Spontaneous-Spin-Polarized 2D π-d Conjugated Frameworks Towards Enhanced Oxygen Evolution Kinetics.

Alternative strategies to design sustainable-element-based electrocatalysts enhancing oxygen evolution reaction (OER) kinetics are demanded to develop affordable yet high-performance water-electrolyzers for green hydrogen production. Here, it is demonstrated that the spontaneous-spin-polarized 2D π-d conjugated framework comprising abundant elements of nickel and iron with a ratio of Ni:Fe = 1:4 with benzenehexathiol linker (BHT) can improve OER kinetics by its unique electronic property. Among the bimetallic NiFex:y-BHTs with various ratios with Ni:Fe = x:y, the NiFe1:4-BHT exhibits the highest OER activity. The NiFe1:4-BHT shows a specific current density of 140 A g-1 at the overpotential of 350 mV. This performance is one of the best activities among state-of-the-art non-precious OER electrocatalysts and even comparable to that of the platinum-group-metals of RuO2 and IrO2. The density functional theory calculations uncover that introducing Ni into the homometallic Fe-BHT (e.g., Ni:Fe = 0:1) can emerge a spontaneous-spin-polarized state. Thus, this material can achieve improved OER kinetics with spin-polarization which previously required external magnetic fields. This work shows that a rational design of 2D π-d conjugated frameworks can be a powerful strategy to synthesize promising electrocatalysts with abundant elements for a wide spectrum of next-generation energy devices.

Cations Determine the Mechanism and Selectivity of Alkaline Oxygen Reduction Reaction on Pt(111).

The proton-coupled electron transfer (PCET) mechanism of the oxygen reduction reaction (ORR) is a long-standing enigma in electrocatalysis. Despite decades of research, the factors determining the microscopic mechanism of ORR-PCET as a function of pH, electrolyte, and electrode potential remain unresolved, even on the prototypical Pt(111) surface. Herein, we integrate advanced experiments, simulations, and theory to uncover the mechanism of the cation effects on alkaline ORR on well-defined Pt(111). We unveil a dual-cation effect where cations simultaneously determine i) the active electrode surface by controlling the formation of Pt-O and Pt-OH overlayers and ii) the competition between inner- and outer-sphere PCET steps. The cation-dependent transition from Pt-O to Pt-OH determines the ORR mechanism, activity, and selectivity. These findings provide direct evidence that the electrolyte affects the ORR mechanism and performance, with important consequences for the practical design of electrochemical systems and computational catalyst screening studies. Our work highlights the importance of complementary insight from experiments and simulations to understand how different components of the electrochemical interface contribute to electrocatalytic processes.

Human-Machine Collaboration for Accelerated Discovery of Promising Oxygen Evolution Electrocatalysts with On-Demand Elements.

A drastically efficient method for identifying electrocatalysts with desirable functionality is a pressing necessity for making a breakthrough in advanced water-electrolyzers toward large-scale green hydrogen production and addressing the significant challenge of carbon neutrality. Despite extensive investigations over the last several centuries, it remains a time-consuming task to identify even one promising affordable electrocatalyst without platinum-group-metal (PGM) for one electrochemical reaction due to its great complexities, particularly for the key anode reaction in the water-electrolyzer of the oxygen evolution reaction (OER). In this study, we demonstrate that a human-machine collaboration based on stepwise-evolving artificial intelligence (se-AI) can significantly shorten the development period of PGM-free multimetal OER electrocatalysts with performance beyond a PGM of RuO2. We were able to reach optimized materials only after 2% experimental trials of the entire candidate pool. The best PGM-free electrocatalyst discovered exhibited excellent activity comparable to RuO2 and, surprisingly, also demonstrated superior stability with a high current density of up to 1000 mA/cm2 at even pH 9.2, which condition is a thermodynamically challenging for typical PGM-free materials. This work illustrates that human's material discovery can be significantly accelerated through collaboration with AI.

Oxide-on-Oxide Porous Electrodes Revealing Superior Reversible Li+-Coupled Electron-Transfer Properties by Unconventional Heterojunction Effects.

