The impact of the alkali cation on the mechanism of the electro-oxidation of ethylene glycol on Pt

Alkaline-Metal Cations Affect Pt Deactivation for the Electrooxidation of Small Organic Molecules by Affecting the Formation of Inactive Pt Oxide

The activity of Pt for the electro-oxidation of several organic molecules changes with the cation of the electrolyte. It has been proposed that the underlying reason behind that effect is the so-called noncovalent interactions between the hydrated cations and adsorbed OH (OHad). However, there is a lack of spectroscopic evidence for this phenomenon, resulting in an incomplete understanding at the microscopic level of these electrochemical processes. Herein, we explore the electro-oxidation of glycerol (EOG) on platinum (Pt) in LiOH, NaOH and KOH using in situ surface-enhanced infrared absorption spectroscopy in the attenuated total reflectance mode (ATR-SEIRAS) and in situ X-ray absorption spectroscopy (XAS). Our results show that the electrolyte cation influences the rate and potential at which adsorbed CO (COad), a catalytic poison, is formed and oxidized. We attribute this to the cation-dependent stability of oxygenated species on the metallic Pt surface and the different intensities of the electric field at the electrode/electrolyte interface. We also demonstrate that the formation of an inactive Pt oxide layer is indirectly also cation-dependent: the formation of this layer is triggered by the cation-dependent oxidative removal of reaction intermediates (for instance, CO). This phenomenon explains the well-known cation-induced differences in the voltammetric profiles, of not just glycerol, but generally of alcohols and polyols.

Alkali Metal Cation-Sulfate Anion Ion Pairs Promoted the Cleavage of C-C Bond During Ethanol Electrooxidation

Direct ethanol fuel cells show great promise as a means of converting biomass ethanol derived from biomass into electricity. However, the efficiency of complete conversion is hindered by the low selectivity in breaking the C-C bond. This selectivity is determined by factors such as the material structure and reaction conditions, including the nature of the supporting electrolyte. Cations serve not only as facilitators of electricity conduction through ion migration but also as influencers of the reaction pathways. In this study, we utilized differential electrochemical mass spectrometry to track the in situ generation of CO2 during potential scanning. The presence of alkali cations led to an enhancement in the CO2 selectivity. In addition, in situ Raman spectroscopy provided evidence of the formation of alkali metal cation-sulfate anion ion pairs. The catalytic activity and CO2 selectivity were found to be directly correlated to the ionic strength of these ion pairs.

Cation Incorporation into Copper Oxide Lattice at Highly Oxidizing Potentials

Electrolyte cations can have significant effects on the kinetics and selectivity of electrocatalytic reactions. We show an atypical mechanism through which electrolyte cations can impact electrocatalyst performance─direct incorporation of the cation into the oxide electrocatalyst lattice. We investigate the transformations of copper electrodes in alkaline electrochemistry through operando X-ray absorption spectroscopy in KOH and Ba(OH)2 electrolytes. In KOH electrolytes, both the near-edge structure and extended fine-structure agree with previous studies; however, the X-ray absorption spectra vary greatly in Ba(OH)2 electrolytes. Through a combination of electronic structure modeling, near-edge simulation, and postreaction characterization, we propose that Ba2+ cations are directly incorporated into the lattice and form an ordered BaCuO2 phase at potentials more oxidizing than 200 mV vs the normal hydrogen electrode (NHE). BaCuO2 formation is followed by further oxidation to a bulk Cu3+-like BaxCuyOz phase at 900 mV vs NHE. Additionally, during reduction in Ba(OH)2 electrolyte, we find both Cu-O bonds and Cu-Ba scattering persist at potentials as low as -400 mV vs NHE. To our knowledge, this is the first evidence for direct oxidative incorporation of an electrolyte cation into the bulk lattice to form a mixed oxide electrode. The oxidative incorporation of electrolyte cations to form mixed oxides could open a new route for the in situ formation of active and selective oxidation electrocatalysts.

