Metal-organic frameworks (MOFs) for hybrid water electrolysis: structure-property-performance correlation.

Hybrid water electrolysis (HWE) is a promising pathway for the simultaneous production of high-value chemicals and clean H2 fuel. Unlike conventional electrochemical water splitting, which relies on the oxygen evolution reaction (OER), HWE involves the anodic oxidation reaction (AOR). The AORs facilitate the conversion of organic or inorganic compounds at the anode into valuable chemicals, while the cathode carries out the hydrogen evolution reaction (HER) to produce H2. Recent literature has witnessed a surge in papers investigating various AORs with organic and inorganic substrates using a series of transition metal-based catalysts. Over the past two decades, metal-organic frameworks (MOFs) have garnered significant attention for their exceptional performance in electrochemical water splitting. These catalysts possess distinct attributes such as highly porous architectures, customizable morphologies, open facets, high electrochemical surface areas, improved electron transport, and accessible catalytic sites. While MOFs have demonstrated efficiency in electrochemical water splitting, their application in hybrid water electrolysis has only recently been explored. In recent years, a series of articles have been published; yet there is no comprehensive article summarizing MOFs for hybrid water electrolysis. This article aims to fill this gap by delving into the recent progress in MOFs specifically tailored for hybrid water electrolysis. In this article, we systematically discuss the structure-property-performance relationships of various MOFs utilized in hybrid water electrolysis, supported by pioneering examples. We explore how the structure, morphology, and electronic properties of MOFs impact their performance in hybrid water electrolysis, with particular emphasis on value-added chemical generation, H2 production, potential improvement, conversion efficiency, selectivity, faradaic efficiency, and their potential for industrial-scale applications. Furthermore, we address future advancements and challenges in this field, providing insights into the prospects and challenges associated with the continued development and deployment of MOFs for hybrid water electrolysis.

Nitrogen substitution induced lattice contraction in nickel nanoparticles for electrochemical hydrogen evolution from simulated seawater.

Herein, we demonstrate a facile method for the introduction of nitrogen in the lattices of nickel nanoparticles to form NiNx (x = 0.13, 0.20, 0.27). X-ray absorption spectroscopy reveals the contraction of the Ni-Ni bond and modulated coordination environment after nitrogen introduction. The NiN0.20 required 87 mV overpotential for -10 mA cm-2 cathodic current density in simulated seawater. The density functional theory calculations revealed favorable EH2Oads and ΔGHads after N-introduction.

Ruthenium Azobis(benzothiazole): Electronic Structure and Impact of Substituents on the Electrocatalytic Single-Site Water Oxidation Process.

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [RuII(Ph-trpy)(Cl)(L)]ClO4, [1]ClO4-[3]ClO4 (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2'-azobis(benzothiazole) ([1]ClO4); L2: 2,2'-azobis(6-methylbenzothiazole) ([2]ClO4); L3: 2,2'-azobis(6-chlorobenzothiazole) ([3]ClO4)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [1]ClO4-[3]ClO4 established selective stabilization of (i) the unperturbed azo (N═N)0 function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L trans to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of [2]ClO4 (CH---O) via the involvement of ClO4- anions. The detailed electrochemical studies revealed metal-based oxidation of [RuII(Ph-trpy)(Cl)(L)]+ (1+-3+) to [RuIII(Ph-trpy)(Cl)(L)]2+ (12+-32+); however, the electronic form of the first reduced state (1-3) could be better represented by its mixed RuII(Ph-trpy)(Cl)(L•-)/RuIII(Ph-trpy)(Cl)(L2-) state. Both native (1+-3+) and reduced (1-3) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, [1]ClO4-[3]ClO4 delivered an electrochemical OER (oxygen evolution reaction) process in alkaline medium on immobilizing them to a carbon cloth support, which divulged an amplified water oxidation feature for [2]ClO4 due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [2]ClO4 could also be rationalized by its lowest Tafel slope (85 mV dec-1) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of [2]ClO4 with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.

