In Situ Analysis of the Facets of Cu-Based Electrocatalysts in Alkaline Media Using Pb Underpotential Deposition.

Establishing relationships between the surface atomic structure and activity of Cu-based electrocatalysts for CO2 and CO reduction is hindered by probable surface restructuring under working conditions. Insights into these structural evolutions are scarce as techniques for monitoring the surface facets in conventional experimental designs are lacking. To directly correlate surface reconstructions to changes in selectivity or activity, the development of surface-sensitive, electrochemical probes is highly desirable. Here, we report the underpotential deposition of lead over three low index Cu single crystals in alkaline media, the preferred electrolyte for CO reduction studies. We find that underpotential deposition of Pb onto these facets occurs at distinct potentials, and we use these benchmarks to probe the predominant facet of polycrystalline Cu electrodes in situ. Finally, we demonstrate that Cu and Pb form an irreversible surface alloy during underpotential deposition, which limits this method to investigating the surface atomic structure after reaction.

Fingerprint Voltammograms of Copper Single Crystals under Alkaline Conditions: A Fundamental Mechanistic Analysis.

A critical step toward the systematic development of electrocatalysts is the determination of the microscopic structure and processes at the electrified solid/electrolyte interface. The major challenges toward this end for experiment and computations are achieving sufficient cleanliness and modeling the complexity of electrochemical systems, respectively. In this sense, benchmarks of well-defined model systems are sparse. This work presents a rigorous joint experimental-theoretical study on the single-crystal (SC) Cu/aqueous interface. Within typical computational uncertainties, we find quantitative agreement between simulated and experimentally measured voltammograms, which allows us to unequivocally identify the *OH adsorption feature in the fingerprint region of Cu(110), Cu(100), and Cu(111) SCs under alkaline conditions. We find the inclusion of hydrogen evolution reaction kinetics in the theoretical model to be crucial for an accurate steady-state description that gives rise to a negligible H* coverage. A purely thermodynamic description of the H* coverage through a Pourbaix analysis would incorrectly lead to a H* adsorption peak. The presented results establish a fundamental benchmark for all electrochemical applications of Cu.

On the Possibilities and Considerations of Interfacing Ultra-High Vacuum Equipment with an Electrochemical Setup.

Efficient electrocatalysts are required in order for electrocatalysis to play a large role in a future largely based on renewable energy sources. To rationally design these catalysts we need to understand the fundamental origin of their activities. In order to elucidate the relationship between catalyst structure and electrochemical behaviour, we investigate well-defined single-crystal catalysts in a UHV chamber interfaced with an electrochemical setup. Using the capabilities of UHV based methods, we can prepare more complex surface structures than it is possible with traditional EC methods and investigate their electrochemical behaviour. We exemplify this by showing results from both clean and intentionally structured Pt(111), Cu(111) and Pt/Cu(111).

Progress and Perspectives of Electrochemical CO2 Reduction on Copper in Aqueous Electrolyte.

To date, copper is the only heterogeneous catalyst that has shown a propensity to produce valuable hydrocarbons and alcohols, such as ethylene and ethanol, from electrochemical CO2 reduction (CO2R). There are variety of factors that impact CO2R activity and selectivity, including the catalyst surface structure, morphology, composition, the choice of electrolyte ions and pH, and the electrochemical cell design. Many of these factors are often intertwined, which can complicate catalyst discovery and design efforts. Here we take a broad and historical view of these different aspects and their complex interplay in CO2R catalysis on Cu, with the purpose of providing new insights, critical evaluations, and guidance to the field with regard to research directions and best practices. First, we describe the various experimental probes and complementary theoretical methods that have been used to discern the mechanisms by which products are formed, and next we present our current understanding of the complex reaction networks for CO2R on Cu. We then analyze two key methods that have been used in attempts to alter the activity and selectivity of Cu: nanostructuring and the formation of bimetallic electrodes. Finally, we offer some perspectives on the future outlook for electrochemical CO2R.

Structure Sensitivity in the Electrocatalytic Reduction of CO2 with Gold Catalysts.

