Strategies to Modulate the Copper Oxidation State Toward Selective C2+ Production in the Electrochemical CO2 Reduction Reaction.

The electrochemical reduction of CO2 to form value-added chemicals receives considerable attention in recent years. Copper (Cu) is recognized as the only element capable of electro-reducing CO2 into hydrocarbons with two or more carbon atoms (C2+), but the low product selectivity of the Cu-based catalyst remains a major technological challenge to overcome. Therefore, identification of the structural features of Cu-based catalysts is of great importance for the highly selective production of C2+ products (ethylene, ethanol, n-propanol, etc.), and the oxidation state of Cu species in the catalysts is found critical to the catalyst performance. This review introduces recent efforts to fine-tune the oxidation state of Cu to increase carbon capture and produce specific C2+ compounds, with the intention of greatly expediting the advance in the catalyst designs. It also points to the remaining challenges and fruitful research directions for the development of Cu-based catalysts that can shape the practical CO2 reduction technology.

Intermetallic Nanoarchitectures for Efficient Electrocatalysis.

Intermetallic structures whose regular atomic arrays of constituent elements present unique catalytic properties have attracted considerable attention as efficient electrocatalysts for energy conversion reactions. Further performance enhancement in intermetallic catalysts hinges on constructing catalytic surfaces possessing high activity, durability, and selectivity. In this Perspective, we introduce recent endeavors to boost the performance of intermetallic catalysts by generating nanoarchitectures, which have well-defined size, shape, and dimension. We discuss the beneficial effects of nanoarchitectures compared with simple nanoparticles in catalysis. We highlight that the nanoarchitectures have high intrinsic activity owing to their inherent structural factors, including controlled facets, surface defects, strained surfaces, nanoscale confinement effects, and a high density of active sites. We next present notable examples of intermetallic nanoarchitectures, namely, facet-controlled intermetallic nanocrystals and multidimensional nanomaterials. Finally, we suggest the future research directions of intermetallic nanoarchitectures.

Polymer-Covered Copper Catalysts Alter the Reaction Pathway of the Electrochemical CO2 Reduction Reaction.

The electrochemical CO2 reduction reaction (CO2RR) has attracted considerable attention recently due to the potential conversion of atmospheric CO2 into useful organic products by utilizing electricity from renewable energy sources. However, the selective formation of desired products only via CO2RR has been elusive due to the presence of a myriad of competing reaction pathways, thus calling for effective strategies controlling the reaction coordinates. The control of binding energies of the reaction intermediate, such as *CO, is pivotal to manipulating reaction pathways, and various attempts have been made to accomplish this goal. Herein, we introduce recent endeavors to increase the catalytic selectivity of Cu-based catalysts by surface modification with polymer coating, which can change the local pH, hydrophilicity/hydrophobicity, reaction concentration, etc. The polymer conjugation also contributed to the enhanced electrocatalytic stability of Cu-based catalysts during the CO2RR. We also point to the remaining challenges and provide perspectives on the further development of Cu-polymer hybrid catalysts for the practical CO2RR.

Chemical Fields: Directing Atom Migration in the Multiphasic Nanocrystal.

ConspectusAtoms in a bulk solid phase are usually trapped to fixed positions and can change their position only under certain conditions (e.g., at a melting point) due to the high energy barrier of migration between positions within the crystal lattice. Contrary to the atoms in the bulk solid phase, however, atoms in nanoparticles can migrate and change their local positions rather easily, enabled by the high surface energies. The energy states of surface atoms of nanoparticles can be altered by surface-binding moieties, which in turn influence the intrananoparticle migration of atoms at the subsurface of nanoparticles. In 2008, this possibility of intrananoparticle migration was demonstrated with RhPd alloy nanoparticles under the different gas environments of reductive CO or oxidative NO. We envisaged that the explosive expansion of well-defined, multiphasic nanoparticle libraries might be realized by specifically dictating the atom migration direction, by modulating the energy state of specific atoms in the multiphasic nanocrystals. The nanoparticle surface energy is a function of a myriad of factors, namely, surface binding moiety, structural features affecting coordination number of atoms such as nanoparticle geometry, steps, and kinks, and the existence of heterointerface with lattice mismatch. Therefore, all these factors affecting atom energy state in the nanoparticle, categorically termed as "chemical field" (CF), can serve as the driving force for purposeful directional movement of atoms within nanoparticles and subsequent reaction. Geometrically well-defined multiphasic nanocrystals present great promises toward various applications with special emphasis on catalysis and thus are worthy synthetic targets. In recent years, we have demonstrated that manipulation of CFs is an effective synthetic strategy for a variety of geometrically well-defined multiphasic nanocrystals. Herein, we classified multiphasic nanocrystals into metallic alloy systems and ionic systems (metal compounds) because the modes of CF are rather different between these two systems. The migration-directing CFs for neutral metallic atoms are mostly based on the local distribution of elements, degree of alloying, or highly energetic structural features. On the other hand, for the ionic system, structural parameters originating from the discrepancy between cations and anions should be more considered; ionic radii, phase stability, lattice strain, anionic frameworks, cation vacancies, etc. can react as CFs affecting atom migration behavior in the multiphasic ionic nanocrystals. We expect that the limits and potentials of CF-based synthesis of multiphasic nanocrystals described in this work will open a wide avenue to diverse material compositions and geometries, which have been difficult or impossible to approach via conventional nanoparticle synthesis schemes.

