Electrochemical etching of MXenes: mechanism, challenges and future outlooks

Abstract

Transition metal carbides, nitrides and carbonitrides, commonly known as MXenes, are an astonishing class of two-dimensional materials, offering versatile surface chemistry, high electrical conductivity, tunable band gaps, and a unique layered morphology, which render them highly attractive for multiple applications ranging from energy storage and conversion to biomedical fields. However, recognising the true potential of MXenes demands precise regulation over their fabrication process and surface functionalization. Traditional MXene fabrication relies on HF acid and fluoride-based etching agents, which pose environmental and safety concerns, subsequently introduce defects and alter surface properties. Consequently, innovative fluoride-free strategies are garnering attention. This review focuses on the eco-friendly electrochemical etching strategy for MXene synthesis, which enriches the MXene surface with a variety of surface terminal groups, such as –O, –OH, and –Cl, varying the electrolyte and etching parameters including their cutting-edge advancements compared to the conventional strategy, highlighting the innovations, challenges, and future outlooks in MXene electrochemical synthesis.

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Shaista Nouseen Shaista Nouseen is a final year doctoral student at the Department of Chemistry and Biochemistry at Mendel University in Brno, Czech Republic, and a researcher at the Future Energy & Innovation Lab at CEITEC Brno, Czech Republic. She completed her master's degree in applied chemistry from India. Her area of research focus includes the synthesis of 2D nanomaterials, their characterisation, and assembly of 3D-printed electrodes and devices for multiple electrochemical applications such as water splitting, nitrate reduction to ammonia generation, supercapacitors, and batteries.

Martin Pumera Martin Pumera has been the Head of the research group at the Future Energy & Innovation Lab at CEITEC Brno, Czech Republic since 2019. He gained his PhD degree in 2001 from Charles University, Prague, Czech Republic. Later, he became a Tenured Group Leader at the National Institute for Materials Science (NIMS), Japan, in 2006, followed by joining Nanyang Technological University, Singapore, as a professor in 2010. His research area varies from quantum materials to 3D printing, electrochemistry, and micro/nanomachines. Martin was given the title of “Highly Cited Researcher” by Clarivate Analytics during the period 2017–2021. Martin published over 950 scientific articles with over 80 000 citations. Martin is proud that 27 of his 125 alumni are group leaders/professor in academia.

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1. Introduction

Two-dimensional (2D) materials have emerged as an intriguing and groundbreaking class of materials that redefine the limits of conventional materials science.^1–10 There are diverse types of 2D materials such as transition metal dichalcogenides (TMDs),¹¹ hexagonal boron nitrides,^12–14 graphene,^15–22 phosphorene,^23–25 layered double hydroxides (LDHs)^26,27 and transition metal carbides (MXenes).^28–37 Among other 2D materials, transition metal carbides, commonly known as MXenes, are exceptional 2D materials due to their unique structural morphology, electrical conductivity, larger surface area, tunable band gaps, versatile surface chemistry, and mechanical robustness.^38–50

MXenes were introduced in 2011 at Drexel University⁵¹ and since then MXenes have been explored for a diverse range of applications, including energy storage,^52–56 like supercapacitors,^57–62 conversion,^63–67 sensing,⁶⁸ electromagnetic interference shielding,^69–71 catalysis,^72–74 and in the biomedical field^75,76 (Fig. 1). As the MXene-based electrodes demonstrate few advantages compared to the conventional carbon-based materials due to their superior conductivity, higher capacitance, and higher energy density, which arise from their unique structure and the ability to combine both electrochemical double layer capacitance (EDLC) and pseudocapacitance, making MXenes ideal for high-performance supercapacitors and energy storage devices.^{54,60–62,77}

Fig. 1 The MXene properties, multiple applications and wide-ranging applications. MXene structure. Reproduced with permission.¹⁷² Copyright 2021, Wiley-VCH.

MXenes are fabricated from their parent ternary layered MAX phases⁷⁸ (Fig. 2A). Conventionally, hydrofluoric acid (HF)/HF-based compounds are used to remove the aluminium metal layer from the parent MAX phase⁷⁹ (Fig. 2B). Recently, Thakur et al. reported the step-by-step fabrication process of MXene,⁸⁰ as shown in Fig. 2C. The MXene chemical structure is composed of M_n+1X_nT_x, where M refers to the number of layers of transition metals (n = 1, 2, 3, or 4), X is represented by carbon and/or nitrogen and T_x is represented by different surface functional groups such as –Cl, –F, –OH, and –O.^81–83 To date, theoretical studies indicate the possibility of different stoichiometric MXenes, and with current progress in the MXene research, the compositions will expand in future.^84–101 MXene family consists of a broad range of promising components, including V₂CT_x, Nb₂CT_x, Mo₂CT_x, Ti₂CT_x, etc.^{84,85,102–109} To date, the most investigated MXene for multiple applications in the field of batteries,^110–112 supercapacitors,^113–120 electro-photo-catalysis,^121–123 biosensing,⁴⁹ and drug delivery¹²⁴ among others is titanium-based MXene, i.e., Ti₃C₂T_x.^106,125

Fig. 2 (A) The illustration of the chemical formula and structure of the MAX phases and their corresponding MXenes. Reproduced with permission.⁷⁸ Copyright 2014, Wiley-VCH. (B) The fabrication process of MXene from the corresponding MAX phase via HF treatment. Reproduced with permission.⁷⁹ Copyright 2012, American Chemical Society. (C) Graphic illustration of the synthesis procedure of the MAX phase and the corresponding MXene. (i) Fabrication of the MAX phase through the reactive pressure-less sintering. (ii) MAX phase fabrication process variables employed in this process. (iii) HCl washing to remove intermetallic impurities. (iv) Pre-etch cleaning of the fabricated MAX phase with HCl by varying the time duration and temperature conditions. (v) The eradication of the aluminium layer from the MAX phase through selective etching. (vi) The etching agent HF–HCl is employed to etch the MAX phase. (vii) Delamination stage to convert the etched multilayered powder into the single-flake MXene. (viii) The delamination variables for the fabrication of the acquired MXene. Reproduced under the terms of the CC-BY license.⁸⁰ Copyright 2023, Wiley-VCH.

However, the main approach for fabricating MXenes is employing HF acid or HF-based compounds as etching agents. The major challenges encountered by employing HF as the etching agent are: (i) the introduction of –F terminal groups on the surface of MXene, which could negatively affect the conductivity and have a detrimental influence on applications such as batteries and supercapacitors.^126–128 (ii) Health hazards for humans occur due to the formation of corrosive and poisonous HF production, as fluoride ions are highly reactive, and extended exposure can easily result in the penetration of fluoride ions into the human body tissue and initiate fatal damage to tissues and organs.^129–133 (iii) The handling of HF requires special personal protective equipment (PPE).^129–133 (iv) The ecological effects due to the formation of corrosive HF are catastrophic for the environment.^130–133 Thus, the requirement to substitute the HF-based compounds for the synthesis of MXene is significant with the non-hazardous, sustainable, and eco-friendly strategies.^134–136

The MXene synthesis journey began with the revolutionary fabrication of Ti₃C₂T_x (MXene) using the traditional method, which involves employing HF as an etching agent to etch A layers from their parent MAX phases. Subsequently, in 2014, the mixture of hydrochloric acid (HCl) + lithium fluoride (LiF)¹³⁷ and later bi-fluoride salts such as NaHF₂, NH₄HF₂, KHF₂, NH₄F, etc. were employed as etching agents to synthesise MXene, which showcased innovative possibilities.^{101,138–143} Moreover, several fluorine-free approaches were explored for MXene fabrication, including the chemical vapour deposition (CVD) method¹⁴⁴ and the electrochemical etching method;^145–151 Jawaid et al. reported a halogen etching method to etch the MAX phases to fabricate corresponding MXenes¹⁵² (Fig. 3A); Wang et al. reported a HCl-assisted hydrothermal etching approach to fabricate MXenes¹⁵³ (Fig. 3B); Wang et al. proposed a low-temperature molten-salt (LTMS) etching approach for the fabrication of Ti₃C₂T_x employing NH₄HF₂ as the etching agent¹⁵⁴ (Fig. 3C). Furthermore, trifluoromethanesulfonic acid solution was also employed to synthesise MXenes (Ti₃C₂T_x)¹⁵⁵ (Fig. 3D).

