Favorable Amorphous-Crystalline Iron Oxyhydroxide Phase Boundaries for Boosted Alkaline Water Oxidation

Recent advances in the synthesis of transition metal hydroxyl oxide catalysts and their application in electrocatalytic oxygen evolution reactions

With the extensive use of fossil energy, people will face the depletion of fossil energy and increasingly severe problems. As a non-polluting, high specific energy density energy source, hydrogen energy is expected to solve this problem by producing hydrogen through electrolysis of water through renewable energy power generation. Water electrolysis technology involves two important half-reactions: the cathode hydrogen evolution reaction (HER) and anode oxygen evolution reaction (OER). The OER is a 4-electron transfer process with a high energy barrier. In order to achieve higher energy conversion, OER catalyst technology is a key part of the process. Researchers have conducted a lot of research into high-performance, high-stability, and highly economical OER catalysts, among which oxyhydroxide (MOOH), as an active substance for OER, has received particular attention. This article provides a timely follow-up to the research on oxyhydroxides, first introducing the two catalytic mechanisms of OER, namely the adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM). Then, strategies are proposed to improve OER catalytic performance by increasing catalytic active surface area/active sites, optimizing intermediate adsorption energy based on the AEM, triggering the LOM, and enhancing catalyst stability. Finally, the challenges and future development directions of MOOH catalysts are analyzed, which provides guidance for the design and preparation of high-performance OER catalysts in the future.

The synthesis and application of crystalline-amorphous hybrid materials

Crystalline-amorphous hybrid materials (CA-HMs) possess the merits of both pure crystalline and amorphous phases. Abundant dangling bonds, unsaturated coordination atoms, and isotropic structural features in the amorphous phase, as well as relatively high electronic conductivity and thermodynamic structural stability of the crystalline phase simultaneously take effect in CA-HMs. Furthermore, the atomic and bandgap mismatch at the CA-HM interface can introduce more defects as extra active sites, reservoirs for promoted catalytic and electrochemical performance, and induce built-in electric field for facile charge carrier transport. Motivated by these intriguing features, herein, we provide a comprehensive overview of CA-HMs on various aspects-from synthetic methods to multiple applications. Typical characteristics of CA-HMs are discussed at the beginning, followed by representative synthetic strategies of CA-HMs, including hydrothermal/solvothermal methods, deposition techniques, thermal adjustment, and templating methods. Diverse applications of CA-HMs, such as electrocatalysis, batteries, supercapacitors, mechanics, optoelectronics, and thermoelectrics along with underlying structure-property mechanisms are carefully elucidated. Finally, challenges and perspectives of CA-HMs are proposed with an aim to provide insights into the future development of CA-HMs.

Interface engineering on super-hydrophilic amorphous/crystalline NiFe-based hydroxide/selenide heterostructure nanoflowers for accelerated industrial overall water splitting at high current density

Designing heterojunction catalysts with energy effects at the interface, particularly combining the surface structure advantages of super-hydrophilic interfaces with the high activity advantages of bimetal synergistic optimisation, is the key to developing economical and efficient industrial electrocatalytic water-splitting catalysts. In this study, a coupled nanoflower-like NiFe(OH)x/(Ni, Fe)Se heterostructure catalyst supported on Ni foam (NF) (NFSe@NFOH/NF) was designed and successfully prepared using hydrothermal and electrodeposition strategies. Owing to the electron interaction at the heterogeneous amorphous (NFOH)/crystalline (NFSe) interface and the bimetallic synergistic effect of Ni and Fe, the prepared NFSe@NFOH/NF exhibited excellent and stable oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalytic properties, with low overpotentials of 214/276 mV at 100 mA⋅cm-2 and 262/340 mV at 500 mA⋅cm-2. The assembled water electrolyser comprising NFSe@NFOH/NF || NFSe@NFOH/NF needed only small voltages of 1.73 and 1.85 V to yield current densities of 100 and 500 mA⋅cm-2, respectively. This study offers an innovative design idea for the rational adoption of interface engineering and amorphous-crystalline engineering techniques to construct catalysts with excellent catalytic activity and stability for electrocatalytic overall water splitting (EOWS) at a high current density, which further facilitates the advancement of sustainable energy technology in the future.

