Enhanced Activity and Acid pH Stability of Prussian Blue-type Oxygen Evolution Electrocatalysts Processed by Chemical Etching

Prussian blue analogue heterointerface on a pre-oxidized copper foam surface for enhanced anodic oxidation performance

We developed CoFe-PBA on a pre-oxidized copper foam surface (CoFe-PBA@A-CF) for electrocatalytic benzyl alcohol oxidation. CoFe-PBA@A-CF decreased the cell voltage by 0.17 V in a two-electrode setup after substituting the OER with the BAOR. The pre-oxidation of CF enhanced the surface area and improved charge transfer, boosting the catalytic performance.

Design of Core@Shell Nanoparticles Based on Gold and Magnetic NiFe Prussian-Blue Analogues Featuring Shape-Dependent Magnetic and Electrochemical Activity

Au@Prussian-Blue analogue (PBA) core@shell nanoparticles (NPs) are highly versatile nanostructures with complementary and shape-dependent properties of interest in the current technologies. However, due to the high reactivity of cyanides toward Au, scarce PBAs have been successfully synthesized in direct contact with Au NPs, leaving the formation of anisotropic Au@PBA NPs as a significant synthetic challenge. Here, we have developed a robust protocol for synthesizing core@shell NPs, composed of a magnetic CsNi[Fe(CN)6] PBA shell grown on individual Au NPs, regardless of the core morphology (spheres, rods, or stars). Specifically, the uniqueness of our protocol lies in the prior Au core functionalization with anchoring molecules that facilitate PBA growth while preventing Au etching and preserving the initial oxidation states of the metals. This has afforded direct growth of ferromagnetic NiIIFeIII PBAs on Au NPs. Moreover, by exploiting the structural mismatch at the Au/PBA interface and the curvature of anisotropic Au templates, we manage to induce a substantial structural strain within the PBA shell. When star-shaped Au nanoparticles are used, a maximum strain of 2.0% is reached. This strain combined with an increased polycrystallinity lead to modifications in the PBA catalytic properties, resulting in a 10-fold improvement in the intrinsic electrocatalytic activity.

Insight into the structural reconstruction of alkaline water oxidation electrocatalysts

The oxygen-evolution reaction (OER) is an indispensable component of various energy storage and conversion electrocatalytic systems. However, the slow reaction kinetics have forced the development of advanced, efficient, and inexpensive OER electrocatalysts to break through the bottleneck of its application. Recently, the structural reconstruction of precatalysts has provided a promising avenue to boost the catalytic activity of electrocatalysts. Structural reconstruction implies atomic rearrangement and composition change of the pristine catalytic materials, which is a very complex process. Therefore, it is very crucial to have a deep understanding of the reconstruction chemical process and then modulate the reconstruction by deliberate design of electrochemical conditions and precatalysts. However, a systematic review of the structural reconstruction process, research methods, influencing factors and structure-performance relationship remains elusive, significantly impeding the further developments of efficient electrocatalysts based on structural reconstruction chemistry. This critical review is dedicated to providing a deep insight into the structural reconstruction during alkaline water oxidation, comprehensively summarizing the basic research methods to understand the structural evolution process and various factors affecting the structural reconstruction process, and providing a reference and basis for regulating the dynamic reconstruction. Moreover, the impact of reconstruction on the structure and performance is also covered. Finally, challenges and perspectives for the future study on structural reconstruction are discussed. This review will offer future guidelines for the rational development of state-of-the-art OER electrocatalysts.

Exploring suitability of solid 3D substrates for designing self-supported electrocatalysts for water splitting

The choice of solid 3D substrates to design electrocatalysts significantly impacts the efficiency and effectiveness of self-supported electrocatalysts used in water splitting. These substrates are pivotal in significantly boosting the performance by providing structural support, facilitating electron transport, and increasing the active surface area. This improvement leads to higher catalytic performance and better stability, ultimately optimizing the electrocatalytic process. The interaction between the substrate and the electrocatalyst can also affect the intrinsic properties of the catalyst, further influencing its performance. Therefore, understanding and optimizing the use of solid 3D substrates is vital for advancing water-splitting technologies. This article explores the critical role of solid 3D substrates in enhancing the activity of self-supported materials for water splitting. By examining recent developments and research in this region, we target to showcase a comprehensive understanding of how different 3D substrates influence electrocatalyst properties and performance and to highlight future directions for optimizing these systems in water-splitting applications.

