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MXenes have superior catalytic activities, because of their excellent electronic properties. In this work, the electronic properties, quantum capacitance, and photocatalytic activity of Pt-doped Zr2CO2 (Pt-VO-Zr2CO2) nanosheets under biaxial strain are investigated by first-principles calculations. Pt-VO-Zr2CO2 are all indirect semiconductors under compressive strain and are direct semiconductors under tensile strain except at 10%. The red shift of Zr-d and O-p orbitals in conduction bands results in the decrease of band gaps under compressive strain. Pt-VO-Zr2CO2 under strain except at −8% are cathode materials in an aqueous system, especially for Pt-VO-Zr2CO2 with +10% strain with the maximum storage charge. Pt-VO-Zr2CO2 under strain can reduce water at pH 0 and perform overall water splitting in a neutral environment. Pt-VO-Zr2CO2 nanosheets with a strain larger than +4% can reduce CO2 to all products, and tensile strain enhances CO2 photocatalysis. The N2 reduction capability of Pt-VO-Zr2CO2 reduces in a neutral environment. [ABSTRACT FROM AUTHOR]

MXenes have wide applications because of the structural flexibility and compositional diversity. Quantum capacitance, electronic and photocatalytic properties of Sc2CO2 monolayer under uniaxial strains are investigated by first‐principles calculation. Sc2CO2 monolayer can withstand stress up to 13.96 N/M and about 14% tensile uniaxial strain limit. Sc2CO2 undergoes a transition from semiconductor to metal at −15% uniaxial strain. Sc2CO2 under uniaxial strains are only used for the water reduction at pH = 0. Sc2CO2 under uniaxial strains of −10%, −7%, −3%, 0%, and 4% within the appropriate pH scope are potential photocatalysts for water redox reaction. Sc2CO2 with −15% uniaxial strain is promising anode material, and Sc2CO2 with tensile uniaxial strain is more potential cathode material. Wide voltage improves the top quantum capacitance at negative bias under uniaxial strain and keeps the type of electrode materials. Quantum capacitance of Sc2C with mixed termination (Sc2CO0.59F1.19(OH)0.22) under uniaxial strain is explored. The introduction of uniaxial strain and mixed termination has little effect on the electrode type of Sc2CO2 MXene. [ABSTRACT FROM AUTHOR]

MXene is widely used as electrode material because of its excellent performance. A new branch, i‐MXene is recently synthesized and confirmed to have superior performance. Density functional theory is used to investigate the structural and electronic properties, magnetic anisotropy and quantum capacitance of Cr2/3Sc1/3CO2 i‐MXene. The results indicate that the three models are stable and have magnetism, which originates from Cr‐d states. Cr2/3Sc1/3CO2 has higher work function and can be used as hole injection electrode. Model 2 prefers in‐plane as the magnetic easy direction, while the magnetic easy‐axis of model 3 is out‐of‐plane. The three models of Cr2/3Sc1/3CO2 are all suitable for anode material in whole voltage. Quantum capacitance, surface storage charges and electrode types at 233, 300, and 353 K are also investigated. [ABSTRACT FROM AUTHOR]

MXenes are promising electrode materials due to their excellent performance. However, the quantum capacitance (Cdiff) and surface storage charge (Q) of bare M2C are little reported theoretically up to now. Herein, Cdiff and Q of 12 M2C MXenes related with 3d, 4d, and 5d transition metal (TM) atoms are investigated in aqueous and ionic/organic systems. All M2C MXenes are metallic. M2C with 5d TM are cathode materials. M2C with 4d TM are also cathode materials except Y2C MXene. For M2C with 3d TM, Sc2C and Mn2C are cathode materials, while Ti2C, V2C, and Cr2C are anode materials, especially for Ti2C and V2C with larger |Qp|/|Qn| and Qp. The broadened voltage maintains the type of electrode materials. W2C is the least promising cathode material in broad voltage. [ABSTRACT FROM AUTHOR]

MXenes have the excellent electrochemical properties as electrode of supercapacitors. Using density functional theory, we investigated the electronic properties and quantum capacitance of Zr2CO2 with atomic swap. The atomic swap results in the appearance of Frenkel‐type defect and Schottky defect. The negative binding energy confirms the stability of the studied systems. The atomic swap makes ZrC1, ZrC2, ZrO1, ZrO2, CO2 systems maintain the indirect semiconductor character, and CO2 system has the direct semiconductor character. The thermionic emission performance of Zr2CO2 MXene is greatly improved by atomic swap. ZrO1 and ZrO2 systems in aqueous system are appropriate anode materials. The wide voltage has little effect on the electrode type for perfect Zr2CO2, ZrO1, ZrO2, CO1, and CO2 systems, but changes ZrC1 and ZrC2 systems from cathode material in aqueous system to anode material in ionic/organic system. The work function and effective mass are also explored. [ABSTRACT FROM AUTHOR]

