Computational screening toward quantum capacitance of transition-metals and vacancy doped/co-doped graphene as electrode of supercapacitors

Functionalised biphenylene and graphenylene: excellent choices for supercapacitor electrodes

Quantum capacitance (CQ) is a crucial parameter that reflects the energy storage capacity of supercapacitors. In this work, we extensively investigate the effect of vacancy induced defects on quantum capacitance of well studied biphenylene (BPN) and graphenylene (GPN) monolayers. Based on density functional theory (DFT), we have systematically studied the consequence of vacancies on structural stability, charge distribution, electronic band structure of pristine systems, and correlated this with the variation of quantum capacitance with applied voltage. The results demonstrate that insertion of a vacancy significantly improves the density of states (DOS) profile around the Fermi level for both structures, attributed to the localisation of charge carriers. This leads to higher CQ values for relatively lower potential, with the highest value of CQ being 221 μF cm-2 for defective BPN. Furthermore, we have calculated the density of surface charge for different voltages to evaluate the adaptability of particular materials as a cathode or anode. It is found that both pristine and defective BPN are more preferable as anode materials. It is noteworthy that GPN and its vacancy induced structure, stand-out as superior candidates for a symmetric supercapacitor in aqueous systems. This work will provide valuable insights to design BPN and GPN-based high performance electrode materials for electric double layer (EDL) supercapacitors.

Element screening of metal sites in Fe-based Prussian blue framework materials for ammonium ion battery applications: a first-principles study

Prussian blue framework materials are expected to be the next generation of electrode materials for commercial batteries because their three-dimensional framework structures facilitate the rapid transport and storage of ions and a variety of redox processes. This work compared the calculations of the model before and after the dispersion correction, and the model considering the effect of van der Waals force was more stable. In addition, the distances between H, C and N atoms were within the range of van der Waals force. Thus it was confirmed that NH4+ was adsorbed on the Ax site in the Prussian blue framework material (AxMa[Mb(CN)6]) by van der Waals interaction, and the charge transfer was mainly achieved by the interaction between the H atom in NH4+ and the N atom in the Prussian blue framework. On this basis, the properties of NH4+ batteries were theoretically screened for the Fe-based Prussian blue analogues (PBAs) with different Ma elements (Ma = Co, Cu, Fe, Mg, Mn, Ni, V or Zn). Considering the regulating effect of different metal elements on the electronic structures of PBAs, MgFe and ZnFe PBAs as the electrode materials of NH4+ batteries are expected to show excellent electrochemical energy storage performance in organic electrolytes.

Theoretical investigation of quantum capacitance of Co-doped α-MnO2 for supercapacitor applications using density functional theory

The rapid depletion of fossil fuels and ever-growing energy demand have led to a search for renewable clean energy sources. The storage of renewable energy calls for immediate attention to the fabrication of efficient energy storage devices like supercapacitors (SCs). As an electrode material for SCs, MnO2 has gained wide research interest because of its high theoretical capacitance, variable oxidation state, vast abundance, and low cost. However, the low electric conductivity of MnO2 limits its practical application. The conductivity of MnO2 can be enhanced by tuning the electronic states through substitution doping with cobalt. In the present work, first principles analysis based on density functional theory (DFT) has been used to examine the quantum capacitance (CQC) and surface charge (Q) of Co-doped MnO2. Doping enhanced the structural stability, electrical conductivity, potential window, and quantum capacitance of α-MnO2. The shortened band gap and localized states near the Fermi level improve the CQC of α-MnO2. For the narrow potential range (-0.4 to 0.4 V), the CQC is observed to increase with doping concentration. The highest CQC value at +0.4 V is observed to be 2412.59 μF cm-2 for Mn6Co2O16 (25% doping), five times higher than that of pristine MnO2 (471.18 μF cm-2). Mn6Co2O16 also exhibits better CQC and 'Q' at higher positive bias. Hence, it can be used as an anode material for asymmetric supercapacitors. All these results suggest better capacitive performance of Co-doped α-MnO2 for aqueous SCs and as an anode material for asymmetric supercapacitors.