Ge-Doped Hematite with FeCoNi-Bi as Cocatalyst for High-Performing Photoelectrochemical Water Splitting

Hematite is a promising photoanode material for photoelectrochemical water-splitting technology. However, the low current density associated with the low conductivity, low charge carrier mobility, and poor oxygen evolution catalytic activity is a challenging issue for the material. In this study, the challenge is addressed by introducing Germanium (Ge) doping, coupled with the use of FeCoNi-Bi as a co-catalyst. Ge doping not only increases the conductivity and charge carrier concentration of the hematite photoanode, but also induces nanopores, thereby expanding its electrochemical reactive surface area to facilitate the oxygen evolution reaction. In the meantime, the FeCoNi-Bi cocatalyst electrodeposited onto the surface of Ge-doped hematite, improves the oxygen evolution reaction performance. As a result, the obtained photoanode achieves a photocurrent density of 2.31 mA cm-2 at 1.23 VRHE, which is three times higher than that of hematite (0.72 mA cm-2). Moreover, a new analytical method is introduced to scrutinize both the positive and negative effects of Ge doping and FeCoNi-Bi cocatalyst on the photoanode performance by decoupling the photoelectrochemical process steps. Overall, this study not only enhances the performance of hematite photoanodes but also guides their rational design and systematic assessment.

Prussian Blue Analogue Glasses for Photoinduced CO2 Conversion

Crystal-to-glass transformation is a powerful approach to modulating the chemical and physical properties of crystals. Here we demonstrate that the glass transformation of cobalt hexacyanoferrate crystals, one of the Prussian blue analogues, increased the concentration of open metal sites and altered the electronic state while maintaining coordination geometries and short-range ordering in the structure. The compositional and structural changes were characterized by X-ray absorption fine structure, energy dispersive X-ray spectroscopy, and X-ray total scattering. The changes contribute to the flat band potential of the glass becoming closer to the redox potential of CO2 reduction. The valence band energy of the glass also shifts, resulting in lower band gap energy. Both the increased open metal sites and the optimal electronic structure upon vitrification enhance photocatalytic activity toward CO2-to-CO conversions (9.9 μmol h-1 CO production) and selectivity (72.4%) in comparison with the crystalline counterpart (3.9 μmol h-1 and 42.8%).

Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

Tunable Photocatalytic Activity of CoFe Prussian Blue Analogue Modified SrTiO3 Core-Shell Structures for Solar-Driven Water Oxidation

This study presents a pioneering semiconductor-catalyst core-shell architecture designed to enhance photocatalytic water oxidation activity significantly. This innovative assembly involves the in situ deposition of CoFe Prussian blue analogue (PBA) particles onto SrTiO3 (STO) and blue SrTiO3 (bSTO) nanocubes, effectively establishing a robust p-n junction, as demonstrated by Mott-Schottky analysis. Of notable significance, the STO/PB core-shell catalyst displayed remarkable photocatalytic performance, achieving an oxygen evolution rate of 129.6 μmol g-1 h-1, with stability over an extended 9-h in the presence of S2O82- as an electron scavenger. Thorough characterization unequivocally verified the precise alignment of the band energies within the STO/PB core-shell assembly. Our research underscores the critical role of tailored semiconductor-catalyst interfaces in advancing the realm of photocatalysis and its broader applications in renewable energy technologies.

Enhanced and synergistic catalytic activation by photoexcitation driven S-scheme heterojunction hydrogel interface electric field

The regulation of heterogeneous material properties to enhance the peroxymonosulfate (PMS) activation to degrade emerging organic pollutants remains a challenge. To solve this problem, we synthesize S-scheme heterojunction PBA/MoS2@chitosan hydrogel to achieve photoexcitation synergistic PMS activation. The constructed heterojunction photoexcited carriers undergo redox conversion with PMS through S-scheme transfer pathway driven by the directional interface electric field. Multiple synergistic pathways greatly enhance the reactive oxygen species generation, leading to a significant increase in doxycycline degradation rate. Meanwhile, the 3D polymer chain spatial structure of chitosan hydrogel is conducive to rapid PMS capture and electron transport in advanced oxidation process, reducing the use of transition metal activator and limiting the leaching of metal ions. There is reason to believe that the synergistic activation of PMS by S-scheme heterojunction regulated by photoexcitation will provide a new perspective for future material design and research on enhancing heterologous catalysis oxidation process.

Metal Recovery from Natural Saline Brines with an Electrochemical Ion Pumping Method Using Hexacyanoferrate Materials as Electrodes

The electrochemical ion pumping device is a promising alternative for the development of the industry of recovering metals from natural sources-such as seawater, geothermal water, well brine, or reverse osmosis brine-using electrochemical systems, which is considered a non-evaporative process. This technology is potentially used for metals like Li, Cu, Ca, Mg, Na, K, Sr, and others that are mostly obtained from natural brine sources through a combination of pumping, solar evaporation, and solvent extraction steps. As the future demand for metals for the electronic industry increases, new forms of marine mining processing alternatives are being implemented. Unfortunately, both land and marine mining, such as off-shore and deep sea types, have great potential for severe environmental disruption. In this context, a green alternative is the mixing entropy battery, which is a promising technique whereby the ions are captured from a saline natural source and released into a recovery solution with low ionic force using intercalation materials such as Prussian Blue Analogue (PBA) to store cations inside its crystal structure. This new technique, called "electrochemical ion pumping", has been proposed for water desalination, lithium concentration, and blue energy recovery using the difference in salt concentration. The raw material for this technology is a saline solution containing ions of interest, such as seawater, natural brines, or industrial waste. In particular, six main ions of interest-Na+, K+, Mg2+, Ca2+, Cl-, and SO42--are found in seawater, and they constitute 99.5% of the world's total dissolved salts. This manuscript provides relevant information about this new non-evaporative process for recovering metals from aqueous salty solutions using hexacianometals such as CuHCF, NiHCF, and CoHCF as electrodes, among others, for selective ion removal.