Defect and doping are effective methods to modulate the physical and chemical properties of materials. In this report, the structural stability, electronic properties, and quantum capacitance (Cdiff) of Zr2CO2 MXene are investigated by the introduction of Si, Ge, Sn, N, B, S, and F atoms. The doping of F, N, and S atoms makes the system undergo the semiconductor‐to‐conductor transition, while the doping of Si, Ge, and Sn atoms maintains the semiconductor characteristics. The B‐doped system can be used as cathode materials, while the systems doped by S, F, N, Sn atoms are promising anode materials of asymmetric supercapacitors, especially for the S‐doped system. The effect of temperature on Cdiff is further explored. The result indicates that the maximum Cdiff of the studied systems gradually decreases with the increasing temperature. Our investigation can provide useful theoretical basis for designing and developing the ideal electrode materials for supercapacitors. [ABSTRACT FROM AUTHOR]

Double traditional metal (TM) Janus MXenes have more superior properties when compared with MXenes. The electronic and optical properties, Bader charge and quantum capacitance of ZrMCO 2 (M = Sc, Ti, V, Cr, Mn, Fe, Y, Zr, Nb, Mo, Hf, Ta, W) are explored by density functional theory (DFT). The stability of these systems is confirmed by cohesive energy. The substitution of Ti/Hf atoms results in the decrease of bandgap of ZrTiCO 2 /ZrHfCO 2 , which is induced by the redshift of conduction band minimum (CBM). ZrMCO 2 (M = Cr, Mn) are magnetic semiconductors and ZrMCO 2 (M = Sc, V, Y, Fe) are magnetic metals. All doping atoms except Hf improve the maximum quantum capacitance of ZrMCO 2 at positive bias. Wide voltage makes ZrMCO 2 (M = Nb, Ta, W) change into anode material and ZrFeCO 2 into cathode material. ZrMCO 2 (M = Sc, Mn, Y, Hf, Zr) and ZrMCO 2 (M = V, Cr) can serve as cathode and anode materials in whole voltage, respectively. Much charge accumulating between Fe and C atoms results in the smallest Fe–C bond among all M − C bonds, which indicates the strongest interaction between Fe and C atoms. Optical properties and work function are further explored. [ABSTRACT FROM AUTHOR]

Inspired by the superior properties of Janus MA 2 Z 4 , the electronic properties, quantum capacitance, and photocatalytic activity of Janus MSiGeN 4 (M=Cr, Hf, Mo, Nb, Ta, Ti, V, W, and Zr) monolayers are explored. The results indicate that TiSiGeN 4 , ZrSiGeN 4 , and HfSiGeN 4 are semiconductors with the bandgap larger than 2 eV. NbSiGeN 4 is a spin-polarization semiconductor with magnetic moment of 0.61 μ B. VSiGeN 4 exhibits the character of half-metal and its spin-up channel hosts a Dirac cone around Fermi level, which is derived from V-d states. The analysis of Bader charge shows that built-in electric field in these systems appears and promotes the charge redistribution across the Janus atomic layer. All systems except NbSiGeN 4 and TaSiGeN 4 are cathode materials because of |Q p |/|Q n | larger than 1 in aqueous system. Broad voltage keeps the electrode type of all systems unchangeable. MSiGeN 4 (M=Hf, Nb, Zr, V, Ti) can perform photocatalytic water splitting, while all MSiGeN 4 can reduce N 2 to NH 3 , but are not good CO 2 photocatalysts. The Bader charge, work function, and optical properties are further explored. [Display omitted] [ABSTRACT FROM AUTHOR]

The electronic and optical properties, quantum capacitance and photocatalytic performance of Zr 2 CO 2 under uniaxial strain are investigated by density functional theory (DFT). The carrier mobility of Zr 2 CO 2 MXene is also explored. Zr 2 CO 2 can withstand the maximum stress of 9.3 N/M and its tensile limit is 18 % strain. All systems have the character of indirect semiconductor and are mechanically stable within elastic limit. The compressive strain significantly increases the hole mobility in x-direction. Electrons in y-direction have higher mobility in free and tensile strain. Tensile strain generally enhances the performance of Zr 2 CO 2 as cathode material, while compressive strain makes the system transfer from cathode to anode material. Zr 2 CO 2 with larger strain than 7 % has excellent CO 2 photocatalysis, CO 2 is reduced to CH 4 , CH 3 OH, HCHO, CO and HCOOH at pH = 0. Zr 2 CO 2 with strain from −7 to 11 % can reduce N 2 to NH 3. Work function and photocatalytic properties at pH = 7 are also explored. • Compressive strain significantly increases the hole mobility in x-direction. • Tensile strain generally enhances the performance of Zr 2 CO 2 as cathode material. • Zr 2 CO 2 with larger strain than 7 % has excellent CO 2 photocatalysis. • Wide potential doesn't affect the electrode type of Zr 2 CO 2 under strain. [ABSTRACT FROM AUTHOR]