Internal spacing of electrodes is a key point for controlling electron-transfer (ET)-related phenomena. However, their disordered porous structures often prevent the observation of microscopic effects. It hampers the development of modern electrochemical theories. The development of model porous electrodes therefore provides an ideal platform to discover intriguing fundamental principles of electrode processes. We developed a new synthetic strategy for all-oxide monolithic ruthenium dioxide (RuO2)/antimony-doped tin oxide (ATO) electrodes with a controlled hierarchically porous structure and oxide-oxide heterojunction. The use of the obtained RuO2/ATO electrodes as model electrodes suppressed influences related to different mass diffusion efficiencies between electrodes with heterojunctions of different types. Then, we showed unconventional oxide-oxide heterojunction effects, improving reversible Li+-coupled electron-transfer properties using model electrodes constituted of various nanostructured (nano-) RuO2 on porous ATO. In addition to the superior electrochemical properties of the nano-RuO2/ATO heterojunction, the quasi-two-dimensional (2D) RuO2/ATO heterojunction led to improved specific capacity at a high rate and longer cycle life. We anticipate that this oxide-oxide heterojunction effect and developed all-oxide model porous electrodes can provide a path to develop advanced reversible energy storage devices.

Superior Multielectron-Transferring Energy Storage by π-d Conjugated Frameworks.

Reversible multielectron-transfer materials are of considerable interest because of the potential impact to advance present electrochemical energy storage technology by boosting energy density. To date, a few oxide-based materials can reach an electron-transfer number per metal-cation (eM ) larger than 2 upon a (de)intercalation mechanism. However, these materials suffer from degradation due to irreversible rearrangements of the cation-oxygen bonds, and are based on precious metals, for example, Ir and Ru. Hence, a design of the non-oxide-based reversible multielectron-transfer materials with abundant elements can provide a promising alternative. Herein, it is demonstrated that the bis(diimino)copper framework can show eM  = 3.5 with cation/anion co-redox mechanism together with a dual-ion mechanism. In this study, the role of the cation-anion interactions is unveiled by using an experiment/theory collaboration applied to a series of the model non-oxide abundant electrode systems based on different metal-nitrogen bonds. These models provide designer multielectron-transfer due to the tunable π-d conjugated electronic structures. It is found that the Cu-nitrogen bonds show a unique reversible rearrangement upon Li-intercalation, and this process responds to acquire a significant reversible multielectron-transfer. This work provides new insights into the affordable multielectron-transfer electrodes and uncovers an alternative strategy to advance the electrochemical energy storage reactions.

Fast-Decoding Algorithm for Electrode Processes at Electrified Interfaces by Mean-Field Kinetic Model and Bayesian Data Assimilation: An Active-Data-Mining Approach for the Efficient Search and Discovery of Electrocatalysts.

The microscopic origins of the activity and selectivity of electrocatalysts has been a long-lasting enigma since the 19th century. By applying an active-data-mining approach, employing a mean-field kinetic model and a statistical approach of Bayesian data assimilation, we demonstrate here a fast decoding to extract key properties in the kinetics of complicated electrode processes from current-potential profiles in experimental and literary data. As the proof-of-concept, kinetic parameters on the four-electron oxygen reduction reaction in the 0.1 M HClO4 solution (ORR: O2 + 4e- + 4H+ → 2H2O) of various platinum-based single-crystal electrocatalysts are extracted from our own experiments and third-party literature to investigate the microscopic electrode processes. Furthermore, data assimilation of the mean-field ORR model and experimental data is performed based on Bayesian inference for the inductive estimation of kinetic parameters, which sheds light on the dynamic behavior of kinetic parameters with respect to overpotential. This work shows that a fast-decoding algorithm based on a mean-field kinetic model and Bayesian data assimilation is a promising data-driven approach to extract key microscopic features of complicated electrode processes and therefore will be an important method toward building up advanced human-machine collaborations for the efficient search and discovery of high-performance electrochemical materials.

