ACS Applied Energy Materials
ISSN / EISSN : 2574-0962 / 2574-0962
Published by: American Chemical Society (ACS) (10.1021)
Total articles ≅ 3,993
Latest articles in this journal
ACS Applied Energy Materials; doi:10.1021/acsaem.1c00685
Analyte concentration effects on the first reduction process of methyl viologens and diquat redox flow battery electrolytes were examined by cyclic voltammetry in aqueous media. A simple one-electron transfer mechanism to form radical cations was detected for diquat, 4,4′-dimethyl diquat, and bis(3-trimethylammonio)-propyl viologen compounds. The radical cations attach to the electrode surface when the source of their electrogeneration is methyl viologen molecules bearing PF6– ions as a counterpart. However, this inner sphere reduction mechanism was not observed in methyl viologen having an I– counterion. For the latter compound, as well as for 5,5′-dimethyl diquat and 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium, a piece of experimental evidence for unexpected, fast, and reversible dimerization interactions between their electrogenerated radical cations is presented. To get information on these bimolecular interactions, a screening methodology (using different levels of theory) was employed in finding suitable dimeric structures and their related interaction energies. By using diquat as a reference system, a relationship between calculated interaction energies and the corresponding experimental dimerization constants was obtained. The examination of redox-active molecules using this experimental and theoretical approach will allow a better selection of redox flow battery electrolytes.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01255
Cathode active materials (CAMs) in state-of-the-art lithium-ion batteries are mostly lithium-transition-metal oxides such as Li(NixCoyMnz)O2 (x + y + z = 1). To achieve optimum cycling stability and performance of the cathode, the extent of degradation processes and side reactions between CAMs and liquid or solid electrolytes has to be minimized. For this purpose, various coating strategies for CAMs have been developed in recent years. The underlying mechanism of the protective function of nanoscale coatings and their role for the enhanced cycling performance are mostly unclear, which is often based on incomplete characterization of the coating. Only a few analytical methods, such as X-ray diffraction, scanning electron microscopy/energy-dispersive X-ray analysis, or X-ray photoelectron spectroscopy, have frequently been used in recent years, which often cannot provide enough information for a reliable and consistent picture of the very thin coating. For this reason, we demonstrate a systematic study on the analytical characterization of coated CAM using additional analytical methods. NCM622 coated with TiO2 by atomic layer deposition is used as a model system and analyzed with SEM/EDX, focused ion beam scanning electron microscopy, scanning transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, low-energy ion scattering, and time-of-flight secondary ion mass spectrometry. The results highlight the advantages and disadvantages of each analytical method for the analysis of typical CAM coatings. The results demonstrate that a combination of the different methods is essential to understand CAM coatings and their properties in full detail.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01573
The applications of Na metal batteries (SMBs) are restricted owing to the capacity attenuation and safety hazards during the cycling process, while a rational design of the electrolyte is critical on solving this problem. In this work, an electrolyte is designed by adding 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE) into a 3.8 M sodium bis(fluorosulfonyl)imide/1,2-dimethoxyethane (NaFSI/DME) electrolyte, forming the localized high-concentration electrolyte (LHCE) for constructing a stable solid electrolyte interface (SEI) for SMBs. Ab initio molecular dynamics (AIMD) results indicate that the solvation degree of Na+ ions with DME molecules in LHCE is lower than that in HCE, which leads to more FSI– anions but less DME molecules to decompose on the Na metal anode. And the TTE could also decompose on the Na metal anode, which synergistically builds a NaF-rich compact SEI with low surface resistance and good mechanical property so that it is favorable for the transportation of Na+ ions and suppression of the Na dendrite growth. Therefore, the optimized LHCE electrolyte in SMBs exhibits an outstanding electrochemical performance. This study provides an updated perspective on the understanding and design of localized high-concentration electrolytes for SMBs.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c00745