• The work functions of Sc 2 CO 2 -R and Sc 2 CF 2 -R increase with the increasing ratio. • All systems are all cathode materials in whole voltage. • Different functional group ratio does not change the electrode type of the mixed configurations. MXenes are two-dimensional transition metal carbides and nitrides, which are promising for supercapacitor applications. The surface of MXenes is usually covered by a mixture of O, OH, and F groups during etching. The effect of mixed termination on the quantum capacitance and work function (WF) of MXenes is investigated by first principles calculation. The WFs of Sc 2 C(OH) 2 -R are smaller than those of Sc 2 CO 2 -R and Sc 2 CF 2 -R when the OH ratio is larger than about 27 %. The quantum capacitances of Sc 2 C MXenes with different ratio of F, O, and OH groups (named as Sc 2 CF 2 -R, Sc 2 CO 2 -R, Sc 2 C(OH) 2 -R) are explored to predict the electrode type of materials. Negative binding energies confirm the stability of the studied mixed systems. The increasing ratio of O group can make the system have decreased TDOS near Fermi level, thus decreasing quantum capacitance. The top quantum capacitance of Sc 2 CO 2 -R and Sc 2 C(OH) 2 -R systems at negative bias increase with the increasing ratio of O/OH group. All systems have larger storage charge at negative potential, and are all potential cathode materials, especially for Sc 2 CO 2 -R and Sc 2 CF 2 -R MXene with much smaller |Q a |/|Q c | than 1 and larger C c. The quantum capacitance under wide voltage is also considered. The effect of mixed termination on the quantum capacitance and work function (WF) of MXenes is investigated by first principles calculation. The WFs of Sc 2 C(OH) 2 -R are smaller than those of Sc 2 CO 2 -R and Sc 2 CF 2 -R when the OH ratio is larger than about 27 %. The top quantum capacitance of Sc 2 CO 2 -R and Sc 2 C(OH) 2 -R systems at negative bias increase with the increasing ratio of O/OH group. All systems have larger storage charge at negative potential, and are all potential cathode materials, especially for Sc 2 CO 2 -R and Sc 2 CF 2 -R MXene. The quantum capacitance under wide voltage is also considered. [Display omitted] [ABSTRACT FROM AUTHOR]

Janus MXene is attracting more and more interest because of its special characteristics. The electronic and optical properties, work function and quantum capacitance of Janus Hf 2 COT (T = -Br, -Cl, –F, –OH, -P, –S, -Se) MXene are investigated using density functional theory. The analysis of binding energies and formation energies indicate their structural stability. The asymmetric functional group changes Hf 2 CO 2 from a semiconductor to metal Hf 2 COT. Hf 2 COF and Hf 2 COOH have larger absorption coefficient and conductivities in infrared region, which have potential applications for optoelectronic devices. The Janus surface groups can effectively increase the quantum capacitance and further change the electrode type of the system. In aqueous system, Hf 2 CO 2 , Hf 2 COBr, Hf 2 COCl and Hf 2 COP are preferred for cathode electrode material, Hf 2 COOH, Hf 2 COS and Hf 2 COSe are preferred for anode material. The quantum capacitance in ionic/organic system is further explored. The electronic properties and quantum capacitance of Hf 2 C with mixed terminations are also investigated. • The asymmetric functional group changes Hf 2 CO 2 from a semiconductor to metal Hf 2 COT. • Janus surface groups can effectively increase the quantum capacitance and change the electrode type of the system. • Hf 2 COOH, Hf 2 COS and Hf 2 COSe are preferred for anode material in aqueous system. • The electronic properties and quantum capacitance of Hf 2 C with mixed terminations are also investigated. [ABSTRACT FROM AUTHOR]

