Mono-/Multinuclear Water Oxidation Catalysts

Elucidating the catalytic mechanisms of O2 generation by [Mn2(μ-O)2(terpy)2(OH2)2]3+ using DFT calculations: a focus on ClO- as oxidant

The experimentally reported Mn(IV)Mn(III) complex [Mn2(μ-O)2(terpy)2(OH2)2]3+ has been observed catalyzing O2 generation with oxidants like ClO- and HSO5-. Previous mechanistic studies primarily focused on O2 generation with HSO5-, concluding that Mn(IV)Mn(III) acts as a catalyst, generating a Mn(IV)Mn(IV)-oxyl species as a key intermediate responsible for O-O bond formation. This computational study employs DFT calculations to investigate whether the catalytic generation of O2 using ClO- follows the same mechanism previously identified with HSO5- as the oxidant, or if it proceeds through an alternate pathway. To this end, we explored multiple pathways using ClO- as the oxidant. Interestingly, our findings confirm that in the case of ClO- as the oxidant, similar to what was observed with HSO5-, the Mn(IV)Mn(IV)-oxyl species indeed plays a crucial role in driving the catalytic evolution of O2 with the potential formation of the binuclear complexes Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH during the reaction. These complexes are reactive in producing O2, with activation free energies of 15.9 and 14.3 kcal mol-1, respectively. However, our calculations revealed that the Mn(IV)Mn(IV)-oxyl complex is significantly more reactive in producing O2 than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, with a lower free energy barrier of 8.1 kcal mol-1. Consequently, even though Mn(IV)Mn(IV)-oxyl is predicted to be present in much lower concentrations than Mn(IV)Mn(IV)-oxy and Mn(IV)Mn(IV)-OH, it emerges as the species acting as the active catalyst for catalytic O2 generation. This study enhances our knowledge of high oxidation state (+3 and +4) manganese chemistry, highlighting its key role in catalysis and paving the way for more efficient Mn-based catalysts with broad applications.

Observation of oxygen evolution over a {Ni12}-cluster-based metal-organic framework

The development of efficient electrocatalysts based on non-noble metals for oxygen evolution reaction (OER) remains an important and challenging task. Multinuclear transition-metal clusters with high structural stability are promising OER catalysts but their catalytic role is poorly understood. Here we report the crystallographic observation of OER activity over robust {Ni12}-clusters immobilised in a porous metal-organic framework, NKU-100, by single-crystal X-ray diffraction as a function of external applied potential. We observed the aggregation of confined oxygen species around the {Ni12}-cluster as a function of applied potential during the electrocatalytic process. The refined occupancy of these oxygen species shows a strong correlation with the variation of current density. This study demonstrates that the enrichment of oxygen species in the secondary co-ordination sphere of multinuclear transition-metal clusters can promote the OER activity.

Promotion of the water oxidation activity of iridium oxide by a nitrogen coordination strategy

The water oxidation reaction is the pivotal half-reaction for photo-/electro-catalytic water splitting. Fabrication of high-efficiency and robust water oxidation is essential to realize wide-scale artificial photosynthesis. Here, we report an efficient strategy to improve the water oxidation activity of iridium oxide by a nitrogen-coordination method. Due to the coordination effect, the iridium oxide can be well dispersed to generate ultra-small nanoparticles and the intrinsic activity can be improved for the water oxidation reaction. This study suggests that high-performance water oxidation catalysts can be constructed based on a nitrogen-coordination strategy.

Water Oxidation at Neutral pH using a Highly Active Copper-Based Electrocatalyst

The sluggish kinetics of the oxygen evolution reaction (OER) at the anode severely limit hydrogen production at the cathode in water splitting systems. Although electrocatalytic systems based on cheap and earth-abundant copper catalysts have shown promise for water oxidation under basic conditions, only very few examples with high overpotential can be operated under acidic or neutral conditions, even though hydrogen evolution in the latter case is much easier. This work presents an efficient and robust Cu-based molecular catalyst, which self-assembles as a periodic film from its precursors under aqueous conditions on the surface of a glassy carbon electrode. This film catalyzes the OER under neutral conditions with impressively low overpotential. In controlled potential electrolysis, a stable catalytic current of 1.0 mA cm^-2 can be achieved at only 2.0 V (vs. RHE) and no significant decrease in the catalytic current is observed even after prolonged bulk electrolysis. The catalyst displays first-order kinetics and a single site mechanism for water oxidation with a TOF (k_cat ) of 0.6 s^-1 . DFT calculations on of the periodic Cu(TCA)₂ (HTCA=1-mesityl-1H-1,2,3-triazole-4-carboxylic acid) film reveal that TCA defects within the film create Cu^I active sites that provide a low overpotential route for OER, which involves Cu^I , Cu^II -OH, Cu^III =O and Cu^II -OOH intermediates and is enabled at a potential of 1.54 V (vs. RHE), requiring an overpotential of 0.31 V. This corresponds well with an overpotential of approximately 0.29 V obtained experimentally for the grown catalytic film after 100 CV cycles at pH 6. However, to reach a higher current density of 1 mA cm^-2 , an overpotential of 0.72 V is required.

