Phosphorus‐Doped Nanocrystalline Diamond for Supercapacitor Application

Interlayer Affected Diamond Electrochemistry

Diamond electrochemistry is primarily influenced by quantities of sp3-carbon, surface terminations, and crystalline structure. In this work, a new dimension is introduced by investigating the effect of using substrate-interlayers for diamond growth. Boron and nitrogen co-doped nanocrystalline diamond (BNDD) films are grown on Si substrate without and with Ti and Ta as interlayers, named BNDD/Si, BNDD/Ti/Si, and BNDD/Ta/Ti/Si, respectively. After detailed characterization using microscopies, spectroscopies, electrochemical techniques, and density functional theory simulations, the relationship of composition, interfacial structure, charge transport, and electrochemical properties of the interface between diamond and metal is investigated. The BNDD/Ta/Ti/Si electrodes exhibit faster electron transfer processes than the other two diamond electrodes. The interlayer thus determines the intrinsic activity and reaction kinetics. The reduction in their barrier widths can be attributed to the formation of TaC, which facilitates carrier tunneling, and simultaneously increases the concentration of electrically active defects. As a case study, the BNDD/Ta/Ti/Si electrode is further employed to assemble a redox-electrolyte-based supercapacitor device with enhanced performance. In summary, the study not only sheds light on the intricate relationship between interlayer composition, charge transfer, and electrochemical performance but also demonstrates the potential of tailored interlayer design to unlock new capabilities in diamond-based electrochemical devices.

Rational Design of Diamond Electrodes

Diamond electrodes stepped onto the stage in the early 1990s for electroanalytical applications. They possess the features of long-term chemical inertness, wide potential windows, low and stable background currents, high microstructural stability at different potentials and in different media, varied activity toward different electroactive species, reliable electrochemical response of redox systems without conventional pretreatment, high resistance to surface fouling in most cases, and possibility of forming composites with different components such as other carbon materials, carbides, and oxidizes. Most diamond electrodes are prepared in microcrystalline or nanocrystalline form using chemical vapor deposition techniques. Starting from diamond films and diamond composites, numerous nanostructured diamond electrodes have also been produced. The features of diamond electrodes are therefore heavily dependent on the growth conditions and post-treatment procures that are applied on diamond electrodes such as introduced dopant(s), surface termination(s), surface functional group(s), added components, and final structure(s). Numerous applications of diamond electrodes have been explored in the fields of electrochemical sensing, electrosynthesis, electrocatalysis, electrochemical energy storage and conversion, devices, and environmental degradation.This Account summarizes our strategies to design different diamond electrodes, including diamond films, diamond composites, as well as their nanostructures. With respect to diamond films, the modulation of their dopant(s) and surface termination(s) as well as the attachment of functional modifier(s) onto their surface are discussed. Electrochemical hydrogenation and oxygenation of diamond electrodes are detailed at an atomic scale. As the examples of designing diamond electrodes at a molecular scale, photochemical and electrochemical attachment of modifier(s) onto diamond electrodes are shown. Moreover, electrochemical grafting of diazonium salts is proposed as a new technique to identify hydrogenated, hydroxylated, and oxygenated terminations of diamond electrodes. The introduction of additional component(s) into a diamond film to form diamond composites is then overviewed, where a hydrogen-induced selective growth model is proposed to elucidate the preparation of diamond/β-SiC composites. Subsequently, the production of various diamond nanostructures from diamond films and composites by means of top-down, bottom-up, and template-free approaches is shown. Electrochemical application examples of diamond electrodes are overviewed, covering direct electrochemistry of natural Cytochrome c on a hydroxylated diamond surface, sensitive electrochemical DNA biosensing on tip-functionalized diamond nanowires, and construction of high-performance supercapacitors using diamond electrodes and redox electrolytes. Our diamond supercapacitors, also named battery-like diamond supercapacitors or diamond supercabatteries, are highlighted since they combine the features of supercapacitors and batteries. Future perspectives of diamond electrodes are outlined, ranging from their rational design and synthesis to their electrochemical applications in different fields.

Electrochemical Capacitors with Confined Redox Electrolytes and Porous Electrodes

Electrochemical capacitors (ECs), including electrical-double-layer capacitors and pseudocapacitors, feature high power densities but low energy densities. To improve the energy densities of ECs, redox electrolyte-enhanced ECs (R-ECs) or supercapbatteries are designed through employing confined soluble redox electrolytes and porous electrodes. In R-ECs the energy storage is based on diffusion-controlled faradaic processes of confined redox electrolytes at the surface of a porous electrode, which thus take the merits of high power densities of ECs and high energy densities of batteries. In the past few years, there has been great progress in the development of this energy storage technology, particularly in the design and synthesis of novel redox electrolytes and porous electrodes, as well as the configurations of new devices. Herein, a full-screen picture of the fundamentals and the state-of-art progress of R-ECs are given together with a discussion and outlines about the challenges and future perspectives of R-ECs. The strategies to improve the performance of R-ECs are highlighted from the aspects of their capacitances and capacitance retention, power densities, and energy densities. The insight into the philosophies behind these strategies will be favorable to promote the R-EC technology toward practical applications of supercapacitors in different fields.

Single-step grown boron doped nanocrystalline diamond-carbon nanograss hybrid as an efficient supercapacitor electrode

Direct synthesis of a nano-structured carbon hybrid consisting of vertically aligned carbon nanograsses on top of boron-doped nanocrystalline diamond is demonstrated and the carbon hybrid is further applied as an electrode material for the fabrication of supercapacitors. The hybrid film combines the dual advantages of sp2 (carbon nanograss) and sp3 (nanocrystalline diamond) bonded carbon, possessing not only the excellent electrical characteristics of sp2 carbon but also the exceptional electrochemical stability of sp3 carbon. As a result, the specific capacitance of the as-prepared hybrid material reaches up to 0.4 F cm-2, one of the highest reported in diamond-based supercapacitors. The entire electrochemical results exhibit enhanced electron transfer efficiency with remarkable stability of 95% of capacitance retention even after 10 000 cycles.