Electrochemical oxygen reduction has garnered attention as an emerging alternative to the traditional anthraquinone oxidation process to enable the distributed production of hydrogen peroxide. Here, we demonstrate a selective and efficient nonprecious electrocatalyst, prepared through an easily scalable mild thermal reduction of graphene oxide, to form hydrogen peroxide from oxygen. During oxygen reduction, certain variants of the mildly reduced graphene oxide electrocatalyst exhibit highly selective and stable peroxide formation activity at low overpotentials (< 10 mV) under basic conditions, exceeding the performance of current state-of-the-art alkaline catalysts. Spectroscopic structural characterization and in situ Raman spectroelectrochemistry provide strong evidence that sp 2-hybridized carbon near-ring ether defects along sheet edges are the most active sites for peroxide production, providing new insight into the electrocatalytic design of carbon-based materials for effective peroxide production.

As energy demand continues to increase, so too do anthropogenic carbon emissions and global temperatures. Renewable energy sources such as solar, wind and hydroelectricity displace fossil fuel carbon emissions and continue to progress to wider deployment. However, long-term (seasonal) energy storage remains a challenge that must be addressed for renewables to meet a major fraction of global energy demand 1 . Carbon dioxide electroreduction to renewable fuels and feedstocks provides an energy storage solution to the seasonal variability of renewable energy sources 2 . When coupled with carbon capture technology, the carbon dioxide reduction reaction (CO 2 RR) offers a means to close the carbon cycle.CO 2 RR electrocatalysts lower energetic barriers to CO 2 reduction by stabilizing intermediates and transition states in the multistep electrochemical reduction process 3 . Copper reduces CO 2 to a wide range of hydrocarbon products such as methane, ethylene, ethanol and propanol 4 . Unfortunately, bulk copper is not selective among various carbon products, and it also suffers Faradaic efficiency (FE) losses to the competing hydrogen evolution reaction.Among possible products, C2+ hydrocarbons are highly sought in view of their commercial value. Ethylene, for example, is a precursor to the production of polyethylene, a major plastic. Selectively producing ethylene over methane circumvents costly paraffin-olefin separation 5 . Developing catalysts that work at ambient conditions to produce C2 selectively over C1 gaseous products will increase the relevance of renewable feedstocks in the chemical sector.Oxide-derived copper is one class of catalyst that has shown enhanced CO 2 RR activity and increased selectivity towards multi-carbon products [6][7][8] . The selectivity of these catalysts is dependent on structural morphology and copper oxidation state 9-17 . Electrochemical reduction of copper oxide catalyst films can lead to grain boundaries, undercoordinated sites and roughened surfaces that are hypothesized to be catalytically active sites 8,18 . Residual oxides, proposed to play a key role in catalysis, may exist after electrochemical reduction 7 . A recent report of oxygen plasma-activated oxide-derived copper catalysts achieved an ethylene FE of 60% at − 0.9 V versus reversible hydrogen electrode (RHE) 9 , with activity attributed to the presence of Cu + species. In situ hard X-ray absorption spectroscopy (hXAS) experiments have suggested stable Cu + species exist at highly negative CO 2 RR potentials of ~− 1.0 versus RHE 9 . However, the presence of Cu + species during CO 2 RR is still the subject of debate; 7,19 and in situ tracking of the copper oxidation state with time resolution during CO 2 RR has remained elusive.Morphological effects of copper nanostructures have a significant effect on the selectivity of CO 2 RR to multi-carbon products [20][21][22][23][24] . Copper catalysts with different morphologies have been synthesized through annealing, chemical treatments on thin films, colloidal synthesis and ...

Electrochemical carbon dioxide recycling provides an attractive approach to synthesizing fuels and chemical feedstocks using renewable energy. On the path to deploying this technology, basic and applied scientific hurdles remain. Integrating catalytic design with mechanistic understanding yields scientific insights and progresses the technology towards industrial relevance. Catalysts must be able to generate valuable carbon-based products with better selectivity, lower overpotentials and improved current densities with extended operation. Here, we describe progress and identify mechanistic questions and performance metrics for catalysts that can enable carbon-neutral renewable energy storage and utilization.

Copper is uniquely active for the electrocatalytic reduction of carbon dioxide (CO) to products beyond carbon monoxide, such as methane (CH) and ethylene (CH). Therefore, understanding selectivity trends for CO electrocatalysis on copper surfaces is critical for developing more efficient catalysts for CO conversion to higher order products. Herein, we investigate the electrocatalytic activity of ultrathin (diameter ∼20 nm) 5-fold twinned copper nanowires (Cu NWs) for CO reduction. These Cu NW catalysts were found to exhibit high CH selectivity over other carbon products, reaching 55% Faradaic efficiency (FE) at -1.25 V versus reversible hydrogen electrode while other products were produced with less than 5% FE. This selectivity was found to be sensitive to morphological changes in the nanowire catalyst observed over the course of electrolysis. Wrapping the wires with graphene oxide was found to be a successful strategy for preserving both the morphology and reaction selectivity of the Cu NWs. These results suggest that product selectivity on Cu NWs is highly dependent on morphological features and that hydrocarbon selectivity can be manipulated by structural evolution or the prevention thereof.

The electrochemical reduction of carbon dioxide (CO 2 RR) offers a compelling route to energy storage and high-value chemical manufacture. The presence of sulfur atoms in catalyst surfaces promotes undercoordinated sites, thereby improving the electrochemical reduction of CO 2 to formate. The resulting sulfurmodulated tin catalysts accelerate CO 2 RR at geometric current densities of 55 mA cm À2 at À0.75 V versus RHE with a Faradaic efficiency of 93%.

