Photosynthesis of formate from CO₂ and water at 1% energy efficiency via copper iron oxide catalysis

Abstract

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 ...

Photocatalytic CO 2 reduction vs. H 2 production: The effects of surface carbon-containing impurities on the performance of TiO 2 -based photocatalysts

“…Hydrogen production by photocatalytic steam reforming of methanol occurred at constant rate, as in previous studies [30,31,39]. The photocatalytic activity results obtained in this reaction with the investigated materials are summarized in Table 1.…”

“…This holds particularly true if carbon doped or high surface area materials synthesized at low temperature without post annealing treatments are used as photocatalysts [14][15][16]. Isotopic CO 2 labelling would be one way to demonstrate that the reaction products arise from carbon dioxide reduction and not from other carbon sources, and it has been employed in a few studies so far [24,28,[30][31][32].…”

“…New promising strategies have been recently proposed for CO 2 photoreduction, including for instance the use of H 2 as electron donor in the presence of irradiated nickel supported on silicaalumina [21], the photothermal approach in fluidized bed reactors [22,23], the use of photogenerated pyridinyl radicals [24,25], the use of metal organic frameworks as visible light harvesting materials [26] or of phthalocyanins and porphirins as sensitizers [27,28], the band gap narrowing of photocatalytic materials with high conduction band [29] and the use of sensitized semiconductors in Z-scheme systems [30]. Most recent reports in the photocatalytic reduction of CO 2 concern the use of small band gap materials such as CuFeO 2 coupled with CuO [31] and of a novel monolithic assembly based on a triple-junction, exhibiting a promising 4.6% efficiency of solar into chemical energy conversion [32].…”

Recent progress on advanced design for photoelectrochemical reduction of CO2 to fuels

“…For instance, a CuFeO 2 /CuO mixed catalyst was synthesized through electroplating (Fig. 7c) [40]. Constructing a setup for CO 2 reduction, the CuFeO 2 /CuO photocathode produced formate with over 90% selectivity under simulated solar light without any external electrical bias (Fig.…”

“…1a). Numerous p-type semiconductors have been exploited and investigated, including silicon [29,30], metal oxides [31][32][33], sulfides [34,35], tellurides [36,37], phosphides [38,39] and others [40,41]. In addition, profiting from the remarkable activity for CO 2 adsorption and activation, metal materials (e.g., Pd, Au, Ag and Cu) usually serve as the cathodes in electrochemical CO 2 reduction [42].…”