pH effect on electrocatalytic reduction of CO2 over Pd and Pt nanoparticles

Adsorbed hydrogen participates in electrocatalytic reduction of CO₂ and competitive hydrogen evolution reaction (HER) simultaneously, and its reaction pathway greatly affects the activity and selectivity of CO₂ reduction. In this work, we investigate pH effect on electrocatalytic reduction of CO₂ over Pd and Pt nanoparticles (NPs) with a similar size in a pH range from 1.5 to 4.2. Pt NPs completely contribute to HER in the pH range. Over Pd NPs, Faradaic efficiency for CO production at − 1.19 V (vs. reversible hydrogen electrode) varies from 3.2% at pH of 1.5 to 93.2% at pH of 4.2, and current density for CO production reaches maximum at pH of 2.2. The significant enhancement of Faradaic efficiency and current density for CO production over Pd NPs at high pH values is attributed to decreased kinetics of hydrogen evolution reaction by increasing hydrogen binding energy and lowered adsorption affinity of CO-like intermediate compared to Pt.     

Electrochemical and FTIR spectroscopic study of CO2 reduction at a nanostructured Cu/reduced graphene oxide thin film

Here we report a facile approach to synthesize a novel nanostructured thin film comprising Cu nanoparticles (NPs) and reduced graphene oxide (rGO) on a glassy carbon electrode (GCE) via the direct electrochemical reduction of a mixture of cupper and graphene oxide (GO) precursors. The effect of the applied potential on the electrochemical reduction of CO₂ was investigated using linear sweep voltammetric (LSV) and chronoamperometric (CA) techniques. Carbon monoxide and formate were found as the main products based on our GC and HPLC analysis. The electrochemical reduction of CO₂ at the Cu/rGO thin film was further studied using in situ ATR-FTIR spectroscopy to identify the liquid product formed at different applied cathodic potentials. Our experimental measurements have shown that the nanostructured Cu/rGO thin film exhibits an excellent stability and superb catalytic activity for the electrochemical reduction of CO₂ in an aqueous solution with a high current efficiency of 69.4% at − 0.6 V vs. RHE, promising for the efficient electrochemical conversion of CO₂ to valuable products.     

Pd Nanostructures with High Tolerance to CO Poisoning in the Formic Acid Electrooxidation Reaction

Pd architectures such as nanobars and nanoparticles were synthetized by the polyol method using di-ethylene glycol as reaction media. The morphology, composition and electrocatalytic properties were investigated by transmission electronmicroscopy (TEM), thermo-gravimetric analysis (TGA), X-ray diffraction (XRD) and electrochemical measurements. The electrocatalytic activity of Pd nanostructures was tested in terms of formic acid electrooxidation reaction (FAOR) in acid media (0.5 M H₂SO₄) and compared with commercial Pd/XC-72 (Pd/C). Results from the electrochemical studies showed that Pdnanobars (PdNB/C) presented higher tolerance to the CO and CO₂ poisoning effect compared with Pd nanoparticles (PdNP/C) and commercial Pd/C. Furthermore, the onset potential toward formic acid electrooxidation at high concentration (1 M) on PdNB/C exhibited a negative shift ca. 100 mV compared with commercial Pd/C. Finally, PdNB/C in the presence of 1 M FA showed a lower poisoning degree compared with commercial Pd/C and PdNP/C.     

A simple cathodic process for carboxylating noble metals and generating new versatile electrode interfaces

The electrochemical reactivity of polarized metals such as platinum, palladium, and rhodium toward carbon dioxide in aprotic dimethylformamide (DMF) solutions of tetramethylammonium tetrafluoroborate (TMABF₄) is presented. The capacity of metals such as Pd and Pt to cathodically insert the electrolytes under superdry conditions (via the generation of organometallic intermediates analogous to Zintl metals) is combined with the concomitant carboxylation of those metals within a potential range from − 1 V to − 2.5 V vs. Ag/AgCl/KCl(sat). Under these conditions, dense surface carboxylation of these precious metals occurs, totally suppressing their catalytic activity. Thick layers of the carboxylated metals (platinum-CO₂^– and palladium-CO₂⁻) are chemically stable and may then be further functionalized for specific applications.     