Acid anion electrolyte effects on platinum for oxygen and hydrogen electrocatalysis

Platinum is an important material with applications in oxygen and hydrogen electrocatalysis. To better understand how its activity can be modulated through electrolyte effects in the double layer microenvironment, herein we investigate the effects of different acid anions on platinum for the oxygen reduction/evolution reaction (ORR/OER) and hydrogen evolution/oxidation reaction (HER/HOR) in pH 1 electrolytes. Experimentally, we see the ORR activity trend of HClO4 > HNO3 > H2SO4, and the OER activity trend of HClO4 [Formula: see text] HNO3 ∼ H2SO4. HER/HOR performance is similar across all three electrolytes. Notably, we demonstrate that ORR performance can be improved 4-fold in nitric acid compared to in sulfuric acid. Assessing the potential-dependent role of relative anion competitive adsorption with density functional theory, we calculate unfavorable adsorption on Pt(111) for all the anions at HER/HOR conditions while under ORR/OER conditions [Formula: see text] binds the weakest followed by [Formula: see text] and [Formula: see text]. Our combined experimental-theoretical work highlights the importance of understanding the role of anions across a large potential range and reveals nitrate-like electrolyte microenvironments as interesting possible sulfonate alternatives to mitigate the catalyst poisoning effects of polymer membranes/ionomers in electrochemical systems. These findings help inform rational design approaches to further enhance catalyst activity via microenvironment engineering.

Role of Noncovalent Interactions on the Electrocatalytic Oxidation of Ethanol in Alkali Metal Hydroxide Solutions

Ethanol is considered to be one of the most promising fuels for fuel cells. However, ethanol fuel cells have a sluggish Faraday efficiency due to complex interactions between the electrolyte, electrode, and ethanol. Recent studies have further suggested that noncovalent interactions originated from the hydrated alkali metal cations and the adsorbed OH_ad at the Pt electrode surface also played an important role in the electron transfer. In this regard, the noncovalent interactions in different alkali metal hydroxide (AMH) solutions have been systematically investigated in this study, and it was observed that the noncovalent interactions could result in the occupation of the Pt electrode surface active sites and sluggish migration of ethanol molecules in the electrical double layer, significantly affecting the electro-oxidation efficiency. Further, it was concluded that the electro-oxidation efficiency in different AMH solutions followed the order of K⁺ > Na⁺ > Rb⁺ > Cs⁺ > Li⁺ due to the noncovalent interactions.

Cation Effects on Interfacial Water Structure and Hydrogen Peroxide Reduction on Pt(111)

The interface between the Pt(111) surface and several MeF/HClO₄ (Me⁺ = Li⁺, Na⁺, or Cs⁺) aqueous electrolytes is investigated by means of cyclic voltammetry and laser-induced temperature jump experiments. Results point out that the effect of the electrolyte on the interfacial water structure is different depending on the nature of the metal alkali cation, with the values of the potential of maximum entropy (pme) following the order pme (Li⁺) < pme (Na⁺) < pme (Cs⁺). In addition, the hydrogen peroxide reduction reaction is studied under these conditions. This reaction is inhibited at low potentials as a consequence of the build up of negative charges on the electrode surface. The potential where this inhibition takes place (E _inhibition) follows the same trend as the pme. These results evidence that the activity of an electrocatalytic reaction can depend to great extent on the structure of the interfacial water adlayer and that the latter can be modulated by the nature of the alkali metal cation.

Real-Time Characterization of the Fine Structure and Dynamics of an Electrical Double Layer at Electrode-Electrolyte Interfaces

The chemisorption of an electrolyte species on electrode surfaces is ubiquitous and affects the dynamics and mechanism of various electrochemical reactions. Understanding of the chemical structure and property of the resulting electrical double layer is vital but limited. Herein, we operando probed the electrochemical interface between a gold electrode surface and a common electrolyte, phosphate buffer, using our newly developed in situ liquid secondary ion mass spectrometry. We surprisingly found that, on the positively charged gold electrode surface, sodium cations were anchored in the Stern layer in a partially dehydrated form by a formation of compact ion pairs with the accumulated phosphate anions. The resulting strong adsorption phase was further revealed to retard the electro-oxidation reaction of ascorbate. This finding addressed one major gap in the fundamental science of electrode-electrolyte interfaces, namely, where and how cations reside in the double layer to impose effects on electrochemical reactions, providing insights into the engineering of better electrochemical systems.