• Zr 2 CO 2 under +8% strain undergoes the indirect-direct semiconductor transition. • Tensile strain can improve CO 2 photocatalysts and enrich photocatalytic CO 2 reduction products. • Zr 2 CO 2 under strains are all potential cathode materials in whole potential. • Zr 2 CO 2 without strain can spontaneously perform HER with ΔG of 0.98 eV and OER with the overpotential of 1.16 eV. The electronic, optical, and photocatalytic properties of Zr 2 CO 2 MXene under biaxial strain are investigated by hybrid HSE06 functional. The quantum capacitance is also studied. The tensile strain limit of Zr 2 CO 2 is about 17%. Zr 2 CO 2 under +8% strain undergoes the indirect-direct semiconductor transition, which is due to blueshift of Zr-d and O-p states. Absorption coefficient is more sensitive to the tensile strain, and tensile strain can effectively improve the solar energy utilization of Zr 2 CO 2 MXene. Zr 2 CO 2 under strains are all potential cathode materials in aqueous and ionic/organic systems. Zr 2 CO 2 under +2% and +3% strains can perform photocatalytic water splitting in acid environment. Tensile strain can improve the CO 2 photocatalysts and enrich the photocatalytic CO 2 reduction products. The N 2 reducing ability of Zr 2 CO 2 is enhanced under tensile strain, but weakened under compressive strain. Zr 2 CO 2 under strain cannot reduce N 2 to N 2 H and N 2 H 2 at pH=0 and 7. The hydrogen evolution reaction and oxygen evolution reaction are further evaluated. [Display omitted] The electronic, optical, photocatalytic properties and quantum capacitance of Zr 2 CO 2 MXene under biaxial strain are investigated by hybrid HSE functional. Zr 2 CO 2 under +8% strain undergoes the indirect-direct semiconductor transition. Tensile strain can effectively improve the solar energy utilization of Zr 2 CO 2 MXene. Zr 2 CO 2 under strains are all potential cathode materials in whole potential. Tensile strain can improve the CO 2 photocatalysts and enrich the photocatalytic CO 2 reduction products. Zr 2 CO 2 without strain can spontaneously perform HER with ΔG of 0.98 eV and OER with the overpotential of 1.16 eV. [ABSTRACT FROM AUTHOR]

• Sc 2 CO 2 , Zr 2 CO 2 and Hf 2 CO 2 monolayers are potential cathode materials. • M 2 CO 2 monolayer gradually transfers from cathode to anode material with the weakening metallicity of TM atom. • The wide voltage has little influence on the electrode type of materials. • Mixed termination has changed the cathode character of Zr 2 CO 2 and Hf 2 CO 2 to anode character in aqueous system. MXenes have drawn great attention as potential electrode material because of the high capacitive performance. However, there are little information about the quantum capacitance (C diff) and surface storage charge (Q) of M 2 C MXene with pure -O and mixed surface termination. In this paper, density functional theory (DFT) is utilized to explore the C diff and Q of M 2 CO 2 monolayers with 3d, 4d and 5d transitional metals (TM) in aqueous and ionic/organic systems. The most stable configurations for the studied systems are confirmed. Sc 2 CO 2 , Zr 2 CO 2 and Hf 2 CO 2 monolayers are potential cathode materials. Ti 2 CO 2 monolayer is a potential anode material. The wide voltage has little influence on the electrode type of materials. M 2 CO 2 monolayers gradually transfer from cathode to anode material with the weakening metallicity of transitional metal atom. M 2 C MXenes with mixed configuration are further explored. M 2 CO 0.42 F 1.19 (OH) 0.39 (M = Sc, Cr, Nb, Ta) are cathode materials in aqueous system, M 2 CO 0.42 F 1.19 (OH) 0.39 (M = Ti, V, Zr, Mo, Hf, W) are suitable for anode materials. [ABSTRACT FROM AUTHOR]

First-principles calculation was adopted to study quantum capacitance, electronic and photocatalytic properties of Sc 2 CO 2 monolayer under biaxial strain. Our study shows that Sc 2 CO 2 can bear the maximum stress of 14.895 N/M. Sc 2 CO 2 with −10% strain undergoes the semiconductor-metal transition due to the blue shift of C-p, Sc-d, and O-p orbitals. The analysis of photocatalytic properties indicates that monolayer Sc 2 CO 2 under the biaxial strain from −6% to 2% can produce spontaneous water splitting reaction in appropriate pH range. The influence of biaxial strain on electrode type is also investigated in aqueous and ionic/organic systems. The results indicate that biaxial strain has little impact on electrode type of materials except −10% strain. Sc 2 CO 2 with −10% strain is suitable for anode material, while Sc 2 CO 2 under other strains is potential cathode material. The introduction of mixed termination has little influence on the electrode type of Sc 2 CO 0·42 F 1·19 (OH) 0.4 under strains. The wide potential has little influence on electrode type of Sc 2 C with pure –O and mixed termination. • Sc 2 CO 2 can bear the maximum stress of 14.895 N/M. • Biaxial strain has little impact on electrode type of materials except 10% strain. • Sc 2 CO 2 under the biaxial strain from −6% to 2% can produce spontaneous water splitting reaction in appropriate pH range. • Mixed termination has little influence on the electrode type of Sc 2 CO 0·42 F 1·19 (OH) 0.4 under strains. [ABSTRACT FROM AUTHOR]

MXenes are a new two-dimensional materials with potential applications in electrochemical energy storage. The electronic and optical properties and quantum capacitance of pristine Ti 2 CO 2 monolayer (PTM) and Ti 2 CO 2 with C vacancy line (CVL) defect are investigated using density functional theory. The stability of the studied systems is confirmed by the binding energy and vacancy formation energy. The introduction of CVL improves the field emission performance because of the low work function. The systems with CVL defect have superior absorption compared to the PTM system in the infrared region. One of the four tested systems with CVL defect has the strongest absorption and optical conductivity in the infrared region. The quantum capacitance and surface storage charge are further explored in aqueous and ionic/organic systems. The top quantum capacitance and surface storage charge of the studied systems at negative and positive bias are all improved due to the introduction of CVL. The studied systems are all suitable as anode material. The extension of voltage does not change the electrode type of materials. • The introduction of CVL improves the field emission performance because of the low work function. • CVL2 system has the strongest absorption and optical conductivity in infrared region. • The introduction of CVL improves the top C diff and Q of the studied systems at negative and positive bias. [ABSTRACT FROM AUTHOR]