Two-Dimensional π-Conjugated Frameworks as a Model System to Unveil a Multielectron-Transfer-Based Energy Storage Mechanism.

ConspectusAlthough electrochemical energy storage is commonplace in our society, further advancements in this technology are indispensable for the transition to a low-carbon society. Recent intensive research has expanded concepts in this field; however, finding one suitable material to obtain a high energy density accomplishing the criteria of next-generation batteries is still a conundrum. To solve this issue, material investigations based on big data combined with artificial intelligence are a present trend. On the contrary, this Account focuses on an alternative approach, i.e., fundamental research to shed light on key basic principles to design new electrode materials and new principles achieving multielectron transfer, which is a key to improve a specific capacity. In addition to the cation-redox mechanism, materials showing the multielectron-transfer mechanism based on cation-/anion-redox can enrich material choices with high theoretical energy densities. The challenge in this mechanism is that a rational design of electrode materials based on microscopic understanding of underlying electrode processes has not been fully achieved so far. This is a key bottleneck in machine-learning approaches as well because the reliability of outputs from an algorithm is dependent on the reliability of data from a corresponding microscopic electrode process. Therefore, uncovering fundamental mechanisms in electrochemical energy storage remains one of the primary goals for the present research. In our series of investigations, we developed concepts for replacing complex practical electrode materials, such as polyanion or Li-rich layered oxides, by simplified model systems based on two-dimensional (2D) π-conjugated frameworks, which are based on purely organic aromatic systems and metal-containing coordination polymers. These materials are relatively simple, but it is still possible to control their complexity of systems in order to mimic certain aspects of structure-property relations in practical electrode materials. In particular, recent studies have shown that we can tune electronic structures of 2D π-conjugated frameworks, which is a key feature to investigate electron-transfer mechanisms, along with the concept of the threefold correlation approach, i.e., the relations in chemical structures, electronic structures, and electrochemical reactions. In this Account, several model studies focusing on microscopic understandings of structure-electrochemical energy storage functions are presented in which we investigate how the structural periodicity and nature of the coordination environment affect their electronic properties and the electrochemical reactions. In particular, we investigate the effects of combinations of linkers and metal ions toward the mechanism of the electrochemical energy storage reaction. We identified few major factors determining the energy storage mechanism of 2D π-conjugated frameworks. Local configurations of coordinate covalent bonding and organic linkers interact with each other, and these effects provide unique electronic states. These electronic states are projections of intriguing electrochemical features in this materials system, such as cation/anion co-redox mechanism, anion-insertion mechanism, or inductive effect. This Account indicates that 2D π-conjugated frameworks can be applied as models to extract fundamental/microscopic principles in the complicated electrode processes, which is linked to practical electrode materials, such as oxides. Therefore, the approach shown here is a powerful tool to unveil microscopic electrochemical energy storage mechanisms, which is indispensable to advance clean energy technology and accelerate decarbonization.

Emergent electrochemical functions and future opportunities of hierarchically constructed metal-organic frameworks and covalent organic frameworks.

Designing spatial and architectural features across from the molecular to bulk scale is one of the most important topics in materials science which has received a lot of attention in recent years. Looking back to the past research, findings on the influences of spatial features denoted as porous structures on the applications related to mass transport phenomena have been widely studied in traditional inorganic materials, such as ceramics over the past two decades. However, due to the difficulties in precise control of the porous structures at the molecular level in this class of materials, the mechanistic understanding of the effects of spatial and architectural features across from the molecular level to meso-/macroscopic scale is still lacking, especially in electrochemical reactions. Further understanding of fundamental electrochemical functions in well-defined architectures is indispensable for the further advancement of key next-generation energy devices. Furthermore, creating periodic porosity in reticular structures is starting to be recognized as an emerging approach to control the electronic structure of materials. In this review, we focus on the investigations on preparing well-defined molecular-level crystalline porous materials known as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) into hierarchically constructed architectures from molecular structures lower than the reticular frameworks to meso-/macroscopic scale structures. By connecting well-defined nanosized porous structures in MOFs/COFs and additional length-scale space or shapes, emergent electrochemical functions towards emerging devices, such as beyond Li-ion batteries including all-solid-state rechargeable batteries, are expected to be obtained. By summarizing recent advancements in synthetic strategies of hierarchically constructed MOF/COF based materials and fundamental investigation of their structural effect in a wide spectrum of electrochemical applications, we highlight the importance and future direction of this developing field of hierarchically constructed MOFs/COFs, while emphasizing the required chemical stability of the MOFs/COFs which meet the use in the game-changing electrochemical devices.