Hydrogen production by electrochemical reaction using ethylene glycol with terephthalic acid

Hydrogen production by electrolysis reaction of ethylene glycol and terephthalic acid in alkaline solution. When ethylene glycol and terephthalic acid were added to water together, more hydrogen was produced.

Recent advances in the electrooxidation of biomass-based organic molecules for energy, chemicals and hydrogen production

The recent developments in biomass-derivative fuelled electrochemical converters for electricity or hydrogen production together with chemical electrosynthesis have been reviewed.

Operando Thin-Layer ATR-FTIR Spectroelectrochemical Radial Flow Cell with Tilt Correction and Borehole Electrode

A novel spectroelectrochemical ATR-FTIR thin-film cell was designed and applied to elucidate the intermediates during electrocatalytic alcohol oxidation. In the novel cell design, the working electrode is positioned coplanar above the internal reflection element (IRE) to ensure uniform electrolyte film thickness at reaction conditions. The depletion of the reactant (i.e., ethanol or ethylene glycol in the case of electrocatalytic alcohol oxidation) is decreased by a specifically designed flow-through glassy carbon borehole electrode embedded in PEEK. The electrolyte can be pumped through the disk-shaped gap between the ring working electrode and the IRE into the borehole via an external peristaltic pump. To ensure a radially uniform electrolyte flow, the working electrode and the internal reflection element need to be aligned in parallel at a well-controlled distance, which was achieved by a three-microelectrode-assisted tilt correction. Tilt correction of this four-electrode ensemble and the IRE was performed by three step-motor-driven micrometer screws that allow adjustment of the electrode orientation. The effect of electrolyte pumping through the borehole electrode was analyzed by performing anodic ethanol oxidation using nickel boride as electrocatalyst. The applicability, reliability, and functionality of the cell was further assessed by oxidizing ethylene glycol and determining the reaction products as a function of the electrolyte flow rate. It is found to be essential to induce forced electrolyte convection into the thin electrolyte layer to achieve well-defined steady-state conditions, as mass transport by diffusion is by far insufficient, resulting in reactant depletion, product accumulation, and local pH changes.

How cations affect the electric double layer and the rates and selectivity of electrocatalytic processes

Electrocatalysis is central to the production of renewable fuels and high-value commodity chemicals. The electrolyte and the electrode together determine the catalytic properties of the liquid/solid interface. In particular, the cations of the electrolyte can greatly change the rates and reaction selectivity of many electrocatalytic processes. For this reason, the careful choice of the cation is an essential step in the design of catalytic interfaces with high selectivity for desired high-value products. To make such a judicious choice, it is critical to understand where in the electric double layer the cations reside and the various distinct mechanistic impacts they can have on the electrocatalytic process of interest. In this perspective, we review recent advances in the understanding of the electric double layer with a particular focus on the interfacial distribution of cations and the cations' hydration states in the vicinity of the electrode under various experimental conditions. Furthermore, we summarize the different ways in which cations can alter the rates and selectivity of chemical processes at electrified interfaces and identify possible future areas of research in this field.

Spectroelectrochemical studies on the effect of cations in the alkaline glycerol oxidation reaction over carbon nanotube-supported Pd nanoparticles

The effects of the alkali cations Na⁺ and K⁺ were investigated in the alkaline electrochemical oxidation of glycerol over Pd nanoparticles (NPs) deposited on functionalized carbon nanotubes (CNTs). The electrocatalytic activity was assessed by cyclic voltammetry revealing a lower overpotential of glycerol oxidation for nitrogen-functionalized Pd/NCNTs compared with oxygen-functionalized Pd/OCNTs. Whereas significantly lower current densities were observed for Pd/OCNT in NaOH than in KOH in agreement with stronger non-covalent interactions on the Pd surface, Pd/NCNT achieved an approximately three-times higher current density in NaOH than in KOH. In situ electrochemistry/IR spectroscopy was applied to unravel the product distribution as a function of the applied potential in NaOH and KOH. The IR spectra exhibited strongly changing band patterns upon varying the potential between 0.77 and 1.17 V vs RHE: at low potentials oxidized C₃ species such as mesoxalate and tartronate were formed predominantly, and with increasing potentials C₂ and C₁ species originating from C–C bond cleavage were identified. The tendency to produce carbonate was found to be less pronounced in KOH. The less favored formation of highly oxidized C₃ species and of carbonate is deduced to be the origin of the lower current densities in the cyclic voltammograms (CVs) for Pd/NCNT in KOH. The enhanced current densities in NaOH are rationalized by the presence of Na⁺ ions bound to the basic nitrogen species in the NCNT support. Adsorbed Na⁺ ions can form complexes with the organic molecules, presumably enhanced by the chelate effect. In this way, the organic molecules are assumed to be bound more tightly to the NCNT support in close proximity to the Pd NPs facilitating their oxidation.