Theoretical Studies on the Quantum Capacitance of Two-Dimensional Electrode Materials for Supercapacitors

In recent years, supercapacitors have been widely used in the fields of energy, transportation, and industry. Among them, electrical double-layer capacitors (EDLCs) have attracted attention because of their dramatically high power density. With the rapid development of computational methods, theoretical studies on the physical and chemical properties of electrode materials have provided important support for the preparation of EDLCs with higher performance. Besides the widely studied double-layer capacitance (C_D), quantum capacitance (C_Q), which has long been ignored, is another important factor to improve the total capacitance (C_T) of an electrode. In this paper, we survey the recent theoretical progress on the C_Q of two-dimensional (2D) electrode materials in EDLCs and classify the electrode materials mainly into graphene-like 2D main group elements and compounds, transition metal carbides/nitrides (MXenes), and transition metal dichalcogenides (TMDs). In addition, we summarize the influence of different modification routes (including doping, metal-adsorption, vacancy, and surface functionalization) on the C_Q characteristics in the voltage range of ±0.6 V. Finally, we discuss the current difficulties in the theoretical study of supercapacitor electrode materials and provide our outlook on the future development of EDLCs in the field of energy storage.

Understanding the Electric Double-Layer Structure, Capacitance, and Charging Dynamics

Significant progress has been made in recent years in theoretical modeling of the electric double layer (EDL), a key concept in electrochemistry important for energy storage, electrocatalysis, and multitudes of other technological applications. However, major challenges remain in understanding the microscopic details of the electrochemical interface and charging mechanisms under realistic conditions. This review delves into theoretical methods to describe the equilibrium and dynamic responses of the EDL structure and capacitance for electrochemical systems commonly deployed for capacitive energy storage. Special emphasis is given to recent advances that intend to capture the nonclassical EDL behavior such as oscillatory ion distributions, polarization of nonmetallic electrodes, charge transfer, and various forms of phase transitions in the micropores of electrodes interfacing with an organic electrolyte or ionic liquid. This comprehensive analysis highlights theoretical insights into predictable relationships between materials characteristics and electrochemical performance and offers a perspective on opportunities for further development toward rational design and optimization of electrochemical systems.

MOF-derived electrochemical catalyst Cu-N/C for the enhancement of amperometric oxygen detection

Electrochemical sensors using ionic liquids as electrolytes for oxygen detection are now getting more and more attention. Recently, an ionic liquid combined with an electrochemically active catalyst system has become popular for boosting the sensing performance of oxygen sensors. In this work, the imidazolyl-based ionic liquid 1-butyl-2,3-dimethylimidazole bis((trifluoromethyl)sulfonyl)imide [Bmmim][TFSI] is first prepared by a facile two-step method. Subsequently, a transition metal and N-codoped porous carbon oxygen reduction electrochemical catalyst Cu-N/C is synthesized by calcining the Cu-doped ZIF-8 precursor and then blending it in different ratios with the ionic liquid [Bmmim][TFSI] as composite electrolytes for oxygen detection. The composite electrolyte Cu-N/C/[Bmmim][TFSI] exhibits increased responses in cyclic voltammetry (CV) and chronoamperometry (CA) relative to that of the pure ionic liquid. Furthermore, the CV and CA data show that 6% Cu-N/C/[Bmmim][TFSI] has the optimum oxygen sensing response with an enhanced reduction peak current, a sensitivity of 0.1678 μA/[% O₂] and a good linear fitting coefficient of 0.9991. In conclusion, the results confirm the success of using Cu-N/C as an electrochemical catalyst composed of the Cu-N/C/[Bmmim][TFSI] electrolyte for improving the responsivity, stability and sensitivity towards a wide range of oxygen concentrations.