Electrochemical conversion of CO2 into energy‐dense liquids, such as formic acid, is desirable as a hydrogen carrier and a chemical feedstock. SnOx is one of the few catalysts that reduce CO2 into ...formic acid with high selectivity but at high overpotential and low current density. We show that an electrochemically reduced SnO2 porous nanowire catalyst (Sn‐pNWs) with a high density of grain boundaries (GBs) exhibits an energy conversion efficiency of CO2‐into‐HCOOH higher than analogous catalysts. HCOOH formation begins at lower overpotential (350 mV) and reaches a steady Faradaic efficiency of ca. 80 % at only −0.8 V vs. RHE. A comparison with commercial SnO2 nanoparticles confirms that the improved CO2 reduction performance of Sn‐pNWs is due to the density of GBs within the porous structure, which introduce new catalytically active sites. Produced with a scalable plasma synthesis technology, the catalysts have potential for application in the CO2 conversion industry.
Against the grain: An electrochemically reduced SnO2 porous nanowire catalyst (Sn‐pNWs) with a high density of grain boundaries (GBs) has an energy conversion efficiency for CO2‐into‐HCOOH that is higher than analogous catalysts of reduced SnO2‐based materials. The high density of GBs within the porous structure is decisive.
Si wire arrays are a promising architecture for solar-energy-harvesting applications, and may offer a mechanically flexible alternative to Si wafers for photovoltaics. To achieve competitive ...conversion efficiencies, the wires must absorb sunlight over a broad range of wavelengths and incidence angles, despite occupying only a modest fraction of the array's volume. Here, we show that arrays having less than 5% areal fraction of wires can achieve up to 96% peak absorption, and that they can absorb up to 85% of day-integrated, above-bandgap direct sunlight. In fact, these arrays show enhanced near-infrared absorption, which allows their overall sunlight absorption to exceed the ray-optics light-trapping absorption limit for an equivalent volume of randomly textured planar Si, over a broad range of incidence angles. We furthermore demonstrate that the light absorbed by Si wire arrays can be collected with a peak external quantum efficiency of 0.89, and that they show broadband, near-unity internal quantum efficiency for carrier collection through a radial semiconductor/liquid junction at the surface of each wire. The observed absorption enhancement and collection efficiency enable a cell geometry that not only uses 1/100th the material of traditional wafer-based devices, but also may offer increased photovoltaic efficiency owing to an effective optical concentration of up to 20 times.
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•The current state-of-the-art of electrochemical conversion of CO2 into fuels.•Unconventional heterogeneous catalysts for CO2 reduction reaction.•Future prospective for advanced ...heterogeneous catalysts for CO2 reduction reaction
The electrochemical conversion of CO2 into energy-rich fuels and chemicals has gained significant interest as a potential strategy for simultaneously mitigating increasing global CO2 concentration and effectively storing intermittent renewable energy from sources such as solar and wind. This process recycles CO2, permitting a carbon-neutral, closed-loop of fuel combustion and waste CO2 reduction to help prevent a rising concentration of this greenhouse gas in the atmosphere. At the same time, intermittent electricity generation can be stored in an energy-dense, portable form in chemical bonds. However, the stability of CO2 makes its conversion kinetically challenging, generally requiring a large overpotential, and thus the efficiency of electrochemical CO2 reduction is strongly dependent on the activity and selectivity of the cathodic electrocatalyst. In this review, we discuss the current state-of-the-art of unconventional heterogeneous catalysts with a focus on activity and product selectivity, even if the CO2 reduction reaction mechanism remains uncertain. Various emerging approaches to enhance the yield of specific products and the overall rate of reaction will also be addressed. Finally, prospects for the development of next-generation catalysts will be discussed.
Ultrathin dual layers of TiO2 and Ni have been used to stabilize polycrystalline BiVO4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO2 ...layers were deposited on BiVO4 thin films by atomic-layer deposition, with Ni deposited onto the TiO2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems.