Controllable atoms implantation for inducing high valency nickel towards optimizing electronic structure for enhanced overall water splitting

Adjusting the electronic structure and intrinsic activity of the active site of the catalyst based on atomic implantation is the crucial to realizing efficient electrochemical water splitting in alkaline media. Thus, we introduce vanadium (V) atoms with abundant vacant d orbitals as dopants into nickel selenides (NiSe), which has abundant variable valence states, and successfully synthesise three-dimensional bi-functional catalysts self-supported on Ni foam (NF). The electron structure characterisation reveals that, compared with the pure NiSe phase, the oxidation states of Ni cations and electron concentration at the Se site in V-NiSe increase due to the V doping. These changes are accompanied by changes in the electronic structure and active sites in V-NiSe. The as-generated V-NiSe nanorods exhibit an optimised electronic structure, high number of active sites and highly rough nanorod array structure with large electrochemically active surface area and in situ growth characteristics of conductive NF. Thus, the as-generated V-NiSe nanorods catalysts exhibit excellent bi-functional catalytic activity, with 50 mA⋅cm-2 at an overpotential of 270.2 and 251.2 mV for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER), respectively, in KOH (1 M). Water electrolysis using V-NiSe as both the anode and cathode requires a cell voltage of 1.76 V to drive 50 mA⋅cm-2, continuously operating for 80 h. This study provides a systematic understanding of the design of transition-metal catalysts using heteroatomic doping to control their electronic structure and catalytic activity.

Crystalline/amorphous composite interface of CoP@Ni/Fe-P as a boosted electrocatalyst for full water splitting

Heterojunction materials have become good candidates for electrocatalysts thanks to their unique physicochemical merits. Herein, a crystalline-amorphous CoP@Ni/Fe-P heterojunction is constructed for whole water splitting. Originating from the strong electronic reaction at the amorphous-crystal interfaces, the electron density of Co, Ni, Fe and P is adjusted, which will optimize the adsorption and desorption energy of intermediates for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and lower the kinetic barrier. The CoP@Ni/Fe-P heterojunction displays overpotentials of 125 and 250 mV to drive a current density of 10 mA cm-2 in 1 M KOH. In addition, the whole water splitting performance requires a cell voltage of 1.56 V to deliver 10 mA cm-2 and shows good stability. This work provides a way to design and prepare transition-metal-based materials with good electrocatalytic activity by constructing a crystalline and amorphous heterojunction.

Ce Site in Amorphous Iron Oxyhydroxide Nanosheet toward Enhanced Electrochemical Water Oxidation

Iron oxyhydroxide has been considered an auspicious electrocatalyst for the oxygen evolution reaction (OER) in alkaline water electrolysis due to its suitable electronic structure and abundant reserves. However, Fe-based materials seriously suffer from the tradeoff between activity and stability at a high current density above 100 mA cm^-2 . In this work, the Ce atom is introduced into the amorphous iron oxyhydroxide (i.e., CeFeO_x H_y ) nanosheet to simultaneously improve the intrinsic electrocatalytic activity and stability for OER through regulating the redox property of iron oxyhydroxide. In particular, the Ce substitution leads to the distorted octahedral crystal structure of CeFeO_x H_y , along with a regulated coordination site. The CeFeO_x H_y electrode exhibits a low overpotential of 250 mV at 100 mA cm^-2 with a small Tafel slope of 35.1 mVdec^-1 . Moreover, the CeFeO_x H_y electrode can continuously work for 300 h at 100 mA cm^-2 . When applying the CeFeO_x H_y nanosheet electrode as the anode and coupling it with the platinum mesh cathode, the cell voltage for overall water splitting can be lowered to 1.47 V at 10 mA cm^-2 . This work offers a design strategy for highly active, low-cost, and durable material through interfacing high valent metals with earth-abundant oxides/hydroxides.

Research Advances in Amorphous-Crystalline Heterostructures Toward Efficient Electrochemical Applications

Interface engineering of heterostructures has proven a promising strategy to effectively modulate their physicochemical properties and further improve the electrochemical performance for various applications. In this context related research of the newly proposed amorphous-crystalline heterostructures have lately surged since they combine the superior advantages of amorphous- and crystalline-phase structures, showing unusual atomic arrangements in heterointerfaces. Nonetheless, there has been much less efforts in systematic analysis and summary of the amorphous-crystalline heterostructures to examine their complicated interfacial interactions and elusory active sites. The critical structure-activity correlation and electrocatalytic mechanism remain rather elusive. In this review, the recent advances of amorphous-crystalline heterostructures in electrochemical energy conversion and storage fields are amply discussed and presented, along with remarks on the challenges and perspectives. Initially, the fundamental characteristics of amorphous-crystalline heterostructures are introduced to provide scientific viewpoints for structural understanding. Subsequently, the superiorities and current achievements of amorphous-crystalline heterostructures as highly efficient electrocatalysts/electrodes for hydrogen evolution reaction, oxygen evolution reaction, supercapacitor, lithium-ion battery, and lithium-sulfur battery applications are elaborated. At the end of this review, future outlooks and opportunities on amorphous-crystalline heterostructures are also put forward to promote their further development and application in the field of clean energy.

