Smart Electrode Surfaces by Electrolyte Immobilization for Electrocatalytic CO2 Conversion.

The activity and selectivity of molecular catalysts for the electrochemical CO2 reduction reaction (CO2RR) are influenced by the induced electric field at the electrode/electrolyte interface. We present here a novel electrolyte immobilization method to control the electric field at this interface by positively charging the electrode surface with an imidazolium cation organic layer, which significantly favors CO2 conversion to formate, suppresses hydrogen evolution reaction, and diminishes the operating cell voltage. Those results are well supported by our previous DFT calculations studying the mechanistic role of imidazolium cations in solution for CO2 reduction to formate catalyzed by a model molecular catalyst. This smart electrode surface concept based on covalent grafting of imidazolium on a carbon electrode is successfully scaled up for operating at industrially relevant conditions (100 mA cm-2) on an imidazolium-modified carbon-based gas diffusion electrode using a flow cell configuration, where the CO2 conversion to formate process takes place in acidic aqueous solution to avoid carbonate formation and is catalyzed by a model molecular Rh complex in solution. The formate production rate reaches a maximum of 4.6 gHCOO- m-2 min-1 after accumulating a total of 9000 C of charge circulated on the same electrode. Constant formate production and no significant microscopic changes on the imidazolium-modified cathode in consecutive long-term CO2 electrolysis confirmed the high stability of the imidazolium organic layer under operating conditions for CO2RR.

Characterization of LiCoO2 nanoparticle suspensions by single collision events.

Transient electrochemical experiments associated with the collisions between hydrothermally synthesized LiCoO2 (LCO) nanoparticles/aggregates of different sizes and a polarized gold ultramicroelectrode (UME) were used as a new additive-free analytical tool applied to Li ion insertion compounds. The size of the LCO nanoparticles/aggregates, ranging from 75 to 450 nm, the diffusion coefficient of the LCO nanoparticles/aggregates in suspension (∼8 × 10-9 cm2 s-1), and the Li ion diffusion coefficient within crystalline LCO nanoparticles (∼1.3 × 10-11 cm2 s-1) were estimated from single collision events. Interestingly, the charge exchanged during each nanoparticle collision was related to the size of the corresponding LCO aggregate, which enables electrochemical sizing distribution measurement displaying evident concordance with optical techniques, including dynamic light scattering (DLS) and cryo-transmission electron microscopy (cryo-TEM). Studying the nanoparticle collision frequency on the UME surface as a function of the LCO nanoparticle concentration allows estimation of the diffusion coefficient of LCO nanoparticles/aggregates in suspension. Finally, from the current decay observed in chronoamperometry after LCO nanoparticle collision on the polarized UME surface, which corresponds to the LCO oxidation (i.e. the Li+ deinsertion reaction), the Li ion diffusion coefficient within the host crystalline material is estimated. This is a key parameter, which controls the cycle lifetime and charge rate in Li ion battery performance. This new approach thus allows a fine description of the nanoparticle properties, which includes sizing as well as estimation of the Li ion diffusion coefficient within the host crystalline material.

Electrocatalytic studies on imidazolium based ionic liquids: defining experimental conditions.

The number of publications devoted to studying electrochemical reactions in room temperature ionic liquids (RTILs) is constantly growing, but very few of them have been devoted to defining proper experimental conditions to obtain reproducible electrochemical results. In this work, we demonstrate that the combination of a proper RTIL purification treatment and a filtered Ar gas stream allow us to obtain featureless voltammograms in [C4mim][BF4], [C4mim][NTf2], and [C4m2im][NTf2], which otherwise present signals associated with different types of impurities such as water and some minor electroactive impurities acquired during the RTIL synthesis process. Moreover, we demonstrate that bubbling Ar, or another inert gas, through the electrolyte in order to purge O2 dissolved in RTILs is one of the major sources of water and O2 impurities incorporated in RTILs within the electrochemical cell. To overcome this source of water uptake, we have incorporated a gas stream purification filter before the gas reaches the RTIL in the electrochemical cell. To illustrate the effect of these impurities in relevant electrocatalytic studies, we study the electrocatalytic reduction of CO2 on Pt nanoparticles and the key role of an appropiate filter when the CO2 gas stream is bubbled within imidazolium based RTILs. Our cyclic voltammetric studies point out that CO2 electroreduction on Pt nanoparticles only presents activity in [C4mim][NTf2] and [C4m2im][NTf2], thus suggesting that the C-2 position on the imidazolium ring is not the key position in CO2 electrochemical reduction. In contrast, the same Pt nanoparticles are inactive towards CO2 electroreduction in [C4mim][BF4], which is a more hydrophilic RTIL.

