In view of the climate changes caused by the continuously rising levels of atmospheric CO2 , advanced technologies associated with CO2 conversion are highly desirable. In recent decades, electrochemical reduction of CO2 has been extensively studied since it can reduce CO2 to value-added chemicals and fuels. Considering the sluggish reaction kinetics of the CO2 molecule, efficient and robust electrocatalysts are required to promote this conversion reaction. Here, recent progress and opportunities in inorganic heterogeneous electrocatalysts for CO2 reduction are discussed, from the viewpoint of both experimental and computational aspects. Based on elemental composition, the inorganic catalysts presented here are classified into four groups: metals, transition-metal oxides, transition-metal chalcogenides, and carbon-based materials. However, despite encouraging accomplishments made in this area, substantial advances in CO2 electrolysis are still needed to meet the criteria for practical applications. Therefore, in the last part, several promising strategies, including surface engineering, chemical modification, nanostructured catalysts, and composite materials, are proposed to facilitate the future development of CO2 electroreduction.

Recent Advances in Inorganic Heterogeneous Electrocatalysts for Reduction of Carbon Dioxide

As a promising approach to achieving two objectives with one strategy, photocatalytic CO2 conversion for C1/C2 “solar fuels” production can provide a package solution to the current global warming and growing energy demand by using inexhaustible solar energy and increasing atmospheric CO2. Although numerous efforts have been made to enhance the CO2 conversion efficiency through developing photocatalysts and CO2 reduction systems in recent years, some challenges still remain in improving the activity and selectivity of the CO2 photoreduction reactions. This review gives an overview of fundamental aspects and recent research advances of heterogeneous photocatalytic CO2 conversion systems in the last 3 years, and the catalysts are categorized as one-step excitation semiconductor systems, one-step excitation photosensitized semiconductor systems, and two-step excitation hybrid systems such as semiconductor heterojunction and Z-scheme systems. Also, some suggestions are given for further confirming that the ca...

Recent Advances in Heterogeneous Photocatalytic CO2 Conversion to Solar Fuels

The gradually increased concentration of carbon dioxide (CO2 ) in the atmosphere has been recognized as the primary culprit for the rise of the global mean temperature. In recent years, development of routes for highly efficient conversion of CO2 has received much attention. This Review describes recent progress on the design and synthesis of solid-state catalysts for the electrochemical reduction of CO2 . The significance of this catalytic conversion is presented, followed by the general parameters for CO2 electroreduction and a summary of the reaction apparatus. We also discuss various types of solid catalysts based on their CO2 conversion mechanisms. We summarize the crucial factors (particle size, surface structure, composition, etc.) determining the performance for electroreduction.

Nanostructured Materials for Heterogeneous Electrocatalytic CO2 Reduction and their Related Reaction Mechanisms

In this work we propose four non-coupled binding energies of intermediates as descriptors, or 'genes', for predicting the product distribution in CO2 electroreduction. Simple reactions can be understood by the Sabatier principle (catalytic activity vs. one descriptor), while more complex reactions tend to give multiple very different products and consequently the product selectivity is a more complex property to understand. We approach this, as a logistical classification problem, by grouping metals according to their major experimental product from CO2 electroreduction: H2, CO, formic acid and beyond CO* (hydrocarbons or alcohols). We compare the groups in terms of multiple binding energies of intermediates calculated by density functional theory. Here we find three descriptors to explain the grouping: the adsorption energies of H*, COOH* and CO*. To further classify products beyond CO*, we carry out formaldehyde experiments on Cu, Ag and Au and combine these results with the literature to group and differentiate alcohol or hydrocarbon products. We find that the oxygen binding (adsorption energy of CH3O*) is an additional descriptor to explain the alcohol formation in reduction processes. Finally, the adsorption energy of the four intermediates, H*, COOH*, CO* and CH3O*, can be used to differentiate, group and explain products in electrochemical reduction processes involving CO2, CO and carbon-oxygen compounds.

Electrochemical CO2 Reduction: A Classification Problem.

Electroreduction of CO2 into hydrocarbons could contribute to alleviating energy crisis and global warming. However, conventional electrocatalysts usually suffer from low energetic efficiency and poor durability. Herein, atomic layers for transition-metal oxides are proposed to address these problems through offering an ultralarge fraction of active sites, high electronic conductivity, and superior structural stability. As a prototype, 1.72 and 3.51 nm thick Co3O4 layers were synthesized through a fast-heating strategy. The atomic thickness endowed Co3O4 with abundant active sites, ensuring a large CO2 adsorption amount. The increased and more dispersed charge density near Fermi level allowed for enhanced electronic conductivity. The 1.72 nm thick Co3O4 layers showed over 1.5 and 20 times higher electrocatalytic activity than 3.51 nm thick Co3O4 layers and bulk counterpart, respectively. Also, 1.72 nm thick Co3O4 layers showed formate Faradaic efficiency of over 60% in 20 h.