Chlorocobaloxime containing N-(4-pyridylmethyl)-1,8-naphthalamide peripheral ligands: synthesis, characterization and enhanced electrochemical hydrogen evolution in alkaline medium.

Two new discrete cobaloxime based complexes with the general formula [ClCo(dioxime)2L] (1 and 2), L1 = N-(4-pyridylmethyl)-1,8-naphthalamide, L2 = 4-bromo-N-(4-pyridylmethyl)-1,8-naphthalamide have been synthesized and characterized by various spectroscopic techniques such as FT-IR, 1H, 13C{1H} NMR and PXRD. The molecular structures of both complexes have also been determined using single crystal X-ray crystallography. The solid state molecular structures revealed distorted octahedral geometry around the Co(III) central metal ion with two dioximes in the equatorial plane and axial positions are occupied by chloro and pyridine nitrogen of N-(4-pyridylmethyl)-1,8-naphthalamide ligands. Both complexes exhibit weaker non-covalent interactions (C-H⋯O, C-H⋯Cl and C-H⋯π(Centroid) in complex 1 whereas C-H⋯O and C-H⋯Br in complex 2) resulting in the formation of dimeric and 1D supramolecular structures. Furthermore, these complexes are immobilized onto the surface of activated carbon cloth (CC) and their electrocatalytic performance for the hydrogen evolution reaction (HER) has been investigated in alkaline and acidic media as well as buffer solution. In alkaline medium, we found that complex 2 exhibited impressive electrocatalytic HER activity and produced a current density of -10 mA cm-2 at an overpotential of 260 mV, whereas complex 1 produced the same current density at an overpotential of 334 mV. An electrochemical impedance spectroscopy (EIS) spectral study revealed the faster charge transfer kinetics of complex 2 than that of complex 1. Similarly, the low Tafel slope (100 mV dec-1) for the HER with complex 2 indicates faster HER kinetics compared to complex 1. The chronoamperometric study showed that complex 2 is stable under electrocatalytic HER conditions for 5 h without losing the initial current density and it has also been established that the complex structure is retained after electrocatalysis.

Deciphering Ligand Controlled Structural Evolution of Prussian Blue Analogues and Their Electrochemical Activation during Alkaline Water Oxidation.

Herein, we have demonstrated the control over the structure of precatalysts to tune the properties of the active catalysts and their water oxidation activity. The reaction of K3 [Fe(CN)6 ] and Na2 [Fe(CN)5 (NO)] with Co(OH)2 @CC produced precatalysts PC-1 and PC-2, respectively, with distinct structural and electronic features. The replacement of the -CN group with strong π-acceptor -NO modulates the electronic and atomic structure of PC-2. As a result, a facile electrochemical transformation of PC-2 into active catalyst Fe-Co(OH)2 -Co(O)OH (AC-2) has been attained only in 15 CV cycles while 600 CV cycles are required for the electrochemical activation of PC-1 into AC-1. The X-ray absorption studies reveal the contraction of the Co-O and Fe-O bond in AC-2 because of the presence of a higher amount of Co3+ and Fe3+ than in AC-1. The high valent Co3+ and Fe3+ modulates the electronic properties of AC-2 and assists in the O-O bond formation, leading to the improved water oxidation activity.

Homoleptic Ni(II) dithiocarbamate complexes as pre-catalysts for the electrocatalytic oxygen evolution reaction.

