A Tandem Electrocatalysis of Sulfur Reduction by Bimetal 2D MOFs

Abstract

polysulfide by the host materials are conventional approaches to tackle the dissolution problem, [3,4] nonetheless, these could only partially alleviate the shuttle effect due to low fraction of host material in the electrode. Using "flooded" electrolyte could dilute the polysulfide solution thus somehow improves the redox kinetics due to enhanced mass/charge transfer, although it inevitably decreases the actual energy density of the battery. [1] Alternatively, recently established proactive electrocatalysis strategy could not only effectively improve the redox kinetics but also provide a "root" solution to solve the polysulfide shuttling. [5,6] The catalyzed sulfur redox reaction showed faster conversion kinetics, especially for the conversion of soluble LiPSs into the insoluble final products, thus preventing the continuous accumulation of LiPSs in electrolyte that is responsible for the shuttle effect. [7,8] An increasing number of electrocatalysts have been applied to Li-S batteries, [5,[9][10][11][12] however, systematic study on the kinetic and underlying basis to evaluate the exact role of catalyst addressing the PS shuttling issues is still overlooked. The classic electrocatalysis process is mostly referred to as heterogeneous catalytic reaction [13] -catalyzing a specific reaction instead of a series of reactions-as exemplified by the wellestablished electrocatalysis in fuel cells and water splitting such as the oxygen reduction reaction, oxygen evolution reaction (OER), and hydrogen evolution reaction. [14,15] The electrolysis of sulfur reduction reaction is much more complicated due to the consecutive multistep reduction reaction of sulfur, which brought huge challenge for the design of catalyst. [16] The diversified intermediate products of sulfur species would dynamically disproportionate or centralize in the electrolyte solution. [17] Moreover, one specific sulfur intermediate acts as both the reactant and product for the adjacent consecutive reductions, indicating that the sulfur species in the electrocatalysis reactions are highly conjugated and coupled. In addition to the multistep conversion reactions, the sulfur conversion reaction also involves multiphase transition because the long-chain polysulfides (Li 2 S x , 4 ≤ x ≤ 8) are soluble whereas the shortchain polysulfides (Li 2 S x , 1 ≤ x ≤ 2) is insoluble in the electrolyte. [18] Only a few catalysis works have paid attention to such complicated conversion reactions but mostly focused on the The diversity and coupling of sulfur redox intermediates and its associated solid-liquid-solid multiphase conversion mechanism pose great challenges in designing a proper electrocatalysts for Li-S batteries. In this report, it is proposed that an ideal catalyst should possess two catalytic centers which catalyze liquid-liquid conversion and liquid-solid conversion in tandem within one structure, with the use of 2D MOF nanosheets with different metal centers for validation. It is uncovered that the Ni-MOF is more effective in catalyzing the reduction of lo...

Modulating Bond Interactions and Interface Microenvironments between Polysulfide and Catalysts toward Advanced Metal–Sulfur Batteries

“…For instance, the 2D integrated bimetal CoNi‐MOF, where the Ni and Co sites were connected via an O bridge (Figure 8i). [ 140 ] The photon energy of Co and Ni in CoNi‐MOF was changed compared to that in single‐metal MOFs, confirming the charge transfer between the Ni–O–Co bridge (Figure 8j). This electron redistribution endowed the unoccupied 3 d e g state of Ni or Co, which would enhance the electrocatalytic activities for polysulfide conversion.…”

“…i-k) The electron transfer between Ni and Co in the CoNi-MOF and corresponding influence on the activation energy barrier for polysulfide catalytic conversion. Reproduced with permission [140]. Copyright 2021, Wiley-VCH.…”

A N-Rich porous carbon nanocube anchored with Co/Fe dual atoms: an efficient bifunctional catalytic host for Li–S batteries

“…Peaks 2.30 V and 2.05 V are related to the reduction S 8 to Li 2 S 4 and following conversion from Li 2 S 4 to Li 2 S, and anodic peaks at 2.31 and 2.4 V are attributed to the oxidation of Li 2 S to Li 2 S 4 and Li 2 S 4 to S 8 , which suggests that the Co and Fe atom catalysts can promote the long-chain and short-chain polysulfide conversion reaction. 65,66 While anchoring Co and Fe atoms on NC, positive shifts in the cathodic peaks and negative shifts in the anodic peaks were generated, which illustrates the enhanced catalytic activity of the S@CoFe-NC on the conversion reaction of the sulfide species. When the symmetric cells comprising of two same NC, Co-NC, Fe-NC and CoFe-NC electrodes using 0.5 M Li 2 S 6 -containing electrolyte were employed, a similar result could be revealed, that is, fast reaction kinetics and high redox charge storage capacity of the CoFe-NC electrode (Fig.…”

Two-dimensional materials for electrocatalysis and energy storage applications

“…Meng et al synthesized single-metal and bimetal MOFs based on Ni and Co ions by controlling the ratios between BDC ligands and Ni or Co ions. 184 The obtained CoNi-MOF displays a flexible morphology of 2D nanosheets, as depicted in the SEM images (Fig. 22a).…”

“…22a). 184 AFM imaging of the CoNi-MOF indicated that the nanosheets had a mean thickness of 4.6 nm (Fig. 22b).…”