An understanding of the influence of structural surface features on electrocatalytic reactions is vital for the development of efficient nanostructured catalysts. Gold is the most active and selective known electrocatalyst for the reduction of CO2 to CO in aqueous electrolytes. Numerous strategies have been proposed to improve its intrinsic activity. Nonetheless, the atomistic knowledge of the nature of the active sites remains elusive. We systematically investigated the structure sensitivity of Au single crystals for electrocatalytic CO2 reduction. Reaction kinetics for the formation of CO are strongly dependent on the surface structure. Under-coordinated sites, such as those present in Au(110) and at the steps of Au(211), show at least 20-fold higher activity than more coordinated configurations (for example, Au(100)). By selectively poisoning under-coordinated sites with Pb, we have confirmed that these are the active sites for CO2 reduction.

Polycrystalline and Single-Crystal Cu Electrodes: Influence of Experimental Conditions on the Electrochemical Properties in Alkaline Media.

Single and polycrystalline Cu electrodes serve as model systems for the study of the electroreduction of CO2 , CO and nitrate, or for corrosion studies; even so, there are very few reports combining electrochemical measurements with structural characterization. Herein both the electrochemical properties of polycrystalline Cu and single crystal Cu(1 0 0) electrodes in alkaline solutions (0.1 m KOH and 0.1 m NaOH) are investigated. It is demonstrated that the pre-treatment of the electrodes plays a crucial role in determining their electrochemical properties. Scanning tunneling microscopy, X-ray photoelectron spectroscopy and cyclic voltammetry are performed on Cu(1 0 0) electrodes prepared under UHV conditions; it is shown that the electrochemical properties of these atomically well-defined electrodes are distinct from electrodes prepared by other methods. Also highlighted is the significant role of residual oxygen and electrolyte convection in influencing the electrochemical properties.

Oxygen evolution on well-characterized mass-selected Ru and RuO2 nanoparticles.

Oxygen evolution was investigated on model, mass-selected RuO2 nanoparticles in acid, prepared by magnetron sputtering. Our investigations include electrochemical measurements, electron microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy. We show that the stability and activity of nanoparticulate RuO2 is highly sensitive to its surface pretreatment. At 0.25 V overpotential, the catalysts show a mass activity of up to 0.6 A mg-1 and a turnover frequency of 0.65 s-1, one order of magnitude higher than the current state-of-the-art.

Study of underpotential deposited Cu layers on Pt(111) and their stability against CO and CO2 in perchloric acid.

The underpotential deposition (UPD) of copper on a Pt(111) electrode and the influence of gas coadsorbates, i.e. CO and CO2, on the thus deposited copper layer were studied in a 0.1 M HClO4 electrolyte by means of EC-STM. By UPD, an atomically flat Cu layer is formed, which exhibits a pseudomorphic (1 × 1) structure. However, it contains several point defects due to which its total coverage is less than a monolayer, in agreement with the measured charge density in the CV curves. Upon exposure to a CO-saturated solution the pseudomorphic structure collapses to a coalescent structure with many vacancy islands. This phase transition is induced by the preferential binding of CO to the Pt(111) surface. In contrast, CO2, which binds stronger to copper, does not affect the pseudomorphic structure of the Cu layer.

Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts.

The identification of the active sites in heterogeneous catalysis requires a combination of surface sensitive methods and reactivity studies. We determined the active site for hydrogen evolution, a reaction catalyzed by precious metals, on nanoparticulate molybdenum disulfide (MoS2) by atomically resolving the surface of this catalyst before measuring electrochemical activity in solution. By preparing MoS2 nanoparticles of different sizes, we systematically varied the distribution of surface sites on MoS2 nanoparticles on Au(111), which we quantified with scanning tunneling microscopy. Electrocatalytic activity measurements for hydrogen evolution correlate linearly with the number of edge sites on the MoS2 catalyst.

Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution.

The electrochemical hydrogen evolution reaction is catalyzed most effectively by the Pt group metals. As H2 is considered as a future energy carrier, the need for these catalysts will increase and alternatives to the scarce and expensive Pt group catalysts will be needed. We analyze the ability of different metal surfaces and of the enzymes nitrogenase and hydrogenase to catalyze the hydrogen evolution reaction and find a necessary criterion for high catalytic activity. The necessary criterion is that the binding free energy of atomic hydrogen to the catalyst is close to zero. The criterion enables us to search for new catalysts, and inspired by the nitrogenase active site, we find that MoS2 nanoparticles supported on graphite are a promising catalyst. They catalyze electrochemical hydrogen evolution at a moderate overpotential of 0.1-0.2 V.