Electrochemical capacitors (ECs) that store charge based on the pseudocapacitive mechanism combine high energy densities with high power densities and rate capabilities. 2D transition metal carbides (MXenes) have been recently introduced as high‐rate pseudocapacitive materials with ultrahigh areal and volumetric capacitances. So far, 20 different MXene compositions have been synthesized and many more are theoretically predicted. However, since most MXenes are chemically unstable in their 2D forms, to date only one MXene composition, Ti3C2T
MXenes, two‐dimensional (2D) transition metal carbides and/or nitrides, possess surface termination groups such as hydroxyl, oxygen, and fluorine, which are available for surface functionalization. Their surface chemistry is critical in many applications. This article reports amine functionalization of Ti3C2T
- Award ID(s):
- 1804285
- NSF-PAR ID:
- 10456286
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 7
- Issue:
- 6
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract x , has shown stable pseudocapacitive charge storage. Here, a cation‐driven assembly process is demonstrated to fabricate highly stable and flexible multilayered films of V2CTx and Ti2CTx MXenes from their chemically unstable delaminated single‐layer flakes. The electrochemical performance of electrodes fabricated using assembled V2CTx flakes surpasses Ti3C2Tx in various aqueous electrolytes. These electrodes show specific capacitances as high as 1315 F cm−3and retain ≈77% of their initial capacitance after one million charge/discharge cycles, an unprecedented performance for pseudocapacitive materials. This work opens a new venue for future development of high‐performance supercapacitor electrodes using a variety of 2D materials as building blocks. -
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Abstract The chemical stability of 2D MXene nanosheets in aqueous dispersions must be maintained to foster their widespread application. MXene nanosheets react with water, which results in the degradation of their 2D structure into oxides and carbon residues. The latter detrimentally restricts the shelf life of MXene dispersions and devices. However, the mechanism of MXene degradation in aqueous environment has yet to be fully understood. In this work, the oxidation kinetics is investigated of Ti3C2T
x and Ti2CTx in aqueous media as a function of initial pH values, ionic strengths, and nanosheet concentrations. The pH value of the dispersion is found to change with time as a result of MXene oxidation. Specifically, MXene oxidation is accelerated in basic media by their reaction with hydroxyl anions. It is also demonstrated that oxidation kinetics are strongly dependent on nanosheet dispersion concentration, in which oxidation is accelerated for lower MXene concentrations. Ionic strength does not strongly affect MXene oxidation. The authors also report that citric acid acts as an effective antioxidant and mitigates the oxidation of both Ti3C2Tx and Ti2CTx MXenes. Reactive molecular dynamic simulations suggest that citric acid associates with the nanosheet edge to hinder the initiation of oxidation. -
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m H2SO4electrolyte as follows: V‐Ti4N3Tx> Cr‐Ti4N3Tx> Mo‐Ti4N3Tx> Mn‐Ti4N3Tx> pristine Ti4N3Txwith overpotentials as low as 330 mV at −10 mA cm−2with a charge‐transfer resistance of 70 Ω. Scanning electrochemical microscopy (SECM) reveals the electrochemical activity of individual MXene flakes. The SECM data corroborate the bulk HER activity trend for M‐Ti4N3Txas well as provide the first experimental evidence that HER results from catalysis on the MXene basal plane. These electrocatalytic results demonstrate a new pathway to tune the electrochemical properties of MXenes for water splitting and related electrochemical applications. -
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Abstract MXenes, a family of 2D transition‐metal carbides and nitrides, have excellent electrical conductivity and unique optical properties. However, MXenes oxidize in ambient conditions, which is accelerated upon heating. Intercalation of water also causes hydrolysis accelerating oxidation. Developing new tools to readily characterize MXenes’ thermal stability can enable deeper insights into their structure–property relationships. Here, in situ spectroscopic ellipsometry (SE) is employed to characterize the optical properties of three types of MXenes (Ti3C2T
x , Mo2TiC2Tx , and Ti2CTx ) with varied composition and atomistic structures to investigate their thermal degradation upon heating under ambient environment. It is demonstrated that changes in MXene extinction and optical conductivity in the visible and near‐IR regions correlate well with the amount of intercalated water and hydroxyl termination groups and the degree of oxidation, measured using thermogravimetric analysis. Among the three MXenes, Ti3C2Tx and Ti2CTx , respectively, have the highest and lowest thermal stability, indicating the role of transition‐metal type, synthesis route, and the number of atomic layers in MXene flakes. These findings demonstrate the utility of SE as a powerful in situ technique for rapid structure–property relationship studies paving the way for the further design, fabrication, and property optimization of novel MXene materials. -
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Abstract MXenes constitute a rapidly growing family of 2D materials that are promising for optoelectronic applications because of numerous attractive properties, including high electrical conductivity. However, the most widely used titanium carbide (Ti3C2T
x ) MXene transparent conductive electrode exhibits insufficient environmental stability and work function (WF ), which impede practical applications Ti3C2Tx electrodes in solution‐processed optoelectronics. Herein, Ti3C2Tx MXene film with a compact structure and a perfluorosulfonic acid (PFSA) barrier layer is presented as a promising electrode for organic light‐emitting diodes (OLEDs). The electrode shows excellent environmental stability, highWF of 5.84 eV, and low sheet resistanceR Sof 97.4 Ω sq−1. The compact Ti3C2Tx structure after thermal annealing resists intercalation of moisture and environmental contaminants. In addition, the PFSA surface modification passivates interflake defects and modulates theWF . Thus, changes in theWF andR Sare negligible even after 22 days of exposure to ambient air. The Ti3C2Tx MXene is applied for large‐area, 10 × 10 passive matrix flexible OLEDs on substrates measuring 6 × 6 cm. This work provides a simple but efficient strategy to overcome both the limited environmental stability and lowWF of MXene electrodes for solution‐processable optoelectronics.