Chemically Etched Prussian Blue Analog-WS2 Composite as a Precatalyst for Enhanced Electrocatalytic Water Oxidation in Alkaline Media

The electrochemical water-splitting reaction is a promising source of ecofriendly hydrogen fuel. However, the oxygen evolution reaction (OER) at the anode impedes the overall process due to its four-electron oxidation steps. To address this issue, we developed a highly efficient and cost-effective electrocatalyst by transforming Co-Fe Prussian blue analog nanocubes into hollow nanocages using dimethylformamide as a mild etchant and then anchoring tungsten disulfide (WS2) nanoflowers onto the cages to boost OER efficiency. The resulting hybrid catalyst-derived oxide demonstrated a low overpotential of 290 mV at a current density of 10 mA cm-2 with a Tafel slope of 75 mV dec-1 in 1.0 M KOH and a high faradaic efficiency of 89.4%. These results were achieved through the abundant electrocatalytically active sites, enhanced surface permeability, and high electronic conductivity provided by WS2 nanoflowers and the porous three-dimensional (3D) architecture of the nanocages. Our research work uniquely combines surface etching of Co-Fe PBA with WS2 growth to create a promising OER electrocatalyst. This study provides a potential solution to the challenge of the OER in electrochemical water-splitting, contributing to UN SDG 7: Affordable and clean energy.

A Dormant Reagent Reaction-Diffusion Method for the Generation of Co-Fe Prussian Blue Analogue Periodic Precipitate Particle Libraries

Liesegang patterns that develop as a result of reaction-diffusion can simultaneously form products with slightly different sizes spatially separated in a single medium. We show here a reaction-diffusion method using a dormant reagent (citrate) for developing Liesegang patterns of cobalt hexacyanoferrate Prussian Blue analog (PBA) particle libraries. This method slows the precipitation reaction and produces different-sized particles in a gel medium at different locations. The gel-embedded particles are still catalytically active. Finally, the applicability of the new method to other PBAs and 2D systems is presented. The method proves promising for obtaining similar inorganic framework libraries with catalytic abilities.

Cobalt-Iron Oxyhydroxide Obtained from the Metal Phosphide: A Highly Effective Electrocatalyst for the Oxygen Evolution Reaction at High Current Densities

The development of high current density anodes for the oxygen evolution reaction (OER) is fundamental to manufacturing practical and reliable electrochemical cells. In this work, we have developed a bimetallic electrocatalyst based on cobalt-iron oxyhydroxide that shows outstanding performance for water oxidation. Such a catalyst is obtained from cobalt-iron phosphide nanorods that serve as sacrificial structures for the formation of a bimetallic oxyhydroxide through phosphorous loss concomitantly to oxygen/hydroxide incorporation. CoFeP nanorods are synthesized using a scalable method using triphenyl phosphite as a phosphorous precursor. They are deposited without the use of binders on nickel foam to enable fast electron transport, a highly effective surface area, and a high density of active sites. The morphological and chemical transformation of the CoFeP nanoparticles is analyzed and compared with the monometallic cobalt phosphide in alkaline media and under anodic potentials. The resulting bimetallic electrode presents a Tafel slope as low as 42 mV dec-1 and low overpotentials for OER. For the first time, an anion exchange membrane electrolysis device with an integrated CoFeP-based anode was tested at a high current density of 1 A cm-2, demonstrating excellent stability and Faradaic efficiency near 100%. This work opens up a way for using metal phosphide-based anodes for practical fuel electrosynthesis devices.

Iron-Locked Hydr(oxy)oxide Catalysts via Ion-Compensatory Reconstruction Boost Large-Current-Density Water Oxidation

Nickel-iron based hydr(oxy)oxides have been well recognized as one of the best oxygen-evolving catalysts in alkaline water electrolysis. A crucial problem, however, is that iron leakage during prolonged operation would lead to the oxygen evolution reaction (OER) deactivation over time, especially under large current densities. Here, the NiFe-based Prussian blue analogue (PBA) is designed as a structure-flexible precursor for navigating an electrochemical self-reconstruction (ECSR) with Fe cation compensation to fabricate a highly active hydr(oxy)oxide (NiFeOx Hy ) catalyst stabilized with NiFe synergic active sites. The generated NiFeOx Hy catalyst exhibits the low overpotentials of 302 and 313 mV required to afford large current densities of 500 and 1000 mA cm-2 , respectively. Moreover, its robust stability over 500 h at 500 mA cm-2 stands out among the NiFe-based OER catalysts reported previously. Various in/ex situ studies indicate that the Fe fixation by dynamic reconstruction process can reinforce the Fe-activated effect on the OER amenable to the industrial-level large current conditions against the Fe leakage. This work opens up a feasible strategy to design highly active and durable catalysts via thermodynamically self-adaptive reconstruction engineering.

