Single Metal Atoms Anchored in Two-Dimensional Materials: Bifunctional Catalysts for Fuel Cell Applications

Insight into Defect Engineering of Atomically Dispersed Iron Electrocatalysts for High-Performance Proton Exchange Membrane Fuel Cell

Atomically dispersed and nitrogen coordinated iron catalysts (Fe-NCs) demonstrate potential as alternatives to platinum-group metal (PGM) catalysts in oxygen reduction reaction (ORR). However, in the context of practical proton exchange membrane fuel cell (PEMFC) applications, the membrane electrode assembly (MEA) performances of Fe-NCs remain unsatisfactory. Herein, improved MEA performance is achieved by tuning the local environment of the Fe-NC catalysts through defect engineering. Zeolitic imidazolate framework (ZIF)-derived nitrogen-doped carbon with additional CO2 activation is employed to construct atomically dispersed iron sites with a controlled defect number. The Fe-NC species with the optimal number of defect sites exhibit excellent ORR performance with a high half-wave potential of 0.83 V in 0.5 M H2 SO4 . Variation in the number of defects allows for fine-tuning of the reaction intermediate binding energies by changing the contribution of the Fe d-orbitals, thereby optimizing the ORR activity. The MEA based on a defect-engineered Fe-NC catalyst is found to exhibit a remarkable peak power density of 1.1 W cm-2 in an H2 /O2 fuel cell, and 0.67 W cm-2  in an H2 /air fuel cell, rendering it one of the most active atomically dispersed catalyst materials at the MEA level.

Analysis of the Scale of Global Human Needs and Opportunities for Sustainable Catalytic Technologies

We analyzed the enormous scale of global human needs, their carbon footprint, and how they are connected to energy availability. We established that most challenges related to resource security and sustainability can be solved by providing distributed, affordable, and clean energy. Catalyzed chemical transformations powered by renewable electricity are emerging successor technologies that have the potential to replace fossil fuels without sacrificing the wellbeing of humans. We highlighted the technical, economic, and societal advantages and drawbacks of short- to medium-term decarbonization solutions to gauge their practicability, economic feasibility, and likelihood for widespread acceptance on a global scale. We detailed catalysis solutions that enhance sustainability, along with strategies for catalyst and process development, frontiers, challenges, and limitations, and emphasized the need for planetary stewardship. Electrocatalytic processes enable the production of solar fuels and commodity chemicals that address universal issues of the water, energy and food security nexus, clothing, the building sector, heating and cooling, transportation, information and communication technology, chemicals, consumer goods and services, and healthcare, toward providing global resource security and sustainability and enhancing environmental and social justice.

Generalizable Descriptors of Highly Sensitive Detection of As(III) over Transition-Metal Single Atoms: A Combined Density Function Theory and Gradient Boosting Regression Approach

Traditional nanomodified electrodes have made great achievements in electrochemical stripping voltammetry of sensing materials for As(III) detection. Moreover, the intermediate states are complicated to probe because of the ultrashort lifetime and complex reaction conditions of the electron transfer process in electroanalysis, which seriously hinder the identification of the actual active site. Herein, the intrinsic interaction of highly sensitive analytical behavior of nanomaterials is elucidated from the perspective of electronic structure through density functional theory (DFT) and gradient boosting regression (GBR). It is revealed that the atomic radius, d-band center (ε_d), and the largest coordinative TM-N bond length play a crucial role in regulating the arsenic reduction reaction (ARR) performance by the established ARR process for 27 sets of transition-metal single atoms supported on N-doped graphene. Furthermore, the database composed of filtered intrinsic electronic structural properties and the calculated descriptors of the central metal atom in TM-N₄-Gra were also successfully extended to oxygen evolution reaction (OER) systems, which effectively verified the reliability of the whole approach. Generally, a multistep workflow is developed through GBR models combined with DFT for valid screening of sensing materials, which will effectively upgrade the traditional trial-and-error mode for electrochemical interface designing.

