CO2 Reduction: From the Electrochemical to Photochemical Approach

Abstract

Increasing CO₂ concentration in the atmosphere is believed to have a profound impact on the global climate. To reverse the impact would necessitate not only curbing the reliance on fossil fuels but also developing effective strategies capture and utilize CO₂ from the atmosphere. Among several available strategies, CO₂ reduction via the electrochemical or photochemical approach is particularly attractive since the required energy input can be potentially supplied from renewable sources such as solar energy. In this Review, an overview on these two different but inherently connected approaches is provided and recent progress on the development, engineering, and understanding of CO₂ reduction electrocatalysts and photocatalysts is summarized. First, the basic principles that govern electrocatalytic or photocatalytic CO₂ reduction and their important performance metrics are discussed. Then, a detailed discussion on different CO₂ reduction electrocatalysts and photocatalysts as well as their generally designing strategies is provided. At the end of this Review, perspectives on the opportunities and possible directions for future development of this field are presented.

Keywords: CO2 reduction; electrocatalysis; nanotechnology; photocatalysis.

Figure 1

Atmospheric CO₂ concentration and corresponding global average temperature since the late 19th century. Red bars indicate temperatures above and blue bars indicate temperatures below the 1901–2000 average temperature. Adopted from the website of National Ocean and Atmospheric Administration (NOAA).3 Copyright 2017, NOAA.

Figure 2

Analogy among a) natural photosynthesis, b) electrochemical synthesis on electrocatalysts powered by a photovoltaic cell, and c) photochemical synthesis on powdery photocatalysts.

Figure 3

Possible reaction pathways for electrocatalytic CO₂RR on metal electrodes in aqueous solutions. Adapted with permission.45 Copyright 1994, Elsevier.

Figure 4

Schematic showing the five fundamental steps in photocatalytic CO₂ reduction. Adopted with permission.20 Copyright 2014, Springer.

Figure 5

a) Faradaic efficiency as a function of potential for major (top), intermediate range (middle), and minor (bottom) products on a metallic Cu surface. Reproduced with permission.36 Copyright 2012, The Royal Society of Chemistry. Particle size dependence of b) current density and c) Faradaic efficiency for different CO₂RR products on Cu NPs; d) population of surface atoms with certain coordination number (CN) as a function of particle diameter. Reproduced with permission.54 Copyright 2014, American Chemical Society. Scanning electron microscope (SEM) images of e) an annealed Cu electrode and f) the same electrode after CO₂RR; g) Faradaic efficiency for CO and HCOOH as a function of potential on polycrystalline Cu and annealed Cu. Reproduced with permission.48 Copyright 2012, American Chemical Society.

Figure 6

a) Transmission electron microscopy (TEM) image of 8 nm Au NPs; b) potential‐dependent Faradaic efficiency for CO on Au NPs with different sizes; c) current densities for CO formation at various potentials. Reproduced with permission.63 Copyright 2013, American Chemical Society. d) Cross‐sectional SEM image and e) high‐magnification TEM image of oxide‐derived Au NPs; f) Faradaic efficiency for CO and formate on oxide‐derived Au NPs in 0.5 m NaHCO₃. Reproduced with permission.65 Copyright 2012, American Chemical Society.

Figure 7

a) SEM image of oxide‐derived Ag; b) Faradaic efficiency for CO on polycrystalline Ag and oxide‐derived Ag. Reproduced with permission.73 c) Cross‐sectional SEM image of an Ag‐IO film; d) potential‐dependent Faradaic efficiency for CO on Ag films with varying roughness factors. Reproduced with permission.75

Figure 8

a,b) Change of the total current density and CO Faradaic efficiency with time on a) untreated Sn and b) etched Sn at −0.7 V versus RHE in 0.5 m NaHCO₃; c) their potential‐dependent Faradaic efficiency for CO and formic acid. Reproduced with permission.65 Copyright 2012, American Chemistry Society. d) High‐magnification TEM image of Sn quantum sheets confined in graphene; e) polarization curves, f) potential‐dependent Faradaic efficiency for formate, and g) chronoamperometry stability at −1.8 V versus SCE on Sn quantum sheets confined in graphene as well as several control samples in 0.1 m NaHCO₃ aqueous solution. Reproduced with permission.79 Copyright 2016, Nature Publishing Group.