Fig. 3 (A) Schematic illustration of the halogen etching route of the MAX phase to fabricate MXenes. (i) A general procedure of etching, purification, delamination and separation for the creation of halogen-terminated MXenes. (ii) The addition of Br₂ to Ti₃AlC₂ in anhydrous cyclohexane produces a deep red solution. (iii) When bromine (Br₂) reacts with the MAX phase aluminium interlayer, the supernatant solution converts into a pale-yellow coloured solution, indicating the exhaustion of Br₂ in the solution and the creation of the AlBr₃ species. AlBr₃ species are rendered inert due to the presence of tetrabutylammonium bromide (TBAX), which acts as the stabiliser. (iv) Subsequently, purification of the MXene crude through continual redispersion in the CHCl₃ nonpolar solvent. (v) The purified MXene is achieved through dispersion and centrifugation in the THF solvent. Reproduced with permission.¹⁵² Copyright 2021, American Chemical Society. (B) The HF-free Mo₂CT_x MXene was fabricated by employing the HCl-assisted hydrothermal etching approach. Reproduced with permission.¹⁵³ Copyright 2021, Wiley-VCH. (C) Diagrammatic representation of the LTMS etching approach to produce the MXene Ti₃C₂T_x employing the NH₄HF₂-LTMS etching method. Reproduced with permission¹⁵⁴ Copyright 2024, Wiley-VCH. (D) Visual representation of the synthesis method of MXene Ti₃C₂T_x from the corresponding MAX phase employing the trifluoromethanesulfonic acid solution. Reproduced with permission.¹⁵⁵ Copyright 2024, Wiley-VCH.

Other routes to fabricate MXenes without HF were explored employing HF-free solutions. For example, Shi et al. reported an ambient stable iodine-assisted etching method to formulate MXenes¹⁵⁶ (Fig. 4A); Li et al. proposed the fabrication of high-purity MXenes through alkali treatment. They etched the MAX phase Ti₃AlC₂ with NaOH in the water solution by varying the etching parameters and etching conditions for the synthesis of fluoride-free-terminated MXene Ti₃C₂ (ref. 157) (Fig. 4B). Liang et al. proposed a photo-Fenton approach to fabricate MXenes and compared it to the conventional MXene fabrication process,¹⁵⁸ as shown in Fig. 4C. Several other HF-free routes were reported, including Lewis acid molten salt etching,^159–162 hydrothermal etching,¹⁶³ UV-induced selective etching,¹⁶⁴ ball-milling,¹⁶⁵ halogen etching (Br₂, I₂, ICl, IBr, etc.),¹⁵² thermal reduction etching,⁸¹ large-scale fabrication of MXene through supercritical etching,¹⁶⁶ and microwave-assisted etching approaches.¹⁶⁷

Fig. 4 (A) Schematic illustration of the fabrication and delamination route of the MXene employing iodine-assisted etching. Reproduced with permission.¹⁵⁶ Copyright 2021. Wiley-VCH, under Creative Commons Attribution-NonCommercial-No Derivatives License. (B) Schematic illustration of the fabrication of MXene and the reaction mechanism of etching the MAX phase with NaOH in the water solution by varying etching parameters and conditions. Reproduced with permission.¹⁵⁷ Copyright 2018, Wiley-VCH. (C) Visual illustration of the synthesis of fluoride-terminated MXene Ti₃C₂ through a traditional approach and a graphic model of P. F. approach for the creation of F-free Ti₃C₂ and their application for the flexible lithium–sulfur batteries, along with the diagrammatic representation of the Fe(III)-oxalato P. F. reaction approach. Reproduced with permission.¹⁵⁸ Copyright 2022, American Chemical Society.

Additionally, several MXene hybrid structure formulations were reported using an HF-free based MXene fabrication route, including the fabrication of MXene–copper/cobalt hybrids through the Lewis acidic molten salt etching for excellent energy storage applications in symmetric supercapacitor devices,¹⁶⁸ as illustrated in Fig. 5A. In addition, Huang et al. proposed the fabrication of MXene/transition metal sulfide (Ti₃C₂T_x/MS_y) heterostructures with interfacial electronic coupling employing a molten salt etching approach. The fabricated Ti₃C₂T_x/MS_y heterostructures were used for sodium storage applications¹⁶⁹ (Fig. 5B). Xuan et al. proposed the intercalation and delamination technique for MXene fabrication by treating the Ti₃AlC₂ MAX phase with the organic base tetramethylammonium hydroxide (TMAOH) solution (Fig. 5C), which leads to the formation of Al(OH)₄⁻-modified and TMA⁺-intercalated MXene. The acquired MXene exhibited high NIR absorption. Thus, they were utilised as the photothermal agent against tumour cells.¹²⁹ Different types of HF-free etching routes for MXene fabrication, technique, etching agent, terminal groups and their applications are outlined in Table 1.

Fig. 5 (A) Schematic illustration of MXene hybrid formulation of Ti₃C₂–Cu/Co. Reproduced with permission.¹⁶⁸ Copyright 2021 Wiley-VCH. (B) A graphic visual of the fabrication procedure of the Ti₃C₂T_x/MS_y heterostructures. Reproduced with permission.¹⁶⁹ Copyright 2022, Wiley-VCH. (C) Graphic demonstration of the intercalation and delamination method for MXene using TMAOH. Reproduced with permission.¹²⁹ Copyright 2016, Wiley-VCH.

Table 1 Different HF-free etching routes of MXene fabrication, techniques, etching agents, terminal groups and applications