CO tolerant Pt/WC methanol electro-oxidation catalyst

Platinum supported on WC (Pt/WC) catalyst (20 wt.% Pt) was synthesized as a new methanol electro-oxidation catalyst. Particle size of 7.5 nm was obtained from X-ray diffraction results and a uniform distribution of particles was observed by transmission electron microscopy. In cyclic voltammetry (CV) measurement, the reduction peak potential of PtO increased from 0.72 V in commercial Pt/C to 0.76 V in Pt/WC. By combining the CV and CO stripping results, spill-over of H⁺ from Pt to WC was observed. Electrochemically active surface area calculated from the desorption area of H⁺ were 11.2 and 5.74 m²/g catalyst for Pt/WC and Pt/C, while those obtained from the desorption area of CO were 4.42 and 6.40 m²/g catalyst, respectively. CO electro-oxidation peak potential greatly decreased from 0.80 V in Pt/C to 0.68 V in Pt/WC. The reaction of WC with water to produce WC–OH could lower to CO electro-oxidation peak potential. Specific activity for methanol electro-oxidation increased from 144 mA/m² in Pt/C to 188 mA/m² in Pt/WC.     

The electrochemical reduction of carbon dioxide (CO2) to methanol in the presence of pyridoxine (vitamin B6)

Experiments aimed at ameliorating carbon dioxide (CO₂) into methanol were explored using pyridoxine, a member of the vitamin B₆ family, to enhance the reduction process. At a platinum electrode, an aqueous solution (pH ≈ 5) of pyridoxine showed a quasi-reversible redox couple with the cathodic peak detected at ca. − 0.55 V vs. Ag/AgCl (3 M KCl) in the presence of CO₂ and argon. An increase in the corresponding cathodic peak current was observed following saturation of the solution with CO₂ using a Pt electrode, but with no detectable reduction current recorded at a glassy carbon electrode for the same system. Confirmation of methanol formation during the pyridoxine-assisted CO₂ reduction was conducted by using gas chromatography analysis of the electrolyzed solutions and faradic yields of ca. 5% were afforded. A combination of the results from the cyclic voltammetry and constant current chronopotentiometry experiments revealed an overpotential of ≤ 200 mV was required. The results indicate a potential utility of pyridoxine as an alternative reagent to the more toxic pyridine during the electrochemical reduction of CO₂.     

Electrochemical reduction of CO2 with clathrate hydrate electrolytes and copper foam electrodes

We report on the first use of clathrate hydrates as electrolyte additive in the electrochemical reduction of carbon dioxide. Clathrate hydrates allow the enrichment of significantly larger volumes of gas than liquids can usually dissolve. Electrolyte solutions containing 10%_mass THF with and without CO₂ containing clathrate hydrates were investigated with a copper-foam working electrode. Our results show that at − 1.0 V vs Ag/AgCl the Faradaic efficiency for the production of CO and further reduced carbonaceous products was 80% with clathrates vs 20% with non-clathrate electrolytes of identical chemical composition at nearly equal temperature.     

Investigation of the electrocatalytic activity of boron-doped diamond electrodes modified with palladium or gold nanoparticles for oxygen reduction reaction in basic medium

The paper reports on the electrocatalytic activity of boron-doped diamond (BDD) electrodes electrochemically modified with palladium (Pd) or gold nanoparticles (Au NPs) towards oxygen reduction reaction (ORR) in alkaline medium. The BDD/Pd NP interface shows a well-defined diffusion-controlled voltammetric oxygen reduction peak at −0.25 V vs. Ag/AgCl. This is more positive than the ORR peak at −0.59 V vs. Ag/AgCl observed on BDD/Au-NP composite electrodes. The ORR proceeds via a four-electron process in both cases.     