2D Metal-Organic Frameworks as Competent Electrocatalysts for Water Splitting

Hydrogen, a clean and flexible energy carrier, can be efficiently produced by electrocatalytic water splitting. To accelerate the sluggish hydrogen evolution reaction and oxygen evolution reaction kinetics in the splitting process, highly active electrocatalysts are essential for lowering the energy barriers, thereby improving the efficiency of overall water splitting. Combining the distinctive advantages of metal-organic frameworks (MOFs) with the physicochemical properties of 2D materials such as large surface area, tunable structure, accessible active sites, and enhanced conductivity, 2D MOFs have attracted intensive attention in the field of electrocatalysis. Different strategies, such as improving the conductivities of MOFs, reducing the thicknesses of MOF nanosheets, and integrating MOFs with conductive particles or substrates, are developed to promote the catalytic performances of pristine MOFs. This review summarizes the recent advances of pristine 2D MOF-based electrocatalysts for water electrolysis. In particular, their intrinsic electrocatalytic properties are detailly analyzed to reveal important roles of inherent MOF active centers, or other in situ generated active phases from MOFs responsible for the catalytic reactions. Finally, the challenges and development prospects of pristine 2D MOFs for the future applications in overall water splitting are discussed.

Regulating the Synergistic Effect in Bimetallic Two-Dimensional Polymer Oxygen Evolution Reaction Catalysts by Adjusting the Coupling Strength Between Metal Centers

Bimetallic electrocatalysts with its superior performance has a broad application prospect in oxygen evolution reaction (OER), but the fundamental understanding of the mechanism of synergistic effect is still limited since there lacks a practical way to decouple the influence factors on the intrinsic activity of active sites from others. Herein, a series of bimetallic Co-Ni two-dimensional polymer (2DP) model OER catalysts with well-defined architecture, monolayer characteristic, were designed and synthesized to explore the influence of the coupling strength between metal centers on OER performance. The coupling strength was regulated by adjusting the spacing between metal centers or the conjugation degree of bridge skeleton. Among the examined 2DPs, CoTAPP-Ni-MF-2DP, which has the strongest coupling strength between metal centers exhibited the best OER performance. These model systems can help to explore the precise structure-performance relationships, which is important for the rational catalyst design at the atomic/molecular levels.

Synergistic Effect of FeII and MnII Ions in Cyano-Bridged Heterometallic Coordination Polymers on Catalytic Selectivity of Benzene Oxygenation to Phenol

A cyano-bridged heterometallic coordination polymer with partial deficiencies of CN^- ligands, [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)], forms open metal sites both on Mn^II and Fe^II ions by liberation of monodentate ligands such as NH₃ and H₂O. [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] exhibits high catalytic activity and selectivity of benzene oxygenation to phenol in the presence of m-chloroperoxybenzoic acid as an oxidant. The postcatalytic spectroscopy of [Mn^II(H₂O)_8/3]_3/2[Fe^II(CN)₅(NH₃)] and catalysis comparison with a physical mixture of [Mn^II(H₂O)₃]₂[Fe^II(CN)₆] and [Fe(H₂O)_3/2]_4/3[Fe(CN)₆], which has open metal sites on both Mn^II and Fe ions separately, indicated that the high activity resulted from high oxidation ability and phenol adsorption ability of Fe^II and Mn^II ions, respectively.

Trade-Off of Metal Sites in Fe-Ni Bimetal Metal-Organic Frameworks for Efficient CO2 Photoreduction with Nearly 100% CO Selectivity

This work disclosed the trade-off effect of two metal sites, which display distinct, key functionalities in naturally occurring and artificial catalysts for developing an advanced CO₂ reduction system. To exploit the metal-organic frameworks (MOFs) as advanced catalysts, we prepared a series of Prussian blue analogues (FeNi_x PBAs) of tunable Ni/Fe molar ratio without changing the oxidation state of Fe and Ni for use as a photocatalyst in the CO₂ reduction reaction (CRR). The FeNi_0.66 PBA gives a superior CO yield rate (14.28 mmol·g^-1·h^-1) with nearly 100% CO selectivity, but the PBA would be basically CRR-inactive without either Ni or Fe. Experimental and calculation studies demonstrate that Fe and Ni display distinct functionalities. Specifically, Fe is an efficient mediator that boosts the electron transfer both from the photosensitizer to FeNi_x PBA and from FeNi_x PBA to CO₂, and Ni serves as the active site for CO₂ adsorption and reduction. Intriguingly, when there is already sufficient Ni in the catalyst, further increase of the Ni content gives marginal gains in the CO₂ adsorption affinity that cannot offset the weakened electron transfer due to the Ni excess. The findings can help advance the design of bimetallic MOF catalysts that mimic naturally occurring bimetallic catalysts.