MXene	Technique	Etching agent	Terminal group	Advantage	Application	Ref
Ti3C2Tx	Molten salt etching	CuCl2/TBAOH	–Cl	High wettability	Lithium-ion battery	174
Ti3C2Tx	Molten salt etching	CuCl2/NaCl/KCl	–Cl and –O	High power performance	Electrochemical energy storage	175
Ti3C2Tx	Molten salt etching	FeCl2·4H2O	–Cl and –O	Remarkably improved electronic conductivity	Sodium storage	169
Ti3C2Tx	Molten salt etching	ZnCl2	–Cl	Improved electrochemical properties	Sodium-ion battery	176
Ti3C2Tx	Lewis acid etching	FeCl3	–Cl	Tunable coordination chemistry	Lithium–sulfur batteries	177
Ti3C2Tx	Lewis acid etching	ZnCl2	–Cl	Element replacement	—	161
Ti3C2Tx	Lewis acid etching	CuCl2, CuBr2, and CuI2	–Cl, –Br, and –I	Lattice expansion	Supercapacitor	160
Ti3C2Tx	Alkali etching	KOH	–OH	Single-layered MXenes	Electrochemical application	178
Ti3C2Tx	Alkali etching	NaOH	–OH and –O	Larger c-lattice parameter	Lithium-ion battery	179
Ti3C2Tx	Alkali etching	KOH	–OH and –O	Improved catalytic activity	Aerobic oxidative desulfurization	180
Ti3C2Tx	Hydrothermal alkali etching	NaOH/hydrazine	–OH	—	Nitrate storage	181
Ti3C2Tx	Hydrothermal alkali etching	NaOH	–OH and –O	High-purity MXene	Supercapacitor	157
Ti3C2Tx	Hydrothermal alkali etching	NaOH and TMAOH	–OH and –O	MXene nanosheets with a small (∼50–100 nm) lateral size	Nitrogen reduction reaction	182
Ti3C2Tx	Hydrothermal alkali etching	NaOH	–OH and –O	Resistant to oxidative degradation	Biocompatible	183
Ta4C3Tx	Acid–alkali etching	HCl/KOH	–OH and –Cl	Biocompatible	Supercapacitor	184
Ti3C2Tx	Chemical ball-milling etching	LiCl and TMAOH	–OH	Larger surface area	Lithium-ion battery	165
Mo2CTx	Microwave-assisted hydrothermal etching	NaOH/Na2S	–OH and –O	MoS2/Mo2CTx hybrid	Hydrogen evolution reaction	185
Ti3C2Tx	Organic base etching	TMAOH	–OH and –O	High catalytic activity	Removal of tetracyclic antibiotics	186
Ti2C–O	Organic base etching	TMAOH	–O	Highly stable	Humidity sensor	187
Ti3C2Tx	Organic base etching	TMAOH	Al(OH)4	Light absorption in the NIR region	Photothermal therapeutics	129
Mo2C	Chemical vapour deposition	—	—	Controlled growth of ultrathin 2D Mo2C crystals on a liquid Cu surface	—	188
Mo2C	Chemical vapour deposition	—	—	Large area and high quality	—	189
Mo2CTx	HCl-assisted hydrothermal etching	HCl	–O and –Cl	High efficiency	Energy storage applications	153
Ti3C2Tx	Halogen/acid etching	I2/HCl	–OH and –O	High stability	Supercapacitor	156
Ti3C2Tx	Halogen/halide etching	IBr, ICl, I2, Br2, and TBAX	Halides	Controlled surface chemistries	—	152
Mo2CTx	Halide etching	HBr solution, LiBr, NaBr, KBr, and NH4Br	–Br	Enhanced photocatalytic performance	H2 production	74
Ti2CTx	Thermal reduction	Mixed argon/hydrogen (Ar/H2, 95/5, v/v) flow temperature between 400 and 900 °C	—	Large-scale MXene production	Lithium-ion storage	81
Ti3C2Tx	Lithiation expansion	Li foil/LiTFSI	–OH and –O	Rapid and scalable synthesis	Supercapacitor	134
Ti3C2Tx	Photo-Fenton approach	Sodium oxalate/FeCl3	–OH and –O	High purity with 95% yield	Lithium–sulfur batteries	158
Mo2CTx	UV-assisted etching	UV light	—	High purity	Lithium-ion batteries and sodium-ion batteries	164
Mo2CTx	UV-assisted phosphoric acid etching	UV light and phosphoric acid	—	Safe etching route	—	190
Ti3C2Tx	Hydrothermal acidic etching	HCl	–OH, –O or –Cl	Enhanced electrochemical properties	Supercapacitor applications	191

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Among different fluoride-free etching strategies, electrochemical etching is garnering significant research attention due to several benefits such as (i) rapid and selective MXene fabrication with tunable morphology through electrochemical etching of A layers.¹⁷⁰ (ii) A straightforward delamination can be achieved with just sonication in electrochemical capacitors, eliminating the need for multiple steps typically required in the conventional etching method.¹⁷¹ (iii) Electrochemical etching of MXene with controlled surface termination, which results in improved electrochemical performance.¹⁷² (iv) Green, sustainable, eco-friendly and less hazardous approach compared to HF-based fabrication of MXenes.^172,173 In summary, the main aim of this review is to provide a comprehensive study of the electrochemical etching of MXenes, including an in-depth study to understand the mechanisms and process to selectively remove A layers. Additionally, different transition metal-based MXenes like Ti, Nb, V, and Mo prepared by the electrochemical etching process are discussed. This review article highlights the various advantages of electrochemical etching, such as sustainability, cost-effectiveness, and environmental friendliness, compared to the conventional fluoride-based strategies. The innovations, trends, obstacles, and future perspectives in the electrochemical etching of MXene fabrication are briefly discussed (Fig. 6).

Fig. 6 The outline of the review article.

2. Mechanism of electrochemical etching

The electrochemical etching approach enables modification of the MXene properties and generates desired patterns/structures on the MXene surface.^192–196 In this approach, MXenes can be synthesised using the corresponding MAX phase as an electrode by selectively etching A layers under a certain applied voltage. Lukatskaya et al. employed an electrochemical etching approach using different electrolytic solutions like 5 wt% NaCl, 10 wt% HCl, and 5 wt% HF and MAX as the precursor to fabricate carbon-derived carbon (CDC). They used three different types of MAX phases—Ti₃AlC₂, Ti₂AlC and Ti₃SiC₂. They acquired cyclic voltammetry (CV) profiles, as illustrated in Fig. 7A. In this approach, a certain voltage is applied, which facilitates the disruption of the M–A bonds in the MAX phase and initiates aluminium layer etching simultaneously.

Fig. 7 (A) (i) Graphic demonstration of fabrication of the CDC from the MAX phase under room temperature conditions. Cyclic voltammograms acquired in HF (triangles) and HCl (circles) when (ii) Ti₃SiC₂, (iii) Ti₃AlC₂, and (iv) Ti₂AlC were employed as the anode. Reproduced with permission.¹⁹⁷ Copyright 2014, Wiley-VCH. (B) Anodic electrochemical etching of the bulk MAX phase Ti₃AlC₂ in the binary aqueous electrolyte solution. (i) Diagrammatic illustration of the electrochemical etching and the delamination procedure. (ii) The electrochemical system employed for the electrochemical etching. (iii) The acquired optical image of the as-received bulk MAX phase Ti₃AlC₂. (iv) Aqueous dispersion of the delaminated MXene Ti₃C₂T_x. (v) X-ray diffraction curves of different MAX phases – Ti₃AlC₂, MXene–Ti₃C₂T_x, and Ti₃C₂T_x sheets. (vi and viii) SEM micrographs of the MAX phases Ti₃AlC₂ and Ti₃C₂T_x. (vii and ix) Cross-sectional HR-TEM micrographs of the MAX phase and MXenes Ti₃AlC₂ and Ti₃C₂T_x, respectively. Reproduced with permission.¹⁹⁹ Copyright 2018, Wiley-VCH.

However, when the voltage rises gradually, it eliminates the transition metal (M) layers, producing amorphous carbon. In summary, this report is crucial for the electrochemical etching of MXene. As it confirms that the regulation of the voltage window and time duration associated with the reaction concerning M and A layers are crucial parameters to selectively eliminate A layers and assist in effective MXene fabrication without the removal of M layers.¹⁹⁷ This method emphasises the importance of the regulation of the etching parameters to obtain optimum conditions for MXene fabrication. Moreover, to better understand the significance of the electrochemical etching parameters, a summary of electrochemical etching techniques, which includes different parameters like electrolytic solution, voltage window, time duration and temperature, is provided in Table 2.