The photocatalytic activity and the quantum capacitance are investigated for Hf 2 CO 2 monolayer under biaxial strain by first-principles calculations. Hf 2 CO 2 monolayer under biaxial strain can reduce H+ to H 2 at pH=0 and has the photocatalytic redox capacity within appropriate pH range. It can also reduce CO 2 and N 2 to different products at pH=0 and 7. Hf 2 CO 2 monolayers under biaxial strain are potential cathode materials for supercapacitors in aqueous electrolyte. Temperature had little effect on the electrode type of the material. [Display omitted] • Hf 2 CO 2 monolayers under strain can reduce H+ to H 2 at pH = 0. • Hf 2 CO 2 monolayers under strain can reduce CO 2 and N 2 to different products at pH = 0 and 7. • Hf 2 CO 2 monolayers under strain are potential cathode materials in aqueous electrolyte. MXenes have great potential applications in energy storage and photocatalyst. The photocatalytic activity and the quantum capacitance of Hf 2 CO 2 monolayer under biaxial strain are investigated in this paper. The result indicates that Hf 2 CO 2 monolayer under biaxial strain can reduce H+ to H 2 at pH = 0 and has the strongest reducing ability at 12% tensile strain. Hf 2 CO 2 monolayer under strain has the photocatalytic capacity for overall water splitting within appropriate pH range. The tensile strain can enhance the CO 2 reduction ability of Hf 2 CO 2 monolayer. The reducing ability of Hf 2 CO 2 for N 2 is improved under tensile strain and reduced under compressive strain. The analysis of exciton binding energy further confirms the potential photocatalytic activity of Hf 2 CO 2 monolayer under strain. Hf 2 CO 2 monolayers under strain are potential cathode materials for supercapacitors in aqueous electrolyte, the wide voltage makes Hf 2 CO 2 monolayer under −6% strain transform into electrode material of symmetric supercapacitors. The effect of temperature on the electrode type of materials is further explored. [ABSTRACT FROM AUTHOR]

The electronic properties and quantum capacitance of pristine Hf 2 CO 2 (PH) and N-doped Hf 2 CO 2 systems are investigated using density functional theory (DFT). All the doped systems have the metallic characteristics. The redshift of C-p, Hf-d, O-p, and Hf-p states in valence band gradually increases with the increasing N-doping concentration. The quantum capacitance and the electrode type of Hf 2 CO 2 monolayer can be effectively modulated by N-doping. PH and PH systems with N-doped concentration of 78% are potential cathode materials, while PH systems with N-doped concentration of 11%, 22%, 33%, 44%, 56%, and 67% are potential anode materials, PH systems with 89% and 100% N-doped concentration are suitable for symmetric electrode materials in aqueous system. The wide voltage changes the electrode type of PH system with N-doped concentration of 78%, 89% and 100%. The electrode type of N-doping MXene with mixed terminations are further explored. • All the doped systems have the metallic characteristics. • N-doping can tune the quantum capacitance and the electrode type of Hf 2 CO 2 monolayer. • The wide voltage changes the electrode type of some N-doped systems. • Quantum capacitance of four systems with mixed terminations are explored. [ABSTRACT FROM AUTHOR]

MXenes are excellent electrode materials and have superior photocatalytic activity. The electronic properties, optical properties, photocatalytic activity and quantum capacitance of Sc 2 CT 2 (T = F, P, Cl, Se, Br, O, Si, S, OH) monolayers are explored by first-principles calculations. All the possible configurations are considered. The results indicate that the introduction of F, Cl, Br, S groups results in the metal-to-indirect-semiconductor transition, while the introduction of O, OH groups makes the metal-to-direct-semiconductor transition. Sc 2 C(OH) 2 monolayer can be applied in optoelectronic fields to improve its power efficiency because of low work function. The analysis of optical properties indicates that Sc 2 CT 2 (T = P, Si) monolayers show the strong absorption performance in infrared region, while Sc 2 CT 2 (T = F, Cl, Se, Br, O, S, OH) monolayers show the strong absorption performance in ultraviolet region. When compared with other systems, Sc 2 CT 2 (T = O, S) monolayers have the better photocatalytic activity within the pH scope of 7.52–14 and 0–0.2, respectively. Our research also indicates that Sc 2 CT 2 (T = F, P, Cl, Se, Br, O, S, OH) monolayers are potential cathode materials in aqueous and ionic/organic systems. • Sc 2 CT 2 (T = P, Si) monolayers show the strong absorption performance in infrared region. • Sc 2 CT 2 (T = O, S) monolayers have good photocatalytic activity at appropriate pH scope. • Sc 2 C and Sc 2 CSi 2 are potential electrode materials for symmetric supercapacitors in aqueous system. • The introduction of F, Cl, Br, S groups results in the metal-to-indirect-semiconductor transition. [ABSTRACT FROM AUTHOR]

