contributed to the performance enhancement of the Pd1Co1/N-rGO catalyst.

Effect of Rh Coverage on CO-Adsorption and -Stripping Behaviors of Rhodium-Adlayer-Modified Platinum Electrodes

Pt/Rh bimetal catalysts are often used in various reactions such as alcohol oxidation and hydrogenation. For comparison, their catalytic activity is often standardized by the real surface area or electrochemical surface area. However, the conventional method of using the electric charge for hydrogen desorption is difficult to apply to Pt/Rh bimetal catalysts because the potential regions of hydrogen desorption for Pt and Rh overlap. In this study, Rh-adlayer-modified Pt (Rhx/Pt) electrodes with different Rh coverages were prepared by underpotential depositions of Cu adatoms at different potentials followed by galvanic replacement of Cu with rhodium, and CO stripping was applied to determine the Rh coverages of these electrodes. No CO that had been adsorbed on a Pt electrode was removed by potentiostatic CO oxidation at 0.75 V vs reversible hydrogen electrode for 15 s in an Ar-saturated 0.5 M sulfuric acid solution at 5 °C, but 85% of CO on an Rh electrode was removed. The Rh coverages for the Rhx/Pt electrodes corrected on the basis of this result were in good agreement with the corresponding Rh coverages estimated from the electric charge for the stripping of the Cu adlayer. Moreover, the Rhx/Pt electrodes exhibited noteworthy CO-stripping and CO-adsorption behaviors. A single CO-stripping cyclic voltammetry peak was observed regardless of Rh coverage, and shifted toward lower potential as the Rh coverage was increased. This result resembled the trend of initiation potentials of chemisorbed oxygen formation, suggesting a bifunctional mechanism. In infrared reflectance–absorption spectra of the CO-adsorbed Rhx/Pt electrodes, a single asymmetric band assigned to “atop” CO was observed irrespective of the Rh coverage, and shifted to lower wavenumbers as the Rh coverage was increased. The d-band center estimated from the valence level spectra of the Rhx/Pt electrodes, which reflected the Pt 5d electronic structure, shifted downward as the Rh coverage was increased.

Isolating the contributions of surface Sn atoms in the bifunctional behaviour of PtSn CO oxidation electrocatalysts

The bifunctional mechanism is well-acknowledged for the promoted CO oxidation on Pt-based bimetallic electrocatalysts. However, the direct identification of the active oxygenated species and the nature and electrochemistry of the second component are still a matter of debate. Herein, Snad-Pt/C catalysts, where Sn ad-atoms are exclusively on the surface of Pt nanoparticles at low coverages ranging from 0.0033 to 0.2 monolayers to avoid sub-surface Sn and alloy formation, were prepared as a model system to resolve these issues using a surface organometallic chemistry approach. Effects of the Sn ad-atoms on CO oxidation were studied by CO stripping voltammograms as a function of Sn coverage. Using in situ XAS measurements, the Sn average oxidation state is estimated to increase from +0.2 to +3.1 as the potential increases from 0 to 0.8 VRHE, with the number of the oxygen neighbours increasing stepwise. Pt4.5-Sn-(OH)1.5 is revealed as the active species responsible for the bifunctional mechanism at low overpotentials and is generated via a redox couple corresponding to Pt4.5-Sn*/Pt4.5-Sn-(OH)1.5.

The effect on the electrocatalytic activity of the chemical interaction of selenium with palladium centers: oxygen reduction and methanol oxidation reactions in alkaline medium

The chemical reactivity of two different selenium precursors (SeO2 and Se) with nanoparticulated palladium was studied in a simple aqueous phase synthesis to generate palladium selenides (Pd x Se y ). As confirmed by XRD, XPS, TEM and energy dispersive spectroscopy analyses, the products generated showed different degrees of selenization according to the nature of the chemical precursor. Such degree of selenization was more important with elemental selenium, in contrast to SeO2. Surface electrochemistry and CO stripping in alkaline medium, clearly revealed the different interactions and stability of Pd x Se y achieved with the Pd/C precursor depending on the selenium source. The electrocatalysis of the oxygen reduction reaction was also influenced by the Se source, first in the different degree of reactivity, and second in the selectivity of the reduction product between H2O and H2O2, as well as the tolerance to the methanol oxidation reaction.

