Carbon Nanotube‐Encapsulated Noble Metal Nanoparticle Hybrid as a Cathode Material for Li‐Oxygen Batteries

Abstract

Although Li‐oxygen batteries offer extremely high theoretical specific energy, their practical application still faces critical challenges. One of the main obstacles is the high charge overpotential caused by sluggish kinetics of charge transfer that is closely related to the morphology of discharge products and their distribution on the cathode. Here, a series of noble metal nanoparticles (Pd, Pt, Ru and Au) are encapsulated inside end‐opened carbon nanotubes (CNTs) by wet impregnation followed by thermal annealing. The resultant cathode materials exhibit a dramatic reduction of charge overpotentials compared to their counterparts with nanoparticles supported on CNT surface. Notably, the charge overpotential can be as low as 0.3 V when CNT‐encapsulated Pd nanoparticles are used on the cathode. The cathode also shows good stability during discharge–charge cycling. Density functional theory (DFT) calculations reveal that encapsulation of “guest” noble metal nanoparticles in “host” CNTs is able to strengthen the electron density on CNT surfaces, and to avoid the regional enrichment of electron density caused by the direct exposure of nanoparticles on CNT surface. These unique properties ensure the uniform coverage of Li2O2 nanocrystals on CNT surfaces instead of localized distribution of Li2O2 aggregation, thus providing efficient charge transfer for the decomposition of Li2O2.

Long life rechargeable Li-O2 batteries enabled by enhanced charge transfer in nanocable-like Fe@N-doped carbon nanotube catalyst

“…Heteroatom doping or hybridization with metal species represents an efficient way to enhance the activity of carbon-based catalysts towards oxygen reduction reaction (ORR)/OER reactions [39][40][41][42]. The presence of heteroatoms (e.g., N) and metal species not only reduces the local work function on carbon surface for better O2 adsorption, but also changes the charge density on the carbon surface via electron transfer effect [16,[43][44][45][46][47][48][49][50][51]. Nevertheless, the understanding on the origin of their activity and the chemical nature of the reaction pathways is still quite limited, which hinders the development of high-performance cathode catalysts.…”

Palladium-Filled Carbon Nanotubes Cathode for Improved Electrolyte Stability and Cyclability Performance of Li-O2Batteries

“…This also confirms retained catalytic activity of Pd when encapsulated inside the CNTs, consistent with previously observations. 36,42,35 In addition, the presence of Pd nanocatalysts in both Pd-filled and Pd-coated CNTs shifts the onset of ORR peak of pristine CNTs from 2.8 V to 2.9 V, showing enhanced cathodic activity. 43 The Pd-filled CNTs also demonstrated 36% increase in first discharge capacity over Pd-coated.…”

“…It is considered that the presence of Pd inside the CNTs strengthens the π electron density on the CNT surface yielding homogeneously distributed nucleation sites for Pd-filled CNTs compared to heterogeneously distributed nucleation sites for Pd-coated CNTs. 35 This delocalization of Li 2 O 2 seeding sites contributes to intimate contact between Li 2 O 2 and CNTs and helps promote the formation of high surface discharge products. At voltage exceeding 3.7 V vs Li/Li + Pd-coated CNTs shows the highest rate of oxidation of electrolyte and electrolyte decomposition products.…”

“…Advanced approaches such as dispersing catalysts in polymeric membrane over the oxygen electrode, chemically binding the catalysts on the surface of CNTs by atomic layer deposition, and encapsulation of catalysts inside carbon cathodes have been developed to overcome the deactivation and agglomeration of catalysts in Li-O 2 batteries. [33][34][35][36][37] In this study, we report the effect of filling CNT with Pd nanoparticle catalysts to assist the ORR/OER reactions in Li-O 2 batteries while minimally destabilizing the electrolyte. To assess the effectiveness of this approach, various electrochemical, spectroscopic, and microscopic techniques have been used.…”

Integrated Porous Cathode made of Pure Perovskite Lanthanum Nickel Oxide for Nonaqueous Lithium–Oxygen Batteries

“…Fore xample,h ierarchical micron-sized mesoporous/macroporous graphene has been used as ac athode material in nonaqueousl ithium-oxygen batteries and delivered an excellent specific discharge capacity of 13 700 mAh g À1 . [5] In addition, many other carbon-based materials have been used for cathodes,w hich include carbon powder, [6] carbon nanotubes, [7] and ordered carbons. [8] However, recent detailed studies of nonaqueous lithium-oxygen batteries suggested the serious instability issue of carbon-based materials:c arbon can react with the discharge product Li 2 O 2 , [9] promote electrolyted ecompositiond uring discharge-chargec ycles, [10] and evend ecompose at charge voltages higher than 3.5 V. [9a] This instability will lead to the formationo ft he irreversible product lithium carbonate (Li 2 CO 3 )o nt he electrode surface.T his product is an insulator and will increase the electron transport resistance to lead to adeterioration of the round-trip efficiency and as horter cycle life.…”