Efficient electroreduction of carbon dioxide has been a widely pursued goal as a sustainable method to produce value‐added chemicals while mitigating greenhouse gas emissions. Processes have been ...demonstrated for the electroreduction of CO2 to CO at nearly 100 % faradaic efficiency, and as a consequence, there has been growing interest in the further electroreduction of carbon monoxide. Oxide‐derived copper catalysts have promising performance for the reduction of CO to hydrocarbons but have still been unable to achieve high selectivity to individual products. A pulsed‐bias technique is one strategy for tuning electrochemical selectivity without changing the catalyst. Herein a pulsed‐bias electroreduction of CO was investigated on oxide‐derived copper catalyst. Increased selectivity for single‐carbon products (i.e., formate and methane) was achieved for higher pulse frequencies (<1 s pulse times), as well as an increase in the fraction of charge directed to CO reduction rather than hydrogen evolution.
Feel the pulse: Variable pulsing frequency in a pulsed‐bias electrochemical CO reduction affects selectivity and favors methane and formate at short pulse times. Pulsed‐bias electrosynthesis enables some control of product selectivity that can be complementary to catalyst design. This type of CO reduction can be coupled to high‐efficiency reduction of CO2 to CO for a two‐step electrosynthesis of useful products from CO2.
Silicon wire arrays, though attractive materials for use in photovoltaics and as photocathodes for hydrogen generation, have to date exhibited poor performance. Using a copper-catalyzed, ...vapor-liquid-solid-growth process, SiCl₄ and BCl₃ were used to grow ordered arrays of crystalline p-type silicon (p-Si) microwires on p⁺-Si(111) substrates. When these wire arrays were used as photocathodes in contact with an aqueous methyl viologen²⁺/⁺ electrolyte, energy-conversion efficiencies of up to 3% were observed for monochromatic 808-nanometer light at fluxes comparable to solar illumination, despite an external quantum yield at short circuit of only 0.2. Internal quantum yields were at least 0.7, demonstrating that the measured photocurrents were limited by light absorption in the wire arrays, which filled only 4% of the incident optical plane in our test devices. The inherent performance of these wires thus conceptually allows the development of efficient photovoltaic and photoelectrochemical energy-conversion devices based on a radial junction platform.
Electrochemical reduction of CO2 to liquid fuels is a promising route to a carbon-neutral, energy-dense storage of intermittent renewable electricity. However, electrocatalysts generally suffer from ...high overpotential and poor selectivity for multi-carbon products such as ethanol, and efforts to enhance such catalysts are limited by scaling relations which inhibit a simultaneous optimization of each elementary electrochemical step. In this work, the multistep proton-coupled electron-transfer reaction for the conversion of CO2 to C2H5OH was strategically divided into two independently optimized steps in a sequential cascade reaction using heterogeneous electrocatalysts to convert CO2 to CO and CO to C2H5OH within a single integrated electrochemical system. The exclusion of CO2 reactant from the second-stage electrolyzer was observed to be critical for maintaining appreciable ethanol selectivity. The cascade system produced C2H5OH at an overall faradaic efficiency of 11.0% at an average applied potential of −0.52 V vs. RHE, making it highly competitive with known single-step electrocatalysts for ethanol production from CO2. This performance was despite limited conversion of the intermediate CO between cascade steps (∼6.4%), and reactor design improvements to enhance the conversion could lead to significantly enhanced ethanol production performance.
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Slurries of semiconductor particles individually capable of unassisted light‐driven water‐splitting are modeled to have a promising path to low‐cost solar hydrogen generation, but they have had poor ...efficiencies. Tandem microparticle systems are a clear direction to pursue to increase efficiency. However, light absorption must be carefully managed in a tandem to prevent current mismatch in the subcells, which presents a possible challenge for tandem microwire particles suspended in a liquid. In this work, a Ni‐catalyzed Si/TiO2 tandem microwire slurry is used as a stand‐in for an ideal bandgap combination to demonstrate proof‐of‐concept in situ alignment of unassisted water‐splitting microwires with an external magnetic field. The Ni hydrogen evolution catalyst is selectively photodeposited at the exposed Si microwire core to serve as the cathode site as well as a handle for magnetic orientation. The frequency distribution of the suspended microwire orientation angles is determined as a function of magnetic field strength under dispersion with and without uplifting microbubbles. After magnetizing the Ni bulb, tandem microwires can be highly aligned in water under a magnetic field despite active dispersion from bubbling or convection.
Tandem semiconductor microwires capable of unassisted solar water‐splitting are magnetically aligned under operational conditions with active dispersion using an external magnetic field acting on a ferromagnetic nickel‐hydrogen evolution catalyst. Aligned microwire slurries may help future tandem systems maintain current matching from a balance of photon absorption between the subcell regions.
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