Table 2 Electrochemical etching parameters of the MXene etching technique, electrolytic solution, voltage window, time duration, and temperature

MXene	Etching technique	Electrode setup	Electrolyte	Voltage	Time	Temperature	Ref.
a In the studies in which the temperature conditions were not mentioned, we considered that the reaction occurred under room temperature conditions.
Ti2CTx	Chronoamperometry	Three-electrode setup	1 M HCl	+0.6 V	1 day	^a	198
Ti2CTx	Chronoamperometry	Three-electrode setup	2 M HCl	+0.6 V	5 days	^a	198
Ti3C2Tx	Potentiometry	Two-electrode setup	1 M NH4Cl + 0.2 M TMAOH	+5 V, actual +2.48 V (vs. SCE)	5 hours	^a	199
Ti2CTx	Thermal assisted	Three-electrode setup	1 M HCl	0.3 V	9 hours	50 °C	200
V2CTx	Thermal assisted	Three-electrode setup	1 M HCl	0.5 V	9 hours	50 °C	200
Cr2CTx	Thermal assisted	Three-electrode setup	1 M HCl	1 V	9 hours	50 °C	200
Ti3C2Tx	—	—	1 M NH4HF2	2.5 V	2	^a	201
Ti3C2Tx	—	—	1 M NH4HF2	5 V	2 hours	^a	201
Ti3C2Tx	—	—	1 M NH4HF2	7.5 V	1.5 hours	^a	201
Ti3C2Cl2	Potentiometry	Three-electrode setup	LiCl–KCl	2.0 V	24 hours	^a	172
Ti2CTx	Chronoamperometry	Three-electrode setup	LiCl and KCl	1.3 V versus ref	24 hours	500 °C	170
Ti3C2Tx	Chronoamperometry	Three-electrode electrochemical	[BMIM][PF6]/MeCN	+3–+7 V vs. Ag wire	5 hours	^a	202
Ti3C2Tx	—	Two-electrode system	0.8 M LiOH and 1.0 M LiCl	5.5 V	5 hours	^a	203
Ti3C2Tx	Cyclic voltammetry scanning	Three-electrode setup	1 M NH4Cl and 0.2 M TMAOH	−0.25 to 0.3 V	5 hours	^a	204
Nb2CTx	—	Three-electrode setup	0.5 M HCl	1 V	4 hours	50 °C	205
V2CTx	Galvanic charge–discharge	Closed coin-type CR2030 cell	21 m LiTFSI + 1 m Zn(OTf)2	400 cycles at 10 A g^−1	—	^a	171
Mo2TiC2	—	Two-electrode system	0.93 M NH4Cl and 0.42 M LiOH	5 V	2 hours	^a	173
Mo2TiC2	—	—	Dilute HCl	0.7 V		55 °C	206
EE-Ti3C2	—	Two-electrode system	HBF4	+1 V	28 hours	35 °C	145
EE-Ti3CN	—	Two-electrode system	HBF4	+1 V	28 hours	35 °C	145

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Sun et al. first reported an electrochemical etching route to fabricate Ti₂C MXene using the Ti₂AlC porous MAX phase electrodes. In this approach, a controlled potential is provided for the electrochemical etching using diluted aqueous hydrochloric acid (HCl) as an etching agent.¹⁹⁸ The MAX phase Ti₂AlC was cut into pieces (0.7 × 3 × 0.1 cm³), and a copper wire was attached to this MAX phase parallelepiped utilising a flash-dry silver paint. To protect the lower part of the copper wire and flash-dry silver paste, an epoxy paste was utilised. In a three-electrode system, cyclic voltammetry profiles were acquired, providing the cycling rate at 20 mV s⁻¹, where the MAX phase parallelepiped acts as the working electrode, Pt foil acts as the counter electrode and Ag/AgCl acts as the reference electrode. After the electrochemical etching procedure, the MAX phase electrodes were rinsed with deionised water to remove the remaining aqueous HCl electrolytes on the electrode surface.

First, in the aqueous HCl electrolyte, the elimination of the aluminium (Al) layers from the Ti₂AlC electrodes occurs, which results in the fabrication of the Ti₂CT_x MXene containing several terminal functional groups like hydroxide (–OH), chloride (–Cl), and oxygen (–O). This reaction mechanism is very similar to the chemical etching of the MAX to MXene using traditional etching approaches, like using HF or LiF/HCl as the etching agent. Subsequently, after the successful elimination of the Al metal from the MAX phase, the functional group –Cl is attached to the surface of the fabricated MXene.¹⁹⁸ The elimination of the Al layer on the MAX phase as the working electrode is shown in eqn (1):

Ti₂AlC + yCl⁻ + (2x + z)H₂O → Ti₂C(OH)_2xCl_yO_z + Al³⁺ + (x + z)H₂ + (y + 3)e⁻ (1)

Simultaneously, at the Pt counter electrode, the subsequent reaction is shown in eqn (2):

Al³⁺ + 3e⁻ → Al (2)

Afterwards, the outer layer of Ti₂CT_x MXene, i.e., Ti₂C(OH)_2xCl_yO_z, goes through additional electrochemical etching, where it is converted into the carbon-derived-carbon (CDC) layer with the functional groups like –O, –OH, and –Cl, owing to the concurrent elimination of both Al and Ti layers. Thus, when Ti is also eliminated, due to the over-etching procedure, it results in a core–shell structure, and TiO₂ is formed at the counter electrode (Pt), as shown in eqn (3). The proposed reaction mechanism of the electrochemical etching process is outlined as follows:

Ti⁴⁺ + 2H₂O → TiO₂ + 4H⁺ (3)

Subsequently, Yang et al. reported another HF-free etching approach to electrochemically etch the MAX Ti₃AlC₂. This approach is based on the anodic corrosion of the MAX phase utilising a binary aqueous solution containing tetramethylammonium hydroxide (TMAOH) and ammonium chloride (NH₄Cl) as electrolytes.¹⁹⁹

Following they performed physical and morphological characterisation to confirm the fabrication of MXene as illustrated in Fig. 7B. According to the experimental study, the following reaction mechanism is proposed:

Ti₃AlC₂ − 3e⁻ + 3Cl⁻ → Ti₃C₂ + AlCl₃ (4)

Ti₃C₂ + 2OH⁻ − 2e⁻ → Ti₃C₂(OH)₂ (5)

Ti₃C₂ + 2H₂O → Ti₃C₂(OH)₂ + H₂ (6)

where eqn (4) plays a crucial role in the eradication of aluminium metal from the MAX phase anode. Next, the –OH terminal group is introduced on the MXene surface, as illustrated in eqn (5) and (6). Furthermore, the intercalation of ammonium ions and the removal of the Al metal are taking place concurrently. The following reaction mechanisms are proposed in eqn (7) and (8). This HF-free electrochemical etching results in the formation of mono- and bilayers of MXenes (Ti₃C₂T_x, T–OH, and –O) with a high yield of up to 90% and sizes of up to 18.6 μm. The obtained MXene exhibited a volumetric capacitance of 439 F cm⁻³ and an areal capacitance of 220 mF cm⁻² with a scan rate of 10 mV s⁻¹ for all solid-state supercapacitor applications.¹⁹⁹ The synthesis of MXene via the electrochemical etching approach opens avenues for more sustainable and eco-friendly ways to fabricate different varieties of MXenes with different terminal groups, resulting in enhanced electrochemical performance.

