Recent Advances in Energy-Related Materials—Special Issue Preface

In recent years, there has been unprecedented emphasis on the development of renewable and sustainable energy sources to produce clean and environmentally friendly energy. With governments and companies setting net-zero targets, considerable effort has been invested in the development and commercialization of renewable energy sources, with the objective of replacing or reducing the utilization of fossil fuels, oil, coal, and natural gas in industry worldwide. The process of hydrogen production through electrolysis, biogas, or solar reactors has been demonstrated to emit zero levels of CO₂ when compared with the utilization of fossil fuel feedstocks. Consequently, the global demand for pure hydrogen in many industrial sectors, as well as the need to reduce CO₂ in the atmosphere, has led to the investigation of new alternative energy sources [1].

The advancement of next-generation energy sources, including fuel cells (FCs), batteries, electrolyzers, and solar cells, has led to their application in a wide range of industrial sectors. Nevertheless, it remains important to investigate their effectiveness, durability, and sustainability in order to improve the speed and ease with which they can be accessed. Energy conversion reactions occur during certain processes, such as the electrooxidation of fuels (e.g., methanol, ethanol, formic acid, sodium borohydride, and hydrazine) and oxygen reduction reactions (ORRs) that occur in fuel cells [2,3]; the oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs) that occur during water splitting [4,5]; and the reduction of carbon monoxide (CO) and carbon dioxide (CO₂), which requires high efficiency [6,7]. Catalytic materials play a significant role in initiating and maintaining the activity that occurs in these reactions while remaining stable. Despite the recognized efficiency of platinum, its high cost and limited availability have prompted the pursuit of novel, efficient, cost-effective, and innovative materials. A significant body of research has focused on the development and investigation of materials that reduce platinum (Pt) content in catalytic materials. Initially, these investigations focused on the combination of two or three noble metals in the same catalysts. This approach aimed to enhance catalytic activity while reducing the amount of metal utilized [8,9]. However, the scientific community has redirected its focus to non-noble metals, or oxides of these metals that have been decorated with noble metal particles. The aim of this endeavor is to engineer highly effective catalytic materials [10,11]. In recent studies, the focus has shifted towards the exploration of alternative materials, including non-noble metals and metal-free substances, as well as novel or modified substrates derived from biomass or waste.

Recent advancements in carbon nanomaterials, including graphene, carbon nanotubes (CNTs), graphitic carbon nitride (g-C₃N₄), N-doped carbon, and three-dimensional (3D) carbon architectures, highlightexciting possibilities for the development of metal-free electrocatalysts for oxygen reduction reactions [12,13]. Nonetheless, preliminary investigations have indicated the potential of transition metal-based materials in alkaline water-splitting processes [14,15].

This Special Issue presents five studies (Contributions 1–5) on innovative non-noble-metal catalytic materials that have been demonstrated to effectively catalyze the oxidation of small molecules, the reduction and evaluation of oxygen reactions, and waste-based combustion processes. Additionally, Contributions 6 and 7 collected and reviewed catalytic materials for use in hydrogen production through water splitting.

In their scientific research, the authors of Contribution 1 investigatedthe catalytic performance of a mixed-metal catalyst containing SiO₂, Al₂O₃, Fe₂O₃, and CaO in the combustion of industrial paper waste (i.e., bark, paper sludge, and rejected paper). This investigation pertains to the secondary utilization of raw materials. Siam Kraft Industry Co., Ltd. (Wangsala, Kanchanaburi, Thailand) supplied the bark, paper sludge, and waste paper rejects used in this research. The metal oxide precursors originated from industrial waste, including clinker, used cement, bentonite clay, and iron mill scale. A mixed-metal oxide catalyst containing78.57% SiO₂, 9.28% Al₂O₃, 4.28% Fe₂O₃, and 7.85% CaO was prepared. The results showed that incorporating a mixed-metal oxide catalyst reduced the burnout temperature by approximately 134 °C for bark, 10 °C for paper sludge, and 10 °C for waste paper rejects. The findings of the study indicated that the mixed-metal oxide enhanced combustion reactivity via the accelerated char combustion of biomass. Furthermore, it was hypothesized that the incorporation of catalyst particles into the waste combustion process elevated H₂ and CO₂ concentrations of gaseous byproducts (i.e., H₂, CO, CH₄, C₂H₄, C₂H₆, and CO₂) and enhanced tar decomposition and the water–gas shift reaction.