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid-liquid interfaces.

Multi-electron, multi-proton transfer is important in a wide spectrum of processes spanning biological, chemical and physical systems. These reactions have attracted significant interest due to both fundamental curiosity and potential applications in energy technology. In this Perspective Review, we shed light on modern aspects of electrode processes in the 21st century, in particular on the recent advances and challenges in multistep electron/proton transfers at solid-liquid interfaces. Ongoing developments of analytical techniques and operando spectrometry at electrode/electrolyte interfaces and reliable computational approaches to simulate complicated interfacial electrochemical reactions enable us to obtain microscopic insights about these complex processes, such as the role of quantum effects in electrochemical reactions. Our motivation in this Perspective Review is to provide a comprehensive survey and discussion of state-of-the-art developments in experiments, materials, and theories for modern electrode process science, as well as to present an outlook for the future directions in this field.

Tailoring the Electrochemical Properties of Two-Dimensional Bis(diimino)metal Coordination Frameworks by Introducing Co/Ni Heterometallic Structures.

Bis(diimino)metal coordination frameworks (MDI, M = transition metal), which are a class of metal organic frameworks with two-dimensional anisotropy, high electric conductivity, and redox activity, are attractive platforms for tailoring electrochemical properties by introducing a heterometallic composition. In this study, we synthesized heterometallic CoxNi1-xDI coordination frameworks for electrochemical energy storage applications and investigated their electrochemical properties by experimental and theoretical techniques. Ni atoms were embedded into CoDI, and the crystal structure of CoxNi1-xDI was modified, especially along the interlayer axis, which activated the kinetically impeded redox reactions accompanied by PF6- insertion/extraction. Furthermore, upon charge/discharge with Li+ transport, CoxNi1-xDI with a specific composition exhibited higher specific capacity (248 mAh g-1) than CoDI and NiDI in the potential window of 1.0-3.5 V versus Li+/Li. Density functional theory calculations indicate that the energy levels of the antibonding orbitals around the metals and interlayer spaces are important factors in tailoring the electrochemical properties of CoxNi1-xDI.

Quantum electrocatalysts: theoretical picture, electrochemical kinetic isotope effect analysis, and conjecture to understand microscopic mechanisms.

The science of electrode processes is attracting enormous interest. Advancing their principles and elucidating the inherent microscopic mechanisms can have a huge impact on the understanding of the fundamental laws of the universe, as well as the knowledge to improve performances of energy conversion/storage devices. Therefore, it has become one of the most important subjects. Based on the recent advancements in the field of quantum electrochemistry, an electrocatalyst enabling the quantum electrode processes, namely, a quantum electrocatalyst, is the focus of this Perspective Review. In particular, quantum-tunneling-driven multielectron/multiproton transfers, in which several electrons and protons are spontaneously and/or sequentially transferred at the electrode/electrolyte interfaces, are mainly discussed. These reactions emerge from the nontrivial interactions between the electrodes, reactants, and solvents; therefore, they are essentially fairly complicated phenomena. Together with the confirmation of the basic experimental tips to reliably measure the electrochemical properties and discussions on how to practically use the electrochemical kinetic isotope effect to analyze complicated energy conversion reactions, this contribution has formulated conjectures with regard to microscopic mechanisms involving key electrode processes, i.e., oxygen reduction reaction and hydrogen evolution reaction, as well as the potential of quantum electrocatalysts toward the further advancement of energy conversion technologies.