The electronic properties, optical properties, and quantum capacitance of bare Hf 2 C and Hf 2 CT 2 (T= -O, -S, -Se, -F, -Cl, –OH) are investigated by density functional theory. All the functionalized Hf 2 CT 2 MXenes have higher optical conductivity than Hf 2 C in the ultraviolet region. The wide voltage has little effect on the type of electrode materials Hf 2 CF 2 , Hf 2 CCl 2 , Hf 2 CO 2 , Hf 2 CS 2 , Hf 2 CSe 2 and Hf 2 C(OH) 2 , while the extension voltage modulates Hf 2 C from anode materials in aqueous system to cathode materials in ionic/organic system. [Display omitted] • The introduction of O groups makes the system undergo the transition from metal to semiconductor. • All the functionalized Hf 2 CT 2 MXenes have higher optical conductivity in the ultraviolet region. • The wide voltage doesn't change the type of electrode materials except Hf 2 C MXene. Density functional theory was used to explore the electronic properties, optical properties, and quantum capacitance of bare Hf 2 C and functionalized Hf 2 CT 2 (T = -O, -S, -Se, -F, -Cl, –OH) MXenes. The introduction of functional group improves the stability of the system because of larger binding energies and negative formation energies. The introduction of O groups makes the system undergo the transition from metal to semiconductor, while the introduction of other groups doesn't change the metal character. All the functionalized Hf 2 CT 2 MXenes have higher optical conductivity than Hf 2 C in deep UV region, and Hf 2 CSe 2 has the strongest absorption and optical conductivity in infrared region among all the studied systems. The introduction of functional groups can effectively modulate the quantum capacitance and the type of electrode materials. Hf 2 CCl 2 , Hf 2 C(OH) 2 , Hf 2 C, and Hf 2 CF 2 are potential anode electrode materials, while Hf 2 CO 2 , Hf 2 CS 2 , and Hf 2 CSe 2 are potential cathode electrode materials. The wide voltage doesn't change the type of electrode materials except Hf 2 C MXene. The work function and Bader charge are further analyzed. In addition, H 2 C MXene with mixed termination is further explored. [ABSTRACT FROM AUTHOR]

Two-dimensional materials have attracted great attention due to their excellent properties. In this paper, we conducted a theoretical study on the atomic swap within Sc 2 CF 2 MXene. The geometric structure, electronic properties and quantum capacitance of Sc 2 CF 2 -S C↔F , Sc 2 CF 2 -S C↔Sc , and Sc 2 CF 2 -S F↔Sc monolayers are investigated by first-principles calculation. Sc 2 CF 2 -S C↔Sc and Sc 2 CF 2 -S F↔Sc monolayers form Frenkel-type defect structure after optimization. Pristine Sc 2 CF 2 is an indirect semiconductor, while the atomic swap has no effect on the semiconductor character of Sc 2 CF 2 -S C↔F , Sc 2 CF 2 -S C↔Sc , and Sc 2 CF 2 -S F↔Sc monolayers. Especially, the atomic swap between C and Sc atoms make Sc 2 CF 2 -S C↔Sc monolayer become a narrow direct semiconductor. The quantum capacitances in aqueous and ionic/organic systems are further investigated. In aqueous and ionic/organic electrolyte, Sc 2 CF 2 and Sc 2 CF 2 -S F↔Sc are more suitable for cathode materials, while Sc 2 CF 2 -S C↔Sc monolayer is suitable for anode materials. Sc 2 CF 2 -S C↔F is transformed from a potential cathode material to a potential electrode material for symmetric supercapacitors with the extension of voltage. The effective mass and work function of the systems are further explored. The electronic properties and quantum capacitance of Sc 2 CT 2 with mixed termination groups are also investigated. • Sc 2 CF 2 -S C↔Sc and Sc 2 CF 2 -S F↔Sc monolayers form Frenkel-type defect structure after optimization. • Sc 2 CF 2 and Sc 2 CF 2 -S F↔Sc are more suitable for cathode materials in aqueous and ionic/organic systems. • The introduction of mixed termination improves the QC as the cathode material in aqueous system. [ABSTRACT FROM AUTHOR]