AlCl₃ + 3NH₃ + 2H₂O ↔ AlO(OH) + 3NH₄⁺ + 3Cl⁻ (7)

AlCl₃ + 2OH⁻ ↔ AlO (OH) + H⁺ + 3Cl⁻ (8)

3. Electrochemical etching of Ti-based MXenes

The thermal electrochemical etching route was reported by Pang et al. for the synthesis of Ti-based MXenes, and they extended this method to other transition metal-based MXenes like Cr and V, calling it a universal method, using a three-electrode system employing HCl as the electrolyte by varying the temperature, time, and voltage parameters. The SEM images are provided to confirm the successful etching of MAX to MXene, varying different parameters (Fig. 8A). In this approach, different MAX phases, such as Cr₂AlC, Ti₂AlC, and V₂AlC, are mixed separately with carbon black to fabricate the corresponding 3D composite MXene electrodes.²⁰⁰ These electrochemically etched MXene electrodes doped with the cobalt ions demonstrate a superior capability for catalysing multiple electrochemical reactions, including the HER, OER, and zinc ion batteries. In addition, Cao et al. fabricated electrochemically etched MXene (Ti₃C₂) with –O/–OH and –F terminal groups under room temperature conditions using 1 M NH₄HF₂ aqueous solution. They provided the following etching parameters, varying voltage and time as follows: 2.5 V for 2 h, 5 V for 2 h, and 7.5 V for 1.5 h, demonstrating that synthesised MXene has a larger surface area and a narrow pore size distribution²⁰¹

Fig. 8 (A) Electrochemical etching reaction mechanism and structural conclusions of the MXene Ti₂CT_x. (i) Anticipated electrochemical etching reaction mechanism route of the MAX phase Ti₂AlC in the HCl electrolyte solution. SEM micrographs of the MXene were obtained under varying HCl, temperature, time and voltage electrochemical etching conditions. (ii) MAX Ti₂AlC, (iii) 1 M/25 °C/9 h/0.3 V, (iv) 1 M/50 °C/3 h/0.3 V without CB, (v) 1 M/50 °C/3 h/0.3 V, (vi) 1 M/50 °C/6 h/0.3 V, and (vii) 1 M/50 °C/9 h/0.3 V. Scale bars: 1 μm. Reproduced with permission.²⁰⁰ Copyright 2019, American Chemical Society. (B) Illustration of the fabrication process of MXene from the corresponding MAX phase through MS-electrochemical etching, followed by the in situ modification of surface terminations. Reproduced with permission.¹⁷² Copyright 2021, Wiley-VCH.

Another major challenge in MXene fabrication is to fabricate MXene with different terminal groups. To resolve this issue, the molten salt assisted electrochemical etching approach for fabrication of a –Cl terminated MXene (Ti₃C₂Cl₂) was reported by Shen et al. In this approach, the terminal groups of MXene are in situ modified from –Cl to –S and/or –O using different inorganic salts, which leads to the shortening of the modification stages and additionally increases the different types of terminal groups in MXenes (Fig. 8B). In this approach, Ti₃AlC₂ acts as the anode and nickel acts as the cathode in the LiCl–KCl salt, and a voltage of 2.0 V is applied at 450 °C. However, the real operational bias on the anode attained 0.365 (V vs. Ag/AgCl). At this stage, the Al metal atoms from the MAX phase are selectively oxidised and eliminated owing to the weaker Ti–Al bonds in comparison to the Ti–C bonds. Subsequently, the oxidised Al forms a bond with the Cl ions present in the electrolytic solution, forming AlCl₃ because of their robust binding capabilities.

Additionally, at 450 °C, the evaporation of AlCl₃ occurs, initiating the driving force for the diffusion of Al outwards. As this etching process is thermodynamically influenced, the applied potential plays a significant role.¹⁷² The acquired O-terminated MXene is an outstanding electrode for energy storage applications (supercapacitor), demonstrating a capacitance of 225 F g⁻¹ at a current density of 1.0 A g⁻¹ with a rate performance of 91.1% at 10 A g⁻¹ and an outstanding capacitance retention of 100%. This synthesis process is considered more sustainable because no acid waste is generated, and the salt can be recycled. The reaction mechanisms for electrochemical etching are depicted below (eqn (9)–(12)):

Anode:

Ti₃AlC₂ + 3Cl⁻ = AlCl₃↑ + Ti₃C₂ + 3e⁻ (9)

Ti₃C₂ + 2Cl⁻ = Ti₃C₂Cl₂ + 2e⁻ (10)

Cathode:

Li⁺(K)⁺ + e⁻ = Li(K) (11)

Overall:

Ti₃AlC₂ + 5LiCl(KCl) = Ti₃C₂Cl₂ + 5Li(K) + AlCl₃↑ (12)

Subsequently, Liu et al. reported a one-pot molten salt electrochemical etching approach. A straightforward technique is to synthesise chlorine-terminated MXene (Ti₂CCl_x) from its precursor elements (Ti, Al, and C). This method significantly simplifies the MXene synthesis procedure, where Ti and Al micro-powders are reacted with the carbon nanotubes (CNTs) and reduced graphene oxide (rGO), which serve as different carbon sources. To fabricate the MAX (Ti₂AlC) phase with tunable morphologies like 1D and 2D structures (Fig. 9A), concurrently, the MAX phase is converted into MXene utilising an electrochemically etching method in a cost-effective LiCl–KCl solution. The pseudocapacitive redox reaction of MXene can be initiated with the introduction of an –O surface terminal group and ammonium persulfate washing (APS), which can lead to a better Li⁺ storage capacity of approximately up to 857 C g⁻¹ (240 mAh g⁻¹) with a high rate capability of 86 mAh g⁻¹ at 120C, thus generating promising anode materials for fast-charging batteries for energy storage applications.¹⁷⁰

Fig. 9 (A) Graphic visual of the production process of the MAX phase Ti₂AlC and MXene E-Ti₂CT_x. (a) The graphic illustration of the setup employed for the electrochemical etching. (b) The diagrammatic representation of the production of 1D and 2D Ti₂AlC structures employing CNTs and rGO as their carbon resources. (c) The Ti, Al, and C pellets were immersed in LiCl/KCl, followed by heating at 950 °C for 1 hour. (d) The fabrication of the MAX phase Ti₂AlC. (e) Following the electrochemical etching process at a voltage of 1.3 V for 24 hours to achieve the E-Ti₂CCl_x MXene. Reproduced with permission¹⁷⁰ Copyright 2022, Wiley-VCH, under Creative Commons Attribution-NonCommercial License.⁷⁸ (B) (a) Diagrammatic representation of the production of MXene EE-Ti₃C₂T_x. (b) Images of MXene dispersion illustrating the Tyndall effect. (c) Images of the MXene films. Reproduced with permission.²⁰³ Copyright 2022, American Chemical Society.

The fabrication of MXenes with controlled fluorine termination is another concern. In this context, Yin et al. introduced synchronous fluorination and mild-electrochemical exfoliation of Ti₃C₂F_x MXene using RTIL [BMIM][PF₆] as the –F source and MeCN. This ionic liquid electrolyte offers a non-aqueous etching environment to prevent MXene oxidation. The fabricated MXene was fluorinated utilising TiF₃ and CF groups. These groups were electrochemically reactive, and they contributed to the electrochemical performance of the fabricated Ti₃C₂F_x. This fluorinated Ti₃C₂F_x anode exhibited excellent cycling stability for lithium-ion battery applications, demonstrating a charge capacity of 329 mAh g⁻¹ (initially) at the current density of 200 mA g⁻¹. Following 500 cycles, the charge capacity of 211 mAh g⁻¹ was maintained. Moreover, in this study, they demonstrated that the fluorination of Ti₃C₂F_x can be regulated through adjusting the fabrication conditions.²⁰²

The use of hazardous organic intercalant agents for the delamination of MXene is a major issue. To resolve this issue, Chen et al. reported a simplified route to fabricate Ti₃C₂ MXene, without using any organic intercalant agent for delamination. In this study, they reported electrochemical etching of the MAX phase utilising a mixture of LiOH and LiCl aqueous solution to produce chlorine-terminated MXene, and later sonication was used to delaminate the obtained MXene (Fig. 9B). The acquired delaminated chlorine-terminated MXene flake sizes range from ∼3.9 nm to ∼3.8 μm in thickness and lateral size, respectively, with the stability of up to 15 days, when dispersed in an aqueous solution.²⁰³ Following that, a vacuum filtration technique was utilised to fabricate MXene films. The obtained filtrate MXene films were used for electrochemical energy storage applications, i.e., supercapacitors and exhibited excellent capacitances of 323.7 F g⁻¹, 1.39 F cm⁻², and 1160 F cm⁻³, respectively, outperforming the conventionally fabricated MXene. According to the experimental results, the following equations are proposed to illustrate the etching mechanism (eqn (13)–(15)):

