Intrinsic Carbon‐Defect‐Driven Electrocatalytic Reduction of Carbon Dioxide

135

104

Renewable‐electricity‐powered electrocatalytic CO2 reduction reactions (CO2RR) have been identified as an emerging technology to address the issue of rising CO2 emissions in the atmosphere. While the CO2RR has been demonstrated to be technically feasible, further improvements in catalyst performance through active sites engineering are a prerequisite to accelerate its commercial feasibility for utilization in large CO2‐emitting industrial sources. Over the years, the improved understanding of the interaction of CO2 with the active sites has allowed superior catalyst design and subsequent attainment of prominent CO2RR activity in literature. This review tracks the evolution of the understanding of CO2RR active sites on different electrocatalysts such as metals, metal‐oxides, single atoms, metal‐carbon, and subsequently metal‐free carbon‐based catalysts. Despite the tremendous research efforts in the field, many scientific questions on the role of various active sites in governing CO2RR activity, selectivity, stability, and pathways are still unanswered. These gaps in knowledge are highlighted and a discussion is set forth on the merits of utilizing advanced in‐situ and operando characterization techniques and machine learning (ML). Using this technique, the underlying mechanisms can be discerned, and as a result new strategies for designing active sites may be uncovered. Finally, this review advocates an interdisciplinary approach to discover and design CO2RR active sites (rather than focusing merely on catalyst activity) in a bid to stimulate practical research for industrial application.

Section: Active Sites In Carbon‐based Catalystssupporting

confidence: 76%

A Disquisition on the Active Sites of Heterogeneous Catalysts for Electrochemical Reduction of CO₂ to Value‐Added Chemicals and Fuel

Daiyan

Saputera

Masood

et al. 2020

135

104

“…The ZnAl-LDH nanosheets show a typical LDH phase with clear reflection peaks at 11.52°, 23.35°, 34.32°, and 39.04°, corresponding to the (003), (006), (012), and (113) reflection of LDHs. The highest ratio of D band to G band of MC-NS (I = 1.11) compared with C-NS (I = 1.03) and C-NP (I = 0.95) indicates more defects on MC-NS, [30,31] which can be verified by nano-Fourier transform infrared (nano-FTIR) spectra. After subsequent pyrolysis and pickling treatment, the original peaks were replaced with two apparent reflections at 44° and 51°, which correspond to the (111) and (200) crystal planes of Co phase.…”

mentioning

confidence: 68%

Polysulfide Confinement and Highly Efficient Conversion on Hierarchical Mesoporous Carbon Nanosheets for Li–S Batteries

Chen

et al. 2019

109

performance of Li-S batteries is still restricted by poor conductivity of sulfur itself and discharge products (Li 2 S/Li 2 S 2 ), which makes hard conversion of sulfur. Additionally, the common shuttle effect of intermediate polysulfide also leads to an obvious capacity loss during charging and discharging. [10][11][12][13][14][15] Thus, new strategies are urgent in addressing the problems of high-capacity sulfur cathodes for Li-S batteries.To design high-performance cathode, dispersing nanostructured sulfur particles on carbon materials have attracted particular interests, which not only stabilize sulfur and intermediates but also promotes electron transfer during charging/ discharging. [16][17][18][19][20][21] Although carbon materials have been the first choice to disperse sulfur particles, the nonpolar carbon suffers a weak binding force to the intermediate product, which results in serious shuttle effect and making it difficult to reuse the polysulfide (Scheme 1a). To address these issues, plenty of work has begun to focus on the modification of nonpolar carbon by introducing heteroatoms (B, N, O, or S). [22][23][24][25] This is because the lone pair of electrons on the heteroatoms forms an electrostatic attraction (Li bond) with the polysulfide, which can effectively prevent shuttle of polysulfides. [26,27] However, the high loading and the easy aggregation of sulfur are hard to be balanced, since sulfur are getting easier to migrate and aggregate at surface of carbon supports, which lead to the low concentration loading or limited sulfur utilization. Therefore, sulfur confinement and simultaneous highly efficient conversion remains a challenge for the next-generation Li-S batteries.Herein, we develop honeycomb-like mesoporous carbon nanosheets (MC-NS) with abundant defects and Co-N-C catalytic site as efficient host for simultaneously confinement and efficient conversion of polysulfides, which shows significantly enhanced performance for Li-S batteries cathode (Scheme 1b). The as-prepared MC-NS exhibits an excellent 2D conductive network with a high specific surface area of 335.4 m 2 g −1 and abundant mesopores that provide high storage space and expansion space for sulfur. Moreover, the density functional theory (DFT) calculation and experiments manifest that the presence of abundant defects on the carbon skeleton MC-Ns Lithium-sulfur (Li-S) batteries have attracted increasing attention due to their extremely high theoretical specific capacity and a promising power density. However, practical applications of Li-S batteries are still limited by the relatively low performance, owing to poor conductivity of sulfur itself and discharge products (Li 2 S/Li 2 S 2 ) as well as the shuttle effect of the intermediate polysulfide. Herein, honeycomb-like mesoporous Co, N-doped carbon nanosheets (MC-NS) with a high specific surface area and abundant defects are developed which, simultaneously enable polysulfide confinement and highly efficient conversion. Moreover, density functional theory calculations and experime...