Microfluidics-Assisted Synthesis of Hierarchical Cu2 O Nanocrystal as C2 -Selective CO2 Reduction Electrocatalyst.

Copper-based catalysts have attracted enormous attention due to their high selectivity for C2+ products during the electrochemical reduction of CO2 (CO2 RR). In particular, grain boundaries on the catalysts contribute to the generation of various Cu coordination environments, which have been found essential for C-C coupling. However, smooth-surfaced Cu2 O nanocrystals generally lack the ability for the surface reorganization to form multiple grain boundaries and desired Cu undercoordination sites. Flow chemistry armed with the unparalleled ability to mix reaction mixture can achieve a very high concentration of unstable reaction intermediates, which in turn are used up rapidly to lead to kinetics-driven nanocrystal growth. Herein, the synthesis of a unique hierarchical structure of Cu2 O with numerous steps (h-Cu2 O ONS) via flow chemistry-assisted modulation of nanocrystal growth kinetics is reported. The surface of h-Cu2 O ONS underwent rapid surface reconstruction under CO2 RR conditions to exhibit multiple heterointerfaces between Cu2 O and Cu phases, setting the preferable condition to facilitate C-C bond formation. Notably, the h-Cu2 O ONS obtained the increased C2 H4 Faradaic efficiency from 31.9% to 43.5% during electrocatalysis concurrent with the morphological reorganization, showing the role of the stepped surface. Also, the h-Cu2 O ONS demonstrated a 3.8-fold higher ethylene production rate as compared to the Cu2 O nanocube.

Mn-Dopant Differentiating the Ru and Ir Oxidation States in Catalytic Oxides Toward Durable Oxygen Evolution Reaction in Acidic Electrolyte.

Designing an efficient and durable electrocatalyst for the sluggish oxygen evolution reaction (OER) at the anode remains the foremost challenge in developing proton exchange membrane (PEM) electrolyzers. Here, a highly active and durable cactus-like nanoparticle with an exposed heterointerface between the IrO2 and the low oxidation state Ru by introducing a trace amount of Mn dopant is reported. The heterostructure fabrication relies on initial mixing of the Ru and Ir phases before electrochemical oxidation to produce a conjoined Ru/IrO2 heterointerface. Benefitting from electron transfer at the heterointerface, the low oxidation state Ru species shows excellent initial activity, which is maintained even after 180 h of continuous OER test. In a half-cell test, the Mn-doped RuIr nanocactus (Mn-RuIr NCT) achieves a mass activity of 1.85 A mgIr+Ru -1 at 1.48 VRHE , which is 139-fold higher than that of commercial IrO2 . Moreover, the superior electrocatalytic performance of Mn-RuIr NCT in the PEM electrolysis system ensures its viability in practical uses. The results of the excellent catalytic performance for acidic OER indicate that the heterostructuring robust rutile IrO2 and the highly active Ru species with a low oxidation state on the catalyst surface drive a synergistic effect.

Pd3 Pb Nanosponges for Selective Conversion of Furfural to Furfuryl Alcohol under Mild Condition.

Alloy structures with high catalytic surface areas and tunable surface energies can lead to high catalytic selectivity and activities. Herein, the synthesis of sponge-like Pd3 Pb multiframes (Pd3 Pb MFs) is reported by using the thermodynamically driven phase segregation, which exhibit high selectivity (93%) for the conversion of furfural to furfuryl alcohol (FOL) under mild conditions. The excellent catalytic performance of the Pd3 Pb MF catalysts is attributed to the high surface area and optimized surface energy of the catalyst, which is associated with the introduction of Pb to Pd. Density functional theory calculations show that the binding energy of FOL to the surface energy-tuned Pd3 Pb MF is sufficiently lowered to prevent side reactions such as over-hydrogenation of FOL.

Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts.