Ultrathin Co3O4 Layers Realizing Optimized CO2 Electroreduction to Formate

Highly selective electrochemical reduction of CO2 to HCOOH on a gallium oxide cathode

We report on a highly improved CO2 to HCOOH conversion system using a tandem photo-electrode (TPE) of InGaN and two Si p-n junctions. To improve its efficiency, narrow-band-gap InGaN was applied as the photo-absorption layer. In the TPE structure, the current matching between GaN-based photo-absorption layer and two Si p-n junctions is crucial for the improvement of the efficiency. The energy conversion efficiency for HCOOH production reached 0.97%, which is greater than average of global biological photosynthetic one.

Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis

Gallium nitride (GaN) has been shown to be a good photocatalyst for not only water splitting but also carbon dioxide (CO2) reduction. To verify the catalytic reaction involved in CO2 reduction, it is essential to confirm that the reaction products only come from dissolved CO2. Here, we report the results of 13CO2-labeling experiments on a GaN-Si based photoelectrochemical system for each reduction product. It was found that the 13C-based CO2 was almost completely converted to formic acid (HCOOH), methane, and ethylene when KCl was used as the electrolyte. In contrast, HCOOH from 12C was observed when KHCO3 electrolyte was used, meaning that HCO3– in the electrolyte partly contributed to CO2 reduction. The energy conversion efficiency and Faradaic efficiency of the respective reduction processes in the present system are also discussed.

Analysis of Products from Photoelectrochemical Reduction of 13CO2 by GaN-Si Based Tandem Photoelectrode

Organic–inorganic halide perovskites (OIHPs) have been intensively studied in recent years for use in solar cells. Many studies have reported the light-induced phenomena of OIHP materials and their solar cells. In this study, we investigated the influence of a hole-transport layer (HTL) on light-induced degradation (LID) of mixed OIHP solar cells, especially at the OIHP/HTL interface, by hard X-ray photoelectron spectroscopy (HAXPES) and impedance spectroscopy. HAXPES shows accumulation of iodine and metallic lead in the vicinity of the OIHP/HTL and electron-transport layer/OIHP interfaces, respectively, after LID. Under light illumination and in the dark after LID, characteristic impedance responses are observed for a sample with a highly damaged OIHP/HTL interface as an intermediate frequency arc and negative capacitance, respectively, because of the electrochemical reaction in the vicinity of the OIHP/HTL interface. Overall, the results indicate that light-induced iodide diffusion to the OIHP/HTL inter...

Influence of a Hole-Transport Layer on Light-InducedDegradation of Mixed Organic–Inorganic Halide Perovskite SolarCells

We have matched the world’s highest open-circuit voltage (Voc: 1.06 V) achieved to date for a layered structure comprised of a glass/tin oxide (SnO2)/hydrogenated amorphous silicon oxide (a-SiO:H) (p–i–n)/back electrode. For the purposes of this study, we adjusted the band gaps of each layer (p–i–n) to improve overall film quality. Fine-tuning of band profiles with reference to activation energy and optical band gap allowed us to offset the conduction band and the valence band of each layer (p–i–n) and thus improve the built-in potential rather than the electron conductivity, Fourier transform infrared spectroscopy, transmittance or reflectance ratio, resulting in a high Voc. To fully exploit the characteristics of wide-band-gap materials and prevent problems with absorbance, we employed commercially available SnO2 in the front transparent conductive oxide instead of zinc oxide. Using our deposition and evaluation technologies to build a wide-band-gap single solar cell, we succeeded in matching the world’s highest Voc of 1.06 V (Eff: 5.38%, Jsc: 8.15 mA/cm2, FF: 0.624).

Takeyuki Sekimoto

Papers

Highly selective electrochemical reduction of CO2 to HCOOH on a gallium oxide cathode

Tandem photo-electrode of InGaN with two Si p-n junctions for CO2 conversion to HCOOH with the efficiency greater than biological photosynthesis

Analysis of Products from Photoelectrochemical Reduction of 13CO2 by GaN-Si Based Tandem Photoelectrode

Influence of a Hole-Transport Layer on Light-InducedDegradation of Mixed Organic–Inorganic Halide Perovskite SolarCells

Designing band offset of a-SiO:H solar cells for very high open-circuit voltage (1.06 V) by adjusting band gap of p–i–n junction