Adsorption of 2,2′-bipyridine at an Au(111)|ionic liquid electrified interface

The adsorption of added 2,2′-bipyridine (2,2′-BP) from 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethanesulfonyl)imide (EMMImNTf₂) at an Au(111) electrode has been investigated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Addition of 2,2′-BP to the ionic liquid clearly modifies the interfacial region as a result of the competition between 2,2′-BP and EMMImNTf₂ to occupy the electrode surface. Within the region of ideal polarizability, the 2,2′-BP adlayer undergoes structural changes, shown by the presence of peaks in the CV curves. Between −0.2 V and + 0.9 V, the capacitance–potential curves obtained from EIS data present a capacity maximum depending strongly on the ac frequency, which is typical pseudo-capacitive behavior indicative of a reorganization of the interfacial layer. At more positive potentials a true capacity value close to 10 μF.cm^− 2 and invariant with the potential suggests that the 2,2′-BP molecules adopt a perpendicular orientation with the nitrogen atoms facing the electrode surface, similar to their adsorption on gold from aqueous solutions.     

Co-deposition of arsenic and arsine on Pt,Cu, and Fe electrodes

An electrochemical investigation of arsenic in aqueous solutions was carried out in order to assess the possibility of removing it by reduction from As(III) to its elemental form. Arsine evolution was significant at potentials below −0.650 V_SCE on Cu, and below −0.728 V_SCE on Pt. As(V) could also be removed on Cu, with a larger evolution of arsine. When a potential equal to −0.560 V_SCE was applied to an iron electrode, arsenic deposition took place simultaneously with iron dissolution, and arsine evolution was negligible.     

Novel electroactive surface functionality from the coupling of an aryl diamine to carbon black

The reaction of 1,2-diaminoanthraquinone with Vulcan XC72 carbon in 4 M HCl produces two distinct surface bound anthraquinone species, with formal potentials of ca. ?0.03 and ?0.19 V vs. SCE. The more positive couple is very stable to electrochemical cycling and has been assigned to the expected benzimidazole linkage. The other wave, which decays over hours of cycling is thought to be due to an amine linkage. This type of linkage also appears to be formed spontaneously when 1,2-diaminoanthraquinone is adsorbed onto Vulcan XC72 from methanol.     

Influence of surface oxygen groups on V(II) oxidation reaction kinetics

The role of surface oxygen groups on the kinetics of the V(II) oxidation reaction was studied on modified glassy carbon (GC) electrodes by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The reaction was found to be sensitive to the presence of oxygen groups on the electrode surface. Higher O/C ratios determined by X-ray photoelectron spectroscopy (XPS) corresponded to higher reactivities and lower charge transfer resistances measured in a 1 M V(II) electrolyte. The stability of an oxidised GC surface was also investigated in a 1 M V(II) electrolyte by potential holding and cycling experiments. It was found that after holding and cycling to successively more negative potentials up to − 0.8 V/RHE, the electrode surface lost its initial reactivity.     

Cathodic carboxylation of gold in thick {Au-CO2}n layers. A model for reversible electrochemical sequestration of CO2

