Toward a mechanistic understanding of electrocatalytic nanocarbon

Defective Carbon Catalysts with Graphitic N-Modified Adjacent Pentagons as Active Sites for Boosted Oxygen Reduction Reaction in Seawater

Seawater electrocatalysis is highly desired for various energy storage and conversion systems, such as water splitting using seawater as an electrolyte and metal fuel cells. However, the adsorption of chloride ions (Cl-) on the active sites of cathodes would worsen the oxygen reduction reaction (ORR) activity and stability, thus lowering the battery performance. Herein, the coupling active sites of graphitic N-regulated adjacent pentagon defects in carbon nanosheets (GAP/CN) were first synthesized by a low-boiling-point metal-mediated partial N-removal strategy. Experimental and theoretical results affirm the advantageous cooperative effect between adjacent pentagons and graphitic N toward the ORR in a harsh seawater environment, where adjacent pentagons act as the authentic highly effective ORR active sites and surrounding graphitic N site serves as the structural regulator to weaken the binding strength of harmful Cl- to prevent catalyst poisoning. As a result, GAP/CN delivers excellent ORR activities in diverse electrolytes, including 0.1 M KOH (half-wave potential of 0.87 V), alkaline artificial seawater (half-wave potential of 0.87 V), and natural seawater (half-wave potential of 0.71 V), and also good long-term stability, which can be comparable to commercial Pt/C. This study provides valuable guidance for the rational design of ORR electrocatalysts for seawater-related energy-conversion devices.

Encapsulated Fe3C boosted electrocatalytic performance for oxygen reduction reaction of N-doped carbon nanotube

The shortcomings of precious metal based catalysts have limited the development of novel energies. So, developing low-cost and high performance transition metal based catalysts is one of the most feasible way to substitute the precious metal based catalysts. In all of the developed catalysts for oxygen reduction reactions (ORR), the iron-based nitrogen doped carbon nanotube (N-CNT) show great promise. In this paper, N-CNT with Fe3C encapsulated based catalysts (Fe3C@N-CNT) were synthesized. The encapsulation structure of Fe3C@N-CNT was confirmed by HAADF-STEM, XANES, XPS, XRD, SEM and other methods. XANES tests show that the doped nitrogen of Fe3C changed the length of Fe-C bond and furtherly influenced the electron transfer between encapsulated Fe3C and outer layer N-CNT. Electrochemical tests showed that the half-wave potential and onset potential ORR of 750 °C calcined Fe3C@N-CNT were 90 mV higher than that of 20 wt% Pt/C. The additional electric field between Fe3C and N-CNT modulated the C-N bond of surface N-CNT and enhanced the catalytic performance for ORR. This paper reveal that constructing encapsulated additional electron providing center is an effective way to design high performance catalysts.

Atomic Ordering-Induced Ensemble Variation in Alloys Governs Electrocatalyst On/Off States

The catalytic behavior of a material is influenced by ensembles─the geometric configuration of atoms on the surface. In conventional material systems, ensemble effects and the electronic structure are coupled because these strategies focus on varying the material composition, making it difficult to understand the role of ensembles in isolation. This study introduces a methodology that separates geometric effects from the electronic structure. To tune the Pd ensemble size on the catalyst surface, we compared the reactivity of structurally different but compositionally identical Pd3Bi intermetallic and solid solution alloys. Pd3Bi intermetallics display no reactivity for methanol oxidation (MOR), while their solid solution counterparts show significant reactivity (0.5 mA cmPd-2). Intermetallics form smaller ensembles (1, 3, 4, and 5 atoms across all low-energy facets), whereas solid solution Pd3Bi has several facets that support larger Pd ensembles, with an average size of 5.25 atoms and up to 6 atoms. A partially ordered Pd3Bi (a mixed phase of intermetallic and solid solution) alloy shows intermediate MOR activity (0.1 mA cmPd-2), confirming that methanol oxidation activity tracks with the average ensemble size. All Pd3Bi alloys maintained similar electronic structures, as confirmed by X-ray photoelectron spectroscopy (XPS) valence band spectroscopy and X-ray absorption near edge structure (XANES) measurements, indicating that reactivity differences arise from variations in the ensemble size induced by differences in the atomic ordering. Our findings offer an approach for designing materials with controllable active site configurations while maintaining the catalyst's electronic structure, thereby enabling more efficient catalyst design.

Multifunctional Strategies of Advanced Electrocatalysts for Efficient Urea Synthesis

The electrochemical reduction of nitrogenous species (such as N2, NO, NO2 -, and NO3 -) for urea synthesis under ambient conditions has been extensively studied due to their potential to realize carbon/nitrogen neutrality and mitigate environmental pollution, as well as provide a means to store renewable electricity generated from intermittent sources such as wind and solar power. However, the sluggish reaction kinetics and the scarcity of active sites on electrocatalysts have significantly hindered the advancement of their practical applications. Multifunctional engineering of electrocatalysts has been rationally designed and investigated to adjust their electronic structures, increase the density of active sites, and optimize the binding energies to enhance electrocatalytic performance. Here, surface engineering, defect engineering, doping engineering, and heterostructure engineering strategies for efficient nitrogen electro-reduction are comprehensively summarized. The role of each element in engineered electrocatalysts is elucidated at the atomic level, revealing the intrinsic active site, and understanding the relationship between atomic structure and catalytic performance. This review highlights the state-of-the-art progress of electrocatalytic reactions of waste nitrogenous species into urea. Moreover, this review outlines the challenges and opportunities for urea synthesis and aims to facilitate further research into the development of advanced electrocatalysts for a sustainable future.