Hybrid nanostructures enriched in a large number of electroactive sites are fascinating electrode materials for the high-performance supercapacitors (SCs). Transition-metal nitrides attract a lot of attention due to their excellent electrochemical performance for energy-related applications. Herein, a three-dimensional (3D) structure is engineered by tailoring nickel–cobalt nitride (NiCo–N) nanoparticles anchored on carbon nanocoils/nickel foam (CNCs/NF) substrates through a facile solvothermal reaction and subsequent annealing under an NH3 atmosphere. The 3D CNCs/NF scaffolds offer a high surface area for the growth of NiCo–N nanoparticles, consequently resulting in the increased electroactive sites for redox reactions. The optimal binder-free hybrid composite electrode (NiCo–N/CNCs/NF at 600 °C) yields a specific capacitance of 5235 F g–1 at 1 A g–1 and a rate capability of 86% at 50 A g–1, and 95.6% capacitance is retained after 3000 cycles. This remarkable electrochemical performance solely corresponds to the synergistic effect of NiCo–N and CNCs/NF, thereby achieving efficacious redox reactions and desirable electronic conductivity. Moreover, the appealing electrochemical performance of the NiCo–N/CNCs/NF hybrid composite paves the way as a promising candidate for SC electrodes.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01207
Bimetallic alloys are important catalysts with enhanced catalytic activities and product selectivities. However, the phase dependence of catalytic activity in bimetallic alloys afford contradictory characters in that one phase catalyzes the main reaction and the other catalyzes the side reaction; this aspect of bimetallic alloy catalysts has not been investigated. In this study, we systematically synthesized NiSn alloys from Ni, which is catalytically active in hydrogen generation, and Sn, which is catalytically active in electrochemical CO2 reduction. The thus-prepared alloys were applied in catalyzing the phase-dependent electrochemical CO2 reduction, and the formate generation mechanism was elucidated. The Faradaic efficiency of formate was found to increase with increasing Sn atomic concentration, and Ni3Sn4 showed higher catalytic activity than only Sn for electrochemical CO2 reduction. Density functional theory calculations revealed that Ni can additionally provide catalytically active sites for formate generation in a suitable phase. Thus, our investigation brings a better understanding of the catalytic activities of bimetallic alloys prepared from metals with different characters for electrochemical CO2 reduction.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01020
Metal oxide charge-transport layers (CTLs) are known to influence the properties and performance of organometal halide perovskite solar cells (PSCs). Accordingly, this work demonstrates, in detail, the crystalline grain growth mechanism of CH3NH3PbI3 (MAPbI3) depending on the hydrophilicity of CTLs such as compact (c)-TiO2, mesoporous (mp)-TiO2, SnO2, and NiOx. Importantly, smaller water contact angles of CTLs (11.5° for SnO2; 21.4° for mp-TiO2; 27.8° for NiOx; and 30.7° for c-TiO2) were linked to larger average grain sizes of a top-layered perovskite film (308.2 nm for SnO2; 266.4 nm for mp-TiO2; 209.7 nm for NiOx; and 185.4 nm for c-TiO2), indicating ‘hydrophilic surface-driven crystalline grain growth’ of MAPbI3 on metal oxides. Furthermore, by estimating the solubility parameter (δ) of CTLs, we explain that, when Δδ = δCTL – δsolvent is large, the MAPbI3 grain size increases because of a limited chance of nucleation during the antisolvent-assisted one-step coating process. However, it is notable that the hydrophilic surface of CTLs may induce instability of MAPbI3 under humidity. Finally, the highest power conversion efficiency (∼19.03%) was obtained when SnO2 served as an electron-transport layer for the planar heterojunction PSCs.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01647
To improve the intrinsic electrochemical properties of LiCoPO4 (LCP), a promising high-energy cathode material, we explored the effects of Zn-doping on the material structure and electrochemical properties of LCP. During the delithiation/lithiation process, besides the reported intermediate phase Li2/3Co1–nZnnPO4, an unreported intermediate phase Li1/2Co1–nZnnPO4 was observed by electrochemical test and in situ X-ray diffraction. The reason for the appearance of the intermediate phase was revealed by Gibbs free energy diagram. Two conjectured configurations of Li1/2Co1–nZnnPO4 were proposed. The redox potential and the conductivity of Zn-doped LCP will increase with doping concentration, resulting in improved rate capability, cycle life, and energy efficiency.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01025