Molecule Confined Isolated Metal Sites Enable the Electrocatalytic Synthesis of Hydrogen Peroxide

The direct synthesis of hydrogen peroxide (H₂ O₂ ) through the two-electron oxygen reduction reaction is a promising alternative to the industrial anthraquinone oxidation process. Selectivity to H₂ O₂ is however limited by the four-electron pathway during oxygen reduction. Herein, it is reported that aminoanthraquinone confined isolated metal sites on carbon supports selectively steer oxygen reduction to H₂ O₂ through the two-electron pathway. Confining isolated NiN_x sites under aminoanthraquinone increases the selectivity to H₂ O₂ from below 55% to above 80% over a wide potential range. Spectroscopy characterization and density functional theory calculations indicate that isolated NiN_x sites are confined within a nanochannel formed between the molecule and the carbon support. The confinement reduces the thermodynamic barrier for OOH* desorption versus further dissociation, thus increasing the selectivity to H₂ O₂ . It is revealed how tailoring noncovalent interactions beyond the binding site can empower electrocatalysts for the direct synthesis of H₂ O₂ through oxygen reduction.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Hybrid Metal-Boron Diatomic Site Embedded in C2 N Monolayer Promotes C-C Coupling in CO2 Electroreduction

Double-atom catalyst (DAC) has gained much interest for its versatile tuning and synergistic effect of dual-atom active sites. Metal (M)-metal (M) diatomic sites, either homo- or heteronuclear, are typically researched. Hybrid metal-non-metal combined sites have rarely been studied and even the viability of such active sites are unknown. Herein, CO₂ electroreduction (CO₂ RR) is explored on M@X-C₂ N (M = Fe, Co, Ni, and Cu; X = S, P, and B) which renders naturally generated M-X diatomic site. Using spin-polarized density functional theory coupled with computational hydrogen electrode model, it is demonstrated that the functionality of hybrid M-B dual-atom center is superior over that of a single- or double-M center in driving CO₂ RR especially C-C coupling. Among metal-boron DACs studies, Fe@B-C₂ N (μ = 2μ_B ) exhibits the lowest free energy barrier of 0.17 eV in C-C coupling whereas Ni@B-C₂ N (μ = 0μ_B ) mainly produces CH₄ with the lowest barrier of 0.42 eV. Hence, the electronic spin state of M can be particularly important in modulating selectivity and C-C coupling barrier in CO₂ RR. Fe@B-C₂ N is predicted as the promising catalyst for CO₂ RR towards C₂₊ products owing partially to its enhanced spin state. The findings can enrich the design strategy of electrocatalysts normally running at ambient conditions.

Enabling multifunctional electrocatalysts by modifying the basal plane of unifunctional 1T'-MoS2 with anchored transition metal single atoms

Multifunctional electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are attractive for overall water-splitting, rechargeable metal-air batteries, and unitized regenerative fuel cells. A single-atom catalyst (SAC) may exhibit additional advantages over its nanoparticle counterpart, and already there have been significant advances in the development of bifunctional and trifunctional SACs for HER, ORR, and OER, but great challenges remain for their rational design. Herein, we propose a strategy to realize multifunctional SACs, i.e., modifying unifunctional materials to introduce new active sites on the surface. Specifically, by virtue of the intrinsic excellent HER performance of 1T'-MoS₂, we theoretically design multifunctional SACs by anchoring appropriate transition-metal single atoms. Intriguingly, 1T'-MoS₂ with supported Co single atoms (Co@MoS₂) are demonstrated to be highly active for both OER and ORR with ultralow overpotentials of less than 0.3 V, ascribed to the moderate chemical activity and unique electronic structure of the Co atomic center. Consequently, combining the intrinsic HER activity of 1T'-MoS₂, Co@MoS₂ is proposed to be promising efficient trifunctional SACs. Further, the phase engineering on SACs is unrevealed and elucidated by comparing the properties of the Co atomic center-supported on 1T'-MoS₂ and 1H-MoS₂. This work provides a feasible strategy for the design of multifunctional SACs for the renewable and sustainable energy technology and provides an insight into the phase engineering on SACs.