Catalytic reduction of CO₂ (saturated in organic polar solvents, e.g. N,N-dimethylfomamide, containing Me₄NX or NaBF₄) was achieved at smooth gold electrodes and at glassy carbon electrodes galvanostatically capped with a thin layer of gold. Under these quite explicit conditions, very sharp reduction steps were observed near − 1.5 V vs. Ag/AgCl. With small cations listed above, an unexpected behavior was observed, a progressive electrode inhibition occurring upon several scans or after a fixed-potential electrolysis at E < − 1.7 V. This phenomenon could be attributed to the insertion of CO₂ into gold, leading to the formation of a thick iono-metallic multi-strata layer (less conducting than pure metal) that grows with the electrode charge. The formation of this new interface is due to the concur of three elements: transient CO₂ anion radical, the metal, and rather small-sized cations (M⁺ = Na⁺ or TMA⁺), the three possibly associated in a form {Au-CO₂⁻,M⁺} apparently very reactive with oxygen, moisture, and with some organic π-acceptors. Upon multi-scans up to − 2.2 V, the thickness of formed layer progressively increases reaching more than 10^− 7 to 10^− 6 mol cm^− 2. Such multi-layers undergo decomposition in the anodic domain at about + 1.7 V liberating CO₂ beforehand trapped in Au. Coulometric analyses demonstrated that insertion (cathodic) and release (anodic) steps are quite equivalent, which permits to consider this process as chemically reversible sequestration of carbon dioxide.     

Comparison of electrocatalytic reduction of CO2 to HCOOH with different tin oxides on carbon nanotubes

A comparative study is reported on the electrocatalytic reduction of CO₂ to HCOOH in aqueous alkaline solution with differently prepared tin-oxide particles on multi-walled carbon nanotubes from SnCl₂ or SnCl₄ precursors. The highest faradaic and energy efficiencies of 64% and 27% were obtained at − 1.40 V vs. SCE with particles that were obtained by KBH₄ reduction from a SnCl₂ precursor. At lower potentials, competitive reduction reactions occur. A SnCl₂ versus SnCl₄ precursor favors retention of a Sn(II) valence state in a surface tin oxyhydroxide surface layer. Different morphologies of the particle agglomerates made little difference in the electrocatalytic selectivity and activity. The two SnO_x/CNT electrodes both showed a current density retention of ~ 70% after a 20-h electrolysis.     

The potentials of zero charge of Pd(1 1 1) and thin Pd overlayers on Au(1 1 1)

The potential of zero charge (pzc) of Pd(1 1 1) has been determined in dilute NaF solutions by measuring the Gouy–Chapman minimum of the double-layer capacity. For a massive Pd(1 1 1) single crystal electrode a pzc of −0.12 V vs. SCE has been found. The corresponding values for thin Pd(1 1 1) overlayers on Au(1 1 1) have also been determined. While the pzc of the first, pseudomorphic Pd layer on Au(1 1 1) is −0.09 V vs. SCE, the pzc of a five monolayers thick Pd film on Au(1 1 1) is practically identical to the pzc of the massive Pd(1 1 1) electrode. By comparing pzc's and work functions for Au(1 1 1) and Pd(1 1 1), the dipole contribution to the potential drop across the Pd(1 1 1)/water interface is estimated.     

Stability of carbon-supported palladium nanoparticles in alkaline media: A case study of graphitized and more amorphous supports

The stability and degradation mechanism of graphitized (Graphene nanosheets) and more amorphous (Vulcan XC-72R) carbon-supported palladium nanoparticles was investigated. Coupling identical-location transmission electron microscopy (ILTEM) and electrochemistry enabled to correlate the distribution of the Pd nanoparticles under accelerated stress test (up to 1000 cycles between 0.1 and 1.23 V vs. RHE, in a 0.1 M NaOH solution at 25 °C) with changes in electrochemical accessible surface area (ECSA). The carbon-supported Pd nanoparticles undergo similar rates of degradation in terms of electrochemical surface areas on both supports. However, their mechanisms of degradation differ: on amorphous carbon, the primary mode of degradation is Pd nanoparticles detachment (and minor agglomeration), whereas on graphitized supports it is more likely their coalescence and dissolution/redeposition. “Bulk” carbon-corrosion is negligible in both cases, as proven by ex situ Raman spectroscopy. So, using a graphitized carbon support (Graphene nanosheets) versus a more amorphous one (Vulcan XC-72R) does not enable to significantly depreciate the Pd/C catalyst degradation in alkaline media.     

