High-Energy-Density Hydrogen-Ion-Rocking-Chair Hybrid Supercapacitors Based on Ti3C2Tx MXene and Carbon Nanotubes Mediated by Redox Active Molecule

MXenes have emerged as promising high-volumetric-capacitance supercapacitor electrode materials, whereas their voltage windows are not wide. This disadvantage prevents MXenes from being made into aqueous symmetric supercapacitors with high energy density. To attain high energy density, constructing asymmetric supercapacitors is a reliable design choice. Here, we propose a strategy to achieve high energy density of hydrogen ion aqueous-based hybrid supercapacitors by integrating a negative electrode of Ti3C2Tx MXene and a positive electrode of redox-active hydroquinone (HQ)/carbon nanotubes. The two electrodes are separated by a Nafion film that is proton permeable in H2SO4 electrolyte. Upon charging/discharging, hydrogen ions shuttle back and forth between the cathode and anode for charge compensation. The proton-induced high capacitance of MXene and HQ, along with complementary working voltage windows, simultaneously enhance the electrochemical performance of the device. Specifically, the hybrid supercapacitors operate in a 1.6 V voltage window and deliver a high energy density of 62 Wh kg(-1), which substantially exceeds those of the state-of-the-art aqueous asymmetric supercapacitors reported so far. Additionally, the device exhibits excellent cycling stability and the all-solid-state planar hybrid supercapacitor displays exceptional flexibility and integration for bipolar cells to boost the capacitance and voltage output. These encouraging results provide the possibility of designing high-energy-density noble-metal-free asymmetric supercapacitors for practical applications

Surface Functional Groups and Interlayer Water Determine the Electrochemical Capacitance of Ti₃C₂<i>T</i>_<i>x</i> MXene

MXenes, an emerging class of conductive two-dimensional materials, have been regarded as promising candidates in the field of electrochemical energy storage. The electrochemical performance of their representative Ti₃C₂<i>T</i>_<i>x</i>, where <i>T</i> represents the surface termination group of F, O, or OH, strongly relies on termination-mediated surface functionalization, but an in-depth understanding of the relationship between them remains unresolved. Here, we studied comprehensively the structural feature and electrochemical performance of two kinds of Ti₃C₂<i>T</i>_<i>x</i> MXenes obtained by etching the Ti₃AlC₂ precursor in aqueous HF solution at low concentration (6 mol/L) and high concentration of (15 mol/L). A significantly higher capacitance was recognized in a low-concentration HF-etched MXene (Ti₃C₂<i>T</i>_<i>x</i>–6M) electrode. <i>In situ</i> Raman spectroscopy and X-ray photoelectron spectroscopy demonstrate that Ti₃C₂<i>T</i>_<i>x</i>–6M has more components of the −O functional group. In combination with X-ray diffraction analysis, low-field ¹H nuclear magnetic resonance spectroscopy in terms of relaxation time unambiguously underlines that Ti₃C₂<i>T</i>_<i>x</i>–6M is capable of accommodating more high-mobility H₂O molecules between the Ti₃C₂<i>T</i>_<i>x</i> interlayers, enabling more hydrogen ions to be more readily accessible to the active sites of Ti₃C₂<i>T</i>_<i>x</i>–6M. The two main key factors (<i>i.e.</i>, high content of −O functional groups that are involved bonding/debonding-induced pseudocapacitance and more high-mobility water intercalated between the MXene interlayers) simultaneously account for the superior capacitance of the Ti₃C₂<i>T</i>_<i>x</i>–6M electrode. This study provides a guideline for the rational design and construction of high-capacitance MXene and MXene-based hybrid electrodes in aqueous electrolytes

Nanosecond Laser Confined Bismuth Moiety with Tunable Structures on Graphene for Carbon Dioxide Reduction

Substrate-supported catalysts with atomically dispersed metal centers are promising for driving the carbon dioxide reduction reaction (CO2RR) to produce value-added chemicals; however, regulating the size of exposed catalysts and optimizing their coordination chemistry remain challenging. In this study, we have devised a simple and versatile high-energy pulsed laser method for the enrichment of a Bi “single atom” (SA) with a controlled first coordination sphere on a time scale of nanoseconds. We identify the mechanistic bifurcation routes over a Bi SA that selectively produce either formate or syngas when bound to C or N atoms, respectively. In particular, C-stabilized Bi (Bi–C) exhibits a maximum formate partial current density of −29.3 mA cm–2 alongside a TOF value of 2.64 s–1 at −1.05 V vs RHE, representing one of the best SA-based candidates for CO2-to-formate conversion. Our results demonstrate that the switchable selectivity arises from the different coupling states and metal-support interactions between the central Bi atom and adjacent atoms, which modify the hybridizations between the Bi center and *OCHO/*COOH intermediates, alter the energy barriers of the rate-determining steps, and ultimately trigger the branched reaction pathways after CO2 adsorption. This work demonstrates a practical and universal ultrafast laser approach to a wide range of metal–substrate materials for tailoring the fine structures and catalytic properties of the supported catalysts and provides atomic-level insights into the mechanisms of the CO2RR on ligand-modified Bi SAs, with potential applications in various fields