Atomic Ni and Cu co-anchored 3D nanoporous graphene as an efficient oxygen reduction electrocatalyst for zinc-air batteries

Highly active, cost-effective and durable electrocatalysts for the oxygen reduction reaction (ORR) are critically important for renewable energy conversion and storage. Here we report a 3D bicontinuous nitrogen doped nanoporous graphene electrocatalyst co-anchoring with atomically dispersed nickel and copper atoms ((Ni,Cu)-NG) as a highly active single-atom ORR catalyst, fabricated by the combination of chemical vapor deposition and high temperature gas transportation. The resultant (Ni,Cu)-NG exhibits an exceptional ORR activity in alkaline electrolytes, comparable to the Pt-based benchmarks, from the synergistic effect of the CuNx and NiNx complexes. Endowed with high catalytic activity and outstanding durability under harsh electrochemical environments, rechargeable zinc-air batteries using (Ni,Cu)-NG as the cathodes show excellent energy efficiency (voltage gap of 0.74 V), large power density (150.6 mW cm-2 at 250 mA cm-2) and high cycling stability (>500 discharge-charge cycles at 10 mA cm-2). This study may pave an efficient avenue for designing highly durable single-atom ORR catalysts for metal-air batteries.

Pulsed Laser in Liquids Made Nanomaterials for Catalysis

Catalysis is essential to modern life and has a huge economic impact. The development of new catalysts critically depends on synthetic methods that enable the preparation of tailored nanomaterials. Pulsed laser in liquids synthesis can produce uniform, multicomponent, nonequilibrium nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomistic structure within the nanoparticle and at its surface. We cover the fundamentals, unique advantages, challenges, and experimental solutions of this powerful technique and review the state-of-the-art of laser-made electrocatalysts for water oxidation, oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidations, followed by laser-made nanomaterials for light-driven catalytic processes and heterogeneous catalysis of thermochemical processes. We also highlight laser-synthesized nanomaterials for which proposed catalytic applications exist. This review provides a practical guide to how the catalysis community can capitalize on pulsed laser in liquids synthesis to advance catalyst development, by leveraging the synergies of two fields of intensive research.

Carbon and Oxygen Coordinating Atoms Adjust Transition Metal Single-Atom Catalysts Based On Boron Nitride Monolayers for Highly Efficient CO2 Electroreduction

Although single-atom catalysts (SACs) with transition metal-nitrogen complexes have been studied widely, investigations that use light-element atoms to adjust the coordination environment of the central metal atoms in metal-nitrogen complexes are still rare but show enormous potential for various electrocatalytic reactions. Herein, we design novel SACs based on monolayer BN adjusted by B, C, or O coordinating atoms as catalysts for the CO₂ reduction reaction (CRR). These SACs are denoted as M@BN_D (BN = monolayer boron nitride; D = B, C, or O atom; M = Co, Cr, Fe, Mn, Mo, Pd, Pt, Ru, V, W, Ni, Zn, Zr, Ag, Au, Cu, or Ti atom) and are investigated as CRR catalysts using density functional theory calculations. Among these structures, we identified some promising candidate catalysts for CRR with impressive low limiting potential (U_L): Pt@BN_C with a U_L of -0.18 for the product CH₄ and Co@BN_C and Au@BN_O with U_L of -0.41 and -0.37 V, respectively, for the product CH₃OH. In particular, Pt@BN_C shows a remarkable reduction in U_L for the product CH₄ compared to any existing catalysts, synthesized or predicted. In addition, the ultralow U_L for CRR on Pt@BN_C was derived from the unique bonding feature between the single metal atom and adsorbates and the modulation of ionic interactions induced by the coordination effect of the C atom.

Polyoxadiazoles as proton exchange membranes for fuel cell application

The number of researches on the ion exchange membrane has increased considerably in recent years showing interest in fuel cell technology for the automobile and portable applications. The most promising fuel cell technology for low-temperature operation (80 °C < T < 150 °C) uses a polymer membrane separating the anode and cathode compartments in an electrochemical cell. Polyoxadiazoles (PODs) belong to a class of heterocyclic polymers, which possess a number of unique properties, such as thermal, mechanical, and chemical resistance. In the present review, numerous ways of POD synthesis are discussed in relation to their functional properties. In addition, different approaches to the elaboration of POD-based composite membranes are discussed in details in order to reveal the structure/properties relationship.