Metal oxide nanomaterials have increasing significance and broad applications in catalysis, ranging from support materials to active catalysts. In this paper, we have demonstrated a facile synthetic strategy to create disordered, high-surface-area metal oxide nanomaterials using biomineralization-inspired methods. Using protamine as a protein template, a range of TiOx–SiOx nanomaterials were synthesized and implemented for catalytic CO2 thermal reduction reactions. The modularity of synthetic options afforded via biomineralization enables increases in surface area, which are ideally suited for subsequent modification to regulate catalytic performance. All materials were thoroughly characterized using a suite of synchrotron scattering and spectroscopic methods. Through these techniques, we have demonstrated that protamine-induced biomineralization results in largely disordered materials with changes in the local atomic structure dependent on the applied synthetic conditions. Protamine removal with treatment under acidic conditions greatly increased material surface area while causing measurable changes in the structure as revealed by X-ray absorption spectroscopy. Upon subsequent hydrogenation, Ti- and Si-based defects were induced in the materials while the disordered nature of the material was still largely retained. Furthermore, we found that the incorporation of Si into TiOx was able to mitigate the known anatase to rutile phase change during the reaction while stabilizing the defect sites. The synthetic strategies described in this work are expected to be translatable to other metal oxide nanomaterial chemical structures, providing a means to control catalytic properties using benign synthetic strategies.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c01405
The lithiation mechanism of tin nanoparticle-based negative electrodes is reported and systematically studied via operando 7Li nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) combined with ex situ 119Sn magic-angle spinning (MAS) NMR. Besides the formation of the Sn-rich phases Li2Sn5 and LiSn, also the Li-richer phase Li7Sn3 is observed in good agreement with the structural evolution of the binary Li–Sn phase diagram. However, the structural investigations using ex situ 119Sn MAS NMR clearly reveal the formation of a disordered LixSn phase with increasing lithiation, possessing the structural fingerprints of Li7Sn3 with no long-range order and a body-centered cubic (bcc) packing of Sn (from XRD). Thus, in contrast to previous studies relying on 7Li NMR only, the formation of any of the Li-rich bulk crystalline Li–Sn phases, Li13Sn5, Li5Sn2, Li7Sn2, and Li17Sn4, could not be confirmed from 119Sn MAS NMR, showing that these Li–Sn phases are not formed under electrochemical operation. From a more general point of view, our approach using ex situ 119Sn MAS NMR demonstrates the possibilities of using the heavier framework ions as reporters of the local structural environments in negative electrodes. This relies on the sensitivity of the isotropic 119Sn shift with respect to the first and second atomic coordination environments, which provides a powerful source of complementary structural information to the typically performed operando 7Li NMR and XRD measurements.
ACS Applied Energy Materials; doi:10.1021/acsaem.1c00714
Lithium-ion batteries have achieved commercial success; however, work remains to increase the capacity and safety of both the anode and cathode electrodes. Organic anodes have the potential to replace conventional graphite anodes because they are abundant, safe, and high-capacity materials. Superlithiated organic anodes achieve capacities in excess of 1500 mA h g–1; however, the mechanism of superlithiation and how it relates to different materials is an open question. Here, we disclose a pyrene-fused azaacene polymer that undergoes superlithiation and exhibits a continuous activation process, whereby the capacity increases with the number of cycles, reaching values up to 1775 mA h g–1 (1535 mA h g–1, subtracting the carbon additive contribution). This high performance is attributed to the stability and extended conjugation afforded by the polymer design. Ex situ studies suggest cycling results in deformation of the electrode structure, from an amorphous electrode material to one with increased crystallinity and sp2 character. Importantly, this superlithiated electrode maintains the same capacity across a 10-fold increase in rate during the activation process, showing that the kinetic limitations of superlithiation can be overcome and suggesting that commercial practical superlithiation anodes are within reach.