This Feature Article describes research on the optical properties of arrays of silver and gold nanoparticles, particles that exhibit localized surface plasmon resonances in the visible and near-infrared. These resonances lead to strong absorption and scattering of light that is strongly dependent on nanoparticle size and shape. When arranged into multidimensional arrays, the nanoparticles strongly interact such that the collective properties can be rationally designed by changing the dimensions of the array (one-, two-, or three-dimensional), interparticle spacing, and array shape or morphology. Emerging from this work is a large body of literature focusing on one-, two-, and three-dimensional arrays, which provide unique opportunities for realizing materials with interesting and unusual photonic and metamaterial properties. Electrodynamics theory provides an accurate description of the optical properties, often based on simple models such as coupled dipoles, effective medium theory, and anomalous diffraction. In turn, simple models and simulation methods allow for the prediction and explanation of a variety of observed optical properties. In one and two dimensions, these tunable optical properties range from extinction spectra that are red- or blue-shifted compared to the isolated particles to lattice plasmon modes that involve strong interactions between localized plasmon resonances in the nanoparticles and photonic modes that derive from Bragg diffraction in the crystalline array. Three-dimensional arrays can exhibit unique effective medium properties, such as negative permittivity that leads to metallic optical response even when there is less than 1% metal content in the array. They also can be rationally designed to have photonic scattering modes dictated and controlled by interactions between nanoscale plasmonic nanoparticles and the mesoscale superlattice crystal habit (i.e., the crystalline size, shape, and morphology). This discussion of plasmonic arrays across multiple dimensions provides a comprehensive description of those factors that can be easily tuned for the design of plasmon-based optical materials.

Using renewable energy to recycle CO provides an opportunity to both reduce net CO emissions and synthesize fuels and chemical feedstocks. It is of central importance to design electrocatalysts that both are efficient and can access a tunable spectrum of products. Syngas, a mixture of carbon monoxide (CO) and hydrogen (H), is an important chemical precursor that can be converted downstream into small molecules or larger hydrocarbons by fermentation or thermochemistry. Many processes that utilize syngas require different syngas compositions: we therefore pursued the rational design of a family of electrocatalysts that can be programmed to synthesize different designer syngas ratios. We utilize in situ surface-enhanced Raman spectroscopy and first-principles density functional theory calculations to develop a systematic picture of CO* binding on Cu-enriched Au surface model systems. Insights from these model systems are then translated to nanostructured electrocatalysts, whereby controlled Cu enrichment enables tunable syngas production while maintaining current densities greater than 20 mA/cm.

Electrochemically upgrading CO2 to carbon-neutral multicarbons (C2+) is a promising technology for CO2 recycling and utilization. Since such transformations involve multiple elementary steps, a tandem strategy becomes attractive as catalysts can be optimized for specific reaction steps independently. Related strategies have been demonstrated under low working current densities; however, the applicability of a tandem strategy towards high-rate CO2 electrolysis to C2+ is unknown. Here, we demonstrate that a Cu-Ag tandem catalyst can enhance the multicarbon production rate in CO2RR by decoupling high-rate CO2 reduction to CO on Ag and subsequent CO coupling on Cu. With the addition of Ag, the partial current towards C2+ over a Cu surface increased from 37 mA/cm 2 to 160 mA/cm 2 at -0.70 V vs RHE in 1M KOH while no mutual interference between two metals was observed. Moreover, the normalized intrinsic activity of C2H4 and C2H5OH in the tandem platform under CO2 reduction conditions is significantly higher than Cu alone under either pure CO2 or CO atmosphere. Our results indicate that the CO-enriched local environment generated by Ag can enhance C2+ formation on Cu beyond CO2 or CO feeding, suggesting possible new mechanisms in a tandem three-phase environment. Manuscriptprove CO2RR catalytic performance. Thus, we conducted post-electrolysis characterization of the tandem Cu500Ag1000 catalyst to determine whether the structure is maintained. Previous works have shown structural and electronic differences owing to strong Ag interactions with Cu: for example, up to 0.8 o Cu(111) peak shift in XRD could be found for a Cu-Ag alloy system 14 whereas up to 0.3 eV Cu 2p3/2 peak shift in XPS was reported for a Cu-Ag dimer. 23 In contrast, no peak shift of Cu or Ag could be observed for the tandem Cu500Ag1000 catalyst in either XRD, XPS or Cu LMM Auger peak after electrolysis, indicating the structural maintenance of this tandem catalyst and absence of electronic interactions between Ag and Cu throughout electrolysis. This absence is likely due to the bulk-like nature of Ag and Cu used, in addition to the mild conditions in which the electrode is fabricated, resulting in thermodynamically favored separation 52 . Importantly, this does not preclude the Ag-Cu surface and interfacial alloying observed in other reports which use more energetic fabrication conditions.2.2 Enhanced CO2RR catalytic performances toward C2+ products over tandem Cu-Ag catalysts. The polarization response curve of Cu500Ag1000 in Fig. 2a shows higher geometric current density than Cu500 or Ag1000 alone under the same potentials. Interestingly, partial current densities toward C2+ products over different catalysts are also observed to be substantially higher for Cu500Ag1000, which cannot be explained simply through the individual contributions of Cu500 and Ag1000 (Fig. 2b). Explicitly, Ag1000 does not contribute to C-C coupling reactions in the potential range from -0.5 V to -0.8 V vs RHE. Thus, all partial current toward C2+ products should come from the C...