Ti₃AlC₂ + 3Cl⁻ + 3e⁻ → Ti₃C₂ + AlCl₃ (13)

Ti₃C₂ + xCl⁻ + (2 − x)OH⁻ − 2e⁻ → Ti₃C₂Cl_x(OH)_2−x (14)

AlCl₃ + 2OH⁻ → AlOOH + H⁺ + 3Cl⁻ (15)

Additionally, the fluoride (−F) terminal group of MXene drastically impacts the charge transfer efficiency and hinders ion access to MXene (Ti₃C₂). To overcome this obstacle, Qian et al. introduced fluoride termination-free Ti₃C₂T_x MXene fabrication. In this study, to electrochemically etch the MAX phase, a three-electrode setup was used employing the cyclic voltammetry technique using 1 M NH₄Cl and 0.2 M TMAOH as the electrolyte.²⁰⁴ An exact potential range of −0.25 to 0.35 V was applied at room temperature for 5 hours with constant stirring of 300 rpm. During this process, an in situ alkaline electrolyte was formed, destroying the Ti–Al bonds and aiding in the formation of –AlO(OH) and –OH terminal groups on the surface of Ti₃C₂T_x MXene (Fig. 10a and b). The SEM images, along with the EDS mapping and XRD pattern, confirm the successful fabrication of MXene (Fig. 10c and d). The fabricated MXene was employed for capacitive deionisation (CDI) device application delivering an outstanding salt elimination capacity of 20.27 mg⁻¹ with an adsorption rate of 1.01 mg g⁻¹ min⁻¹ because of the improved ion transport capability and hydrophilicity of etched Ti₃C₂T_x.

Fig. 10 (a) Schematic illustration of the fabrication process and morphological structure of the fluoride-free electrochemical etched MXene Ti₃C₂T_x. (b) The three-electrode configuration was employed to acquire the CV scan for the MAX electrode with a scan rate of 20 mV s⁻¹, (c) SEM and elemental mapping analysis images, and (d) XRD patterns of MXene and MAX phase. Reproduced with permission.²⁰⁴ Copyright 2023, American Chemical Society.

Additionally, Chan et al. reported low fluoride electrochemical etching of two titanium-based MAX phases, Ti₃AlC₂ and Ti₃AlCN, to formulate the corresponding MXene Ti₃C₂ and Ti₃CN employing HBF₄ as the electrolyte solution. The electrochemical etching process occurs in a graphite crucible, where it also acts as the current collector to the MAX phase powder and a platinum (Pt) wire serves as the cathode electrode.¹⁴⁵ In this electrochemical etching procedure, no binder is required as the etching of the MAX phases Ti₃AlC₂ and Ti₃AlCN is possible in its powder state, as all the MAX particles are settled down on the graphite crucible at the time of the electrochemical etching procedure (Fig. 11). The reaction mechanism is proposed as follows for this electrochemical etching route (eqn (16)–(18)):

Fig. 11 Schematic illustration of the low-fluoride electrochemical etching fabrication of MXene along with the TEM micrograph of delaminated MXene and SEM micrographs of MXene powder. Reproduced with permission.¹⁴⁵ Copyright 2024, The Royal Society of Chemistry under Creative Commons Attribution 3.0 Unported License.

Anode:

Ti₃AlC₂(s) + 3BF₄⁻(aq.) → Ti₃C₂(s) + Al³⁺(aq.) + 3F⁻(aq.) + BF₃(g) + e⁻ (16)

Cathode:

H⁺(aq.) + e⁻ → 1/2H₂(g) (17)

Overall:

Ti₃AlC₂(s) + 3HBF₄(aq.) → Ti₃C₂(s) + 3BF₃(g) + AlF₃(s) + 3/2H₂(g) (18)

According to the reaction mechanism, during the electrochemical etching process, the selective anodic dissolution of the aluminium metal occurs on the MAX phase Ti₃AlC₂ with the tetrafluoroborate ion. This electrochemical etching pathway reduces the etching time. Thus, the release of the HF gas is minimal. Furthermore, the electrochemically etched MXenes are tested for the energy storage application for lithium-ion batteries and demonstrate similar electrochemical performance compared to the chemically etched MXene. Following that, Zheng et al. reported electrochemical synthesis of TiC and carbon-derived carbon using Ti₃SiC₂ in CaCl₂ molten salt at 900 °C with potentials 2.5 V and 3.0 V, respectively, illustrating that non-Al precursors can be employed to fabricate MXene using an electrochemical etching strategy.²⁰⁷

Besides, novel techniques can be explored to synthesise MXene via electrochemical etching; for example, based on our previous work, we formulated a 3D-printed MAX/PLA electrode and investigated its in situ electrochemical etching process to convert into an electrochemically active 3DP-etched MAX electrode. In this process, the electrodes were washed with distilled water and used directly for electrochemical analysis, without any separation from the polymer, which exhibits promising electrochemical performance, highlighting the capability of direct use of these electrodes for various applications, unlike the traditional carbon-based electrodes, which require additional formulation steps and lack precision. However, several challenges persist in obtaining large quantities of pure MXene, as separation from polymer is a challenging process.³⁷

4. Electrochemical etching of Nb, V, Mo-based MXene

The most extensively studied MAX phase until now is the titanium transition metal-based MAX phase. Thus, exploration of other MAX phases and their corresponding MXenes is becoming more prominent. In this context, Song et al. reported an electrochemical etching technique to fabricate Nb₂CT_x MXene using the Nb₂AlC MAX phase as the precursor. The electrochemical etching was performed in a three-electrode setup utilising 0.5 M HCl as the electrolyte by providing a voltage of under 1 V for 4 hours.²⁰⁵ The aluminium metal was selectively etched via anodization at a temperature of 50 °C. The electrochemical etching reaction mechanism for Nb₂AlC is proposed in eqn (19):

Nb₂AlC + yCl⁻ + (2x + z)H₂O → Nb₂C(OH)_2xCl_yO_z + Al³⁺ + (x + z)H₂↑ + (y + 3)e⁻ (19)

The synthesised electrochemically etched MXenes were purified. The fluoride-free Nb₂CT_x–acetylcholinesterase (AChE)-based electrochemical biosensors were assembled for phosmet detection. These fabricated materials provide a limit of detection of as low as 0.046 ng mL⁻¹. This fluoride-free electrochemically etched MXenes have advantages such as improved electron transfer potential and better enzyme activity compared to the traditional HF-based etched MXenes (Fig. 12A).

Fig. 12 (A) Graphic illustration of the electrochemical exfoliation and delamination routes for the MAX phase (Nb₂AlC) through the electrochemical etching method, followed by demonstrating the enzyme inhibition effect for phosmet detection employing an electrochemically etched MXene Nb₂CT_x/AChE biosensor. Reproduced with permission.²⁰⁵ Copyright 2020, Wiley-VCH under CC-BY License. (B) A graphic visual for the electrochemical etching procedure of Mo₂TiC₂ MXene fabrication from their corresponding MAX phase. The two-electrode configuration is employed, where Pt foil or carbon cloth acts as the cathode electrode and Mo₂TiAlC₂ MAX phase block acts as the anode electrode, followed by using an NH₄Cl and LiOH mixture as the electrolyte. Reproduced with permission.¹⁷³ Copyright 2023, Wiley-VCH.