Constructing FeNiPt@C Trifunctional Catalyst by High Spin-Induced Water Oxidation Activity for Zn-Air Battery and Anion Exchange Membrane Water Electrolyzer

Developing cost-efficient trifunctional catalysts capable of facilitating hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) activity is essential for the progression of energy devices. Engineering these catalysts to optimize their active sites and integrate them into a cohesive system presents a significant challenge. This study introduces a nanoflower (NFs)-like carbon-encapsulated FeNiPt nanoalloy catalyst (FeNiPt@C NFs), synthesized by substituting Co2+ ions with high-spin Fe2+ ions in Hofmann-type metal-organic framework, followed by carbonization and pickling processes. The FeNiPt@C NFs catalyst, characterized by its nitrogen-doped carbon-encapsulated metal alloy structure and phase-segregated FeNiPt alloy with slight surface oxidization, exhibits excellent trifunctional catalytic performance. This is evidenced by its activities in HER (-25 mV at 10 mA cm-2), ORR (half-wave potential of 0.93 V), and OER (294 mV at 10 mA cm-2), with the enhanced water oxidation activity attributed to the high-spin state of the Fe element. Consequently, the Zn-air battery and anion exchange membrane water electrolyzer assembled by FeNiPt@C NFs catalyst demonstrate remarkable power density (168 mW cm-2) and industrial-scale current density (698 mA cm-2 at 1.85 V), respectively. This innovative integration of multifunctional catalytic sites paves the way for the advancement of sustainable energy systems.

Design of Superior Electrocatalysts for Proton-Exchange Membrane-Water Electrolyzers: Importance of Catalyst Stability and Evolution

By virtue of the high energy conversion efficiency and compact facility, proton exchange membrane water electrolysis (PEMWE) is a promising green hydrogen production technology ready for commercial applications. However, catalyst stability is a challenging but often-ignored topic for the electrocatalyst design, which retards the device applications of many newly-developed electrocatalysts. By defining catalyst stability as the function of activity versus time, we ascribe the stability issue to the evolution of catalysts or catalyst layers during the water electrolysis. We trace the instability sources of electrocatalysts as the function versus time for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acid and classify them into internal and external sources. Accordingly, we summarize the latest studies for stability improvements into five strategies, i. e., thermodynamic stable active site construction, precatalyst design, support regulation, superwetting electrode fabrication, and catalyst-ionomer interface engineering. With the help of ex-situ/ in-situ characterizations and theoretical calculations, an in-depth understanding of the instability sources benefits the rational development of highly active and stable HER/OER electrocatalysts for PEMWE applications.

Catalytic selectivity of nanorippled graphene

Experiments have shown that nanoscale ripples in a graphene membrane exhibit unexpectedly high catalytic activity with respect to hydrogen dissociation. Nonetheless, the catalytic selectivity of nanorippled graphene remains unknown, which is an equally important property for assessing a catalyst's potential and its fit-for-purpose applications. Herein, we examine the catalytic selectivity of nanorippled graphene using a model reaction of molecular hydrogen with another simple but double-bonded molecule, oxygen, and comparing the measurement results with those from splitting of hydrogen molecules. We show that although nanorippled graphene exhibits a high catalytic activity toward hydrogen dissociation, the activity for catalyzing the hydrogen-oxygen reaction is quite low, translating into a strong catalytic selectivity. The latter reaction involves the reduction of oxygen molecules by the dissociated hydrogen adatoms, which requires additional energy cost and practically determines the selectivity. In this sense, the well-established information about reactions in general of atomic hydrogen with many other species in the literature could potentially predict the selectivity of nanorippled graphene as a catalyst. Our work provides implications for the catalytic properties of nanorippled graphene, especially its selectivity. The results would be important for its extension to a wider range of reactions and for designer technologies involving hydrogen.

Modulating the Density of Catalytic Sites in Multiple-Component Covalent Organic Frameworks for Electrocatalytic Carbon Dioxide Reduction

It is generally assumed that the more metal atoms in covalent organic frameworks (COFs) contribute to higher activity toward electrocatalytic carbon dioxide reduction (CO2RR) and hindered us in exploring the correlation between the density of catalytic sites and catalytic performances. Herein, we have constructed quantitative density of catalytic sites in multiple COFs for CO2RR, in which the contents of phthalocyanine (H2Pc) and nickel phthalocyanine (NiPc) units were preciously controlled. With a molar ratio of 1/1 for the H2Pc and NiPc units in COFs, the catalyst achieved the highest selectivity with a carbon monoxide Faradaic efficiency (FECO) of 95.37% and activity with a turnover frequency (TOF) of 4713.53 h-1. In the multiple H2Pc/NiPc-COFs, the electron-donating features of the H2Pc units provide electron transport to the NiPc centers and thus improved the binding ability of CO2 and intermediates on the NiPc units. The theoretical calculation further confirmed that the H2Pc units donated their electrons to the NiPc units in the frameworks, enhanced the electron density of the Ni sites, and improved the binding ability with Lewis acidic CO2 molecules, thereby boosting the CO2RR performance. This study provides us with new insight into the design of highly active catalysts in electrocatalytic systems.