Nanoframe alloy structures represent a class of high-performance catalysts for the oxygen reduction reaction (ORR), owing to their high active surface area, efficient molecular accessibility, and nanoconfinement effect. However, structural and chemical instabilities of nanoframes remain an important challenge. Here, we report the synthesis of PtCu nanoframes constructed with an atomically ordered intermetallic structure (O-PtCuNF/C) showing high ORR activity, durability, and chemical stability. We rationally designed the O-PtCuNF/C catalyst by combining theoretical composition predictions with a silica-coating-mediated synthesis. The O-PtCuNF/C combines intensified strain and ligand effects from the intermetallic PtCu L11 structure and advantages of the nanoframes, resulting in superior ORR activity to disordered alloy PtCu nanoframes (D-PtCuNF/C) and commercial Pt/C catalysts. Importantly, the O-PtCuNF/C showed the highest ORR mass activity among PtCu-based catalysts. Furthermore, the O-PtCuNF/C exhibited higher ORR durability and far less etching of constituent atoms than D-PtCuNF/C and Pt/C, attesting to the chemically stable nature of the intermetallic structure.

Catalytic Nanoframes and Beyond.

The ever-increasing need for the production and expenditure of sustainable energy is a result of the astonishing rate of consumption of fossil fuels and the accompanying environmental problems. Emphasis is being directed to the generation of sustainable energy by the fuel cell and water splitting technologies. Accordingly, the development of highly efficient electrocatalysts has attracted significant interest, as the fuel cell and water splitting technologies are critically dependent on their performance. Among numerous catalyst designs under investigation, nanoframe catalysts have an intrinsically large surface area per volume and a tunable composition, which impacts the number of catalytically active sites and their intrinsic catalytic activity, respectively. Nevertheless, the structural integrity of the nanoframe during electrochemical operation is an ongoing concern. Some significant advances in the field of nanoframe catalysts have been recently accomplished, specifically geared to resolving the catalytic stability concerns and significantly boosting the intrinsic catalytic activity of the active sites. Herein, general synthetic concepts of nanoframe structures and their structure-dependent catalytic performance are summarized, along with recent notable advances in this field. A discussion on the remaining challenges and future directions, addressing the limitations of nanoframe catalysts, are also provided.

Carbon Transition-metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors.

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.

Pt2+-Exchanged ZIF-8 nanocube as a solid-state precursor for L10-PtZn intermetallic nanoparticles embedded in a hollow carbon nanocage.

Nanoparticles with an atomically ordered alloy phase have received enormous attention for application as catalysts in fuel cells because of their unique electronic properties resulting from unusually strong d-orbital interactions between two metal components. However, the synthesis of intermetallic nanoparticles requires a high reaction temperature, thus necessitating the protection of nanoparticles with inorganic layers to prevent aggregation of nanoparticles during synthesis. The protective layer needs to be removed later for application as a catalyst, which is a cumbersome process. Herein, a novel synthetic strategy is reported for the preparation of L10-PtZn intermetallic nanoparticles by utilizing Pt2+-exchanged ZIF-8 nanocubes as a solid-state precursor. The Pt2+-exchanged ZIF-8 phase plays a dual role as a metal ion source for L10-PtZn nanoparticles and as a carbonaceous matrix that restrains the aggregation of nanoparticles during thermal treatment. The L10-PtZn nanoparticles embedded in a hollow carbon nanocage obtained from one-step annealing of Pt2+-exchanged ZIF-8 showed better electrocatalytic activity and durability toward methanol oxidation under acidic electrolyte conditions than those obtained from commercial Pt/C catalysts.

Reactive nanotemplates for synthesis of highly efficient electrocatalysts: beyond simple morphology transfer.

Efficient electrocatalysts for energy conversion in general, and fuel cell operation and water electrolysis in particular, are pivotal for carbon-free hydrogen production. While the requirements of successful electrocatalysts include a high number density of catalytically active sites, high surface-to-volume ratio, inherently high catalytic activity, and robustness of the catalyst surface structure under harsh operating conditions, it is extremely difficult to synthesize nanocatalysts that could possess all these structural characteristics. Nanotemplate-mediated synthesis, namely, the coating or filling of a template with a desired material phase followed by the removal of the template, has captured the interest of researchers because of the ease of creating hollow-structured nanocatalysts with a high surface to volume ratio. Recent studies, however, have revealed that nanotemplates could be more than just passive supports because they greatly affect catalytic performance by creating an unusual synergy between the substrate and catalyst and by providing dopants to the actual catalyst phase owing to their reactive nature. In this review, we discuss the most notable recent advances in the nanotemplate-based synthesis of electrocatalysts as well as the unusual effects of nanotemplates on the performance of nanocatalysts. We also provide an outlook for this fledgling field so that future research efforts could be focused on the development of practically useful electrocatalysts that could shape the future of energy technologies.