Model electrocatalysts for the oxidation of rechargeable electrofuels - carbon supported Pt nanoparticles prepared in UHV

Isopropanol (IPA) can be used as a rechargeable electrofuel. In this approach, IPA is oxidized to acetone (ACE) in a direct alcohol fuel cell and the formed ACE is subsequently back-converted to IPA in a heterogeneously catalyzed process. To study the electrochemical reaction mechanisms of the IPA oxidation at the molecular level, appropriate and well-defined model electrocatalysts are necessary. In this work we prepare such model electrocatalysts by surface science methods in ultra-high vacuum (UHV). The catalysts consist of well-defined platinum nanoparticles on carbon supports. As carbon support, we use flat highly ordered pyrolytic graphite (HOPG) and thin (20 nm) magnetron sputtered carbon films on a polycrystalline gold substrate. In a first step, we characterize the model electrocatalysts and investigate their stability in-situ with complementary methods, i.e. by electrochemical scanning tunneling microscopy (EC-STM), electrochemical on-line inductively coupled plasma mass spectrometry (ICP-MS) and CO stripping experiments followed by electrochemical infrared reflection absorption spectroscopy (EC-IRRAS). We determined a stability window ranging from -0.65 VRHE to 1.15 VRHE for both sample types, independent of the presence or absence of IPA in the electrolyte. In the second step, we study the oxidation of IPA on tPt nanoparticles using differential electrochemical mass spectrometry (DEMS) and EC-IRRAS. The onset of IPA oxidation is observed at 0.3 VRHE. ACE is formed with high selectivity, while we identify traces of CO2 as the only side-product formed at higher potentials. However, we do not observe any formation of adsorbed CO. A direct comparison of these results with previous work on Pt(111) suggests that low coordinated Pt sites and size effects play a subordinate role for IPA oxidation on Pt electrocatalysts.

Surface-enriched Pd/C electrocatalysts with a low load of Pt prepared by surface redox reactions for the oxygen reduction reaction

Pt–Pd/C bimetallic catalysts with low platinum contents were synthesized by surface redox reactions. In this method, hydrogen preadsorbed on the palladium surface of the parent catalyst (Pd/C) acts as a reducer of the PtCl6− species, allowing a selective deposition and closer interaction between both metals (Pt–Pd). The physicochemical study was carried out by XRD, HAADF-STEM, HRTEM, EDS, and electronic interaction between Pt–Pd was characterized by XPS. The electrochemical characterizations by CV, CO stripping, and steady-state LSV toward ORR using RDE indicate that 1%Pt–Pd/C sample exhibited the best results. The present work demonstrates that the selective incorporation of Pt in low quantities on the Pd surface increases the interaction between both metals, producing electronic interactions, synergetic effects, and an enhanced electrocatalytic activity toward the ORR.

The Relationships among Hydrogen Adsorption, CO Stripping, and Selectivity of CO2 Reduction on Pd Nanoparticles

The low formation overpotential and strong adsorption ability of CO on palladium surface sites constrain the lower potential limit and current density of the electrochemical reduction of CO2 (CO2ER) to formate on Pd although this reaction has been considered one of the most effective methods for CO2 recycling. Among various factors, the participation of adsorbed hydrogen atoms seems to be a key factor affecting the selectivity of CO2ER on Pd. This article discusses the relationship between the selectivity and hydrogen adsorption on the Pd nanoparticles/carbon (Pd/XC72) catalyst through two kinds of systems: gas diffusion electrode (GDE) and rotating ring disk electrode (RRDE). The main product of the CO2ER on Pd/XC72 is changed from formate to CO when both the terrace and step sites of Pd particles are under a low coverage of adsorbed H atoms. In addition, the progress of CO self-poisoning on the Pd/XC72 catalyst and the peak corresponding to the oxidation of COOH* on the Pt ring electrode are clearly observed by the electrochemical analysis methods. The high selectivity of CO generation from the CO2ER on the Pd/XC72 catalyst is attributed to the progressive adsorption of CO which inhibits the participation of Hads during the CO2ER.

