Advanced Catalytic Materials for Renewable Energy Sources

The accelerating global economy requires a substantial quantity of energy, and the emissions resulting from the combustion of fossil fuels are contributing to the exacerbation of climate-related issues due to the greenhouse effect. For several decades, research has been conducted on the potential replacement of fossil fuels, including oil, coal, and natural gas. In recent times, there has been a notable increase in scientific research and development efforts focused on the advancement of renewable and sustainable energy production from next-generation fuel cells (FC), batteries, electrolysers, and solar cells [1,2,3]. Researchers are intensively studying a range of fuel cells, including hydrogen (HFC), alkaline (AFC), direct liquid (DLFC), proton exchange membrane (PEMFC), and solid oxide (SOFC) fuel cells. The investigation of the mechanisms and kinetics of energy conversion reactions, such as the electro-oxidation of fuels (methanol, ethanol, formic acid, sodium borohydride, and hydrazine), as well as the conversion of carbon monoxide (CO), oxygen reduction (ORR), oxygen evolution (OER), hydrogen evolution (HER), and carbon dioxide (CO₂), related to the diminishing of noble metals used toward these reactions, is of great importance [4,5,6,7]. Platinum is well known for its efficacy as catalysts in the aforementioned energy conversion reactions [8,9]. Nevertheless, due to its high cost, non-renewability resources, and other disadvantages, such as its contamination of the electrode surface or the influence of the solution medium, there is a need to identify new catalytic materials that can replace platinum or, at the very least, reduce its content in catalysts. Therefore, scientists globally are striving to develop novel, high-efficiency materials for use in the generation of renewable and sustainable energy sources, as well as in the integration of innovative materials in efficient energy conversion and storage systems.

In order to ascertain their suitability for use in fuel cell reactions, a number of different elements, materials, and compounds have been subjected to investigation. In the beginning, scientists employed alternative noble metals, including gold (Au), palladium (Pd), and their alloys, such as PtPd or PtAu, to deposit metals on carbon or other substrates in order to investigate them in energy conversion reactions [10,11]. This was performed with the objective of reducing the platinum content of the electrocatalysts. Nevertheless, these catalysts remained expensive and continued to be affected by the challenges of solution medium influence and surface poisoning. Consequently, in order to obtain low-cost, high-efficiency, and high-performance catalysts for accelerating electrochemical reactions in energy conversion systems, scientists gradually directed their attention to various kinds of materials, including transition earth metals, metal oxides, nitrides, oxynitrides, carbonitrides, metal chalcogenides, carbon-based non-noble metals, and metal-free catalysts [12,13,14,15].

This Special Issue presents a variety of catalytic materials and different synthesis approaches for the fabrication of catalysts, which are essential for the efficient conversion of energy.

The hydrogen fuel cell and hydrogen as a fuel constituted the initial basis for the development of subsequent fuel cells. The scientific and technological solutions that have been achieved have already permitted the utilization of hydrogen fuel cell prototypes for a variety of purposes, including in the automotive and energy industries, as well as in the production of portable electronic devices. Nevertheless, the production of hydrogen is a costly process, and the storage of this gas presents a significant challenge. Furthermore, hydrogen is highly flammable and dependent on fossil fuels. In view of these considerations, the scientific community has identified the investigation of new hydrogen storage as a priority area of research. Sodium borohydride (sodium borohydride anion BH₄⁻) has been the subject of considerable research interest due to its potential for generating highly pure hydrogen on demand or undergoing direct oxidation in a FC [16,17].

The scientific research presented by Contributions 1, 5, 7, and 8 was based on the oxidation of the borohydride anion BH₄⁻. In general, alkaline fuel cells were proposed as a potential solution due to the less corrosive nature of an alkaline environment, which ensures greater longevity. Furthermore, the intrinsically faster kinetics of the oxygen reduction reaction (ORR) in an AFC permits the use of non-noble and low-cost metal electrocatalysts, including silver, cobalt, nickel, and others. This renders the AFC a potentially low-cost technology in comparison to other fuel cells that employ platinum catalysts (see references [18,19]). Contribution 1 presented a catalyst, AuCeO₂/C, prepared by microwave irradiation, for the oxidation of borohydride and reduction in oxygen. The authors obtained an AuCeO₂/C catalyst with an Au loading and electrochemically active surface area of Au nanoparticles (AuNPs) equal to 71 µg cm⁻² and 0.05 cm², respectively. They then proceeded to compare this new catalyst with the Au/C catalyst, which exhibited an Au loading and electrochemically active surface area of AuNPs equal to 78 µg cm⁻² and 0.19 cm², respectively. The objective of this research was to enhance the catalytic activity of the Au/C catalyst through the utilization of CeO₂. The AuCeO₂/C catalyst exhibited a current density ca. 4.5 times higher for the oxidation of sodium borohydride than that observed for the bare Au/C catalyst. Furthermore, the onset potential for the oxygen reduction reaction (0.96 V) on the AuCeO₂/C catalyst was comparable to that of the commercial Pt/C catalyst (0.98 V).

