Efficient Homogeneous Electrocatalytic Water Oxidation by a Manganese Cluster with an Overpotential of Only 74 mV

Hexagonal Mo Bronze: Single Crystal Structures, Electrocatalytic Hydrogen Evolution, and Proton Conductivity

Molybdenum trioxide (MoO3) is a well-known transition metal oxide that has drawn much attention as a functional material having numerous applications. However, a vast majority of studies have primarily focused on α-MoO3, the thermodynamically stable polymorph of MoO3. This present work encompasses the synthesis of single crystals of two metastable hexagonal MoO3 described by the formulas {Mn0.03Na0.01}@[Mo0.93VIMo0.07VO3] (1) and {Cu0.01Na0.01}@[Mo0.97VIMo0.03VO3] (2), their comprehensive structural characterization by single-crystal X-ray crystallography, and routine spectral and microscopic studies. Interestingly, compound 1 acts as an efficient electrocatalyst for the hydrogen evolution reaction (HER) as well as an effective proton conductor in comparison to the performance of compound 2. The HER activity of compound 1 is characterized by an overpotential of 340 mV@1 mA cm-2 and a low Tafel slope of 75 mV/decade. The catalyst (compound 1) displays a Faradaic efficiency of 88% with a turnover frequency of 2.9 s-1. The proton conductivity value of this compound (1) is determined to be 4.9 × 10-3 S cm-1 at 55 °C under 98% relative humidity; the relevant proton conduction is operated by the Grotthuss mechanism with an activation energy of 0.17 eV.

Homogeneous Water Oxidation Catalyzed by First-Row Transition Metal Complexes: Unveiling the Relationship between Turnover Frequency and Reaction Overpotential

The utilization of earth-abundant low-toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first-row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

The Role of the -OH Groups within Mn12 Clusters in Electrocatalytic Water Oxidation

The formidable reactivity of the oxygen-evolving center near photosystem II is largely based on its protein environment that stabilizes it during catalysis. Inspired by this concept, the water-soluble Mn₁₂ clusters Mn₁₂ O₁₂ (O₂ CC₆ H₃ (OH)₂ )₁₆ (H₂ O)₄ (3,5DHMn₁₂ ) and Mn₁₂ O₁₂ (O₂ CC₆ H₃ (OH)₃ )₁₆ (H₂ O)₄ (3,4,5THMn₁₂ ) were developed as efficient electrocatalysts for water oxidation. In this work, the role of the -OH groups in the electrocatalytic process was explored by describing the structural and electrocatalytic properties of two new Mn₁₂ clusters, 3,4DHMn₁₂ and 2,3DHMn₁₂ , having one -OH group in the meta position relative to the benzoate-Mn moiety, and one at the para or ortho position, respectively. The Mn centers in 3,4DHMn₁₂ were discovered to have lower oxidation potential compared with those in 2,3DHMn₁₂ , and thus, 3,4DHMn₁₂ can catalyze water oxidation with higher rate and TON than 2,3DHMn₁₂ . Hence, the role of the -OH groups in the electrocatalysis was established, being involved in electronic stabilization of the Mn centers or in proton shuttling.

A synthetic manganese-calcium cluster similar to the catalyst of Photosystem II: challenges for biomimetic water oxidation

Herein, we report the synthesis, characterization, crystal structure, density functional theory calculations, and water-oxidizing activity of a pivalate Mn-Ca cluster. All of the manganese atoms in the cluster are Mn(iv) ions and have a distorted MnO6 octahedral geometry. Three Mn(iv) ions together with a Ca(ii) ion and four-oxido groups form a cubic Mn3CaO4 unit which is similar to the Mn3CaO4 cluster in the water-oxidizing complex of Photosystem II. Using scanning electron microscopy, transmission electron microscopy, energy dispersive spectrometry, extended X-ray absorption spectroscopy, chronoamperometry, and electrochemical methods, a conversion into nano-sized Mn-oxide is observed for the cluster in the water-oxidation reaction.

Homogeneous Electrochemical Water Oxidation at Neutral pH by Water-Soluble NiII Complexes Bearing Redox Non-innocent Tetraamido Macrocyclic Ligands

Water oxidation is the bottleneck reaction in artificial photosynthesis. Exploring highly active and stable molecular water oxidation catalysts (WOCs) is still a great challenge. In this study, a water-soluble Ni^II complex bearing a redox non-innocent tetraamido macrocyclic ligand (TAML) is found to be an efficient electrocatalyst for water oxidation in neutral potassium phosphate buffer. Controlled-potential electrolysis experiments show that it can sustain at a steady current of approximately 0.2 mA cm^-2 for >7 h at 1.75 V versus normal hydrogen electrode (NHE) without the formation of NiO_x . Electrochemical and spectroelectrochemical tests show that the redox-active ligand, as well as HPO₄ ^2- in the buffer, participate in the catalytic cycle. More importantly, catalytically active intermediate [Ni^III (TAML^2- )-O^. ] is formed via several proton-coupled electron transfer processes and reacts with H₂ O with the assistance of base to release molecular oxygen. Thus, the employment of redox non-innocent ligands is a useful strategy for designing effective molecular WOCs.