Design the PdCu/TaN[sbnd]C electrocatalyst with core-shell structure having high efficiency for methanol and formic acid oxidation reactions

For the first time, PdCu/TaNC catalyst with a special core-shell structure is assembled, which exhibits high catalytic performance for methanol oxidation reaction (MOR) in alkaline medium (1326.95 A gPd−1) and formic acid oxidation reaction (FAOR) in acidic medium (1052.63 A gPd−1), 6.3 and 4.9 times those of the commercial Pd/C, respectively. According to the results of Tafel, Electrochemical Impedance Spectroscopy (EIS), CO stripping and chronoamperometry (CA), it is revealed that the reaction kinetics, anti-CO poisoning ability and stability of PdCu/TaNC for MOR and FAOR are all enhanced significantly. Density Functional Theory (DFT) calculations also indicate that the adsorption intensity of CO is weakened and the binding intensity of OH is enhanced on the Pd3Cu1/TaN(001) surface, which are beneficial to the removal of adsorbed CO and enhancement of the anti-CO poisoning ability. Combined with the characterization analysis and theoretical calculations, the good catalytic performance of PdCu/TaNC for MOR and FAOR can be attributed to the unique electronic structure and morphology from the effective interface, geometric effects and proper d-band center.

Nanoporous Pd-Cu thin films as highly active and durable catalysts for oxygen reduction in alkaline media

A facile electrochemical etching approach was developed and implemented to enhance the electrocatalytic performance of Pd-Cu films for oxygen reduction reaction (ORR) in alkaline media. The nanoporous (np) Pd-Cu was synthesized through electrochemical de-alloying of pre-deposited Pd-Cu films. Scanning electron microscopy (SEM) with energy dispersive spectroscopy was employed to confirm the residual amount of Cu in the np structure. The accordingly prepared np catalysts showed excellent ORR activity up to 1.11 A/mgPd at a potential of 0.9 V vs. RHE which is around 22-times and 6.2-times higher than that of a plain Pd film and commercial Pd/C catalyst, respectively. The ORR activity enhancement is attributed to the unique porous structure, large Pd surface area, and modified electronic structure of Pd with residual Cu as confirmed by the SEM, hydrogen underpotential deposition, and CO stripping characterizations. The np Pd-Cu catalyst also showed excellent durability which is manifested by a negligible decrease in the half-wave potential (4 mV negative shift) after 10,000 potential cycles in alkaline media. These findings provide insights into the rational design of an electrocatalyst's structure utilizing an electrochemical de-alloying method to achieve high atomic utilization and improved catalytic performance.

Au Clusters on Pd Nanosheets Selectively Switch the Pathway of Ethanol Electrooxidation: Amorphous/Crystalline Interface Matters

AbstractThe development of ethanol electrooxidation catalysts with high activity and robust stability is essential for the commercialization of direct ethanol fuel cells. However, because of their low C1 selectivity, the electrocatalytic efficiency of catalysts for complete ethanol oxidation is still far from satisfying. Herein, a novel 2D PdAu heterogeneous catalyst for enhancing C1 selectivity to achieve highly efficient ethanol oxidation through phase and interphase engineering is designed. It is found that owing to the plentiful amorphous/crystalline interphases, the selectivity of C1 pathway on PdAu heterocatalyst can be improved up to 33.2% at a low potential, 10.4 times higher than commercial Pd/C (3.2%). Furthermore, 89.1% of initial mass activity of PdAu HNS can be retained after the accelerated degradation test of 2000 potential cycles, much higher than those of Pd NS (39.3%), commercial Pd/C (34.4%), and Pt/C (11.4%). The CO stripping and in situ Fourier transform infrared experiments reveal that 2D PdAu heterocatalyst, with intricate design of the amorphous Pd domain and the crystalline Au cluster interface, has better antipoisoning properties and stronger CC bond cleavage ability than pure Pd nanosheets. Density functional theory calculations further demonstrate that the introduction of Au clusters switches on the electroactivity of amorphous Pd as the electron pump to accomplish the complete oxidation of ethanol, in which the selectivity for C1 pathway is significantly boosted whereas the typical C2 pathway is substantially blocked.