Exploring the Potential of MBenes Supercapacitors: Fluorine-Free Synthesized MoAl1–xB with Ultrahigh Conductivity and Open Space

The present study describes the synthesis of multilayered MBenes MoAl1–xB with different degrees of Al deintercalation using a mild, fluorine-free approach of dilute alkali to remove Al from MoAlB. We propose an etching route and compare it to conventional fluoride etching products. Additionally, the study explores the possible application and energy storage mechanism of MBenes in supercapacitors, marking the first investigation of its kind. At room temperature, 1/24-MoAl1–xB with terminal groups −OH exhibits ∼25% Al removal in 1 wt % NaOH for 24 h, outperforming traditional etching technology. Increasing the Al removal exposed more open space, resulting in a higher capacitance. Compared to LiF/HCl-MoAl1–xB (etched by LiF + HCl), 1/24-MoAl1–xB has a higher energy storage capability. The multilayered 1/24-MoAl1–xB film electrode exhibits ultrahigh conductivity with a rapid relaxation time of 0.97 s and high areal capacitance (2006.60 mF cm–2) while maintaining 80.2% capacitance after 5000 cycles. The MoAl1–xB all-solid-state supercapacitor (ASSS) delivers a high capacitance of 741.6 mF cm–2 at 1 mV s–1 for a single electrode and exhibits stable capacitance even at a 90° bending angle, highlighting its potential practical use. Our research represents an important step in the synthesis of MBenes and highlights their potential applications in supercapacitors.


Capacitance
In a 3-electrode set, the areal capacitance for the film electrode could be calculated by where CS (mF cm -2 ), and CV (mF cm -3 ) are the areal capacitance and volumetric capacitance, respectively. GCD/CV means calculating from GCD or CV curves and single-electrode/device represents the capacitance for single electrode or the entire device. I, A, Δt, ΔV, and T are the applied current (mA), the area of the electrodes (cm 2 ), discharging time (s), the potential window (V), and the thickness of the film electrode (cm), respectively ∫ ( ) is the integrated area of the enclosed CV curve, and v is CV scan rate (V s -1 ). 3 Use Equation S9~10 to calculate the energy density E and power density P for ASSS: 1,

Diffusion coefficient
Based on the EIS results, the diffusion coefficient (D) can be analyzed by fitting the real part (Zre) of the impedance with the square root of the radial frequency (ω -1/2 ) as in the following Equation S11, S12: Where R is the gas constant, T is the absolute temperature, S is the surface area of the electrode, n is the number of electronic transfers, F is the Faraday constant, C is the H + ion concentration, and σ is the Warburg factor related to Zre.

Complex model of capacitance
The complex model of capacitance was used to further confirm the impedance behavior.
the relaxation time constant can be calculated according to the peak position for C"(ω) from the equation τ0=1/f0, where f0 is the frequency.

Electrochemical kinetics analysis
1) CV method 4 Electrochemical kinetics analysis has been preliminarily analyzed by the CV method: 5 4 = (S15) Where i represents the current, v represents the sweep rate, and a as well as b are the adjustable coefficients. The b-value is determined from the slope of the plot of log(i) versus log(υ).

2) Dunn's method
The surface-controlled contribution to the overall current response was further quantified by conducting Dunn's method. The current response is proportional to the scan rate for a surface-controlled process, while the current response is proportional to the square root of the scan rate for a diffusion-controlled process as follows: 4, 5 Where i(v) is the current response under a fixed potential, k1v represents the surfacecontrolled contribution, k2v 1/2 represents the diffusion-controlled contribution, and v is the scan rate, where k1 and as k2 are constants.

XPS Analysis for MoAlB and 1/24-MoAl1-xB 2.1 MoAlB
The species belonging to the MoAlB compound were extracted from the highresolution spectra fitting shown in Figure 4 (lower spectra) and surface oxides. 8 The high-resolution spectra of the Al 2p region were fitted by two components MoAlB and Al2O3, the first belongs to Al species in the MoAlB compound 7 and the second belongs to Al2O3 surface oxide. 9 While the high-resolution XPS spectra 5 of B 1s were fitted by 2 components MoAlB and B2O3, the first is assigned to B species in the MoAlB compound 7 and the second is assigned to B2O3 surface oxide. 10 As for the high-resolution spectra of O 1s, it was fitted by two components Al oxide 9 and Mo oxide 11 .