Contribution 7 presents a nickel thin-film catalyst consisting of nanorods prepared on a glass microscope slides by the magnetron sputtering method as a promising candidate for the generation of hydrogen. The author investigated the nickel catalyst for use in the catalytic hydrolysis process of dimethylamine borane (DMAB) and lithium borohydride (LiBH₄). The results demonstrated that the hydrolysis reactions of DMAB and LiBH₄ exhibited first-order kinetic behaviour. The activation energy values of DMAB and LiBH₄ were calculated to be 40.0 kJ mol⁻¹ and 63.74 kJ mol⁻¹, respectively. The activation entropy values were calculated to be −152 J mol⁻¹ K⁻¹ and −75.74 J mol⁻¹ K⁻¹ for DMAB and LiBH₄, respectively. Similarly, the activation enthalpy values were calculated to be 37.34 kJ mol⁻¹ and 62.45 kJ mol⁻¹ for DMAB and LiBH₄, respectively.

Meanwhile Contribution 8 employed the use of PET bottle waste to develop cost-effective and highly efficient catalysts for energy conversion. The researchers have employed PET bottle waste to derive nitrogen-doped graphene (NG), which has been identified as a valuable catalyst support. The preparation of NG involved a one-pot thermal decomposition process of mineral water waste bottles with urea at 800 °C. Subsequently, NG/Pt electrocatalysts with Pt loadings as low as 0.9 wt.% and 1.8 wt.% were prepared via a straightforward reduction method in an aqueous solution at room temperature. The electrochemical behaviour of the newly prepared NG/Pt electrocatalysts was investigated in the context of the borohydride oxidation reaction (BOR) in a solution of 0.03 M of NaBH₄ in 2 M NaOH, as well as in a direct borohydride/peroxide fuel cell (DBFC). The findings demonstrated that the NG/Pt_1 catalyst displayed enhanced activity, as evidenced by a number of exchanged electrons of ca. 2.7. Furthermore, the DBPFC demonstrated a maximum power density of 45 mW cm⁻² at 25 °C when operated at three different temperatures (25, 35, and 45 °C) employing an NG/Pt_1 catalyst. This value exhibited a significant improvement with an increase in temperature, reaching a peak power density of 75 mW cm⁻² at 45 °C. Furthermore, a mass-specific power density of up to 15.8 W mg_Pt⁻¹ was achieved. The findings of this research demonstrate that a variety of materials could be employed in the preparation of catalysts for use in recycling and environmental applications. It is anticipated that the utilization of the NG/Pt composite in fuel cells will facilitate a reduction in costs and an increase in the utilisation of electrochemical energy devices.

Finally, in his short review, Contribution 5 collected data on the advantages and disadvantages of borohydride oxidation recently described in the literature. The author justifies that concentrations of NaBH₄ and any catalysts, as well as the form in which they are introduced into the reaction, the changes in temperature throughout the hydrogen generation process, and the amount of heat released by the reaction are all significant factors influencing the reaction rate. Therefore, further research on hydrogen generation remains relevant and valuable.

A water splitting method of generating clean hydrogen was investigated by researchers as well. Electrochemical water splitting is regarded as a crucial technique for the efficient generation of hydrogen, which can help address energy shortages and environmental concerns. However, the advancement of this approach is currently constrained by the slow oxygen evolution reaction (OER). To address the slow reaction kinetics of OER, there is a significant focus on the exploration of low-cost and efficient electrocatalysts, which is a crucial aspect of the advancement of electrochemical water splitting [20,21]. Two more catalysts were proposed in this Special Issue for the investigation of oxygen evolution reaction (OER). Contribution 2 offered a perovskite-structured FeTiO₃ catalyst prepared via a facile one-step solvothermal method using ionic liquid as templates. Perovskite-type oxides have gained significant attention due to their solid crystal structure, high electronic conductivity and catalytic activity [22]. In their work, the authors demonstrated that the onset potential of the reaction could be reduced to 1.45 V with a current density of 30 mA/cm² by employing FeTiO₃ in the OER process. This outcome was found to be comparable to that achieved by the benchmark IrO₂ catalyst. Furthermore, the catalyst demonstrated remarkable stability for a period of 6 h when tested in a 1M KOH solution. Accordingly, the authors posit that their prepared FeTiO₃ perovskite could be considered as an effective catalyst for water electrolyser technology in the future. Contribution 9 presents NiFe layered double hydroxides (denoted as FNH) as a robust electrocatalyst for OER. The authors presented a synthesis of NiFe layered double hydroxide on a Ni foam substrate via a facile hydrothermal method. This method enables the construction of FNH catalyst in situ on a substrate without the use of a binder, thereby enhancing electron transfer and maintaining stability. Furthermore, it allows for the uniform dispersion and vertical growth of the catalyst. Alternatively, the morphology and chemical composition of FNH can be modified by the appropriate addition of the Fe element during the synthesis process. The authors report that the FNH catalyst they prepared demonstrated an overpotential of 386.8 mV to deliver 100 mA cm⁻² for OER, exhibited good cycling stability after continuous operation for 28 h and CV scanning for 5000 cycles. The authors ascribe the efficiency of the FNH catalyst to the unique vertical nanosheet arrays and 3D self-supported substrate with a high specific area and open-pore structure, as well as the introduction of the Fe element and the accompanying regulation in the electronic structure of the Ni element due to the iron-induced electron redistribution.