Three papers in this Special Issue pertain to the investigation of fuel oxidation and oxygen reduction reactions in fuel cells and batteries. An investigation of new catalytic materials was undertaken, with the objective of enhancing the activity of these reactions. Recently, researchers have noted the efficiency of nitrogen-modified carbon (N-doped/C) for use as a substrate for preparing catalytic materials combined with non-noble metals. Contributions 3 and 4 present N-doped/C catalysts supported by MnO and cerium (Ce) particles for ORRs in alkaline and acidic media.

Furthermore, Contribution 3 presents an N-doped carbon substrate that was synthesized from coffee waste and then prepared as a Mn-Xerogel via sol–gel synthesis. During the investigation, the authors developed a catalyst with small 27 nm MnO particles distributed on the N-doped/C surface. The prepared MnO/N-CC-5 catalyst demonstrated high stability, comparable to the technologically advanced Pt/C catalyst, in an alkaline 0.1 M KOH solution for ORRs. The optimized MnO/N-CC-5 catalyst, with an ECSA of 4.07 m²/g, exhibited a half-wave potential of 0.78 V vs. RHE. Furthermore, after 5000 potential cycles, the MnO/N-CC-5 catalyst exhibited only a 10 mV loss ofhalf-wave potential and retained 96% of its current under continuous chronoamperometric stability for over 18 h. The authors conclude that the synergistic effect of N-doped carbon and MnO nanoparticles enhances dispersion and prevents coalescence during the high-temperature treatment of xerogel-N-CC. Furthermore, the improved electronic transfer and stabilization of MnO nanoparticles atthe N-doped sites enhanced ORR activity and stability under electrochemical conditions.

In Contribution 4, Ce was identified as a promising electrocatalyst for use in combination with N-doped carbon. The unique f-orbitals of Ce induce distinctive electronic behavior in the catalyst, helping to form stable coordination structures with N-doped carbons. In addition, Ce has an excellent ability to scavenge the OH^● produced during ORRs, therefore helping to stabilize the catalyst. Ce/N-C-1, 2, 3, and 4 catalysts were synthesized through a metal–organic framework and pyrolysis strategy, corresponding to Ce loadings of 24, 48, 72, and 96 mg of Ce(NO₃)₂precursor. The researchers’ findings, as outlined in the extant literature, demonstrate a direct correlation of ORR activity withE_1/2 in a 0.1 M HClO₄ electrolyte, as well as with Ce content. The observed increase in both metrics is attributable to the rise in Ce atom concentration within the catalyst, thereby substantiating their hypothesis. The Ce/N-C-3 catalyst, which was optimized, exhibited an increase in E_1/2 potential of 150 mV in comparison to the N-C substrate. The Ce/N-C-3 catalyst delivered a half-wave potential of 0.68 V vs. RHE, demonstrating acceptable stability with a loss of 70 mV after 5000 potential cycles, compared to a loss of 110 mV for the Pt/C (10 wt.%) catalyst. Following 30 days of uninterrupted operation of DCMFCs, in which Ce/N-C-3 was utilized as the cathode catalyst, in dual-chamber microbial fuel cells, a volumetric power density of approximately 300 mW m⁻³ was observed, accompanied by organic matter degradation of 74%. The Pt/C (10 wt.%) catalyst demonstrated a half-wave potential of 0.74 V for ORRs in this study. However, the non-precious Ce/N-C-3 catalyst signified that the obtained ORR activity was still significant. Future studies should endeavor to increase ORR activity by incorporating additional metal active sites composed of other rare earth metals and transition metals.