Quantum proton tunneling in multi-electron/-proton transfer electrode processes.

Quantum proton tunneling (QPT) in two representative multi-electron/-proton transfer electrode processes, i.e. hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), was investigated using polycrystalline platinum (pcPt) and gold (pcAu) electrodes at 298 kelvin (K). To observe quantum effects in the electrode processes, the hydrogen/deuterium kinetic isotope effect constant ratio (≡KH/D) was measured in various conditions. For the HER in both acidic and alkaline conditions, results show that the pcPt exhibits a negligible or weak QPT evident by the small value of KH/D (1 < KH/D < 3), which indicates that the semiclassical transition state theory (SC-TST) scheme dominates the rate-determining step (RDS). For pcAu in an alkaline condition, the KH/D was a small value of ca. 1 at a low η region around 0.2 V. However, at a high η region >0.6 V, a high KH/D (>13) was obtained. These results suggest a transition of the electrode process from SC-TST to a full QTP in the RDS on increasing the overpotential. For ORR with pcPt, KH/D higher than the theoretical maximum in SC-TST was observed in the alkaline condition at a low overpotential region. A primitive but robust theoretical analysis suggests that the QPT governs the rate-determining step of ORR in this condition. However, this full QPT path transits to the classical in a higher overpotential region. Therefore, contrary to the HER on pcAu in alkaline, the electrode process shows a transition from a full QPT to SC-TST on increasing the overpotential. No QPT in ORR on a pcPt electrode was observed in an acidic condition. This report describes that the QPT in surface electrochemical systems is strongly affected by the choice of system. Although several systems show a clear manifestation of QPT in the electrode processes and also primitive interpretations can be made of these observations, deriving a fine molecular-level picture of the results including several complicated effects remains challenging. However, the observations suggest that selection of a full QPT path might be affected strongly by different microscopic proton transfer mechanisms, i.e. proton transfer from hydronium ion or water molecules.

Quantum-to-Classical Transition of Proton Transfer in Potential-Induced Dioxygen Reduction.

We report an observation of a quantum tunneling effect in a proton-transfer (PT) during potential-induced transformation of dioxygen on a platinum electrode in a low overpotential (η) region at 298 K. However, this quantum process is converted to the classical PT scheme in the high η region. Therefore, there is a quantum-to-classical transition of the PT (QCT-PT) process as a function of the potential, which is confirmed by theoretical analysis. This observation indicates that the quantum tunneling governs the multistep electron-proton-driven transformation of dioxygen in the low η condition.

Multielectron-Transfer-based Rechargeable Energy Storage of Two-Dimensional Coordination Frameworks with Non-Innocent Ligands.

The metallically conductive bis(diimino)nickel framework (NiDI), an emerging class of metal-organic framework (MOF) analogues consisting of two-dimensional (2D) coordination networks, was found to have an energy storage principle that uses both cation and anion insertion. This principle gives high energy led by a multielectron transfer reaction: Its specific capacity is one of the highest among MOF-based cathode materials in rechargeable energy storage devices, with stable cycling performance up to 300 cycles. This mechanism was studied by a wide spectrum of electrochemical techniques combined with density-functional calculations. This work shows that a rationally designed material system of conductive 2D coordination networks can be promising electrode materials for many types of energy devices.

Two-Dimensional Corrugated Porous Carbon-, Nitrogen-Framework/Metal Heterojunction for Efficient Multielectron Transfer Processes with Controlled Kinetics.

