Direct conversion of CO2into carbon‐neutral fuels or industrial chemicals holds a great promise for renewable energy storage and mitigation of greenhouse gas emission. However, experimentally finding an electrocatalyst for specific final products with high efficiency and high selectivity poses serious challenges due to multiple electron transfer, complicated intermediates, and numerous reaction pathways in electrocatalytic CO2reduction. Here, an intrinsic descriptor that correlates the catalytic activity with the topological, bonding, and electronic structures of catalytic centers on M–N–C based single‐atom catalysts is discovered. The “volcano”‐shaped relationships between the descriptor and catalytic activity are established from which the best single‐atom catalysts for CO2reduction are found. Moreover, the reaction mechanisms, intermediates, reaction pathways, and final products can also be distinguished by this new descriptor. The descriptor can also be used to predict the activity of the single‐atom catalysts for electrochemical reactions such as hydrogen evolution, oxygen reduction and evolution reactions in fuel cells and water‐splitting. These predictions are confirmed by the experimental results for onset potential and Faraday efficiency. The design principles derived from the descriptors open a door for rational design and rapid screening of highly efficient electrocatalysts for CO2conversion as well as other electrochemical energy systems.
Electrochemical CO2reduction offers a compelling route to mitigate atmospheric CO2concentration and store intermittent renewable energy in chemical bonds. Beyond C1, C2+feedstocks are more desirable due to their higher energy density and more significant market need. However, the CO2‐to‐C2+reduction suffers from significant barriers of CC coupling and complex reaction pathways. Due to remarkable tunability over morphology/pore architecture along with great feasibility of functionalization to modify the electronic and geometric structures, carbon materials, serving as active components, supports, and promoters, provide exciting opportunities to tune both the adsorption properties of intermediates and the local reaction environment for the CO2reduction, offering effective solutions to enable CC coupling and steer C2+evolution. However, general design principles remain ambiguous, causing an impediment to rational catalyst refinement and application thrusts. This review clarifies insightful design principles for advancing carbon materials. First, the current performance status and challenges are discussed and effective strategies are outlined to promote C2+evolution. Further, the correlation between the composition, structure, and morphology of carbon catalysts and their catalytic behavior is elucidated to establish catalytic mechanisms and critical factors determining C2+performance. Finally, future research directions and strategies are envisioned to inspire revolutionary advancements.
more » « less- Award ID(s):
- 1804326
- NSF-PAR ID:
- 10368049
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Energy Materials
- Volume:
- 12
- Issue:
- 24
- ISSN:
- 1614-6832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
Conventional lithium-ion batteries are unable to meet the increasing demands for high-energy storage systems, because of their limited theoretical capacity. 1 In recent years, intensive attention has been paid to enhancing battery energy storage capability to satisfy the increasing energy demand in modern society and reduce the average energy capacity cost. Among the candidates for next generation high energy storage systems, the lithium sulfur battery is especially attractive because of its high theoretical specific energy (around 2600 W h kg-1) and potential cost reduction. In addition, sulfur is a cost effective and environmentally friendly material due to its abundance and low-toxicity. 2 Despite all of these advantages, the practical application of lithium sulfur batteries to date has been hindered by a series of obstacles, including low active material loading, poor cycle life, and sluggish sulfur conversion kinetics. 3 Achieving high mass loading cathode in the traditional 2D planar thick electrode has been challenged. The high distorsion of the traditional planar thick electrodes for ion/electron transfer leads to the limited utilization of active materials and high resistance, which eventually results in restricted energy density and accelerated electrode failure. 4 Furthermore, of the electrolyte to pores in the cathode and utilization ratio of active materials. Catalysts such as MnO 2 and Co dopants were employed to accelerate the sulfur conversion reaction during the charge and discharge process. 5 However, catalysts based on transition metals suffer from poor electronic conductivity. Other catalysts such as transition metal dopants are also limited due to the increased process complexities. . In addition, the severe shuttle effects in Li-S batteries may lead to fast failures of the battery. Constructing a protection layer on the separator for limiting the transmission of soluble polysulfides is considered an effective way to eliminate the shuttle phenomenon. However, the soluble sulfides still can largely dissolve around the cathode side causing the sluggish reaction condition for sulfur conversion. 5 To mitigate the issues above, herein we demonstrate a novel sulfur electrode design strategy enabled by additive manufacturing and oxidative vapor deposition (oCVD). Specifically, the electrode is strategically designed into a hierarchal hollow structure via stereolithography technique to increase sulfur usage. The active material concentration loaded to the battery cathode is controlled precisely during 3D printing by adjusting the number of printed layers. Owing to its freedom in geometry and structure, the suggested design is expected to improve the Li ions and electron transport rate considerably, and hence, the battery power density. The printed cathode is sintered at 700 °C at N 2 atmosphere to achieve carbonization of the cathode during which intrinsic carbon defects (e.g., pentagon carbon) as catalytic defect sites are in-situ generated on the cathode. The intrinsic carbon defects equipped with adequate electronic conductivity. The