Jack H. Baricuatro

The Evolution of the Polycrystalline Copper Surface, First to Cu(111) and Then to Cu(100), at a Fixed CO2RR Potential: A Study by Operando EC-STM

A study based on operando electrochemical scanning tunneling microscopy (EC-STM) has shown that a polycrystalline Cu electrode held at a fixed negative potential, -0.9 V (vs SHE), in the vicinity of CO2 reduction reactions (CO2RR) in 0.1 M KOH, undergoes stepwise surface reconstruction, first to Cu(111) within 30 min, and then to Cu(100) after another 30 min; no further surface transformations occurred after establishment of the Cu(100) surface. The results may help explain the Cu(100)-like behavior of Cu(pc) in terms of CO2RR product selectivity. They likewise suggest that products exclusive to Cu(100) single-crystal electrodes may be generated through the use of readily available inexpensive polycrystalline Cu electrodes. The study highlights the dynamic nature of heterogeneous electrocatalyst surfaces and also underscores the importance of operando interrogations when structure-composition-reactivity correlations are intended.

Engineering Cu surfaces for the electrocatalytic conversion of CO2: Controlling selectivity toward oxygenates and hydrocarbons

Significance Anthropogenic global warming necessitates the development of renewable carbon-free and carbon-neutral technologies for the future. Electrochemical CO2 reduction is one such technology that has the potential to impact climate change by enabling sustainable routes for the production of fuels and chemicals. Whereas the field of CO2 reduction has attracted great interest, current state-of-the-art electrocatalysts must be improved in product selectivity and energy efficiency to make this pathway viable for the future. Here, we investigate how controlling the surface structure of copper electrocatalysts can guide CO2 reduction activity and selectivity. We show how the coordination environment of Cu surfaces influences oxygenate vs. hydrocarbon formation, providing insights on how to improve selectivity and energy efficiency toward more valuable CO2 reduction products. In this study we control the surface structure of Cu thin-film catalysts to probe the relationship between active sites and catalytic activity for the electroreduction of CO2 to fuels and chemicals. Here, we report physical vapor deposition of Cu thin films on large-format (∼6 cm2) single-crystal substrates, and confirm epitaxial growth in the <100>, <111>, and <751> orientations using X-ray pole figures. To understand the relationship between the bulk and surface structures, in situ electrochemical scanning tunneling microscopy was conducted on Cu(100), (111), and (751) thin films. The studies revealed that Cu(100) and (111) have surface adlattices that are identical to the bulk structure, and that Cu(751) has a heterogeneous kinked surface with (110) terraces that is closely related to the bulk structure. Electrochemical CO2 reduction testing showed that whereas both Cu(100) and (751) thin films are more active and selective for C–C coupling than Cu(111), Cu(751) is the most selective for >2e− oxygenate formation at low overpotentials. Our results demonstrate that epitaxy can be used to grow single-crystal analogous materials as large-format electrodes that provide insights on controlling electrocatalytic activity and selectivity for this reaction.

Operando Spectroscopic Analysis of CoP Films Electrocatalyzing the Hydrogen-Evolution Reaction

Transition metal phosphides exhibit high catalytic activity toward the electrochemical hydrogen-evolution reaction (HER) and resist chemical corrosion in acidic solutions. For example, an electrodeposited CoP catalyst exhibited an overpotential, η, of -η < 100 mV at a current density of -10 mA cm-2 in 0.500 M H2SO4(aq). To obtain a chemical description of the material as-prepared and also while effecting the HER in acidic media, such electrocatalyst films were investigated using Raman spectroscopy and X-ray absorption spectroscopy both ex situ as well as under in situ and operando conditions in 0.500 M H2SO4(aq). Ex situ analysis using the tandem spectroscopies indicated the presence of multiple ordered and disordered phases that contained both near-zerovalent and oxidized Co species, in addition to reduced and oxygenated P species. Operando analysis indicated that the active electrocatalyst was primarily amorphous and predominantly consisted of near-zerovalent Co as well as reduced P.

A DEMS Study of the Reduction of CO 2 , CO, and HCHO Pre-Adsorbed on Cu Electrodes: Empirical Inferences on the CO 2 RR Mechanism