Synergistic effect of oxidation etching and phase transformation triggered by controllable ion-bath microenvironments toward constructing ultra-thin porous nanosheets for accelerated industrial water splitting at high current density

Precisely tailoring the structure of inorganic materials at the micron and nanometer scales, especially in collaboration with component customization to design efficient, stable and low-cost transition-metal-based catalysts for industrial electrocatalytic water splitting (EWS) is a key renewable energy technology, but still facing a daunting challenge. Here, the controllable escape of Ni atom is adopted to disturb the hydrothermal ion-bath environment, thereby resulting in the coexistence of high valence Ni and Fe ions. Combined with a one-step hydrothermal coordination strategy, the timeline-adjusted ion-bath microenvironment can effectively trigger the phase transformation of carbonate hydroxide hydrate nanosheets (NFCH) to nickel ferrite intercalated NFCH ultra-thin porous nanosheets (NF-CH-O). Thanks to the high-energy phase boundary synergistic effect and the rapid mass transfer advantages of ultra-thin porous nanostructures, the as-prepared NF-CH-O nanosheets exhibit remarkable oxygen and hydrogen evolution reaction (OER/HER) catalytic activity and stability, with low overpotentials of 207/191 mV at 50 mA cm^-2, respectively, as well as the activity retention for 100 h. The alkaline water electrolyzer set up with NF-CH-O as both anodic and cathodic electrodes only requires a cell potential of 1.688 V to reach 50 mA cm^-2 in a continuous operation of 100 h. More impressively, NF-CH-O only requires overpotentials of 266, 292 mV and 1.877 V to drive high current densities up to 500 mA cm^-2 for OER, HER and EWS, respectively, and exhibits excellent stability with a reduction in the activity of less than 10% over cycles of more than 65 h. This work highlights the room-temperature controllable ion-bath oxidative etching strategy to design efficient bifunctional catalysts with ultra-thin porous structure and high-current-density activity. Meanwhile, combined with the advantages of direct growth on the substrate for mass production, such meticulous consideration of nanostructured design will be more competitive in the H₂-production industry.

Construction of CoP/Co2P Coexisting Bifunctional Self-Supporting Electrocatalysts for High-Efficiency Oxygen Evolution and Hydrogen Evolution

Development of a low cost, high activity, and stable nonprecious metal bifunctional catalyst for electrocatalytic water cracking is a hot topic and big challenge. In this paper, we prepared a nitrogen-doped carbon nanotube (NCNT)-enhanced three-dimensional self-supported electrocatalyst with CoP and Co₂P coexistence by a two-step strategy of high-temperature carbonization and low-temperature phosphorylation. Furthermore, the induced three-dimensional carbon network skeleton facilitates rapid charge transfer. In addition, the active sites of the carbon foam (CF) are greatly increased by the construction of hollow structures. As a bifunctional electrocatalyst, CoP/Co₂P/NCNT@CF exhibited excellent catalytic activity for both hydrogen evolution reaction and oxygen evolution reaction in alkaline media, requiring low overpotentials of 133 and 289 mV to obtain a current density of 10 mA cm^-2, respectively. Additionally, the synthesized catalysts also exhibit good long-term stability, maintaining high catalytic activity after 20 h of continuous operation. We also confirmed the main driving force to improve the electron transfer between the heterostructures of Co and P by XPS spectra. The excellent electrocatalytic performance can be attributed to the close synergy between the highly active CoP/Co₂P/NCNT and CF. This study provides a new strategy for the design of highly active bifunctional self-supporting electrocatalysts.

Recent Advances in Design of Electrocatalysts for High-Current-Density Water Splitting

Electrochemical water splitting technology for producing "green hydrogen" is important for the global mission of carbon neutrality. Electrocatalysts with decent performance at high current densities play a central role in the industrial implementation of this technology. This field has advanced immensely in recent years, as witnessed by many types of catalysts designed and synthesized toward industriallyrelevant current densities (>200 mA cm^-2 ). By discussing recent advances in this field, several key aspects are summarized that affect the catalytic performance for high-current-density electrocatalysis, including dimensionality of catalysts, surface chemistry, electron transport path, morphology, and catalyst-electrolyte interplay. The multiscale design strategy that considers these aspects comprehensively for developing high-current-density electrocatalysts are highlighted. The perspectives on the future directions in this emerging field are also put forward.