sintered 3D cathode is then transferred to the oCVD chamber for depositing a thin PEDOT layer as a protection layer to restrict dissolutions of sulfur compounds in the cathode. Density functional theory calculation reveals the electronic state variance between the structures with and without defects, the structure with defects demonstrates the higher kinetic condition for sulfur conversion. To further identify the favorable reaction dynamic process, the in-situ XRD is used to characterize the transformation between soluble and insoluble polysulfides, which is the main barrier in the charge and discharge process of Li-S batteries. The results show the oCVD coated 3D printed sulfur cathode exhibits a much higher kinetic process for sulfur conversion, which benefits from the highly tailored hierarchal hollow structure and the defects engineering on the cathode. Further, the oCVD coated 3D printed sulfur cathode also demonstrates higher stability during long cycling enabled by the oCVD PEDOT protection layer, which is verified by an absorption energy calculation of polysulfides at PEDOT. Such modeling and analysis help to elucidate the fundamental mechanisms that govern cathode performance and degradation in Li-S batteries. The current study also provides design strategies for the sulfur cathode as well as selection approaches to novel battery systems. References: Bhargav, A., (2020). Lithium-Sulfur Batteries: Attaining the Critical Metrics. Joule 4 , 285-291. Chung, S.-H., (2018). Progress on the Critical Parameters for Lithium–Sulfur Batteries to be Practically Viable. Advanced Functional Materials 28 , 1801188. Peng, H.-J.,(2017). Review on High-Loading and High-Energy Lithium–Sulfur Batteries. Advanced Energy Materials 7 , 1700260. Chu, T., (2021). 3D printing‐enabled advanced electrode architecture design. Carbon Energy 3 , 424-439. Shi, Z., (2021). Defect Engineering for Expediting Li–S Chemistry: Strategies, Mechanisms, and Perspectives. Advanced Energy Materials 11 . Figure 1more » « less
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Abstract The development of low‐cost and efficient electrocatalysts for nitrogen reduction reaction (NRR) at ambient conditions is crucial for NH3synthesis and provides an alternative to the traditional Harber‐Bosch process. Herein, by means of density functional theory (DFT) computations, the catalytic performance of a series of single metal atoms supported on graphitic carbon nitride (g‐C3N4) for NRR is evaluated. Among all the candidates, the Gibbs free energy change of the potential‐determining step for five single‐atom catalysts (SACs), namely Ti, Co, Mo, W, and Pt atoms supported on g‐C3N4monolayer, is lower than that on the Ru(0001) stepped surface. In particular, the single tungsten (W) atom anchored on g‐C3N4(W@g‐C3N4) exhibits the highest catalytic activity toward NRR with a limiting potential of −0.35 V via associative enzymatic pathway, and can well suppress the competing hydrogen evolution reaction. The high NRR activity and selectivity of W@g‐C3N4are attributed to its inherent properties, such as significant positive charge and large spin moment on the W atom, excellent electrical conductivity, and moderate adsorption strength with NRR intermediates. This work opens up a new avenue of N2reduction for renewable energy supplies and helps guide future development of single‐atom catalysts for NRR and other related electrochemical process.
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null (Ed.)Electrochemical reduction of CO 2 into value-added fuels and chemicals driven by renewable energy presents a potentially sustainable route to mitigate CO 2 emissions and alleviate the dependence on fossil fuels. While tailoring the electronic structure of active components to modulate their intrinsic reactivity could tune the CO 2 reduction reaction (CO 2 RR), their use is limited by the linear scaling relation of intermediates. Due to the high susceptibility of the CO 2 RR to the local CO 2 concentration/pH and mass transportation of CO 2 /intermediates/products near the gas–solid–liquid three-phase interface, engineering catalysts’ morphological and interfacial properties holds great promise to regulate the CO 2 RR, which are irrelevant with linear scaling relation and possess high resistance to harsh reaction conditions. Herein, we provide a comprehensive overview of recent advances in tuning CO 2 reduction electrocatalysis via morphology and interface engineering. The fundamentals of the CO 2 RR and design principles for electrode materials are presented firstly. Then, approaches to build an efficient three-phase interface, tune the surface wettability, and design a favorable morphology are summarized; the relationship between the properties of engineered catalysts and their CO 2 RR performance is highlighted to reveal the activity-determining parameters and underlying catalytic mechanisms. Finally, challenges and opportunities are proposed to suggest the future design of advanced CO 2 RR electrode materials.more » « less
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Abstract The electrochemical CO2reduction reaction (CO2RR) to syngas represents a promising solution to mitigate CO2emissions and manufacture value‐added chemicals. Palladium (Pd) has been identified as a potential candidate for syngas production via CO2RR due to its transformation to Pd hydride under CO2RR conditions, however, the pre‐hydridized effect on the catalytic properties of Pd‐based electrocatalysts has not been investigated. Herein, pre‐hydridized Pd nanocubes (PdH0.40) supported on carbon black (PdH0.40NCs/C) are directly prepared from a chemical reduction method. Compared with Pd nanocubes (Pd NCs/C), PdH0.40NCs/C presented an enhanced CO2RR performance due to its less cathodic phase transformation revealed by the in situ X‐ray absorption spectroscopy. Density functional theory calculations revealed different binding energies of key reaction intermediates on PdH0.40NCs/C and Pd NCs/C. Study of the size effect further suggests that NCs of smaller sizes show higher activity due to their more abundant active sites (edge and corner sites) for CO2RR. The pre‐hydridization and reduced NC size together lead to significantly improved activity and selectivity of CO2RR.