Binder-Free Fabrication of Prussian Blue Analogues Based Electrocatalyst for Enhanced Electrocatalytic Water Oxidation

Developing a cost-effective, efficient, and stable oxygen evolution reaction (OER) catalyst is of great importance for sustainable energy conversion and storage. In this study, we report a facile one-step fabrication of cationic surfactant-assisted Prussian blue analogues (PBAs) M_x[Fe(CN)₅CH₃C₆H₄NH₂]∙yC₁₉H₃₄NBr abbreviated as SF[Fe-Tol-M] (where SF = N-tridecyl-3-methylpyridinium bromide and M = Mn, Co and Ni) as efficient heterogeneous OER electrocatalysts. The electrocatalysts have been characterized by Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). In the presence of cationic surfactant (SF), PBAs-based electrodes showed enhanced redox current, high surface area and robust stability compared to the recently reported PBAs. SF[Fe-Tol-Co] hybrid catalyst shows superior electrochemical OER activity with a much lower over-potential (610 mV) to attain the current density of 10 mA cm⁻² with the Tafel slope value of 103 mV·dec⁻¹ than that for SF[Fe-Tol-Ni] and SF[Fe-Tol-Mn]. Moreover, the electrochemical impedance spectroscopy (EIS) unveiled that SF[Fe-Tol-Co] exhibits smaller charge transfer resistance, which results in a faster kinetics towards OER. Furthermore, SF[Fe-Tol-Co] offered excellent stability for continues oxygen production over extended reaction time. This work provides a surface assisted facile electrode fabrication approach for developing binder-free OER electrocatalysts for efficient water oxidation.

Lowering the Water Oxidation Overpotential by Spin-Crossover in Cobalt Hexacyanoferrate

The oxygen evolution reaction (OER) is limited by the inherent linear scaling relationships of its reaction intermediates. Manipulating the spin configuration of the water oxidation intermediates allows us to overcome these constraints. Cobalt hexacyanoferrate (CoFe-PB) is an efficient and robust water oxidation catalyst and further known as a magnetic switch. Its versatile electronic structure renders it a potential candidate for magnetic tuning of the OER. Herein, we used first-principles density functional theory calculations to describe the OER on two different CoFe-PB model systems and evaluated the possibility for spin-crossover (SCO) of their resting states. We show that SCO during OER can significantly lower the overpotential by 0.7 V, leading to an overpotential of around 0.3 V, which is in agreement with the experimentally measured value. Applying an external potential >1.5 V vs SHE, the SCO-assisted pathway becomes largely favored and most likely the predominant reaction pathway.

"Plug and Play" Photosensitizer-Catalyst Dyads for Water Oxidation

We present a simple and easy-to-scale synthetic method to plug common organic photosensitizers into a cyanide-based network structure for the development of photosensitizer-water oxidation catalyst (PS-WOC) dyad assemblies for the photocatalytic water oxidation process. Three photosensitizers, one of which absorbs red light similar to P680 in photosystem II, were utilized to harvest different regions of the solar spectrum. Photosensitizers are covalently coordinated to CoFe Prussian blue structures to prepare PS-WOC dyads. All dyads exhibit steady water oxidation catalytic activities throughout a 6 h photocatalytic experiment. Our results demonstrate that the covalent coordination between the PS and WOC group not only enhances the photocatalytic activity but also improves the robustness of the organic PS group. The photocatalytic activity of "plug and play" dyads relies on several structural and electronic parameters, including the position of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the PS with respect to the HOMO level of the catalytic site, the intensity and wavelength of the absorption band of the PS, and the number of catalytic sites.

Freestanding Metal-Organic Frameworks and Their Derivatives: An Emerging Platform for Electrochemical Energy Storage and Conversion

Metal–organic frameworks (MOFs) have recently emerged as ideal electrode materials and precursors for electrochemical energy storage and conversion (EESC) owing to their large specific surface areas, highly tunable porosities, abundant active sites, and diversified choices of metal nodes and organic linkers. Both MOF-based and MOF-derived materials in powder form have been widely investigated in relation to their synthesis methods, structure and morphology controls, and performance advantages in targeted applications. However, to engage them for energy applications, both binders and additives would be required to form postprocessed electrodes, fundamentally eliminating some of the active sites and thus degrading the superior effects of the MOF-based/derived materials. The advancement of freestanding electrodes provides a new promising platform for MOF-based/derived materials in EESC thanks to their apparent merits, including fast electron/charge transmission and seamless contact between active materials and current collectors. Benefiting from the synergistic effect of freestanding structures and MOF-based/derived materials, outstanding electrochemical performance in EESC can be achieved, stimulating the increasing enthusiasm in recent years. This review provides a timely and comprehensive overview on the structural features and fabrication techniques of freestanding MOF-based/derived electrodes. Then, the latest advances in freestanding MOF-based/derived electrodes are summarized from electrochemical energy storage devices to electrocatalysis. Finally, insights into the currently faced challenges and further perspectives on these feasible solutions of freestanding MOF-based/derived electrodes for EESC are discussed, aiming at providing a new set of guidance to promote their further development in scale-up production and commercial applications.

