Overcoming immiscibility toward bimetallic catalyst library

Bimetallics are emerging as important materials that often exhibit distinct chemical properties from monometallics. However, there is limited access to homogeneously alloyed bimetallics because of the thermodynamic immiscibility of the constituent elements. Overcoming the inherent immiscibility in bimetallic systems would create a bimetallic library with unique properties. Here, we present a nonequilibrium synthesis strategy to address the immiscibility challenge in bimetallics. As a proof of concept, we synthesize a broad range of homogeneously alloyed Cu-based bimetallic nanoparticles regardless of the thermodynamic immiscibility. The nonequilibrated bimetallic nanoparticles are further investigated as electrocatalysts for carbon monoxide reduction at commercially relevant current densities (>100 mA cm^-2), in which Cu_0.9Ni_0.1 shows the highest multicarbon product Faradaic efficiency of ~76% with a current density of ~93 mA cm^-2. The ability to overcome thermodynamic immiscibility in multimetallic synthesis offers freedom to design and synthesize new functional nanomaterials with desired chemical compositions and catalytic properties.

SO2-Induced Selectivity Change in CO2 Electroreduction

Electrochemical conversion of carbon dioxide (CO₂) to value-added chemicals has attracted much attention in recent years as a potential alternative to fossil resources. Although significant works have studied the influence of impurities in the electrolyte (e.g., metal ions), few studies have been performed to understand the influence of gaseous impurities in CO₂ electroreduction. Herein, we study the effects of sulfur dioxide (SO₂) on Ag-, Sn-, and Cu-catalyzed CO₂ electrolysis in a flow-cell electrolyzer in near-neutral electrolyte, representing a broad range of CO₂ reduction catalysts. We show that the presence of SO₂ impurity reduces the efficiency of converting CO₂ due to the preferential reduction of SO₂. In the cases of Ag and Sn, the effect of SO₂ impurity was reversible and the catalytic activities of both catalysts were recovered. On the contrary, a shift in selectivity toward formate accompanied by a suppression of multicarbon (C₂₊) products was observed on Cu catalyst, demonstrating that Cu is highly sensitive to SO₂ impurity. Our results suggest that CO₂ obtained from direct air capture technologies or biorefineries could be more suitable for Cu-catalyzed CO₂ electrolysis as these CO₂ sources would be relatively cleaner (SO₂-free) than fossil-derived sources such as power plants and can be directly coupled with distributed renewable energy sources such as wind and solar.

A Highly Porous Copper Electrocatalyst for Carbon Dioxide Reduction

Electrochemical reduction of carbon dioxide (CO₂ ) is an appealing approach toward tackling climate change associated with atmospheric CO₂ emissions. This approach uses CO₂ as the carbon feedstock to produce value-added chemicals, resulting in a carbon-neutral (or even carbon-negative) process for chemical production. Many efforts have been devoted to the development of CO₂ electrolysis devices that can be operated at industrially relevant rates; however, limited progress has been made, especially for valuable C₂₊ products. Herein, a nanoporous copper CO₂ reduction catalyst is synthesized and integrated into a microfluidic CO₂ flow cell electrolyzer. The CO₂ electrolyzer exhibits a current density of 653 mA cm^-2 with a C₂₊ product selectivity of ≈62% at an applied potential of -0.67 V (vs reversible hydrogen electrode). The highly porous electrode structure facilitates rapid gas transport across the electrode-electrolyte interface at high current densities. Further investigations on electrolyte effects reveal that the surface pH value is substantially different from the pH of bulk electrolyte, especially for nonbuffering near-neutral electrolytes when operating at high currents.

Photoelectrochemical Carbon Dioxide Reduction Using a Nanoporous Ag Cathode

Solar fuel production from abundant sources using photoelectrochemical (PEC) systems is an attractive approach to address the challenges associated with the intermittence of solar energy. In comparison to electrochemical systems, PEC cells directly utilize solar energy as the energy input, and if necessary, then an additional external bias can be applied to drive the desired reaction. In this work, a PEC cell composing of a Ni-coated Si photoanode and a nanoporous Ag cathode was developed for CO2 conversion to CO. The thin Ni layer not only protected the Si wafer from photocorrosion but also served as the oxygen evolution catalyst. At an external bias of 2.0 V, the PEC cell delivered a current density of 10 mA cm(-2) with a CO Faradaic efficiency of ∼70%. More importantly, a stable performance up to 3 h was achieved under photoelectrolysis conditions, which is among the best literature-reported performances for PEC CO2 reduction cells. The photovoltage of the PEC cell was estimated to be ∼0.4 V, which corresponded to a 17% energy saving by solar energy utilization. Postreaction structural analysis showed the corrosion of the Ni layer at the Si photoanode/catalyst interface, which caused performance degradation under prolonged operations. A stable oxygen evolution catalyst with a robust interface is crucial to the long-term stability of PEC CO2 reduction cells.

Synthesis of Nanoporous Metals, Oxides, Carbides, and Sulfides: Beyond Nanocasting

Nanoporous metal-based solids are of particular interest because they combine a large quantity of surface metal sites, interconnected porous networks, and nanosized crystalline walls, thus exhibiting unique physical and chemical properties compared to other nanostructures and bulk counterparts. Among all of the synthetic approaches, nanocasting has proven to be a highly effective method for the syntheses of metal oxides with three-dimensionally ordered porous structures and crystalline walls. A typical procedure involves a thermal annealing process of a porous silica template filled with an inorganic precursor (often a metal nitrate salt), which converts the precursor into a desired phase within the silica pores. The final step is the selective removal of the silica template in either a strong base or a hydrofluoric acid solution. In the past decade, nanocasting has become a popular synthetic approach and has enabled the syntheses of a variety of nanoporous metal oxides. However, there is still a lack of synthetic methods to fabricate nanoporous materials beyond simple metal oxides. Therefore, the development of new synthetic strategies beyond nanocasting has become an important direction. This Account describes new progress in the preparation of novel nanoporous metal-based solids for heterogeneous catalysis. The discussion begins with a method called dealloying, an effective method to synthesize nanoporous metals. The starting material is a metallic alloy containing two or more elements followed by a selective chemical or electrochemical leaching process that removes one of the preferential elements, resulting in a highly porous structure. Nanoporous metals, such as Cu, Ag, and CuTi, exhibit remarkable electrocatalytic properties in carbon dioxide reduction, oxygen reduction, and hydrogen evolution reactions. In addition, the syntheses of metal oxides with hierarchical porous structures are also discussed. On the basis of the choice of hard template, nanoporous metal oxides with bimodal pore size distributions can be obtained. Combining nanocasting with chemical etching, a cobalt oxide with a hierarchical porous structure was synthesized, which possessed a surface area up to 250 m(2) g(-1), representing the highest surface area reported to date for nanoporous cobalt oxides. Lastly, this Account also covers the syntheses of nanoporous metal carbides and sulfides. The combination of in situ carburization and nanocasting enabled the syntheses of two ordered nanoporous metal carbides, Mo2C and W2C. For nanoporous metal sulfides, an "oxide-to-sulfide" synthetic strategy was proposed to address the large volume change issue of converting metal nitrate precursors to metal sulfide products in nanocasting. The successful syntheses of ordered nanoporous FeS2, CoS2, and NiS2 demonstrated the feasibility of the "oxide-to-sulfide" method. Concluding remarks include a summary of recent advances in the syntheses of nanoporous metal-based solids and a brief discussion of future opportunities in the hope of stimulating new interests and ideas.