1 Solar conversion of carbon dioxide and water to value-added chemicals remains a challenge. A 2 number of solar-active catalysts have been reported but still suffer from low selectivity, poor 3 energy efficiency, and instability, and fail to drive simultaneous water oxidation. Herein, we 4 report CuFeO 2 and CuO mixed p-type catalysts fabricated via a widely employed electroplating 5 of earth-abundant cupric and ferric ions followed by annealing under atmospheric air. The 6 composite electrodes exhibited onset potentials at +0.9 V vs. RHE in CO 2 -purged bicarbonate 7 solution and converted CO 2 to formate with over 90% selectivity under simulated solar light (Air 8 Mass 1.5, 100 mW⋅cm −2 ). Wired CuFeO 2 /CuO photocathode and Pt anode couples produced 9 formate over 1 week at a solar-to-formate energy conversion efficiency of ~1% (selectivity 10 >90%) without any external bias while O 2 was evolved from water. Isotope and nuclear magnetic 11 resonance analyses confirmed the simultaneous production of formate and O 2 at the stand-alone 12 couples. Solar CO 2 recycling has received wide attention primarily to address global CO 2 emission and to 1 convert CO 2 and water to value-added chemicals. 1-3 Despite a long research history over the past 2 four decades, 4,5 the technology remains in an early stage, with low CO 2 conversion efficiency 3 and selectivity. CO 2 is highly stable and has limited solubility in water, and its reduction requires 4 multiple proton-coupled electron transfers, resulting in a range of carbon intermediates (C1 -5 C3) 2,6 as well as a larger amount of H 2 over CO 2 conversion products. 7-9 6 For the realization of solar CO 2 recycling, the system of interest should be operated 7 sustainably, which requires the development of not only energy-efficient and cost-effective 8 materials but also stand-alone, complete reaction processes (CO 2 reduction and water oxidation) 9 operating for long periods without any external bias. 10-12 A range of semiconductors (mostly p-10 types) have been studied for CO 2 conversion, including GaP, 4 InP, 5 GaAs, 13 Si, 8,14 Cu 2 O, 15-18 and 11 CuFeO 2 , 19,20 all of which have narrow bandgaps (E g ) and sufficient Fermi levels (E F ) capable of 12 reducing CO 2 . Although promising, these materials inherently require potential biases to drive 13 the CO 2 reduction reaction and compete with other metallic electrodes, 21 whereas complete 14 reactions (CO 2 reduction and water oxidation) have been rarely demonstrated due to large 15 overpotentials. Photocathode-photoanode couples have been demonstrated to operate, 11 yet the 16 syntheses of materials are complicated and the energy conversion efficiency is low (max. 0.14%). 17 We have searched for high-efficiency, low-cost, and scalable p-type materials and found 18 that CuFeO 2 and CuO mixed materials meet all requirements. To our surprise, this material 19 converted CO 2 to formate with selectivity greater than 90% over 1 week and simultaneously 20 produced molecular oxygen via water oxidation when simply ...
Polymeric carbon nitride modified with selected heteroatom dopants was prepared and used as a model photocatalyst to identify and understand the key mechanisms required for efficient photoproduction of H2O2 via selective oxygen reduction reaction (ORR). The photochemical production of H2O2 was achieved at a millimolar level per hour under visible‐light irradiation along with 100 % apparent quantum yield (in 360–450 nm region) and 96 % selectivity in an electrochemical system (0.1 V vs. RHE). Spectroscopic analysis in spatiotemporal resolution and theoretical calculations revealed that the synergistic association of alkali and sulfur dopants in the polymeric matrix promoted the interlayer charge separation and polarization of trapped electrons for preferable oxygen capture and reduction in ORR kinetics. This work highlights the key features that are responsible for controlling the photocatalytic activity and selectivity toward the two‐electron ORR, which should be the basis of further development of solar H2O2 production.
A hybrid photocatalytic system, which is based on a mixed-phase cadmium sulfide matrix composed of nanoparticulate cubic-phase CdS (c-CdS) with average particle diameters of 13 nm and a bandgap energy of 2.6 eV, is coupled with bulk-phase hexagonal CdS (hex-CdS) that has a bandgap energy of 2.3 eV and is interlinked with elemental platinum deposits. The resulting hybrid nanocomposite catalysts are photocatalytically efficient with respect to hydrogen gas production from water with visible light irradiation at λ > 420 nm. Rates of H 2 production approaching 1.0 mmol-H 2 g -1 h -1 are obtained with a c-CdS/Pt/hex-CdS composite photocatalyst, in the presence of a mixed sodium sulfide and sodium sulfite background electrolyte system at pH 14. In contrast, the same composite produces H 2 a rate of 0.15 mmol g -1 h -1 at pH 7 in a water-isopropanol solvent system. The relative order of reactivity for the synthesized hybrid catalysts was found to be c-CdS/ Pt/hex-CdS > Pt/c-CdS/hex-CdS > Pt/hex-CdS > hex-CdS > c-CdS/hex-CdS > quantum-sized c-CdS. A mechanism involving enhanced lifetimes of electron-hole trapping states that are dependent on the surface chemistry of hydrated CdS involving surface hydroxyl (>CdOH) and sulfhydryl groups (>CdSH) are invoked.
Solar-driven hydrogen peroxide (H2O2) production presents unique merits of sustainability and environmental friendliness. Herein, efficient solar-driven H2O2 production through dioxygen reduction is achieved by employing polymeric carbon nitride framework with sodium cyanaminate moiety, affording a H2O2 production rate of 18.7 μmol h −1 mg−1 and an apparent quantum yield of 27.6% at 380 nm. The overall photocatalytic transformation process is systematically analyzed, and some previously unknown structural features and interactions are substantiated via experimental and theoretical methods. The structural features of cyanamino group and pyridinic nitrogen-coordinated soidum in the framework promote photon absorption, alter the energy landscape of the framework and improve charge separation efficiency, enhance surface adsorption of dioxygen, and create selective 2e− oxygen reduction reaction surface-active sites. Particularly, an electronic coupling interaction between O2 and surface, which boosts the population and prolongs the lifetime of the active shallow-trapped electrons, is experimentally substantiated.
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