Four new functionalized Ni(II) dithiocarbamate complexes of the formula [Ni(Lx)2] (1-4) (L1 = N-methylthiophene-N-3-pyridylmethyl dithiocarbamate, L2 = N-methylthiophene-N-4-pyridylmethyl dithiocarbamate, L3 = N-benzyl-N-3-pyridylmethyl dithiocarbamate, and L4 = N-benzyl-N-4-pyridylmethyl dithiocarbamate) have been synthesized and characterized by IR, UV-vis, and 1H and 13C{1H} NMR spectroscopic techniques. The solid-state structure of complex 1 has also been determined by single crystal X-ray crystallography. Single crystal X-ray analysis revealed a monomeric centrosymmetric structure for complex 1 in which two dithiocarbamate ligands are bonded to the Ni(II) metal ion in a S^S chelating mode resulting in a square planar geometry around the nickel center. These complexes are immobilized on activated carbon cloth (CC) and their electrocatalytic performances for the oxygen evolution reaction (OER) have been investigated in aqueous alkaline solution. All the complexes act as pre-catalysts for the OER and undergo electrochemical anodic activation to form Ni(O)OH active catalysts. Spectroscopic and electrochemical characterization revealed the existence of the interface of molecular complex/Ni(O)OH, which acts as the real catalyst for the OER. The active catalyst obtained from complex 2 showed the best OER activity achieving 10 mA cm-2 current density at an overpotential of 330 mV in 1.0 M aqueous KOH solution.

Ruthenium-Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru-Ag Coordination Polymer and Its Enhanced Water-Splitting Feature.

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[RuIII(acac)2(µ-bis-η1-N,η1-N-BTD)2AgI(ClO4)]ClO4}n (3) via the oxidation of the monomeric building block cis-[RuII(acac)2(η1-N-BTD)2] (1) by AgClO4 (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric cis-[RuII(acac)2(η1-N-BTD)2] (1) and [RuII(acac)2(η1-N-BTD)(CH3CN)] (2) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor RuII(acac)2(CH3CN)2 in refluxing CH3CN. Molecular identities of 1-3 were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric 3. Ru(III) (S = 1/2)-derived 3 displayed metal-based anisotropic EPR with ⟨g⟩/Δg = 2.12/0.56 and paramagnetically shifted 1H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of 1n and 2n (n = 1+, 0, 1-) determined a metal-based (RuII/RuIII) oxidation and BTD-based reduction (BTD/BTD•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of 3 with respect to 1 could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of 3 were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric 3 was highlighted by its degradation to the η1-N-BTD coordinated monomeric fragment 1, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric 1 and polymeric 3 exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of 3 in comparison to 1.

Tuning the properties of CoFe-layered double hydroxide by vanadium substitution for improved water splitting activity.

Here, we demonstrate the enhanced water splitting activity of CoFe-LDH by vanadium substitution. Volcano plots are obtained for intrinsic water oxidation and hydrogen evolution with increasing vanadium substitution, and the best catalyst V0.3-CoFe-LDH achieved 10 mA cm-2 current density for H2 and O2 evolution merely at 98 mV and 240 mV overpotential, respectively.

Electrochemical transformation of Prussian blue analogues into ultrathin layered double hydroxide nanosheets for water splitting.

Herein, we demonstrate a template directed route for the synthesis of self-supported cobalt-iron based Prussian blue analogues (PBAs). The PBAs are electrochemically transformed into layered double hydroxides to produce excellent water oxidation and hydrogen evolution activity, while the overall water splitting is attained at a cell voltage of 1.58 V to reach 20 mA cm-2 current density.

Designing Self-Supported Metal-Organic Framework Derived Catalysts for Electrochemical Water Splitting.

To achieve efficient water splitting, it is essential to develop catalysts with high electrochemical performance, enhanced durability and tunable properties. Most of the transition metal-based catalysts employed for the water splitting have been fabricated on the solid-electrode support by using binder, which decreases the activity and durability of the catalyst system. In this respect, self-supported metal organic framework (MOF) derived catalysts have been introduced with enhanced catalytic activity and mechanical stability for the electrochemical water splitting. The self-supported MOF derived catalysts exhibit improved electronic conductivity, high electrochemical surface area, enhanced mechanical stability and strong catalyst-support interaction. Moreover, these catalysts possess highly porous and hollow structure with designed morphology and multi-metallic composition. Recently, a tremendous effort has been provided to explore this newly growing field and new dimensions and directions have been achieved. Looking at this point, we have described here the basic principles of catalyst design from self-supported MOF, structural and interface engineering by controlling the electronic structure of the catalysts to improve the water splitting activity. In addition, the challenges and difficulties associated with this field have been pointed out and addressed for the future progress in this field.