The effective abatement of atmospheric carbon through its conversion via electrochemical reduction to pure and oxygenated hydrocarbon fuels relies on the ability to control product selectivity at viable current densities and faradaic efficiencies. One critical aspect is the choice of the electrode and, in the CO_2-reduction electrocatalyst landscape, copper sits as the only metal known to deliver a remarkable variety of reduction products other than carbon monoxide and formic acid. However, much better catalyst performance is needed. The overall energy efficiency of copper is less than 40 %, and its nominal overvoltage at benchmark current densities remains unacceptably large at ca. 1 V. The diversity of the product distribution also becomes a major inconvenience in the likelihood that only one product is desired; unless, of course, if the selectivity window for such product is already known. Several experimental parameters influence the product selectivity of the CO_2 reduction reactions (hereafter referred to as CO_2RR); the more obvious include the composition and the crystal structure of the catalyst surface, the applied potential, the solution pH, and the supporting electrolyte. The documentation, at the atomic level, of the mechanistic origins of the CO_2RR selectivity of copper demands a systematic combination of ex situ, in situ, and operando techniques to interrogate the electrode surface, pristine and modified, prior to, during, and after the reduction reaction; the task includes not only the analysis of reaction-product distributions but also the identification of surface intermediates that serve as the precursor states for each reaction pathway. We recently studied the nature of well-defined Cu(hkl) single-crystal surfaces that, similar to “real-world” catalysts, were handled in air. Such investigation is pertinent since Cu is a well-known scavenger of molecular oxygen; hence, CO_2RR electrocatalysis must first contend with the initial presence of multilayers of disordered copper oxides. It was found that the oxides are actually easily reduced electrochemically back to the metal; in addition, even if the oxided single-crystal surface is severely disordered, cathodic reduction completely regenerates the original ordered structure. Most recently, we discovered that a polycrystalline Cu electrode held at a fixed negative potential in the CO_2RR region in KOH, undergoes stepwise surface reconstruction, first to Cu(111) and then to Cu(100). The results help explain the Cu(100)-like behavior of Cu(pc) in terms of CO_2RR product selectivity. In the work described in this Letter, we have applied differential electrochemical mass spectrometry (DEMS) of pre-adsorbed reactants and intermediates as a complementary experimental approach in the study of the mechanistic pathways for the Cu-catalyzed CO_2 reduction reactions; the reactant was CO_2 and the intermediates were CO and HCHO. The reduction products monitored by mass spectrometry were H_2, CO (from CO_2), CH_4, H_2C=CH_2 and CH_3CH_2OH.

Overlayer Au-on-W Near-Surface Alloy for the Selective Electrochemical Reduction of CO2 to Methanol: Empirical (DEMS) Corroboration of a Computational (DFT) Prediction

It is now widely known from extensive studies [1–3] over the past few decades on the heterogeneous electrochemical reduction of carbon dioxide in aqueous solutions that, across the vast landscape of CO_2-reduction electrocatalysts, copper stands alone as the single metal that can deliver a remarkable variety of products; unpredictably, however, the product distribution does not include methanol [1–5]. The overall energy conversion efficiency of Cu, defined [6] as the ratio of the free energy of the products generated and that consumed in the electrochemical reduction, is only 30 to 40 %, and the overpotential of Cu at benchmark current densities remains unacceptably large, ca. −1.4 V [1, 6]. The diversity of the product distribution also becomes a major hurdle if only one product is coveted. The desire for catalysts that can perform better than Cu, especially in the generation of methanol, a liquid transportation fuel, and feedstock for direct fuel cells, is thus understandable.

Immobilization-Enabled Proton Reduction Catalysis by a Di-iron Hydrogenase Mimic

We have long been interested in the influence of surface immobilization on the electrochemical integrity of redox-active moieties [1–5]. Our studies have shown that, if the electroactive group itself is directly chemisorbed on (coordinated to) the electrode surface, profound alterations result in both the thermodynamics and kinetics of the electron transfer processes; the oxidative chemisorption of the iodide anion (to zerovalent iodine atoms) or the hydroquinone molecule (to benzoquinone) are prototypical examples. The changes are more subtle and less dramatic if the electroactive site is only a pendant moiety tethered to the surface via an anchor group; mercapto hydroquinone bound exclusively via the –SH group is a well-known specimen. We recently extended our investigations to include enzyme-inspired molecular electrocatalysts in which the multinuclear reactive site may require a certain entatic state to carry out its catalytic function; the anticipation is that the motion-restricted surface-tethered species would suffer diminished catalytic activity. The results are described in this brief communication.

Heterogenization of a Water-Insoluble Molecular Complex for Catalysis of the Proton-Reduction Reaction in Highly Acidic Aqueous Solutions

Our long-held interest in the resiliency of electrochemical functionalities upon surface immobilization has herded us from directly chemisorbed electroactive moieties, to anchor group-leashed redox-active couples and to surface-tethered enzyme-inspired molecular catalysts. The latter represent the most intricate because the electrocatalytic activities involve mixed-valence states and may require certain entatic (fractionally rotated) configurations. In this regard, we recently investigated the proton-reduction electrocatalysis by hydrogenase-inspired di-iron complexes at polycrystalline and (111)-faceted Au electrodes.

Addendum to Immobilization-Enabled Proton-Reduction Catalysis by a Di-iron Hydrogenase Mimic

It may perhaps be conjectured that the mercapto-free di-iron hydrogenase complex, (μ-pdt)[Fe(CO)3][Fe(CO)2(PPh3)], is inactive towards proton reduction because it is decomposed upon exposure to the Au electrode and that the decomposition products poison the catalysis. If decomposition and catalyst inhibition actually transpired, it would only be because the interaction between the surface and the molecular fragment is much stronger than the bond between that fragment and the constituents within the intact complex. The presence of the chemisorbed residue can thus be established by simple cyclic voltammetry of the Au electrode in 1.0 M H2SO4 after immersion in pure acetonitrile and after exposure to the hydrogenase mimic dissolved in acetonitrile. The electrode is rinsed in sulfuric acid solution prior to the voltammetric experiments. The results are shown in Fig. 1. The fact that the two sets of current-potential curves are superimposable provides clear evidence that the mercapto-free complex is neither decomposed nor chemisorbed when exposed to the Au electrode surface. Fig. 1 Cyclic voltammogram in 1.0 M H2SO4 of a smooth polycrystalline Au electrode exposed to pure acetonitrile (spittled line) and to acetonitrile that contained 0.5 mM of (μ-pdt)[Fe(CO)3][Fe(CO)2(PPh3)] (solid line). Potential sweep rate, 10 mV s