MXenes are widely applied in electrodes of supercapacitors because of the higher volume/surface ratio. In this paper, density functional theory (DFT) is used to investigate the quantum capacitance (C diff) and surface storage charge (Q) of F-functionalized M 2 C (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W) MXenes in aqueous and ionic/organic systems. Three possible configurations for F termination are explored, and M-top configuration with F on top of M atom is the most stable. The results indicate that Sc 2 CF 2 is the most suitable cathode material of asymmetric supercapacitor in aqueous and ionic/organic systems, while V 2 CF 2 is the most preferred anode material of asymmetric supercapacitor. Ti 2 CF 2 and Zr 2 CF 2 are electrode materials of symmetric supercapacitor in aqueous and ionic/organic systems. The extension of the voltage modulates Mo 2 CF 2 from the electrode material of symmetric capacitance in aqueous system to the most suitable anode material in ionic/organic system. Nb 2 CF 2 and Ta 2 CF 2 are potential cathode materials in aqueous system, but change to the suitable anode materials in ionic/organic system. Our findings provide the theoretical foundation to develop MXene-based supercapacitors and energy-storage devices. • M-top configuration with F on top of M atom is the most stable structure. • V 2 CF 2 is the most preferred anode material of asymmetric supercapacitor. • The extension of the voltage can effectively modulate the electrode type. • V 2 CT 2 with mixed termination is suitable as the anode material of asymmetric supercapacitor in aqueous and ionic/organic systems. [ABSTRACT FROM AUTHOR]

The effect of the biaxial strain on electronic structure and quantum capacitance of NH 3 adsorption on pristine Hf 2 CO 2 MXene has been theoretically investigated by density functional theory. TH-PH system under −2% strain changes from chemical to physical adsorption and is suitable to be used for NH 3 detection due to the high sensitivity and the short recovery time. The strain can generally improve the maximum C diff of Hf 2 CO 2 MXene. [Display omitted] • TH-PH system under −2% strain exhibits the outstanding selectivity and sensitivity to NH 3. • TH-PH system under −9% strain undergoes the transition from semiconductor to metal. • Strain can generally improve the maximum C diff of Hf 2 CO 2 MXene. • Hf 2 CO 2 under strain is a potential cathode material for asymmetric supercapacitors. The adsorption of NH 3 molecule on Hf 2 CO 2 monolayer is investigated by first-principles calculation. The possible adsorption sites are all considered. The results show that TH-PH system (N atom of NH 3 molecule on the top site over Hf atom) is the most suitable adsorption system among all the possible configurations. The recovery times of TH-PH system are very short in the strain range from −2% to +2%. TH-PH system under −2% strain changes from chemical adsorption to physical adsorption and is suitable to be used for NH 3 detection due to the high sensitivity and short recovery time. The analysis of the effective mass indicates that applying strain can improve electron transfer in the valence band. The quantum capacitance of TH-PH system under strain is further explored. Hf 2 CO 2 under strain is a potential cathode material for asymmetric supercapacitors. The introduction of the strain can generally improve the maximum C diff of Hf 2 CO 2 MXene. [ABSTRACT FROM AUTHOR]

First-principles calculations are used to systematically investigate the structural, electronic properties and quantum capacitance of Ti 2 CO 2 monolayer with different oxygen vacancy concentration (OVC). The band unfolding technique is performed to obtain the effective band structure in the primitive cell. 32 configurations with OVC of 5.56%, 11.11%, and 16.67% are confirmed. Based on the binding energies and vacancy formation energy, three configurations with one oxygen vacancy at C site, two oxygen vacancies at (A, F) and three oxygen vacancies at (I, V, 5) are further investigated. Pristine Ti 2 CO 2 is an indirect semiconductor with the band gap of 0.2871 eV. The introduction of two or three oxygen vacancies leads to the electronic phase transition from semiconductor to metal. The further analysis indicates that the red shift of Ti-d state results in the decrease of band gap for Ti 2 CO 2 with OVC of 5.56%, while Ti-d and O-p states broadening result in the metallization of the systems with OVC of 11.11% and 16.67%. The introduction of oxygen vacancy can generally improve the quantum capacitance and surface storage charge of perfect Ti 2 CO 2 , especially for Ti 2 CO 2 with OVC of 5.56%. The results could provide useful guidance for the practical applications of Ti 2 CO 2 with different OVC in nanoelectronic devices. • 32 configurations with OVC of 5.56%, 11.11%, and 16.67% are confirmed. • The introduction of single Vo results in the decrease of band gap. • The introduction of Vo improves the quantum capacitance of perfect Ti 2 CO 2. [ABSTRACT FROM AUTHOR]