3D Spatial Combination of CN Vacancy-Mediated NiFe-PBA with N-Doped Carbon Nanofibers Network Toward Free-Standing Bifunctional Electrode for Zn-Air Batteries

Constructing flexible free-standing electrodes with efficient bifunctional performance is significant for improving the performance of flexible Zinc-air batteries. Herein, a flexible free-standing bifunctional electrode (N2 -NiFe-PBA/NCF/CC-60) is constructed by the 3D spatial combination of CN vacancy-mediated NiFe Prussian Blue Analogue (NiFe-PBA) and N-doped carbon nanofibers (NCF) rooted on carbon cloth (CC). The in situ formed CN vacancies by N2 -plasma activation tune the local coordination environment and electronic structure of Ni-Fe active sites in NiFe-PBA, thus improving the oxygen evolution reaction (OER) catalytic intrinsic activity, and restraining the loss of Fe element during OER process. The combination of NiFe-PBA and NCF presents a 3D interworking network structure, which exhibits a large specific surface and excellent electrical conductivity, thus guaranteeing sufficient, stable, and efficient oxygen reduction reaction (ORR)/OER active sites. Therefore, the N2 -NiFe-PBA/NCF/CC-60 electrode delivers high-efficiency OER activity with a low overpotential (270 mV at 50 mA cm-2 ) and excellent ORR performance with a positive potential of 0.89 V at 5 mA cm-2 . The N2 -NiFe-PBA/NCF/CC-60 based Zn-air batteries display outstanding discharge/charge stability for 2000 cycles. Meanwhile, the corresponding flexible Zn-air batteries with satisfactory mechanical properties exhibit a low voltage gap of 0.52 V at 1.0 mA cm-2 .

Mechanistic Insights into the Interplay between Ion Intercalation and Water Electrolysis in Aqueous Batteries

Improving electrolyte stability to suppress water electrolysis represents a basic principle for designing aqueous batteries. Herein, we investigate counterintuitive roles that water electrolysis plays in regulating intercalation chemistry. Using the Na_xFe[Fe(CN)₆]∥NaTi₂(PO₄)₃ (x < 1) aqueous battery as a platform, we report that high-voltage overcharging can serve as an electrochemical activation approach to achieving concurrent Na-ion intercalation and an electrolytic oxygen evolution reaction. When the cell capacity is intrinsically limited by deficient cyclable Na ions, the electrolytic water oxidation on the cathode allows for extra Na-ion intercalation from the electrolyte to the NaTi₂(PO₄)₃ anode, leading to a major increase in cyclable Na ions and specific capacity. The parasitic oxygen generation and potential transition-metal dissolution, as proved by our synchrotron and imaging tools, can be significantly mitigated with a simple reassembling approach, which enables stable electrochemical performance and sheds light on manipulating ion intercalation and water electrolysis for battery fast charging and recycling.

An Autonomous Device for Solar Hydrogen Production from Sea Water

Hydrogen production from water electrolysis is one of the most promising approaches for the production of green H2, a fundamental asset for the decarbonization of the energy cycle and industrial processes. Seawater is the most abundant water source on Earth, and it should be the feedstock for these new technologies. However, commercial electrolyzers still need ultrapure water. The debate over the advantages and disadvantages of direct sea water electrolysis when compared with the implementation of a distillation/purification process before the electrolysis stage is building in the relevant research. However, this debate will remain open for some time, essentially because there are no seawater electrolyser technologies with which to compare the modular approach. In this study, we attempted to build and validate an autonomous sea water electrolyzer able to produce high-purity green hydrogen (>90%) from seawater. We were able to solve most of the problems that natural seawater electrolyses imposes (high corrosion, impurities, etc.), with decisions based on simplicity and sustainability, and those issues that are yet to be overcome were rationally discussed in view of future electrolyzer designs. Even though the performance we achieved may still be far from industrial standards, our results demonstrate that direct seawater electrolysis with a solar-to-hydrogen efficiency of ≈7% can be achieved with common, low-cost materials and affordable fabrication methods.