Single Transition Metal Atom Bound to the Unconventional Phase of the MoS2 Monolayer for Catalytic Oxygen Reduction Reaction: A First-Principles Study

Supported single-atom catalysts (SACs) have received a lot of attention due to their super-high atom utilization and outstanding catalytic performance. However, the instability of the supported transition-metal (TM) atoms hampers their widespread applications. Exploration of an appropriate substrate to stabilize the supported single atom is crucial for the future implementation of SACs. In recent years, two-dimensional materials have been proposed as possible substrates due to their large specific surface areas, but their chemically inert surfaces are difficult to stabilize TM atoms without defecting or doping. Herein, by means of systematic first-principles calculations, we demonstrate that the defect-free MoS₂ monolayer in the unconventional phase (1T') can effectively immobilize single TM atoms owing to its unique electrophilic property as compared to the conventional 2H phase. As a prototype probe, we investigated oxygen reduction reaction (ORR) catalyzed by a total of 21 single TM atoms stabilized on 1T'-MoS₂ and successfully screened out two candidates, Cu and Pd@1T'-MoS₂, which have a low overpotential of 0.41 and 0.32 V respectively, outperforming most of the previously reported ORR catalysts. Furthermore, we reveal that the adsorption energy of the ORR intermediate, *OH, provides an excellent descriptor to assess the ORR activity, which is further determined by the d-band center of the supported TM adatoms, thus being a great advantage for future design of stable and high-performance SACs.

Single Atom Catalysts for Fuel Cells and Rechargeable Batteries: Principles, Advances, and Opportunities

Owing to the energy crisis and environmental pollution, developing efficient and robust electrochemical energy storage (or conversion) systems is urgently needed but still very challenging. Next-generation electrochemical energy storage and conversion devices, mainly including fuel cells, metal-air batteries, metal-sulfur batteries, and metal-ion batteries, have been viewed as promising candidates for future large-scale energy applications. All these systems are operated through one type of chemical conversion mechanism, which is currently limited by poor reaction kinetics. Single atom catalysts (SACs) perform maximum atom efficiency and well-defined active sites. They have been employed as electrode components to enhance the redox kinetics and adjust the interactions at the reaction interface, boosting device performance. In this Review, we briefly summarize the related background knowledge, motivation and working principle toward next-generation electrochemical energy storage (or conversion) devices, including fuel cells, Zn-air batteries, Al-air batteries, Li-air batteries, Li-CO₂ batteries, Li-S batteries, and Na-S batteries. While pointing out the remaining challenges in each system, we clarify the importance of SACs to solve these development bottlenecks. Then, we further explore the working principle and current progress of SACs in various device systems. Finally, future opportunities and perspectives of SACs in next-generation electrochemical energy storage and conversion devices are discussed.

In silico discovery of active, stable, CO-tolerant and cost-effective electrocatalysts for hydrogen evolution and oxidation

Various databases of density functional theory (DFT) calculations for materials and adsorption properties are currently available. Using the Materials Project and GASpy databases of material stability and binding energies (H* and CO*), respectively, we evaluate multiple aspects of catalysts to discover active, stable, CO-tolerant, and cost-effective hydrogen evolution and oxidation catalysts. Finally, we suggest a few candidate materials for future experimental validations. We highlight that the stability analysis is easily obtainable but provides invaluable information to assess thermodynamic and electrochemical stability, bridging the gap between simulations and experiments. Furthermore, it reduces the number of expensive DFT calculations required to predict catalytic activities of surfaces by filtering out unstable materials.

Versatile Applications of Metal Single-Atom @ 2D Material Nanoplatforms

Recently, emerging 2D material-supported metal single-atom catalysts (SACs) are receiving enormous attention in heterogeneous catalysis. Due to their well-defined, precisely located metal centers, unique metal-support interaction and identical coordination environment, these catalysts serve as excellent models for understanding the fundamental issues in catalysis as well as exhibiting intriguing practical applications. Understanding the correlations between metal-support combinations and the catalytic performance at the atomic level can be achieved on the SACs@2D materials nanoplatforms. Herein, recent advances of metal SACs on various types of 2D materials are reviewed, especially their exciting applications in the fields of chemicals, energy, and the environment. Based on the summary and perspectives, this work should contribute to the rational design of perfect metal SACs with versatile properties.