Long-Term Stability Challenges and Opportunities in Acidic Oxygen Evolution Electrocatalysis

Polymer electrolyte membrane water electrolysis (PEMWE) has been regarded as a promising technology for renewable hydrogen production. However, acidic oxygen evolution reaction (OER) catalysts with long-term stability impose a grand challenge in its large-scale industrialization. In this review, critical factors that may lead to catalyst's instability in couple with potential solutions are comprehensively discussed, including mechanical peeling, substrate corrosion, active-site over-oxidation/dissolution, reconstruction, oxide crystal structure collapse through the lattice oxygen-participated reaction pathway, etc. Last but not least, personal prospects are provided in terms of rigorous stability evaluation criteria, in situ/operando characterizations, economic feasibility and practical electrolyzer consideration, highlighting the ternary relationship of structure evolution, industrial-relevant activity and stability to serve as a roadmap towards the ultimate application of PEMWE.

Tuning active sites for highly efficient bifunctional oxygen electrocatalysts of rechargeable zinc-air battery

High activity, excellent durability, and low-cost oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalysts are highly required for rechargeable zinc (Zn)-air batteries. Herein, we designed an electrocatalyst by integrating the ORR active species of ferroferric oxide (Fe₃O₄) and the OER active species of cobaltous oxide (CoO) into the carbon nanoflower. By well regulating and controlling the synthesis parameters, Fe₃O₄ and CoO nanoparticles were uniformly inserted into the porous carbon nanoflower. This electrocatalyst can reduce the potential gap between the ORR and OER to 0.79 V. The Zn-air battery assembled with it exhibited an open-circuit voltage of 1.457 V, a stable discharge of 98 h, a high specific capacity of 740 mA h g^-1, a large power density of 137 mW cm^-2, as well as good charge/discharge cycling performance, exceeding the performance of platinum/carbon (Pt/C). This work provides references for exploring highly efficient non-noble metal oxygen electrocatalysts by tuning ORR/OER active sites.

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Shell isolated nanoparticle enhanced Raman spectroscopy for mechanistic investigation of electrochemical reactions

Electrochemical conversion of abundant resources, such as carbon dioxide, water, nitrogen, and nitrate, is a remarkable strategy for replacing fossil fuel-based processes and achieving a sustainable energy future. Designing an efficient and selective electrocatalysis system for electrochemical conversion reactions remains a challenge due to a lack of understanding of the reaction mechanism. Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is a promising strategy for experimentally unraveling a reaction pathway and rate-limiting step by detecting intermediate species and catalytically active sites that occur during the reaction regardless of substrate. In this review, we introduce the SHINERS principle and its historical developments. Furthermore, we discuss recent SHINERS applications and developments for investigating intermediate species involved in a variety of electrocatalytic reactions.

Optimizing Microenvironment of Asymmetric N,S-Coordinated Single-Atom Fe via Axial Fifth Coordination toward Efficient Oxygen Electroreduction

Single-atom catalysts (SACs) are attractive candidates for oxygen reduction reaction (ORR). The catalytic performances of SACs are mainly determined by the surrounding microenvironment of single metal sites. Microenvironment engineering of SACs and understanding of the structure-activity relationship is critical, which remains challenging. Herein, a self-sacrificing strategy is developed to synthesize asymmetric N,S-coordinated single-atom Fe with axial fifth hydroxy (OH) coordination (Fe-N₃ S₁ OH) embedded in N,S codoped porous carbon nanospheres (FeN/SC). Such unique penta-coordination microenvironment is determined by cutting-edge techonologies aiding of systematic simulations. The as-obtained FeN/SC exhibits superior catalytic ORR activity, and showcases a half-wave potential of 0.882 V surpassing the benchmark Pt/C. Moreover, theoretical calculations confirmed the axial OH in FeN₃ S₁ OH can optimize 3d orbitals of Fe center to strengthen O₂ adsorption and enhance O₂ activation on Fe site, thus reducing the ORR barrier and accelerating ORR dynamics. Furthermore, FeN/SC containing H₂ O₂ fuel cell performs a high peak power density of 512 mW cm^-2 , and FeN/SC based Znair batteries show the peak power density of 203 and 49 mW cm^-2 in liquid and flexible all-solid-state configurations, respectively. This study offers a new platform for fundamentally understand the axial fifth coordination in asymmetrical planar single-atom metal sites for electrocatalysis.