Surface structured platinum electrodes for the electrochemical reduction of carbon dioxide in imidazolium based ionic liquids.

The direct CO2 electrochemical reduction on model platinum single crystal electrodes Pt(hkl) is studied in [C2mim(+)][NTf2(-)], a suitable room temperature ionic liquid (RTIL) medium due to its moderate viscosity, high CO2 solubility and conductivity. Single crystal electrodes represent the most convenient type of surface structured electrodes for studying the impact of RTIL ion adsorption on relevant electrocatalytic reactions, such as surface sensitive electrochemical CO2 reduction. We propose here based on cyclic voltammetry and in situ electrolysis measurements, for the first time, the formation of a stable adduct [C2mimH-CO2(-)] by a radical-radical coupling after the simultaneous reduction of CO2 and [C2mim(+)]. It means between the CO2 radical anion and the radical formed from the reduction of the cation [C2mim(+)] before forming the corresponding electrogenerated carbene. This is confirmed by the voltammetric study of a model imidazolium-2-carboxylate compound formed following the carbene pathway. The formation of that stable adduct [C2mimH-CO2(-)] blocks CO2 reduction after a single electron transfer and inhibits CO2 and imidazolium dimerization reactions. However, the electrochemical reduction of CO2 under those conditions provokes the electrochemical cathodic degradation of the imidazolium based RTIL. This important limitation in CO2 recycling by direct electrochemical reduction is overcome by adding a strong acid, [H(+)][NTf2(-)], into solution. Then, protons become preferentially adsorbed on the electrode surface by displacing the imidazolium cations and inhibiting their electrochemical reduction. This fact allows the surface sensitive electro-synthesis of HCOOH from CO2 reduction in [C2mim(+)][NTf2(-)], with Pt(110) being the most active electrode studied.

Enabling circular economy by N-recovery: Electrocatalytic reduction of nitrate with cobalt hydroxide nanocomposites on copper foam treating low conductivity groundwater effluents.

Fertilizers play a vital role in the food-energy-water nexus. The traditional method of artificial nitrogen fixation to produce ammonia is a high-energy intensive centralized process that has caused an imbalance of the N-cycle due to the release of N-species to water. Electrocatalytic nitrate reduction (ENR) to ammonia is a promising N-resource recovery alternative that can enable the circular reuse of ammonia in decentralized settings. However, the primary challenge is identifying selective and affordable electrocatalysts. Identifying electrodes that rely on something other than platinum-group metals is required to surpass barriers associated with using expensive and endangered elements. In this study, an earth-abundant bimetallic catalyst, Cu/Co(OH)x, prepared and optimized by electrodeposition, demonstrates superior ammonia production. Under environmentally relevant conditions of 30 mg NO3--N L-1, Cu/Co(OH)x showed higher ammonia production than pristine Cu foam with 0.7 and 0.3 mmol NH3 gcat-1 h-1, respectively. The experimental evaluation demonstrated direct reduction and catalytic hydrogenation mechanisms in Cu/Co(OH)x sites. Leaching analyses suggest that Cu/Co(OH)x has outstanding stability with negligible metal concentration below the maximum contaminant level for both Cu and Co. These results provide a framework for using earth-abundant materials in ENR with comparable efficiency and energy consumption to platinum-group materials.

Electrocatalytic Conversion of CO2 to Formate at Low Overpotential by Electrolyte Engineering in Model Molecular Catalysis.