Catalysis-Hub.org, an open electronic structure database for surface reactions

We present a new open repository for chemical reactions on catalytic surfaces, available at https://www.catalysis-hub.org . The featured database for surface reactions contains more than 100,000 chemisorption and reaction energies obtained from electronic structure calculations, and is continuously being updated with new datasets. In addition to providing quantum-mechanical results for a broad range of reactions and surfaces from different publications, the database features a systematic, large-scale study of chemical adsorption and hydrogenation on bimetallic alloy surfaces. The database contains reaction specific information, such as the surface composition and reaction energy for each reaction, as well as the surface geometries and calculational parameters, essential for data reproducibility. By providing direct access via the web-interface as well as a Python API, we seek to accelerate the discovery of catalytic materials for sustainable energy applications by enabling researchers to efficiently use the data as a basis for new calculations and model generation.

Prediction of Stable and Active (Oxy-Hydro) Oxide Nanoislands on Noble-Metal Supports for Electrochemical Oxygen Reduction Reaction

Developing cost-effective oxygen electrocatalysts with high activity and stability is key to their commercialization. However, economical earth-abundant catalysts based on first-row transition-metal oxides suffer from low electrochemical stability, which is difficult to improve without compromising their activity. Here, using density functional theory calculations, we demonstrate that noble-metal supports lead to bifunctional enhancement of both the stability and the oxygen reduction reaction (ORR) activity of metal (oxy-hydro) oxide nanoislands. We observe a significant stabilization of supported nanoislands beyond the intrinsic stability limits of bulk phases, which originates from a favorable lattice mismatch and reductive charge transfer from oxophilic supports. We discover that interfacial active sites (located between the nanoisland and the support) reinforce the binding strength of reaction intermediates, hence boosting ORR activity. Considering that both stability and activity lead to discovery of CoOOH|Pt, NiOOH|Ag, and FeO₂|Ag as viable systems for alkaline ORR, we then use a multivariant linear regression method to identify elementary descriptors for efficient screening of promising cost-effective nanoisland|support catalysts.

Noble metal supported hexagonal boron nitride for the oxygen reduction reaction: a DFT study

Discovering active, stable and cost-effective catalysts for the oxygen reduction reaction (ORR) is of utmost interest for commercialization of fuel cells. Scarce and expensive noble metals such as Pt and Pd are the state-of-the-art active ORR catalysts but suffer from low stability against CO poisoning. Hexagonal boron nitride (h-BN) is a particularly attractive material due to its low cost and stability; however, it suffers from intrinsic low activity toward the ORR in the pristine form as a result of its inherently low conductivity with a large band gap of ∼5.5 electron volts. During the past few years, several strategies such as using metal supports, metal doping and atomic vacancies have been reported to significantly increase the conductivity, thereby promoting the ORR activity. Herein we use density functional theory calculations to systematically study these strategies for activating inert h-BN and further examine the stability against CO poisoning. We show that noble metals, such as Ag, Pd, and Pt, require boron (B) or nitrogen (N) vacancies to reasonably activate h-BN toward the ORR. For example, Pd supported h-BN with B-vacancies exhibits significantly high ORR activity. All three examined metal supported h-BNs are predicted to be stable against CO poisoning. These results demonstrate that supporting h-BN on noble metals is a promising strategy to increase the stability against CO poisoning while maintaining high ORR activity.

Recent advancements in Pt-nanostructure-based electrocatalysts for the oxygen reduction reaction

A comprehensive evaluation of Pt-nanostructure-based electrocatalysts for the oxygen reduction reaction.

On the active site for electrocatalytic water splitting on late transition metals embedded in graphene

Transition-metal atoms embedded in nitrogen-doped graphene can be used for electrocatalytic water splitting, but there are open questions regarding the identity of the active site.