The material choice for efficient electrocatalysts is limited because it is necessary to be highly active as well as highly stable. One direction to solve this issue is to understand elementary steps of electrode processes and build an unconventional strategy for a conversion of inert and, therefore, stable materials into efficient catalysts. Herein, we propose a simple concept for obtaining catalysts from inert and hence stable materials by forming their heterojunctions, namely, covering inert Au with corrugated carbon-nitrogen-based two-dimensional porous frameworks. It shows more than 10 times better activity for the hydrogen evolution reaction than for the pure Au surface, and it also demonstrates the high catalytic activity for the oxygen reduction reaction (ORR) via an effective four-electron reduction mechanism, which is different from the usual two-electron reduction typical for ORR on Au surfaces. This activity induced by formation of a heterojunction was analyzed by a conjugation of computational and experimental methods and found to originate from alternative efficient reaction pathways that emerged by the corrugated porous framework and the Au surface. This work provides not only the method for creating active surface but also the knowledge on elementary steps of such complicated multielectron transfer reactions, thereby leading to intriguing strategies for developing energy conversion reactions based on materials which had never been considered as catalysts before.

Carbon- and Nitrogen-Based Organic Frameworks.

This Account provides an overview of organic, covalent, porous frameworks and solid-state materials mainly composed of the elements carbon and nitrogen. The structures under consideration are rather diverse and cover a wide spectrum. This Account will summarize current works on the synthetic concepts leading toward those systems and cover the application side where emphasis is set on the exploration of those systems as candidates for unusual high-performance catalysis, electrocatalysis, electrochemical energy storage, and artificial photosynthesis. These issues are motivated by the new global energy cycles and the fact that sustainable technologies should not be based on rare and expensive resources. We therefore present the strategic design of functionality in cost-effective, affordable artificial materials starting from a spectrum of simple synthetic options to end up with carbon- and nitrogen-based porous frameworks. Following the synthetic strategies, we demonstrate how the electronic structure of polymeric frameworks can be tuned and how this can modify property profiles in a very unexpected fashion. Covalent triazine-based frameworks (CTFs), for instance, showed both enormously high energy and high power density in lithium and sodium battery systems. Other C,N-based organic frameworks, such as triazine-based graphitic carbon nitride, are suggested to show promising band gaps for many (photo)electrochemical reactions. Nitrogen-rich carbonaceous frameworks, which are developed from C,N-based organic framework strategies, are highlighted in order to address their promising electrocatalytic properties, such as in the hydrogen evolution reaction, oxygen reduction reaction (ORR), and oxygen evolution reaction (OER). With careful design, those materials can be multifunctional catalysts, such as a bifunctional ORR/OER electrocatalyst. Although the majority of new C,N-based materials are still not competitive with the best (usually nonsustainable candidates) for each application, the framework/N approach as such is still in its infancy and has already moved organic materials to regions where otherwise only traditional noble metals or special inorganic semiconductors are found. As one potential way to enhance the properties of polymeric frameworks, the idea of catalysts having unique active surfaces based on Mott-Schottky heterojunctions and related concepts are addressed. In order to integrate all of the above versatile subjects from synthesis to applications on C,N-based organic frameworks, we begin the discussion with synthetic concepts and strategies for these frameworks to distinguish these systems from typical covalent organic frameworks based on boron oxide rings. Next we focus on the semiconducting properties of C,N-based organic frameworks in order to show a continuous transition between CTFs and other systems, such as graphitic carbon nitrides. At the end, applications of these materials are shown by highlighting their properties in electrochemical energy storage and photo- and electrocatalysis.

A general salt-templating method to fabricate vertically aligned graphitic carbon nanosheets and their metal carbide hybrids for superior lithium ion batteries and water splitting.

The synthesis of vertically aligned functional graphitic carbon nanosheets (CNS) is challenging. Herein, we demonstrate a general approach for the fabrication of vertically aligned CNS and metal carbide@CNS composites via a facile salt templating induced self-assembly. The resulting vertically aligned CNS and metal carbide@CNS structures possess ultrathin walls, good electrical conductivity, strong adhesion, excellent structural robustness, and small particle size. In electrochemical energy conversion and storage such unique features are favorable for providing efficient mass transport as well as a large and accessible electroactive surface. The materials were tested as electrodes in a lithium ion battery and in electrochemical water splitting. The vertically aligned nanosheets exhibit remarkable lithium ion storage properties and, concurrently, excellent properties as electrocatalysts for hydrogen evolution.