Figure 9

a) CV curves of WSe₂ NFs, bulk MoS₂, Ag NPs, and bulk Ag in CO₂‐saturated EMIM‐BF₄/H₂O solution; b) potential‐dependent Faradaic efficiency for CO and H₂ on WSe₂ NFs; c) CO formation TOF of WSe₂ NFs, bulk MoS₂, and Ag NPs; d) schematic showing an artificial leaf with WSe₂ cocatalyst for reducing CO₂ to CO under light illumination. e) Product formation rates under different light illumination intensities using the WSe₂/IL cocatalyst system. Reproduced with permission.102 Copyright 2016, American Association for the Advancement of Science.

Figure 10

a) Production rates of CO and CH₄ on three TiO₂ nanocrystal polymorphs (anatase, rutile, and brookite). Reproduced with permission.120 Copyright 2012, American Chemical Society. b) Schematic of anatase TiO₂ with different percentages of {101}, {001}, and {010} facets and c–e) SEM images of corresponding synthetic products. Reproduced with permission.122

Figure 11

a) Diffuse reflectance spectra of pure TiO₂, C‐doped TiO₂, S‐doped TiO₂, and N‐doped TiO₂ showing the prominent effect of anion doping. Reproduced with permission.175 Copyright 2008, American Chemical Society. b) Calculated density of state (DOS) of pure TiO₂ and N‐doped TiO₂ with different concentrations of O vacancies. Reproduced with permission.176 Copyright 2009, American Chemical Society.

Figure 12

a) TEM image of W₁₈O₄₉ nanowires for selectively reducing CO₂ to CH₄. Reproduced with permission.182 b,c) SEM and TEM images of Zn₂GeO₄ nanoribbons. Reproduced with permission.134 Copyright 2010, American Chemical Society. d) Schematic of the photocatalytic CO₂ reduction to methanol on the single‐unit‐cell Bi₂WO₆ layers; e) TEM image of Bi₂WO₆ layers; f) methanol formation rate on Bi₂WO₆ layers and bulk Bi₂WO₆, g) stability of methanol formation on Bi₂WO₆ layers. Reproduced with permission.183

Figure 13

a) Schematic of the conventional type‐II heterojunction photocatalyst. Reproduced with permission.40 Copyright 2017, Elsevier. b) SEM image of ZnIn₂S₄/TiO₂; c) comparison of CH₄ yield from photocatalytic CO₂ reduction on 1) ZnIn₂S₄, 2) TiO₂, 3) ZnIn₂S₄/TiO₂, 4) Au/ZnIn₂S₄/TiO₂, and 5) Ag/ZnIn₂S₄/TiO₂ after UV–vis irradiation for 4 h. Reproduced with permission.188 d) Proposed VB and CB alignment for the anatase/rutile interface. Reproduced with permission.197 Copyright 2013, Nature Publishing Group. e) Schematic {001}/{101} surface heterojunction. Reproduced with permission.199 Copyright 2014, American Chemical Society. f) Schematic of CO₂ photoreduction over the CsPbBr₃ QD/GO photocatalyst. Reproduced with permission.202 Copyright 2017, American Chemical Society.

Figure 14

a) Scanning tunneling microscope (STM) image of CO₂ molecules adsorbed on TiO₂ (110) plane. Reproduced with permission.206 Copyright 2011, American Chemical Society. b) Schematic showing the formation of coordinatively unsaturated ZnAl‐LDH nanosheets; c) TEM image of coordinatively unsaturated ZnAl‐LDH nanosheets; d) charge density distribution for the valence band maximum of V_o‐doped ZnAl‐LDH; e) time‐dependent CO yields on different ZnAl‐LDH samples. Reproduced with permission.159

Figure 15

a) Schematic of photocatalytic CO₂ reduction on nanostructured TiO₂ films deposited with Pt cocatalyst particles of varying sizes. Different alignments between TiO₂ band structure and Pt work function is suggested to be responsible for the observed different photocatalytic activities. Reproduced with permission.211 Copyright 2012, American Chemical Society. b) High‐resolution TEM image of an Au−Cu nanoparticle deposited on the TiO₂ surface as the cocatalyst for selectively reducing CO₂ to CH₄. Reproduced with permission.212 Copyright 2014. American Chemical Society. c) Schematic showing the M. thermoacetica–CdS hybrid system for the photosynthetic conversion of CO₂ to acetic acid. Reproduced with permission.213 Copyright 2016, American Association for the Advancement of Science.

Figure 16

a) Schematic of Z‐scheme photocatalytic mechanism. Reproduced with permission.216 b) Schematic of the Z‐scheme system for water splitting and CO₂ reduction by coupling Pt‐loaded metal sulfide and CoO_x/BiVO₄ using RGO as the solid state electron mediator. Reproduced with permission.220 Copyright 2014, American Chemical Society.