• The doping of 3d TM atoms can effectively modulate the magnetism of pristine Sc 2 CF 2. • Mn@PS is an excellent anode material for aqueous and ionic/organic systems. • The doping of Ti, V, Cr, and Mn atoms changes the type of electrode material for aqueous systems. • PS, VS, and Y@PS are suitable for cathode materials for aqueous and ionic/organic systems. • Re@PS system is suitable for aqueous system, but not preferable for ionic/organic systems. MXenes are potential electrode materials in supercapacitors. However, the theoretical investigation of the quantum capacitance (C Q) for transitional-metal (TM) doped MXenes is scarce, and the types of electrode are unclear because of TM doping. Herein, C Q and surface storage charge (Q) of 13 kinds of 3d, 4d, and 5d TM atoms and vacancy doped Sc 2 CF 2 , named as TM@PS and VS, are investigated theoretically. The doping of 3d TM atoms can effectively modulate the magnetism of pristine Sc 2 CF 2. C Q and Q of Sc 2 CF 2 -based electrode materials are effectively improved and the type of electrode materials is changed because of TM doping. For aqueous and ionic/organic systems, Mn@PS is an excellent anode material, while PS, VS, and Y@PS are more suitable for cathode materials of asymmetric supercapacitors. V@PS, Zr@PS, Nb@PS, Hf@PS, and Ta@PS are more suitable for anode materials in ionic/organic system. Fe@PS is a suitable electrode material of symmetric supercapacitors with high C Q and Q in aqueous system, but a cathode material of asymmetric supercapacitors in ionic/organic system. The results can provide useful information to design and develop the high-performance electrode materials. [ABSTRACT FROM AUTHOR]

The electronic structure, surface charge storage and quantum capacitance of pristine and vacancy-defected Ti 2 CO 2 MXene have been theoretically investigated by density functional theory. The adsorption of Li atom on PT monolayer makes the system undergo the semiconductor-metal transition, while the adsorption of Li atom on VT monolayer doesn't change the metal nature. The introduction of C vacancy improves the quantum capacitance of the Li-adsorbed VT monolayers at the zero-voltage region. [Display omitted] • The adsorption processes of Li atom on systems are exothermic and favorable to adsorption. • The adsorption of Li atom make pristine Ti 2 CO 2 undergo the semiconductor-metal transition. • The introduction of C-vacancy improves the quantum capacitance of the Li-adsorbed systems at 0 V. • The magnetism is observed for VT-LH system, with the total magnetic moments of 0.18 μ B. The electronic structure, surface charge storage and quantum capacitance of lithium adsorption on pristine and vacancy-defected Ti 2 CO 2 MXene are theoretically investigated by density functional theory. Negative adsorption energies (E ads) of pristine Ti 2 CO 2 (PT) and C-vacancy Ti 2 CO 2 (VT) monolayers adsorbed by Li atom indicate that the adsorption processes of Li atom on systems are exothermic and favorable to adsorption. The most stable configurations are confirmed for the Li-adsorbed PT and VT monolayers. Pristine Ti 2 CO 2 (PT) monolayer is a semiconductor with the bandgap of 0.2035 eV, while C-vacancy Ti 2 CO 2 (VT) monolayer has the metallic nature. The adsorption of Li atom on PT monolayer makes the system undergo the semiconductor-metal transition, while the adsorption of Li atom on VT monolayer doesn't change the metal nature. The magnetism is observed for the system with Li atom directly adsorbed on top of hollow site (VT-LH), and the total magnetic moments is 0.18 μ B. Charge density differences of the adsorption of Li atom on PT and VT monolayers are further explored. The introduction of C-vacancy improves the quantum capacitances of the Li-adsorbed systems at 0 V, and Li-adsorbed PT and VT monolayers have high quantum capacitance because of the large density of states near Fermi level, and are suitable for cathode material. [ABSTRACT FROM AUTHOR]

Density functional theory calculation is used to investigate the effect of atomic vacancy on stability, electronic and optical properties, quantum capacitance and surface charge storage of pristine and vacancy-defected Zr 2 CO 2 MXenes. The introduction of O vacancy results in the decrease of band gap, while the introduction of C and Zr vacancies results in the semiconductor-metal transition. The introduction of Zr vacancy results in the appearance of magnetism, which is contributed by O-p y , O-p x , C-p z , Zr-d z2 , Zr-d yz states. The analysis of optical properties indicates that the introduction of the vacancy can make the absorption spectra of the systems more sensitive to the infrared light, and reflectivity is improved, especially in the infrared region. For asymmetric supercapacitor, pristine Zr 2 CO 2 MXene with Zr vacancy can be considered as the potential cathode material, while pristine Zr 2 CO 2 MXenes with C and O vacancy are potential anode materials through the analysis of quantum capacitance and surface charge storage. The increase mechanism of quantum capacitance is further explored through the analysis of density of states. • The introduction of C and Zr vacancy results in the semiconductor-metal transition. • The introduction of Zr vacancy results in the appearance of magnetism, which is contributed by O-p y , O-p x , C-p z , Zr-d z2 , Zr-d yz states. • The absorption spectra of studied systems are more sensitive to the infrared light because of the introduction of vacancy. • PZ-V Zr is a promising cathode material, while PZ-V C and PZ-V O MXenes are potential anode materials. [ABSTRACT FROM AUTHOR]