An electrolyte engineering strategy was developed for CO2 reduction into formate with a model molecular catalyst, [Rh(bpy)(Cp*)Cl]Cl, by modifying the solvent (organic or aqueous), the proton source (H2 O or acetic acid), and the electrode/solution interface with imidazolium- and pyrrolidinium-based ionic liquids (ILs). Experimental and theoretical density functional theory investigations suggested that π+ -π interactions between the imidazolium-based IL cation and the reduced bipyridine ligand of the catalyst improved the efficiency of the CO2 reduction reaction (CO2 RR) by lowering the overpotential, while granting partial suppression of the hydrogen evolution reaction. This allowed tuning the selectivity towards formate, reaching for this catalyst an unprecedented faradaic efficiency (FEHCOO -) ≥90 % and energy efficiency of 66 % in acetonitrile solution. For the first time, relevant CO2 conversion to formic acid/formate was reached at low overpotential (0.28 V) using a homogeneous catalyst in acidic aqueous solution (pH=3.8). These results open up a new strategy based on electrolyte engineering for enhancing carbon balance in CO2 RR.

Overcoming barriers for nitrate electrochemical reduction: By-passing water hardness.

Water matrix composition impacts water treatment performance. However, matrix composition impacts have rarely been studied for electrochemical water treatment processes, and the correlation between the composition and the treatment efficiency is lacking. This work evaluated the electrochemical reduction of nitrate (ERN) using different complex water matrices: groundwater, brackish water, and reverse osmosis (RO) concentrate/brine. The ERN was conducted using a tin (Sn) cathode because of the high selectivity towards nitrogen evolution reported for Sn electrocatalysts. The co-existence of calcium (Ca2+), magnesium (Mg2+), and carbonate (CO32-) ions in water caused a 4-fold decrease in the nitrate conversion into innocuous nitrogen gas due to inorganic scaling formation on the cathode surface. XRF and XRD analysis of fouled catalyst surfaces detected brucite (Mg(OH)2), calcite (CaCO3), and dolomite (CaMg(CO3)2) mineral scales formed on the cathode surface. Surface scaling created a physical barrier on the electrode that decreased the ERN efficiency. Identifying these main sources of ERN inhibition was key to devising potential fouling mitigation strategies. For this reason, the chemical softening pre-treatment of a real brackish water was conducted and this significantly increased nitrate conversion and faradaic efficiency during subsequent ERN treatment, leading to a lower electric energy consumption per order. Understanding the ionic foulant composition responsible for influencing electrochemically-driven technologies are the first steps that must be taken to move towards niche applications such as decentralized ERN. Thus, we propose either direct ERN implementation in regions facing high nitrate levels in soft waters, or a hybrid softening/nitrate removal system for those regions where high nitrate and high-water hardness appear simultaneously.

Imaging structure sensitive catalysis on different shape-controlled platinum nanoparticles.

The structure sensitive catalytic activity for oxygen reduction reaction (ORR) on shape-controlled Pt nanoparticles (NPs) is directly imaged using scanning electrochemical microscopy (SECM). We synthesize and compare four types of Pt NPs: spherical, cubic, hexagonal, and tetrahedral-octahedral. Our SECM images show the hexagonal Pt NPs displaying the highest activity for ORR in two acid electrolytes. Meanwhile, cubic and tetrahedral-octahedral NPs drastically change their activity depending on specific adsorption of the different anions in solution. The NPs morphology produces predominant crystallographic planes at the surface of these shape-controlled Pt NPs, which are responsible for their different catalytic activity. Our results translate the studies on Pt single crystal electrodes present in the literature into Pt NPs that are useful as a catalyst in real fuel cells.

Scanning electrochemical microscopy screening of CO2 electroreduction activities and product selectivities of catalyst arrays.

The electroreduction of CO2 is one of the most investigated reactions and involves testing a large number and variety of catalysts. The majority of experimental electrocatalysis studies use conventional one-sample-at-a-time methods without providing spatially resolved catalytic activity information. Herein, we present the application of scanning electrochemical microscopy (SECM) for simultaneous screening of different catalysts forming an array. We demonstrate the potential of this method for electrocatalytic assessment of an array consisting of three Sn/SnOx catalysts for CO2 reduction to formate (CO2RF). Simultaneous SECM scans with fast scan (1 V s-1) cyclic voltammetry detection of products (HCOO-, CO and H2) at the Pt ultramicroelectrode tip were performed. We were able to consistently distinguish the electrocatalytic activities of the three compositionally and morphologically different Sn/SnOx catalysts. Further development of this technique for larger catalyst arrays and matrices coupled with machine learning based algorithms could greatly accelerate the CO2 electroreduction catalyst discovery.