Influence of Fe-clustering on the water oxidation performance of two-dimensional layered double hydroxides

Among the two-dimensional (2D) materials family, layered double hydroxides (LDHs) represent a key member due to their unparalleled chemical versatility. In particular, Fe-based LDHs are distinguished candidates due to their high efficiency as oxygen evolution reaction (OER) electrocatalysts. Herein, we have selected MgFe-based LDH phases as model systems in order to decipher whether Fe-clustering exerts an effect on the OER performance. For that, we have optimized hydrothermal synthesis by using triethanolamine (TEA) as the chelating agent. The magnetic characterisation allows us to identify the Fe-clustering degree by following both magnetic susceptibility as well as magnetization values at 2 K. Thanks to this, we demonstrated that TEA induces an increment in Fe-clustering. Electrochemical OER measurements show that both samples behave identically by using glassy carbon electrodes. Interestingly, when the samples are tested in the most commonly employed electrode, nickel foam, striking differences arise. The sample exhibiting a lower Fe-clustering behaves as a better electrocatalyst with a reduction of the overpotential values of more than 50 mV to reach 100 mA cm^-2, as a consequence of a favoured surface transformation of MgFe-LDHs phases into more reactive oxyhydroxide NiFe-based phases during the electrochemical tests. Hence, this work alerts about the importance of the electrocatalyst-electrode collector interactions which can induce misinterpretations in the OER performance.

Controllable atom implantation for achieving Coulomb-force unbalance toward lattice distortion and vacancy construction for accelerated water splitting

Engineering lattice strain, especially when it combines with the lattice distortion and vacancy (LDV) induced by locally unbalanced Coulomb forces (LUCFs), can improve the local coordination environment of atoms to achieve synergistically active sites and dynamic regulation of electrocatalysts, which are beneficial to high-performance bifunctional water splitting. Considering that Ni-based selenides possess abundant variable valence states, the Nb/Fe diatomic heterogeneous spin states are purposely introduced to produce LUCFs for improving the electronic coordination environment of the materials. The as-prepared Nb/Fe co-doped Ni-Ses (NbFe-Ni_xSe_y) electrocatalyst exhibits the prominent oxygen and hydrogen evolution reaction (OER/HER) properties, with low overpotentials of 237 and 226 mV at 50 mA cm^-2, respectively. The alkaline water electrolyzer with NbFe-Ni_xSe_y as both anodic and cathodic electrodes only requires a cell potential of 1.7 V to reach 50 mA cm^-2 in a continuous operation of 50 h. This work provides a new insight to regulate the electronic structure of advanced catalyst materials at the atomic level through LUCFs-induced LDV and further push forward the energy conversion technology.

Spin-sate reconfiguration induced by alternating magnetic field for efficient oxygen evolution reaction

Oxygen evolution reaction (OER) plays a determining role in electrochemical energy conversion devices, but challenges remain due to the lack of effective low-cost electrocatalysts and insufficient understanding about sluggish reaction kinetics. Distinguish from complex nano-structuring, this work focuses on the spin-related charge transfer and orbital interaction between catalysts and intermediates to accelerate catalytic reaction kinetics. Herein, we propose a simple magnetic-stimulation approach to rearrange spin electron occupation in noble-metal-free metal-organic frameworks (MOFs) with a feature of thermal-differentiated superlattice, in which the localized magnetic heating in periodic spatial distribution makes the spin flip occur at particular active sites, demonstrating a spin-dependent reaction pathway. As a result, the spin-rearranged Co_0.8Mn_0.2 MOF displays mass activities of 3514.7 A g_metal⁻¹ with an overpotential of ~0.27 V, which is 21.1 times that of pristine MOF. Our findings provide a new paradigm for designing spin electrocatalysis and steering reaction kinetics.

The oxygen evolution reaction in magnetic catalysts is related with their spin configuration. Here, the authors propose a magnetic-stimulation method to rearrange spin electron occupation in thermal-differentiated superlattices.

Interfacial Engineering-Triggered Bifunctionality of CoS2 /MoS2 Nanocubes/Nanosheet Arrays for High-Efficiency Overall Water Splitting

Searching for high-efficiency nonprecious bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is paramount for the advancement of water electrolysis technologies and the associated renewable energy devices. Modulation of electronic structure of an electrocatalyst via heterointerface engineering represents an efficient strategy to improve its electrocatalytic performance. Herein, a feasible hydrothermal synthesis of a novel heterostructured catalyst was demonstrated, comprising CoS₂ nanocubes and vertically aligned MoS₂ nanosheet arrays directly grown on flexible and conductive carbon cloth (CC) substrate (denoted as CoS₂ /MoS₂ @CC). Thanks to the elaborate interface engineering and vertically aligned nanosheet arrayed architecture, the resultant self-supported CoS₂ /MoS₂ @CC electrode possessed enriched exposed active sites, modulated electronic configuration, multidimensional mass transport channels, and outstanding mechanical strength, thereby affording exceptional electrocatalytic performances towards the HER and OER in alkaline electrolyte with overpotentials of 71 and 274 mV at 10 mA cm^-2 , respectively. In addition, a two-electrode electrolyzer assembled by CoS₂ /MoS₂ @CC required a cell voltage of 1.59 V at 10 mA cm^-2 with nearly 100 % faradaic efficiency and remarkable durability, showing great potential for scalable and economical water electrolysis.