Toward more efficient photochemical CO2 reduction: Use of scCO2 or photogenerated hydrides

Rhenium(I) and ruthenium(II) complexes have been successfully used for photochemical CO₂ reduction to CO or formate. However, a typical turnover frequency for such reactions is <20 h^?1 and the formation of reduced species beyond CO or formate is very limited. In the case of the rhenium(I) bipyridyl tricarbonyl system, the key intermediate has been shown to decay with a first-order dependence on [CO₂] to produce CO, which is the rate-determining step. The limited concentration of dissolved CO₂ in organic solvents results in extremely slow CO₂ reduction. To improve the reaction rate, we prepared new CO₂-soluble rhenium(I) bipyridine complexes bearing fluorinated alkyl ligands and investigated their photophysical properties in CH₃CN and supercritical CO₂. We also investigated the properties of a metal complex with an NAD⁺ model ligand, [Ru(bpy)₂(pbn)]²⁺ (bpy = 2,2′-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), and prepared the corresponding NADH-like complex [Ru(bpy)₂(pbnHH)]²⁺ upon MLCT excitation followed by reductive quenching. This species can be used as a renewable hydride donor. The electrochemical and photochemical properties, and the reactivity of these species toward CO₂ reduction were investigated.     

Graphene nanoplatelets supported metal nanoparticles for electrochemical oxidation of hydrazine

Graphene nanoplatelets have been applied as the support to electrodeposit monometallic Au and Pd nanoparticles as well as bimetallic Au–Pd nanoparticles. These nanoparticles have been characterized with scanning electron microscope, energy dispersive X-ray spectroscopy, X-ray diffraction spectroscopy, and electrochemical techniques. They are further utilized as the catalysts for electrochemical oxidation of hydrazine. The oxidation peak potential is − 0.35 and 0.53 V (vs. SCE) when monometallic Pd and Au nanoparticle are used as the catalysts. When bimetallic nanoparticles are applied as the catalyst, their composition affects the peak potential and peak current for the oxidation of hydrazine. Higher oxidation current is achieved when bimetallic Au–Pd nanoparticles with an atomic ratio of 3:1 are deposited on graphene nanoplatelets. Metal nanoparticle-loaded graphene nanoplatelets are thus novel platforms for electrocatalytic, electroanalytical, environmental, and related applications.     

A topographic view of the Pt(1 1 1) surface at the electrochemical interface in the presence of carbon monoxide

In this communication we present topographic images of the Pt(1 1 1) surface in CO saturated 0.1 M HClO₄, obtained by scanning tunneling microscopy.The topography presents two different structures, depending on the CO adsorption potential (E_ad = 0.15 V or E_ad = 0.5 V vs RHE). For adsorption at 0.15 V the system presents a heterogeneous appearance, which totally covers the surface and impedes the observation of steps on the substrate surface. When CO is adsorbed at 0.5 V large clusters forming chains along the steps are observed. These aggregates can be, tentatively, correlated with the H-bonded water structure suggested earlier on the basis of FTIR spectroscopy. The clusters have inhibitory effects on CO oxidation.     

Electro-reduction of cuprous chloride powder to copper nanoparticles in an ionic liquid

Cyclic voltammetry of the CuCl powder in a cavity microelectrode revealed direct electro-reduction in solid state in 1-butyl-3-methylimidazolium hexafluorophosphate. Potentiostatic electrolysis of the salt powder (attached to a current collector) in the ionic liquid produced Cu nanoparticles as confirmed by X-ray diffraction, energy dispersive X-ray analysis, scanning and transmission electron microscopy. The particle size decreased down to 10 nm when the electrode potential was shifted from −0.9 V to −1.8 V (versus Ag/Ag⁺). The electro-reduction and the nanoparticle formation mechanisms were investigated in the ionic liquid and also in aqueous 0.1 mol L⁻¹ KClO₄ in which larger Cu particles were obtained.