Additionally, Sheng et al. recently reported a sustainable approach using two-electrode configurations to fabricate 3D MXene electrodes from the precursor Mo₂TiAlC₂ MAX phase¹⁷³ (Fig. 12B). They employed a synergistic blend of cathodic electrophoretic deposition and anodic electrochemical etching of the Al layer from the corresponding MAX phase. This process is less time-consuming compared to the conventional MXene fabrication process, as the MXene (Mo₂TiC₂) was obtained without the use of an ultrasound and large organic molecule intercalation reagent treatment. Within few minutes, MXene (Mo₂TiC₂) was deposited onto the cathode part (carbon cloth or platinum). Additionally, this route is beneficial as less acid waste is generated compared to the traditional MXene fabrication process. Because it allows an effective way to separate the fabricated MXene (Mo₂TiC₂) from the electrolytic solution, as it is directly uniformly deposited onto the cathode part. This electrochemically etched Mo₂TiC₂ deposited on the carbon cloth surface can be directly employed as a 3D MXene electrode electrocatalyst material for energy conversion applications like the HER.

The conventional multistep process of synthesis of MXene from MAX using toxic chemical reagents, followed by device fabrication, is time-consuming. Thus, to tackle this issue, Li et al. introduced a straightforward method by employing the V₂AlC MAX phase as the cathode, Zn metal as the anode and 21 M LiTFSI + 1 M Zn (OTf)₂ as the electrolyte solution, for the fabrication of a closed coin-type CR2030 device for battery testing application.¹⁷¹ Because of the selection of F-enriched solution as the electrolyte, it promotes the exfoliation of the V₂AlC MAX phase, resulting in the formation of V₂CT_X MXene inside the battery cell directly, as confirmed by the morphological characterisation of the MXene cathode after the electrochemical performance of about 400 cycles at 10 A g⁻¹, as illustrated in the SEM image in Fig. 13a. It is demonstrated that the microstructure of the MXene cathode alters significantly, where the V₂AlC MAX phase particles are converted into V₂CT_X MXene, which is evident in the magnified SEM image in Fig. 13b.

Fig. 13 Physical and morphological characterisation of the in situ produced MXene V₂CT_X and their reaction mechanism. (a and b) SEM micrographs of the cathode, (c) transmission electron microscopy (TEM) micrograph. (d) TEM micrograph in the HADDF mode for EDS mapping analysis; insets display the V, C, O, and Zn elements related to MXene. (e) High-resolution TEM micrograph. (f) AFM micrograph. (g) XRD patterns at different cycles. (h) Wide survey XPS spectra. (i) Raman spectra at different cycles. (j) Graphic demonstration of the in situ electrochemical etching process reaction mechanism. Reproduced with permission.¹⁷¹ Copyright 2020, Wiley-VCH.

This conversion of V₂AlC MAX to V₂CT_X MXene is similar to the conventional wet chemistry etching process, with the smoother outer surface of V₂CT_X MXene without any impurities, and the lateral size of V₂CT_X MXene ranges between 1 and 5 μm. The A layers are removed along with the –O, –F and –OH terminals formed in a parallel direction to (001), which results in the preservation of original lamellar structures. The TEM images illustrated in Fig. 13c show classic electron beam transparent characteristics. Additionally, the etching of aluminium layers was confirmed utilising EDS mapping with the Al content less than 0.37 atomic%, as shown in Fig. 13d. Furthermore, the high-resolution transmission electron microscopy analysis (HR-TEM) shows the ordered lattice fringe in Fig. 13e, further confirming the high crystallisation of the in situ electrochemical etched V₂CT_X MXene.

Additionally, the atomic force microscopy (AFM) images illustrated in Fig. 13f show that the thickness of the V₂CT_X MXene is concentrated at 8.5 nm, suggesting that the number of layers is five or seven. The X-ray diffraction further proves the phase transition from V₂AlC MAX to V₂CT_X MXene, as the XRD pattern is obtained after 5, 150, and 400 cycles. At 5 cycles, the V₂AlC MAX phase and additive (polyvinylidene, carbon cloth and carbon black) diffraction peaks are presently displayed in Fig. 13g. Nonetheless, after increasing the number of cycles, i.e., at 150 cycles, the peaks of the V₂AlC MAX phase are diminishing, suggesting the conversion from V₂AlC MAX to V₂CT_X MXene. Following 400 cycles, the V₂AlC MAX phase peak is barely visible, specifically in the peak regions of 13.3° and 41.3°, which are associated with the (002) and (103) crystal planes. It can be suggested that after 400 cycles, the V₂AlC MAX phase is successfully exfoliated. To better understand the etching of Al metal, the survey X-ray photoelectron spectroscopy (XPS) spectra were obtained, as displayed in Fig. 13h, which validated the XRD results, confirming the eradication of Al metal at 73 eV and the addition of a new peak at 685 eV after 400 cycles. This conversion of V₂AlC MAX to V₂CT_X MXene was further validated by Raman spectra, as illustrated in Fig. 13g. The Raman peaks associated with the V₂AlC MAX phase at 158, 239, and 258 cm⁻¹ were diminishing. While the Raman peaks at 114, 139, 262, and 298 cm⁻¹ corresponding to V₂CT_X MXene were dominating. The following reaction mechanism was proposed, based on the observations (eqn (20)).

V₂AlC + yF⁻¹ + (2x + z)H₂O − (y + 3)e⁻¹ → V₂C(OH)_2xF204_yO_z + Al³⁺(Al₂O₃, AlF₃) (20)

So, Fig. 13j displays the breaking of the V–Al bonds in the V₂AlC MAX phase cathode as it was attacked by F⁻¹, which is present in the electrolytic solution, resulting in the etching of Al layers and forming V₂CT_X MXene. In the single-step procedure, the battery device undergoes three phases: MAX exfoliation, electrode oxidation and redox of V₂O₅. This device can be directly used for battery testing applications while all the reactions are undergoing, and it is observed that the electrochemical performance keeps increasing. Moreover, this battery device exhibited excellent electrochemical performance for a zinc ion battery, demonstrating cycling stability of 4000 cycles with the rate performance of 97.5 mAh g⁻¹ at 64 A g⁻¹. According to this study, this in situ electrochemically etched MXene with its high capacity outperform the other reported vanadium-based zinc ion batteries.

This single-step, straightforward green fabrication process of MXene, followed by device assembly, prevents any contamination from outside and expands the applications of MXene in the field of aqueous energy storage devices. Overall, the electrochemical etching strategies hold particular importance in the realm of MXenes, contributing towards a harmless and more precisely controlled approach to synthesize MXene. These strategies have garnered significant attention due to their ability to avoid the dangers related to the fluorine compounds while producing MXene with tunable properties. A summary of electrochemical etching of MXene, including the technique, etching agent, terminal groups, advantages and their applications, is provided in Table 3.