Despite a substantial body of research exploring the potential of renewable energy sources using hydrogen, the scientific community has recently shifted its focus towards the identification of alternative fuel sources for fuel cells. Consequently, there was a rapid increase in the investigation of direct liquid fuel cells, in which low-mass alcohols, glucose, formic acid and other liquids were used as fuels [23,24,25]. Consequently, the necessity for the development of catalytic materials capable of efficiently oxidizing these types of fuel has concomitantly increased.

Contribution 3 undertook an investigation into the direct oxidation of methanol using ternary metal catalysts. In this study, the authors present the synthesis of graphene (GR)-supported Pt:Co:Mn, Pt:Co:Ru, and Pt:Co:Mo catalysts with the molar ratios of metals equal to 1:3:1, 1:2:2, and 7:2:1, respectively. This was achieved through a one-pot microwave-assisted synthesis. All ternary catalysts exhibited enhanced electrocatalytic activity for the methanol oxidation reaction in comparison to the bare Pt/GR catalyst, thereby substantiating the hypothesis that the incorporation of additional metals into the catalysts improves their efficiency. The most efficient electrochemical characteristics in this study were demonstrated by the PtCoMn/GR catalyst, which exhibited a current density value of 144.5 mA cm⁻². This value was up to 4.8 times higher than that observed for the PtCoRu(1:2:2)/GR, PtCoMo(7:2:1)/GR, and bare Pt/GR catalysts.

Meanwhile, Contribution 6 investigated the direct oxidation reaction of glucose on Au(NiMo)/Ti catalysts, providing a rationale for the selection of glucose and underscoring the significance of these studies. The authors constructed 3D-structured NiMo coatings on a Ti surface via the widely used electrodeposition method and decorated them with varying amounts of very small Au crystallites by galvanic displacement (Au(NiMo)/Ti). As anticipated, the modification of NiMo/Ti catalysts with small amounts of Au (i.e., 1.8, 2.3, and 3.9 µg_Au cm⁻²) led to an improvement in the stability of the catalysts and a significant enhancement in their performance, as evidenced by a lower onset potential and higher anodic peak currents in the electrooxidation of glucose. Furthermore, the Au(NiMo)/Ti anode with an Au loading of 3.9 µg_Au cm⁻² in a constructed direct glucose–hydrogen peroxide (C₆H₁₂O₆-H₂O₂) single fuel cell exhibited the highest peak power density of 8.75 mW cm⁻² and the highest specific peak power density of 2.24 mW µg_Au⁻¹ for glucose oxidation at 25 °C.

The aforementioned studies demonstrate the necessity for a multitude of diverse catalysts capable of facilitating specific types of reactions. A review of the contemporary scientific literature reveals a concerted effort to develop catalytic materials that are straightforward to synthesize, cost-effective, and environmentally benign. Consequently, Contribution 4 examines the utilisation of carbon nanomaterials derived from biowaste in the synthesis of polymer nanocomposites (PNCs). A considerable amount of attention has been directed towards various polymers, including thermoplastics, thermosetting polymers, elastomers, and their blends filled with biowaste-based carbon nanomaterials attracted a lot of attention due to their enhanced mechanical properties. The authors put forth the proposition that the pyrolysis process represents an environmentally benign methodology for the synthesis of such carbon nanomaterials. The PNCs synthesized from biowaste-derived carbon nanoparticles and polymer matrix, including those derived from coconut shell, wood apple shell, bamboo biochar, sugarcane bagasse, groundnut shell, and oil palm, demonstrate favourable results in ultimate tensile strength. In conclusion, a multitude of factors exert a significant influence on the synthesis of composites derived from biowaste-based, porous nanoparticles. This review by Contribution 4 provides a comprehensive framework for researchers interested in further investigation in this field.

In summary, this Special Issue demonstrates that there is no singular approach or construction methodology that can be considered the optimal catalyst. The investigation of diverse methodologies, substrates, and combinations of elements has resulted in the development of efficient, cost-effective, and environmentally benign products that can serve as catalytic materials in a range of energy conversion reactions.

We wish to express our most sincere thanks to all authors for their invaluable contributions to this Special Issue. It is encouraging to note that the original research papers published in this Special Issue demonstrate that, gradually, scientists around the world are identifying materials that can replace Pt and other precious metals in renewable energy sources, thereby facilitating their broader application.