Carbon-neutral energy cycles using alcohols

We demonstrated carbon-neutral (CN) energy circulation using glycolic acid (GC)/oxalic acid (OX) redox couple. Here, we report fundamental studies on both catalyst search for power generation process, i.e. GC oxidation, and elemental steps for fuel generation process, i.e. OX reduction, in CN cycle. The catalytic activity test on various transition metals revealed that Rh, Pd, Ir, and Pt have preferable features as a catalyst for electrochemical oxidation of GC. A carbon-supported Pt catalyst in alkaline conditions exhibited higher activity, durability, and product selectivity for electrooxidation of GC rather than those in acidic media. The kinetic study on OX reduction clearly indicated that OX reduction undergoes successive two-electron reductions to form GC. Furthermore, application of TiO₂ catalysts with large specific area for electrochemical reduction of OX facilitates the selective formation of GC.

The effect of the alkali metal cation on the electrocatalytic oxidation of formate on platinum

Non-covalent interactions between hydrated alkali metal cations and oxygenated species on platinum considerably impact the mechanism of formate electro-oxidation.

The electro-oxidation of ethylene glycol on platinum over a wide pH range: oscillations and temperature effects

We report a comprehensive study of the electro-oxidation of ethylene glycol (EG) on platinum with emphasis on the effects exerted by the electrolyte pH, the EG concentration, and temperature, under both regular and oscillatory conditions. We extracted and discussed parameters such as voltammetric activity, reaction orders (with respect to [EG]), oscillation's amplitude, frequency and waveform, and the evolution of the mean electrode potential at six pH values from 0 to 14. In addition, we obtained the apparent activation energies under several different conditions. Overall, we observed that increasing the electrolyte pH results in a discontinuous transition in most properties studied under both voltammetric and oscillatory regimes. As a relevant result in this direction, we found that the increase in the reaction order with pH is mediated by a minimum (~ 0) at pH = 12. Furthermore, the solution pH strongly affects all features investigated, c.f. the considerable increase in the oscillatory frequency and the decrease in the, oscillatory, activation energy as the pH increase. We suggest that adsorbed CO is probably the main surface-blocking species at low pH, and its absence at high pH is likely to be the main reason behind the differences observed. The size of the parameter region investigated and the amount of comparable parameters and properties presented in this study, as well as the discussion that followed illustrate the strategy of combining investigations under conventional and oscillatory regimes of electrocatalytic systems.

Experimental assessment of the sensitiveness of an electrochemical oscillator towards chemical perturbations

In this study we address the problem of the response of a (electro)chemical oscillator towards chemical perturbations of different magnitudes. The chemical perturbation was achieved by addition of distinct amounts of trifluoromethanesulfonate (TFMSA), a rather stable and non-specifically adsorbing anion, and the system under investigation was the methanol electro-oxidation reaction under both stationary and oscillatory regimes. Increasing the anion concentration resulted in a decrease in the reaction rates of methanol oxidation and a general decrease in the parameter window where oscillations occurred. Furthermore, the addition of TFMSA was found to decrease the induction period and the total duration of oscillations. The mechanism underlying these observations was derived mathematically and revealed that inhibition in the methanol oxidation through blockage of active sites was found to further accelerate the intrinsic non-stationarity of the unperturbed system. Altogether, the presented results are among the few concerning the experimental assessment of the sensitiveness of an oscillator towards chemical perturbations. The universal nature of the complex chemical oscillator investigated here might be used for reference when studying the dynamics of other less accessible perturbed networks of (bio)chemical reactions.