Further, Contribution5 proposes the use of an Fe@NiCo₂O₄/NF composite for the oxidation of small molecules, such as urea, ethanol, and ethylene glycol. Firstly, nickel and cobalt metals with a molar ratio of 1:2 were deposited on the surface of nickel foam (NF) via the hydrothermal method to obtain the NiCo₂O₄/NF substrate. Subsequently, iron was electrochemically deposited onto the NiCo₂O₄/NF substrate using a solution containing 0.1 M FeSO₄, 0.4 M H₃PO₄, and 0.5 M Na₂SO₄. The efficiency of the obtained Fe@NiCo₂O₄/NF catalyst was studied electrochemically in solutions of 1.0 M KOH and 1.0 M fuel (i.e., urea, ethanol, and ethylene glycol). The authors identified and discussed the conditions for high Fe@NiCo₂O₄/NF electrode reactivity during the electrochemical oxidation of urea, ethanol, and ethylene glycol. The activity of the catalyst was found to be associated with its structural and morphological characteristics. The electrode demonstrated discernible resistance when subjected to small organic compounds. Consequently, no alterations in the electric current were identified during the 5 h oxidation period.

Currently, the global demand for pure hydrogen is approximately 120 million tons per year. However, low-emission hydrogen continues to play a marginal role in this demand, with less than 1 million tons produced in 2023 [1]. This indicates a significant needfor research related to reactions capable of producing hydrogen, as well as processes involving the splitting of water or seawater. Today, the process of separating water into hydrogen and oxygen using renewable electricity has become fundamental to the environmentally sustainable production of hydrogen. Therefore, current rapid economic development necessitates the creation of novel, advanced, cost-effective, and efficient materials and technologies in this field. A substantial body of research has focused on identifying catalysts that could be sufficiently effective for specific reactions. Thus, it would be advantageous to establish a comprehensive overview of previously examined catalysts.

Contributions 6 and 7 recently summarized the advantages and disadvantages of hydrogen generation reactions and seawater splitting processes based on the literature published in the last few years. These review papers summarize recent studies on the application of various types of catalytic materials in water splitting processes. Contribution 6 provides a comprehensive analysis of the foundational principles of water electrolysis. This analysis presents a thorough review of HERs and OERs, as well as an examination of the concepts underlying proton-exchange membrane electrolyzer technology at both the cell and block levels. Contribution 7 delineates the primary challenges associated with seawater splitting processes and concludes that alkaline seawater electrolysis is a promising technology for producing green hydrogen in the future, as it does not necessitate a complex desalination process. However, both contributors affirm once again that the success of direct water electrolysis technology—an advanced method of producing green hydrogen—hinges on the design and development of efficient catalysts.

Although catalysts made from many different earth-abundant transition metals (e.g., Ni, Co, W, and Fe) have been identified as efficient materials for hydrogen evolution and water splitting processes [15,16,17], the advancement of this approach is still limited by the slow speed on oxygen evolution reactions [18,19]. Developing low-cost, efficient electrocatalysts is crucial for advancing electrochemical water splitting. Contribution 2 presents a cobalt–phosphorus (Co-P) catalyst for hydrogen and oxygen evolution reactions. A simple and expeditious electroless deposition technique was employed to obtain Co-P₅/Cu, Co-P₈/Cu, and Co-P₁₁/Cu catalysts containing 5, 8, and 11% P, respectively. The authors of the study provide a rationale for the activity of the catalyst, attributing it to the specific content of the material in question. In a 1.0 M KOH solution, it was demonstrated that the Co-P coating with 11 wt% P exhibited the lowest HER overpotential value of 98.9 mV, obtaining a current density of 10 mA cm⁻², in comparison to the Co-P coatings with 8 wt% (107.6 mV) and 5 wt% (165.9 mV) P. Conversely, the lowest OER overpotential (378 mV) was observed for the Co-P coating with 8 wt% P, which exhibited a current density of 10 mA cm⁻², in contrast to the Co-P coatings containing 5 wt% (400 mV) and 11 wt% (413 mV) P.

In summary, the results of the studies presented in this second Special Issue on Recent Advances in Energy-Related Materials demonstrate the variety and necessity of stable, efficient, and cost-effective catalysts that can facilitate specific types of reactions. Moreover, the continuous development of environmentally friendly energy sources and technologies in the field of energy conversion is imperative.

We extend our gratitude to all of the contributing authors, whose contributions to this Special Issue are invaluable. Our thorough review of the original scientific articles and reviews published in this Special Issue reveals that scientists around the world are making significant progress in environmental protection and sustainability. Furthermore, these scientists have demonstrated their ability to create new, efficient materials using secondary raw materials while avoiding the use of precious metals in renewable energy sources, which will undoubtedly lead to the wider application of these materials.