Hydrogen peroxide production in the oxygen reduction reaction at different electrocatalysts as quantified by scanning electrochemical microscopy.

We present data for H2O2 production at eight different materials tested as electrocatalysts for the oxygen reduction reaction (ORR) in 0.5 M H2SO4 (Hg, Au, Ag, Cu, Pt, Pd, Pd80Co20, and Au60Cu40) using scanning electrochemical microscopy (SECM) as an alternative to the widely used rotating ring-disk electrode (RRDE) method. The amount of H2O2 is related to the total number of electrons, n, found in the O2 reduction reaction, with n = 2 showing only H2O2 production and n = 4 showing no H2O2 formation. From the SECM study Hg shows n close to 2, whereas Pt and Pd80Co20 show n-values near 4. The other materials show intermediate n-values as a function of potential.

Electrocatalytic activity of Pd-Co bimetallic mixtures for formic acid oxidation studied by scanning electrochemical microscopy.

The electrochemical oxidation of formic acid was studied by the tip generation-substrate collection (TG-SC) mode of scanning electrochemical microscopy (SECM), extending the number of applications of SECM in electrocatalysis. Formic acid was generated at a Hg on Au ultramicroelectrode (UME) tip by reduction of CO(2) in a 0.1 M KHCO(3) solution saturated with this gas. The electrocatalytic activity of different Pd-Co bimetallic compositions was evaluated using a Pd-Co electrocatalyst array formed by spots deposited onto glassy carbon (GC) as a SECM substrate. The SECM tip, which generated a constant formic acid flux, was scanned above the array and the oxidation current generated when formic acid was collected by active electrocatalytic spots was displayed as a function of tip position. This generated a SECM image that showed the electrocatalytic activity of each spot. SECM screening identified Pd(50)Co(50) (Pd/Co = 50:50, atomic ratio) as a better electrocatalyst toward the formic acid oxidation than pure Pd or Pt in 0.1 M KHCO(3) solution and this result was confirmed by cyclic voltammetry. Positive feedback was observed for the most active compositions of Pd-Co which suggests fast reaction kinetics and chemical reversibility during the oxidation of formic acid to CO(2). Moreover this feedback increases the contrast between active and non-active spots in this imaging mode.

Scanning electrochemical microscopy. 60. Quantitative calibration of the SECM substrate generation/tip collection mode and its use for the study of the oxygen reduction mechanism.

The substrate generation/tip collection (SG/TC) mode of scanning electrochemical microscopy (SECM) coupled with linear voltammetry is proposed as a way to quantify reaction intermediates generated in the solution at small substrates (100 mum diameter). The collection efficiency (CE) for SG/TC mode depends on the collector tip radius (a), the tip/substrate distance (d), and the size of the insulating glass sheath surrounding the collector tip (RG). In this work, we present experimental and simulated calibration CE values for different SG/TC geometries. Results of digital simulations in axial 2-D symmetry with the tip approaching a planar substrate are shown and fit experimental results obtained using ferrocenemethanol as a redox mediator very well. This model assumes that the mediator reacts under stationary-state conditions and undergoes diffusion-controlled electron transfer without any heterogeneous or homogeneous kinetic complications. Empirical equations for all SG/TC geometries reported here are provided as a convenient way to predict the maximum CE value for any given distance within the calibration range. Hydrogen peroxide quantification during the oxygen reduction reaction (ORR) at a Hg on Au electrode in acid pH was carried out using the SG/TC mode of SECM to demonstrate the utility of this technique in determining the number of electrons transferred (n) in the ORR. The results (n = 2.12-2.19) clearly point out the predominance of the two-electron pathway over the four-electron pathway when ORR takes place at this electrode material. Therefore, this work presents a powerful alternative to the rotating ring-disk electrode (RRDE) as means of obtaining mechanistic information by calculating the number of electrons transferred during an electrochemical reaction.

Paired electrosynthesis of cyanoacetic acid.

Cyanoacetic acid is formed by cathodic reduction of CO(2) and anodic oxidation of the tetraalkylammonium salt anion; the process is conduced in acetonitrile using a divided cell with a medium porosity glass-frit diaphragm. A mechanism for this paired electrochemical reaction is proposed.