Voltage-Driven Chemical Reactions Enable the Synthesis of Tunable Liquid Ga-Metal Nanoparticles.

Nanosized particles of liquid metals are emerging materials that hold promise for applications spanning from microelectronics to catalysis. Yet, knowledge of their chemical reactivity is largely unknown. Here, we study the reactivity of liquid Ga and Cu nanoparticles under the application of a cathodic voltage. We discover that the applied voltage and the spatial proximity of these two particle precursors dictate the reaction outcome. In particular, we find that a gradual voltage ramp is crucial to reduce the native oxide skin of gallium and enable reactive wetting between the Ga and Cu nanoparticles; instead, a voltage step causes dewetting between the two. We determine that the use of liquid Ga/Cu nanodimer precursors, which consist of an oxide-covered Ga domain interfaced with a metallic Cu domain, provides a more uniform mixing and results in more homogeneous reaction products compared to a physical mixture of Ga and Cu NPs. Having learned this, we obtain CuGa2 alloys or solid@liquid CuGa2@Ga core@shell nanoparticles by tuning the stoichiometry of Ga and Cu in the nanodimer precursors. These products reveal an interesting complementarity of thermal and voltage-driven syntheses to expand the compositional range of bimetallic NPs. Finally, we extend the voltage-driven synthesis to the combination of Ga with other elements (Ag, Sn, Co, and W). By rationalizing the impact of the native skin reduction rate, the wetting properties, and the chemical reactivity between Ga and other metals on the results of such voltage-driven chemical manipulation, we define the criteria to predict the outcome of this reaction and set the ground for future studies targeting various applications for multielement nanomaterials based on liquid Ga.

Alloying as a Strategy to Boost the Stability of Copper Nanocatalysts during the Electrochemical CO2 Reduction Reaction.

Copper nanocatalysts are among the most promising candidates to drive the electrochemical CO2 reduction reaction (CO2RR). However, the stability of such catalysts during operation is sub-optimal, and improving this aspect of catalyst behavior remains a challenge. Here, we synthesize well-defined and tunable CuGa nanoparticles (NPs) and demonstrate that alloying Cu with Ga considerably improves the stability of the nanocatalysts. In particular, we discover that CuGa NPs containing 17 at. % Ga preserve most of their CO2RR activity for at least 20 h while Cu NPs of the same size reconstruct and lose their CO2RR activity within 2 h. Various characterization techniques, including X-ray photoelectron spectroscopy and operando X-ray absorption spectroscopy, suggest that the addition of Ga suppresses Cu oxidation at open-circuit potential (ocp) and induces significant electronic interactions between Ga and Cu. Thus, we explain the observed stabilization of the Cu by Ga as a result of the higher oxophilicity and lower electronegativity of Ga, which reduce the propensity of Cu to oxidize at ocp and enhance the bond strength in the alloyed nanocatalysts. In addition to addressing one of the major challenges in CO2RR, this study proposes a strategy to generate NPs that are stable under a reducing reaction environment.

Tailoring Morphology and Elemental Distribution of Cu-In Nanocrystals via Galvanic Replacement.

The compositional and structural diversity of bimetallic nanocrystals (NCs) provides a superior tunability of their physico-chemical properties, making them attractive for a variety of applications, including sensing and catalysis. Nevertheless, the manipulation of the properties-determining features of bimetallic NCs still remains a challenge, especially when moving away from noble metals. In this work, we explore the galvanic replacement reaction (GRR) of In NCs and a copper molecular precursor to obtain Cu-In bimetallic NCs with an unprecedented variety of morphologies and distribution of the two metals. We obtain spherical Cu11In9 intermetallic and patchy phase-segregated Cu-In NCs, as well as dimer-like Cu-Cu11In9 and Cu-In NCs. In particular, we find that segregation of the two metals occurs as the GRR progresses with time or with a higher copper precursor concentration. We discover size-dependent reaction kinetics, with the smaller In NCs undergoing a slower transition across the different Cu-In configurations. We compare the obtained results with the bulk Cu-In phase diagram and, interestingly, find that the bigger In NCs stabilize the bulk-like Cu-Cu11In9 configuration before their complete segregation into Cu-In NCs. Finally, we also prove the utility of the new family of Cu-In NCs as model catalysts to elucidate the impact of the metal elemental distribution on the selectivity of these bimetallics toward the electrochemical CO2 reduction reaction. Generally, we demonstrate that the GRR is a powerful synthetic approach beyond noble metal-containing bimetallic structures, yet that the current knowledge on this reaction is challenged when oxophilic and poorly miscible metal pairs are used.

The Native Oxide Skin of Liquid Metal Ga Nanoparticles Prevents Their Rapid Coalescence during Electrocatalysis.

