Electroreduction of carbon dioxide (CO(2))--a key component of artificial photosynthesis--has largely been stymied by the impractically high overpotentials necessary to drive the process. We report an electrocatalytic system that reduces CO(2) to carbon monoxide (CO) at overpotentials below 0.2 volt. The system relies on an ionic liquid electrolyte to lower the energy of the (CO(2))(-) intermediate, most likely by complexation, and thereby lower the initial reduction barrier. The silver cathode then catalyzes formation of the final products. Formation of gaseous CO is first observed at an applied voltage of 1.5 volts, just slightly above the minimum (i.e., equilibrium) voltage of 1.33 volts. The system continued producing CO for at least 7 hours at Faradaic efficiencies greater than 96%.

Conversion of carbon dioxide (CO2) into fuels is an attractive solution to many energy and environmental challenges. However, the chemical inertness of CO2 renders many electrochemical and photochemical conversion processes inefficient. We report a transition metal dichalcogenide nanoarchitecture for catalytic electrochemical CO2 conversion to carbon monoxide (CO) in an ionic liquid. We found that tungsten diselenide nanoflakes show a current density of 18.95 milliamperes per square centimeter, CO faradaic efficiency of 24%, and CO formation turnover frequency of 0.28 per second at a low overpotential of 54 millivolts. We also applied this catalyst in a light-harvesting artificial leaf platform that concurrently oxidized water in the absence of any external potential.

), and examined their performance for BP exfoliation (see Section S1, Supporting Information). Initially, a chunk of black phosphorous crystal (0.02 mg mL −1 ) was immersed into different solvents and was sonicated for 15 h (total input energy -1 MJ). We noticed that aprotic and polar solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) are appropriate solvents for the synthesis of atomically thin BP nanofl akes and can produce uniform and stable dispersions after the sonication (see Section S2, Supporting Information). The solutions were then centrifuged and their supernatants were carefully collected by a pipette. Figure 1 A shows the BP nanofl ake dispersions in DMSO and DMF after sonication for 15 h (left image) and after the centrifugation (right image), having concentrations up to 10 µg mL −1 (see Experimental Section).As suggested by experimental [ 15 ] and theoretical [ 16,17 ] reports, BP atomic layers have a thickness dependent direct bandgap ranging from ≈0.3 eV in bulk to more than 1 eV in monolayer. Typically, optical absorption spectroscopy is a robust and reliable method to determine the bandgap of semiconductors in solution form. We used this technique to characterize our dispersed nanofl akes in DMF and DMSO solutions with a focus on the near-IR (NIR) range (Wavelength of 830-2400 nm) where the peaks associated with the optical band gap of atomically thin BP nanofl akes are likely to occur. Interestingly, in both DMF and DMSO solutions several spectral peaks were observed in the NIR range at ≈1.38, ≈1.23, ≈1.05, ≈0.85, and ≈0.72 eV (labeled as numbers 1-5 in Figure 1 B) which are believed to be associated with the enhanced light absorption by mono-, to fi ve-layers thick BP nanofl akes, respectively. These results are in a good agreement with the position of photoluminescence peaks reported for mono-to fi ve-layers thick BP fl akes obtained by mechanical exfoliation. [ 12,22 ] The smaller peaks at 1.38 and 1.23 eV compared to other peaks implies that the yields of mono-and bilayers are lower than other atomic layers.We also measured the normalized absorption intensity over the characteristic length of the cell ( A / l ) at λ = 1176 nm ( E = 1.05 eV) for DMF and DMSO solutions at different concentrations ( C ). As suggested by the Lambert-Beer law ( A / l = αC , where α is the extinction coeffi cient), a linear trend was observed for A / l versus concentration (Figure 1 C), suggesting well-dispersed nanofl akes in both solutions. The extinction coeffi cients for DMF and DMSO solutions were extracted to be α = 4819 and 5374 mL mg −1 m −1 , respectively. The BP fl ake size distribution was also analyzed by dynamic light scattering (DLS) spectroscopy and the average fl ake sizes were determined to be ≈190 and ≈532 nm for the DMF and DMSO solutions, respectively (Figure 1 D).2D nanomaterials such as graphene and transition metal dichalcogenides (TMDCs) have shown outstanding potential in many fi elds such as fl exible electronics, [ 1 ] sensing, [ 2,3 ] and optics, [ 4 ] due to their...

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies of Li-O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li-O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li-O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

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