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AbstractOwing to the rapidly increasing consumption of fossil fuels, finding clean and reliable new energy sources is of the utmost importance. Thus, developing highly efficient and low‐cost catalysts for electrochemical reactions in energy conversion devices is crucial. Single‐atom catalysts (SACs) with maximum metal atom utilization efficiency and superior catalytic performance have attracted significant attention, especially for electrochemical reactions. However, because of the highly unsaturated coordination environment, the stability of SACs can be a challenge for practical applications. In this review, we will summarize the strategies to increase the stability of SACs and synthesizing stable SACs, as well as the application of SACs in electrochemical reactions. Finally, we offer a perspective on the development of advanced SACs through rational design and a deeper understanding of SACs with the help of in situ or operando techniques in electrochemical reactions.

AbstractA fuel cell is an energy conversion device that can continuously input fuel and oxidant into the device through an electrochemical reaction to release electrical energy. Although noble metals show good activity in fuel cell‐related electrochemical reactions, their ever‐increasing price considerably hinders their industrial application. Improvement of atom utilization efficiency is considered one of the most effective strategies to improve the mass activity of catalysts, and this allows for the use of fewer catalysts, saving greatly on the cost. Thus, single‐atom catalysts (SACs) with an atom utilization efficiency of 100% have been widely developed, which show remarkable performance in fuel cells. In this review, we will describe recent progress on the development of SACs for membrane electrode assembly of fuel cell applications. First, we will introduce several effective routes for the synthesis of SACs. The reaction mechanism of the involved reactions will also be introduced as it is highly determinant of the final activity. Then, we will systematically summarize the application of Pt group metal (PGM) and nonprecious group metal (non‐PGM) catalysts in membrane electrode assembly of fuel cells. This review will offer numerous experiences for developing potential industrialized fuel cell catalysts in the future.

AbstractNa‐O2 batteries are advantageous as the candidates of next‐generation electric vehicles due to their ultrahigh theoretical energy density and have attracted enormous attention recently. Tremendous efforts have been devoted to improve the Na‐O2 battery performance by designing advanced electrodes with various carbon‐based materials. Carbon materials used in Na‐O2 batteries not only function as the air electrode to provide active sites and accommodate discharge products but also as Na anode protectors against dendrite growth and chemical/electrochemical corrosion. In this review, we mainly focus on the application of various carbon‐based materials in Na‐O2 batteries and highlight their advances. The scientific understanding on the fundamental design of the material microstructure and chemistry in relation to the battery performance are summarized. Finally, perspectives on enhancing the overall battery performance based on the optimization and rational design of carbon‐based cell components are also briefly anticipated.

AbstractThe high‐temperature pyrolysis process for preparing M–N–C single‐atom catalyst usually results in high heterogeneity in product structure concurrently contains multiscale metal phases from single atoms (SAs), atomic clusters to nanoparticles. Therefore, understanding the interactions among these components, especially the synergistic effects between single atomic sites and cluster sites, is crucial for improving the oxygen reduction reaction (ORR) activity of M–N–C catalysts. Accordingly, herein, we constructed a model catalyst composed of both atomically dispersed FeN4 SA sites and adjacent Fe clusters through a site occupation strategy. We found that the Fe clusters can optimize the adsorption strength of oxygen reduction intermediates on FeN4 SA sites by introducing electron‐withdrawing –OH ligands and decreasing the d‐band center of the Fe center. The as‐developed catalyst exhibits encouraging ORR activity with half‐wave potentials (E1/2) of 0.831 and 0.905 V in acidic and alkaline media, respectively. Moreover, the catalyst also represents excellent durability exceeding that of Fe–N–C SA catalyst. The practical application of Fe(Cd)–CNx catalyst is further validated by its superior activity and stability in a metal–air battery device. Our work exhibits the great potential of synergistic effects between multiphase metal species for improvements of single‐atom site catalysts.

AbstractApplications of lithium–sulfur (Li–S) batteries are still limited by the sluggish conversion kinetics from polysulfide to Li2S. Although various single‐atom catalysts are available for improving the conversion kinetics, the sulfur redox kinetics for Li–S batteries is still not ultrafast. Herein, in this work, a catalyst with dual‐single‐atom Pt‐Co embedded in N‐doped carbon nanotubes (Pt&Co@NCNT) was proposed by the atomic layer deposition method to suppress the shuttle effect and synergistically improve the interconversion kinetics from polysulfides to Li2S. The X‐ray absorption near edge curves indicated the reversible conversion of Li2Sx on the S/Pt&Co@NCNT electrode. Meanwhile, density functional theory demonstrated that the Pt&Co@NCNT promoted the free energy of the phase transition of sulfur species and reduced the oxidative decomposition energy of Li2S. As a result, the batteries assembled with S/Pt&Co@NCNT electrodes exhibited a high capacity retention of 80% at 100 cycles at a current density of 1.3 mA cm−2 (S loading: 2.5 mg cm−2). More importantly, an excellent rate performance was achieved with a high capacity of 822.1 mAh g−1 at a high current density of 12.7 mA cm−2. This work opens a new direction to boost the sulfur redox kinetics for ultrafast Li–S batteries.

AbstractSodium‐ion batteries (NIBs) have become an ideal alternative to lithium‐ion batteries in the field of electrochemical energy storage due to their abundant raw materials and cost‐effectiveness. With the progress of human society, the requirements for energy storage systems in extreme environments, such as deep‐sea exploration, aerospace missions, and tunnel operations, have become more stringent. The comprehensive performance of NIBs at low temperatures (LTs) has also become an important consideration. Under LT conditions, challenges such as increased viscosity of electrolyte, abnormal growth of solid electrolyte interface, and poor contact between collector and electrode materials emerge. The aforementioned issues hinder the diffusion kinetics of sodium ions (Na+) at the electrode/electrolyte interface and cause rapid degradation of battery performance. Consequently, the optimization of electrolyte composition and cathode/anode materials becomes an effective approach to improve LT performance. This review discusses the conduction behavior and limiting factors of Na+ in both solid electrodes and liquid electrolytes at LT. Furthermore, it systematically reviews the recent research progress of LT NIBs from three aspects: cathode materials, anode materials, and electrolyte components. This review aims to provide a valuable reference for developing high‐performance LT NIBs.