Thin Films and Coatings for Energy Storage and Conversion: From Supercapacitors and Batteries to Hydrogen Generators

The transition to a green economy is becoming an important challenge for sustainable economic growth. Thus, there is a need for novel innovative structures and solutions for effective energy storage and conversion. New materials such as metal oxides, 2D metal chalcogenides, or carbon-based materials with unique properties will increase the performance and efficiency of these systems. The development, synthesis, and research of these materials and material-based coatings are key directions in the development of new types of supercapacitors, Li-ion/Na-ion batteries, and hydrogen or oxygen generators with remarkable properties and performance.

Supercapacitors are efficient and sustainable energy storage devices, which are distinctive due to their higher power density and fast charge/discharge rates. The main challenge preventing their wider use is the increase in the energy density to values comparable to those of secondary batteries and fuel cells. The supercapacitor structure for energy storage requires a large specific surface area to achieve high performance. Engineering of the preparation and material properties of structures on the nanoscale is essential for achieving a better performance of energy storage devices [1,2]. With the high specific surface area and good wettability, ions in the electrolyte could easily be captured thereupon, participating in electrode reaction and making full use of electrode active materials.

There are many published studies describing the synthesis and use of metal chalcogenides or carbon-based nanomaterial coatings to increase the performance of supercapacitors. In the work of Tomar et al. [3], hexagonal WSe₂ thin-film electrodes were deposited on graphite sheets using a DC magnetron sputtering technique at a low temperature of 200 °C. The hexagonal nanoflakes-like morphology provided a porous structure with a larger surface area that more than doubled the electrochemical performance of the WSe₂ thin films in terms of capacitance and energy density. In the study of Sugianto et al. [4], a graphene oxide (GO/ZnO) composite was synthesized by the hydrothermal technique using various ratio compositions of GO/ZnO. The characterization revealed different morphologies of the GO/ZnO compound, where ZnO-nanorods developed on the GO surface showed the best electrochemical properties. Dai et al. [5] synthesized NiS₂ microflowers with integrated NiS₂ nanoparticles and mesopores using a facile two-step method: a hydrothermal process coupled with subsequent sulfidation. The single-phase NiS₂ microflowers obtained provided efficient conduction and a unique nano/microstructure, which contributed to improving electrochemical properties such as specific capacity and rate performance. Wang et al. [6] revealed an improvement in specific capacitance by enhancing the layer spacing of multi-layered nanosheets of 1T-MoS₂ and 2H-MoS₂ electrodes. The improvement was achieved by better intercalation during the expansion of interlayer spacing. Electrodes achieved outstanding cycle stability, where the capacitance retention remained above 84% after 30,000 cycles.

Two-dimensional (2D) materials offer the design of different structures with an enhanced specific surface area. Due to the unique properties of 2D materials, many studies were published regarding their fabrication and utilization for energy storage and conversion applications. In the work of Varghese et al. [7], the LiV₃O₈ thin films were deposited on the stainless-steel substrate by the spray pyrolysis method at temperatures below 500 °C. The effect of the molar concentration of raw materials (Li/V) was studied with the highest specific discharge capacitance value of 695 F g⁻¹ at a current density of 0.25 A g⁻¹. Thin NiCo₂O₄ nanoparticles were densely coated on carbon nanotubes (CNTs) in the work of Cheng et al. [8]. Core shell structures were formed by coating 2D metal oxide on CNTs. A significant improvement in specific capacitance and stability was achieved using the nanoparticle coating of NiCo₂O₄ compared to the classical hybrid of NiCo₂O₄ nanosheets in CNTs. Azadmanjiri et al. [9] modified the surface of 2D Ti₃C₂T_X MXene with chalcogen elements (S, Se, and Te). They investigated the effect of surface nanoscale modification on the supercapacitive performance of electrodes. Electrodes with chalcogen coatings obtained significantly higher capacitance than pure Ti₃C₂T_X MXene, showing great potential in different coating modifications.

Karmakar et al. [10] created novel phase Q-carbon thin films via pulsed laser annealing of amorphous diamond-like carbon. Different carbon structures such as filaments, clusters, and microdots were formed by varying the laser energy density. These structures showed particularly stable electrochemical performance for energy applications. Chen et al. [11] developed ultrathin manganese dioxide nanosheets as an effective binder-free supercapacitor electrode by sequential growth of manganese dioxide and PANI in carbon fibers. High specific capacitance of 654 F g⁻¹ and energy density of 27.9 Wh kg⁻¹ were obtained.

The development of Li-ion batteries has become the driving force for growth of applications in electronics, and especially in e-mobility. However, further improvement of batteries is still required to make electric vehicles fully competitive with combustion-based vehicles. The main aims of this research are to increase the energy density and lifetime and to make the batteries safer. The thin coatings of progressive materials that form anodes, electrodes, or artificial solid electrolyte interfaces (SEI) play a significant role in the research of Li/Na ion batteries. For this purpose, techniques such as atomic layer deposition (ALD), pulse laser deposition (PLD), thermal evaporation, or sputtering techniques can find use in the preparation of coatings for Li/Na ion batteries [12]. Among these techniques, ALD has proven capable of preparing ultrathin TiO₂ and Al₂O₃ artificial SEI layers and increasing the cycling lifetime of Li ion batteries [13,14]. Other materials, such as metal diselenide and functional coatings based on these materials, have shown perspective utilized as an anode electrode for Li ion batteries [15] and stabilizing coatings for high-voltage lithium-ion battery cathodes [16]. Thin-film coating has also been implemented in emerging battery technologies such as thin-film solid-state batteries [17] and anode-free batteries [18], which offer new possibilities for the use of battery technologies in electronics.

Another promising area of application for thin films and coatings based on new materials is water electrolysers and hydrogen generation. The use of noble metals prevents the development of a sustainable hydrogen infrastructure. Transition metal chalcogenides (TMCs) are promising candidates for replacing noble metals as earth-abundant electrocatalysts for water splitting [19]. The doping of the surface area can modulate the nanostructured morphology and increase the electrochemical active area associated with exposing more accessible active sites [20].

In the work of Mehmood et al. [21], nickel sulfide with a nanostructured feature has been suggested as a promising electrocatalyst for energy conversion and storage with high performance due to its nanorod-like morphology. A single-step growth of nickel sulfide via a hydrothermal approach was configured for the oxygen evolution reaction (OER) and superior supercapacitor electrodes.

Wang et al. reported [22] the direct growth of MoS₂, WS₂, and NbS₂, a carbon paper substrate, via chemical vapor deposition. The effects of the deposition temperature and gas flow rate were studied by means of their morphology and structure. WS₂ nanosheets were assembled into 1D nanofiber, leading to the superb HER activity.