Liquid metals (LMs) have been used in electrochemistry since the 19th century, but it is only recently that they have emerged as electrocatalysts with unique properties, such as inherent resistance to coke poisoning, which derives from the dynamic nature of their surface. The use of LM nanoparticles (NPs) as electrocatalysts is highly desirable to enhance any surface-related phenomena. However, LM NPs are expected to rapidly coalesce, similarly to liquid drops, which makes their implementation in electrocatalysis hard to envision. Herein, we demonstrate that liquid Ga NPs (18 nm, 26 nm, 39 nm) drive the electrochemical CO2 reduction reaction (CO2RR) while remaining well-separated from each other. CO is generated with a maximum faradaic efficiency of around 30% at -0.7 VRHE, which is similar to that of bulk Ga. The combination of electrochemical, microscopic, and spectroscopic techniques, including operando X-ray absorption, indicates that the native oxide skin of the Ga NPs is still present during CO2RR and provides a barrier to coalescence during operation. This discovery provides an avenue for future development of Ga-based LM NPs as a new class of electrocatalysts.

Copper Nanocrystal Morphology Determines the Viability of Molecular Surface Functionalization in Tuning Electrocatalytic Behavior in CO2 Reduction.

Molecular surface functionalization of metallic catalysts is emerging as an ever-developing approach to tuning their catalytic performance. Here, we report the synthesis of hybrid catalysts comprising copper nanocrystals (CuNCs) and an imidazolium ligand for the electrochemical CO2 reduction reaction (CO2RR). We show that this organic modifier steers the selectivity of cubic CuNCs toward liquid products. A comparison between cubic and spherical CuNCs reveals the impact of surface reconstruction on the viability of surface functionalization schemes. Indeed, the intrinsic instability of spherical CuNCs leads to ejection of the functionalized surface atoms. Finally, we also demonstrate that the more stable hybrid nanocrystal catalysts, which include cubic CuNCs, can be transferred into gas-flow CO2RR cells for testing under more industrially relevant conditions.

Composition effects of electrodeposited Cu-Ag nanostructured electrocatalysts for CO2 reduction.

The electrochemical carbon dioxide reduction (CO2RR) on Cu-based catalysts is a promising strategy to store renewable electricity and produce valuable C2+ chemicals. We investigate the CO2RR on Cu-Ag nanostructures that have been electrodeposited in a green choline chloride and urea deep eutectic solvent (DES). We determine the electrochemically active surface area (ECSA) using lead underpotential deposition (UPD) to investigate the CO2RR intrinsic activity and selectivity. We show that the addition of Ag on electrodeposited Cu primarily suppresses the production of hydrogen and methane. While the production of carbon monoxide slightly increases, the partial current of the total C2+ products does not considerably increase. Despite that the production rate of C2+ is similar on Cu and Cu-Ag, the addition of Ag enhances the formation of alcohols and oxygenates over ethylene. We highlight the potential of metal electrodeposition from DES as a sustainable strategy to develop bimetallic Cu-based nanocatalysts for CO2RR.

The spatial distribution of cobalt phthalocyanine and copper nanocubes controls the selectivity towards C2 products in tandem electrocatalytic CO2 reduction.

The coupling of CO-generating molecular catalysts with copper electrodes in tandem schemes is a promising strategy to boost the formation of multi-carbon products in the electrocatalytic reduction of CO2. While the spatial distribution of the two components is important, this aspect remains underexplored for molecular-based tandem systems. Herein, we address this knowledge gap by studying tandem catalysts comprising Co-phthalocyanine (CoPc) and Cu nanocubes (Cucub). In particular, we identify the importance of the relative spatial distribution of the two components on the performance of the tandem catalyst by preparing CoPc-Cucub/C, wherein the CoPc and Cucub share an interface, and CoPc-C/Cucub, wherein the CoPc is loaded first on carbon black (C) before mixing with the Cucub. The electrocatalytic measurements of these two catalysts show that the faradaic efficiency towards C2 products almost doubles for the CoPc-Cucub/C, whereas it decreases by half for the CoPc-C/Cucub, compared to the Cucub/C. Our results highlight the importance of a direct contact between the CO-generating molecular catalyst and the Cu to promote C-C coupling, which hints at a surface transport mechanism of the CO intermediate between the two components of the tandem catalyst instead of a transfer via CO diffusion in the electrolyte followed by re-adsorption.

Investigation of Ethylene and Propylene Production from CO2 Reduction over Copper Nanocubes in an MEA-Type Electrolyzer.

Previous work carried out in fully liquid environments and at low current densities has demonstrated that highly uniform faceted copper nanocrystals display different selectivity profiles in CO2 reduction compared to polycrystalline copper. As part of ongoing upscaling efforts, it is a matter of interest to investigate whether the high selectivity toward ethylene of copper nanocubes, which show a preferential (100) orientation, is maintained in gas-fed electrolyzers, thus enabling the energy-efficient production of this valuable commodity chemical at industrially relevant current densities. In this work, we assessed the electrochemical CO2 reduction reaction performance of highly uniform copper nanocubes loaded onto gas diffusion electrodes (GDEs) in a zero-gap device. The copper nanocube-loaded GDEs maintained high Faradaic efficiencies toward ethylene at elevated total current densities, resulting in higher overall partial current densities toward this product compared to benchmark electrodes. Interestingly, CO2 reduction to propylene, albeit with low partial current densities, was also observed. However, double-layer capacitance measurements revealed that the performance observed at high current densities is significantly influenced by electrode flooding. The findings of this study can inform future efforts geared toward optimizing the electrodes with this promising class of catalysts.