“…The nature of edge defects is also important, with Deng et al finding that armchair type edges in graphene nanoribbons were inactive toward the ORR, whereas zig‐zag edges showed activity with low energy barriers for the mechanistic steps of the ORR . Wang et al recently discovered that intrinsic carbon defects could lead to high activity for CO 2 reduction, concluding that sp 2 defects (pentagonal and octagonal) were more important than edge defects, and that perhaps the importance of heteroatom doping in metal‐free catalysis needs to be rethought. Very recent work from Jia et al selectively created edge pentagonal defects in highly ordered pyrolytic graphite (HOPG) by removing pyridinic N atoms and showed excellent activity toward the ORR, again suggesting that the importance of heteroatom doping might be over stated in the previous understanding of the community.…”

Section: Heteroatom Doping Defects and Hybridizationmentioning

confidence: 99%

“…Therefore, we suggest the use of advanced surface modification methods to introduce the co‐doping effect in 3D carbons for increasing the active sites and introducing the synergistic effect for ORR. Note that the activation of π electrons in carbon gets affected if the lone‐pair of electrons from N is neutralized by the vacant orbital of B that overall affects the co‐doping efficiency and the performance in ORR . It is therefore important to note that doped heteroatoms such as N and B are separately bonded to C and not bonded with each other.…”

Section: D Carbons For the Orrmentioning

confidence: 99%

“…Apart from the N‐induced positively charged C atoms, commonly considered as the active center for both ORR and OER, P species are also key in promoting the electrocatalytic activity. When C 3 N 4 is doped with P, three out of its five valence electrons form covalent bonds with neighbors N to adopt a forced planar structure and the remaining lone pair can delocalize to the π‐conjugated heptazine ring, leaving positively charged P centers …”

Section: D Carbons For Bifunctional Oxygen Electrocatalysis (Oer Andmentioning

confidence: 99%

See 1 more Smart Citation

3D Carbon Materials for Efficient Oxygen and Hydrogen Electrocatalysis

Sobrido

Jervis

Periasamy

et al. 2019

125

Sustainable energy production at an acceptable cost is key for its widespread application. At present, noble metals and metal oxides are the most widely used for electrocatalysis, but they suffer from low selectivity, poor durability, and scarcity. Because of this, metal‐free carbons have become the subject of great interest as promising alternative electrocatalysts for energy conversion and storage devices, and remarkable progress has been accomplished in the advance of metal‐free carbons as electrocatalysts for renewable energy technologies. Particularly interesting are 3D porous carbon architectures, which exhibit outstanding features for electrocatalysis applications, including broad range of active sites, interconnected porosity, high conductivity, and mechanical stability. This review summarizes the latest advances in 3D porous carbon structures for oxygen and hydrogen electrocatalysis. The structure–performance relationship of these materials is consequently rationalized and perspectives on creating more efficient 3D carbon electrocatalysts are suggested.