On-Demand Atomic Hydrogen Provision by Exposing Electron-Rich Cobalt Sites in an Open-Framework Structure toward Superior Electrocatalytic Nitrate Conversion to Dinitrogen

Electrocatalytic nitrate (NO₃^-) reduction to N₂ via atomic hydrogen (H*) is a promising approach for advanced water treatment. However, the reduction rate and N₂ selectivity are hindered by slow mass transfer and H* provision-utilization mismatch, respectively. Herein, we report an open-framework cathode bearing electron-rich Co sites with extraordinary H* provision performance, which was validated by electron spin resonance (ESR) and cyclic voltammetry (CV) tests. Benefiting from its abundant channels, NO₃^- has a greater opportunity to be efficiently transferred to the vicinity of the Co active sites. Owing to the enhanced mass transfer and on-demand H* provision, the nitrate removal efficiency and N₂ selectivity of the proposed cathode were 100 and 97.89%, respectively, superior to those of noble metal-based electrodes. In addition, in situ differential electrochemical mass spectrometry (DEMS) indicated that ultrafast *NO₂^- to *NO reduction and highly selective *NO to *N₂O or *N transformation played crucial roles during the NO₃^- reduction process. Moreover, the proposed electrochemical system can achieve remarkable N₂ selectivity without the additional Cl^- supply, thus avoiding the formation of chlorinated byproducts, which are usually observed in conventional electrochemical nitrate reduction processes. Environmentally, energy conservation and negligible byproduct release ensure its practicability for use in nitrate remediation.

Shape-controlled Mn–Fe PBA derived micromotors for organic pollutant removal

A new strategy is employed to prepare Mn–Fe PBA derived oxide micromotors with excellent motion performances through co-precipitation and heat treatment, which can be used for organic pollutant degradation with recycling and reusing advantages.

Old Materials for New Functions: Recent Progress on Metal Cyanide Based Porous Materials

Cyanide is the simplest ligand with strong basicity to construct open frameworks including some of the oldest compounds reported in the history of coordination chemistry. Cyanide can form numerous cyanometallates with different transition metal ions showing diverse geometries. Rational design of robust extended networks is enabled by the strong bonding nature and high directionality of cyanide ligand. By virtue of a combination of cyanometallates and/or organic linkers, multifunctional framework materials can be targeted and readily synthesized for various applications, ranging from molecular adsorptions/separations to energy conversion and storage, and spin-crossover materials. External guest- and stimuli-responsive behaviors in cyanide-based materials are also highlighted for the development of the next-generation smart materials. In this review, an overview of the recent progress of cyanide-based multifunctional materials is presented to demonstrate the great potential of cyanide ligands in the development of modern coordination chemistry and material science.

Electrodeposited cobalt hexacyanoferrate electrode as a non-enzymatic glucose sensor under neutral conditions

A CoFe Prussian blue analogue (CoFe PB) modified FTO electrode, prepared via a facile electrodeposition method, is investigated as a non-enzymatic glucose sensor under neutral conditions. The electrode exhibits a linear detection of glucose in the 0.1-8.2 mmol/L range with a detection limit of 67 μM, a sensitivity of 18.69 μA/mM.cm², and a fast response time of less than 7 s under neutral conditions. Its stability is confirmed with both electrochemical experiments and characterization studies performed on the pristine and post-mortem electrode. We also conducted a comprehensive electrochemical analysis to elucidate the identity of the active site and the glucose oxidation mechanism on the Prussian blue surface.

Interface engineering triggered by carbon nanotube-supported multiple sulfides for boosting oxygen evolution

Finding an efficient, stable and cheap oxygen evolution reaction (OER) catalyst is very important for renewable energy conversion systems. There are relatively few related research reports due to the thermodynamic instability of transition metal sulfides (TMSs) at the oxidation potential and these are usually focused on single metal sulfides or bimetal sulfides. Metal sulfide mixture systems are rarely studied. The fabrication of a TMS/TMS interface is a feasible method to improve the kinetics of the OER. Here, we constructed TMS hybrid electrocatalysts with multiple phase interfaces for the oxygen evolution reaction, named S-CoFe/CNTs. The results show that the S-CoFe/CNT catalyst exhibits a low overpotential of 258 mV to achieve a current density of 10 mA cm^-2, and has high activity in the OER process. Meanwhile, the catalyst also shows a low Tafel slope (69 mV dec^-1) and good stability. This can be attributed to the synergistic catalysis of the multiphase interface in the catalyst and the rapid electron transfer pathway brought by CNTs. The new strategy for the synthesis of catalysts containing the TMS/TMS interface provides a new idea and method for the development of efficient and practical water splitting catalysts.

Advances in Understanding the Electrocatalytic Reconstruction Chemistry of Coordination Compounds

Coordination compounds including mainstream metal-organic frameworks and Prussian blue analogues receive extensive researches when they directly serve as electrocatalysts. Their reconstruction phenomena, that are closely associated with actual contributions and intrinsic catalytic mechanisms, are expected to be well summarized. Here, the recent advances in understanding reconstruction chemistry of coordination compounds are reviewed, including their main classifications and structural properties, reconstruction phenomena in electrocatalysis (e.g., oxygen/hydrogen evolution reaction, CO₂ reduction), influence factors of reconstruction parameters (e.g., reconstruction rate and reconstruction degree), and reconstruction-performance correlation. It is outlined that the reconstruction processes are influenced by electronic structure of coordination compounds, pH and temperature of testing solution, and applied potentials. The characterization techniques reflecting the evolution information before and after catalysis are also introduced for reconstruction-related mechanistic study. Finally, some challenges and outlooks on reconstruction investigations of coordination compounds are proposed, and the necessity of studying and understanding of these themes under actual working conditions of devices is highlighted.