Table 3 The electrochemical etching of MXene, including the technique, etching agent, terminal groups, advantages and applications

MXene	Technique	Etching agent	Terminal group	Advantage	Application	Ref
Ti2CTx	Electrochemical etching	HCl	–OH, –O, and –Cl	Dilute HCl solution	—	198
Ti3C2Tx	Electrochemical etching	NH4Cl + TMAOH	–OH and –O	High yield (over 90%) mono- and bilayers	Solid-state supercapacitor	199
Ti2CTx, Cr2CTx, and V2CTx	Electrochemical etching	HCl	–OH, –O, and –Cl	Universal strategy	Hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and rechargeable battery	200
Ti3C2Tx	Electrochemical etching	NH4HF2	–F and –O/–OH	Room temperature	—	201
Ti3C2Tx	Molten-salt-assisted electrochemical etching		–Cl to –O and/or –S	The surface terminations can be in situ modified	Supercapacitor	172
Ti2C	Electrochemical etching	LiCl/KCl	–Cl and –O	1D and 2D tuned morphology	Li-ion storage	170
Ti3C2Tx	Electrochemical etching	[BMIM][PF6]/MeCN	–F and –O/–OH	Controllable fluorination	Lithium-ion batteries	202
Ti3C2Tx	Electrochemical etching	LiOH and LiCl	–Cl	Delamination via sonication	Supercapacitor/zinc ion hybrid capacitor	203
Ti3C2Tx	Electrochemical etching	NH4Cl + TMAOH	–AlO(OH)– and –OH–	High salt-removal capacity	Capacitive deionisation (CDI)	204
Nb2CTx	Electrochemical etching	HCl	—	Chemical stability and biocompatibility	Electrochemical biosensors	205
V2CTX	Electrochemical etching	C2F6LiNO4S2 salt (LiTFSI) and Zn(CF3SO3)2 salt (Zn(OTf)2)	–F	All-in-one protocol	Zinc-ion battery	171
Mo2TiC2	Electrochemical etching	LiOH/NH4Cl	–OH, –O, and –Cl	Deposition on the cathode plate	Hydrogen evolution reaction	173
MoTi2C	Electrochemical etching	HCl	–OH, –O, and –Cl	The active site to capture S ions	Hydrogen evolution reaction	206
Ti3C2 and Ti3CN	Electrochemical etching	Tetrafluoroboric acid	—	Larger lateral dimension of MXene	Lithium-ion supercapacitors	145

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5. Conclusion, challenges, and future outlooks

This review article emphasises the comprehensive study of the electrochemical etching route for the fabrication of MXenes, centering on its fundamental mechanism and the different parameters that impact the electrochemical etching of the aluminium layer from the MAX phase to fabricate the corresponding MXene. The electrochemical etching synthesis of MXene has attracted significant research attention as a substitute for the conventional HF-based etching route, owing to its capability to produce MXene with varying terminal groups like –O, –OH, and –Cl.

Additionally, several benefits of electrochemical etching fabrication processes are highlighted, such as this approach being green, sustainable, less waste-generating, and cost-effective, without using any tedious process. Moreover, emphasis is given to the exploration of MXenes derived from other than titanium-based MAX phases, as to date, most reported studies are on titanium-based MXenes fabricated through electrochemical etching. This review article focuses on the electrochemical etching fabrication process of MXenes like Nb₂CT_x, V₂CT_X, and Mo₂TiC₂ and their outstanding performance for multiple applications in diverse fields, from biomedical to energy storage and conversion, highlighting their huge potential for the future.

Despite the ongoing research in the production of electrochemically etched MXenes, it exhibits various obstacles that need to be resolved for efficient production and real-world applications. The fundamental issues lie in the tuning of the etching parameters, including the selection of a suitable electrolyte solution, duration of etching and voltage window, to attain a uniformly delaminated MXene, concurrently preventing the over-etching of the MXene and maintaining its structural integrity. Because the inappropriate etching parameters can cause defects in MXene, reduced electrical conductivity, stability and residual aluminium impurities, which severely impede its electrochemical performance. One of the most crucial parameters to decide the properties of obtained MXenes, their oxidation resistance and intercalation capability, is through the optimisation of the electrolytic solution. Thus, the selection of the proper electrolytic solution is a crucial parameter.

Another major challenge is mass-scale production, while electrochemical etching provides a sustainable and safer route to fabricate MXene compared to chemical etching. But retaining the consistency when the production is large-scale is still challenging. Moreover, several obstacles, including oxidation of MXene and long-term stability after the electrochemical etching, need to be resolved, as the MXene oxidation leads to a compromise in the electrical and mechanical properties of MXene, resulting in poor electrochemical performance in various electrochemical applications, from energy storage to conversion and biomedical fields. These issues can be resolved by addressing the appropriate selection of electrolytes and the post-etching process to ensure the manufacturing of high-quality MXene for the advancement of MXene's future research. The electrochemical etching strategy has been garnering significant attention since its discovery, as evidenced by the increase in the number of publications each year (Fig. 14).

Fig. 14 The number of publications on electrochemically etched MXenes vs. years (search performed on Google Scholar using the keywords ‘electrochemical etching of MXene’).

The future direction of the electrochemical etching route to fabricate MXene lies in the evolution of the current technologies. To enhance the MXene fabrication efficiency, through optimisation of the MXene properties and terminal groups designed for specific applications, exploration of novel electrolytes, scalability, stability, integration of modern technology with more eco-friendly routes (Fig. 15).

Fig. 15 The outline of the review article.

(1) The MXene fabrication efficiency could be enhanced through the modification of MXene’s electrical and mechanical properties, by modulating the electrochemical etching parameters to attain precise control over the MXene structure, selective etching of the A layer and integrating the specific terminal groups for specific applications to boost the MXene electrochemical competence.

(2) Real-time monitoring methods like in situ spectroscopy and theoretical studies like density functional theory (DFT) may be beneficial to better understand the electrochemical etching route at a molecular scale, letting the researchers modulate the electrochemical etching conditions for enhanced consistency and high-quality MXene.

(3) Exploring novel electrolyte solutions like green solvents and neutral electrolytes for electrochemical etching of selective A-layers that are efficient, less toxic and sustainable could be a promising approach to fabricate electrochemically etched MXene. Most of the conventional approaches are dependent on corrosive acidic electrolytes, which pose safety threats.

(4) The mass-scale economic production of MXene for industrial applications should be the focus of the scientific community. To design and innovate the electrochemical etching routes with advanced features, to avail the MXenes commercially, high-throughput production methods like roll-to-roll manufacturing and large-batch fabrication must be explored.

(5) Improving the stability of the electrochemically fabricated MXene is another potential direction for future studies for its long-term practical applications as degradation and oxidation of MXene severely impede its electrochemical activity.

(6) Integrating different strategies to fabricate different types of MXene composite with other 2D materials like Prussian blue frameworks, COFs, MOFs, and HOF materials^208–212 could be explored in future. Additionally, proper storage conditions and stability of fabricated MXene needs to be improved, which can lead to enhanced electrochemical performance. Thus, resulting MXene could be more appropriate for various purposes like sensing, catalysis, bio-medical, electrochemical energy conversion and storage applications. With the current progress in this area of MXene fabrication, electrochemical etching is projected to become one of the leading techniques for MXene production, encouraging their extensive employment in next-generation expertise.

Conflicts of interest

The authors of the review article possess no conflict of interest.

Data availability

Data will be available at the public repository ZENODO.

Acknowledgements

The present review article was sponsored by ERDF/ESF TECHSCALE (No. CZ.02.01.01/00/22_008/0004587) and co-sponsored by the European Union in REFRESH – Research Excellence For REgion Sustainability/High-tech Industries (project number CZ.10.03.01/00/22_003/0000048) by Operational Programme Just Transition. M. P. acknowledges project ANGSTROM for funding. Project ANGSTROM was selected in the Joint Transnational Call 2023 of M-ERA.NET 3, which is an EU-funded network of about 49 funding organisations (Horizon 2020 grant agreement No 958174). This project “Advancing Supercapacitors with Plasma-designed Multifunctional Hybrid Materials” (no. TQ05000001) is co-financed from the state budget by the Technology Agency of the Czech Republic under the SIGMA Programme within the M-ERA-NET 3 Call 2023. This project/result was funded under the National Recovery Plan from the European Recovery and Resilience Facility.

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