Boosting Electrocatalytic Water Oxidation by Creating Defects and Lattice-Oxygen Active Sites on Ni-Fe Nanosheets

Layered nickel-iron oxide/hydroxide nanosheets have proven to be the most efficient catalyst for the water oxidation reaction. Introducing structural defects to the nanosheets is a particularly attractive method for increasing the number of active sites and tailoring the intrinsic electronic properties. Herein, defects were introduced on Ni-Fe nanosheets through sequentially electrodoping and dedoping the surface of the material with tetramethylammonium ions. The as-prepared defect-rich Ni-Fe nanosheets showed an enhanced catalytic performance for the oxygen evolution reaction (OER) compared with conventional NiFe layered double hydroxides (LDHs), exhibiting an overpotential of only 172 mV at the current density of 10 mA cm^-2 . The relationship between pH and OER activity indicated that the lattice oxygens participated in the catalytic OER process as active sites. This work provides new insights into the understanding of the structure-activity relationship of layered materials and helps to develop new methods to implement defects on such frameworks aided by organic molecules.

In Situ Growth of Amorphous Fe(OH)3 on Nickel Nitrate Hydroxide Nanoarrays for Enhanced Electrocatalytic Oxygen Evolution

Development of highly efficient electrocatalyst for the oxygen evolution reaction (OER) is urgently demanded by the clean hydrogen energy. Herein, in order to further boost the OER activity of metal nitrate hydroxide materials, amorphous Fe(OH)₃ layer is in situ grown on nickel nitrate hydroxide (NiNH) nanoarrays supported on nickel foam (NF) through an interfacial hydrolysis approach, where the loading amount of the Fe(OH)₃ can be simply manipulated by the hydrolysis time. Taking advantage of the synergy of Fe(OH)₃ and NiNH, the optimized Fe(OH)₃@NiNH/NF sample shows a very promising electrocatalytic OER activity in 1 M KOH solution, requiring a very low overpotential of 212 mV vs. reversible hydrogen electrode (RHE) to deliver a geometrical catalytic current density of 100 mA cm^-2 and a low Tafel slope of 49 mV dec^-1. This work provides a new strategy for boosting the electrocatalytic activity of metal hydroxide nitrates through the interface engineering.

Surface-assembled non-noble metal nanoscale Ni-colloidal thin-films as efficient electrocatalysts for water oxidation

A highly operative and inexpensive water oxidation scheme using an efficient nanoscale electrocatalyst is vastly demanded for optimum H₂ production, CO₂ reduction, and has attracted increased attention for chemical energy conversion. We present here a simple route to make efficient electrocatalytic colloidal nanoparticles of nickel out of mere metal ions in a simple borate buffer system. The simple and annealed Ni-colloidal nanoparticles (Ni-CNPs) resulted in a facile transformation into ultrafine films, which further activated the catalysts, while initiating OER just at the overpotential η = 250 mV (1.48 V vs. RHE) under benign conditions. They also showed high porosity and favorable kinetics while displaying impressive Tafel slopes of just 51 mV dec⁻¹, and a high TOF value of 0.79 s⁻¹ at 0.35 V was observed for Ni-CNPs/FTO₅₀₀. These electrocatalysts also showed long-term stability during the bulk water electrolysis experiment conducted for a continuous 20 hours without notable catalytic degradation, which ensures their economic benefits. The electrochemical data, CVs, kinetic study, short-term durability, extended catalytic stability, SEM analysis, and other supporting data provide compelling evidence that these non-precious, metal-based, electroactive, catalytic, colloidal thin-films (simple and annealed) with nanoscale morphological attributes presented promising catalytic performance under the conditions used herein.

Highly applied and accessible electrocatalytic system derived from simple Ni-colloids has been explored to facilely derive kinetically sluggish water oxidation reaction. Ni-catalysts also present well-balanced kinetics of OER and high durability.