Environmentally Benign Formation of Nickel Hexacyanoferrate-Derived Mesoframes for Heterogeneous Catalysis

The tetramethylammonium hydroxide (TMAH)-controlled alkaline etching of nickel hexacyanoferrate (NiHCF) mesocrystals is explored. The alkaline etching enables the formation of hollow framework structures with an increased surface area, the exposure of active Ni and Fe sites and the retention of morphology. The ambient reaction conditions enable the establishment of a sustainable production. Our work reveals novel perspectives on the eco-friendly synthesis of hollow and colloidal superstructures for the efficient degradation of the organic contaminants rhodamine-B and bisphenol-A. In the case of peroxomonosulfate (PMS)-mediated bisphenol-A degradation, the rate constant of the etched mesoframes was 10,000 times higher indicating their significant catalytic activity.

The activation of inert NiFe Prussian Blue analogues to boost oxygen evolution reaction activity

The inert sites of Prussian Blue Analogue (PBA) seriously affected its electrocatalytic activity and application, how to activate inert sites in PBA to fulfill effective oxygen evolution reduction (OER) is a major challenge. Herein, Mo substituted Fe sites and S doped in inert PBA were designed and synthesized by hydrothermal method to enhance structural stability and OER activity. PBA-SMo/NF shows the optimum activity with a low overpotential of 252 and 294 mV for harvesting current density 20 and 100 mA cm^-2, respectively, and exhibits excellent durability under high current density. Theoretical calculation of H₂O adsorption energy and Bader charges reveals that Mo sites in PBA-SMo possess favourable H₂O adsorption kinetics. More important, Gibbs free energy diagram and DOS show PBA-SMo have lower energy barriers for OER and better conductivity. This work provides a kind of guidance for the design and optimization of PBA for broad applications.

Enhancing the electrocatalytic activity and stability of Prussian blue analogues by increasing their electroactive sites through the introduction of Au nanoparticles

Prussian blue analogues (PBAs) have been proven as excellent Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) in acidic, neutral and alkaline media. Further improvements can be achieved by increasing their electrical conductivity, but scarce attention has been paid to quantify the electroactive sites of the electrocatalyst when this enhancement occurs. In this work, we have studied how the chemical design influences the specific density of electroactive sites in different Au-PBA nanostructures. Thus, we have first obtained and fully characterized a variety of monodisperse core@shell hybrid nanoparticles of Au@PBA (PBA of Ni^IIFe^II and Co^IIFe^II) with different shell sizes. Their catalytic activity is evaluated by studying the OER, which is compared to pristine PBAs and other Au-PBA heterostructures. By using the coulovoltammetric technique, we have demonstrated that the introduction of 5-10% of Au in weight in the core@shell leads to an increase in the electroactive mass and thus, to a higher density of active sites capable of taking part in the OER. This increase leads to a significant decrease in the onset potential (up to 100 mV) and an increase (up to 420%) in the current density recorded at an overpotential of 350 mV. However, the Tafel slope remains unchanged, suggesting that Au reduces the limiting potential of the catalyst with no variation in the reaction kinetics. These improvements are not observed in other Au-PBA nanostructures mainly due to a lower contact between both compounds and the Au oxidation. Hence, an Au core activates the PBA shell and increases the conductivity of the resulting hybrid, while the PBA shell prevents Au oxidation. The strong synergistic effect existing in the core@shell structure evidences the importance of the chemical design for preparing PBA-based nanostructures exhibiting better electrocatalytic performances and higher electrochemical stabilities.

Comprehensive Understandings into Complete Reconstruction of Precatalysts: Synthesis, Applications, and Characterizations

Reconstruction induced by external environment (such as applied voltage bias and test electrolytes) changes catalyst component and catalytic behaviors. Investigations of complete reconstruction in energy conversion recently receive intensive attention, which promote the targeted design of top-performance materials with maximum component utilization and good stability. However, the advantages of complete reconstruction, its design strategies, and extensive applications have not achieved the profound understandings and summaries it deserves. Here, this review systematically summarizes several important advances in complete reconstruction for the first time, which includes 1) fundamental understandings of complete reconstruction, the characteristics and advantages of completely reconstructed catalysts, and their design principles, 2) types of reconstruction-involved precatalysts for oxygen evolution reaction catalysis in wide pH solution, and origins of limited reconstruction degree as well as design strategies/principles toward complete reconstruction, 3) complete reconstruction for novel material synthesis and other electrocatalysis fields, and 4) advanced in situ/operando or multiangle/level characterization techniques to capture the dynamic reconstruction processes and real catalytic contributors. Finally, the existing major challenges and unexplored/unsolved issues on studying the reconstruction chemistry are summarized, and an outlook for the further development of complete reconstruction is briefly proposed. This review will arouse the attention on complete reconstruction materials and their applications in diverse fields.