Bound manganese oxides capable of reducing the bacteriochlorophyll dimer of modified reaction centers from Rhodobacter sphaeroides

A biohybrid model system is described that interfaces synthetic Mn-oxides with bacterial reaction centers to gain knowledge concerning redox reactions by metal clusters in proteins, in particular the Mn₄CaO₅ cluster of photosystem II. The ability of Mn-oxides to bind to modified bacterial reaction centers and transfer an electron to the light-induced oxidized bacteriochlorophyll dimer, P⁺, was characterized using optical spectroscopy. The environment of P was altered to obtain a high P/P⁺ midpoint potential. In addition, different metal-binding sites were introduced by substitution of amino acid residues as well as extension of the C-terminus of the M subunit with the C-terminal region of the D1 subunit of photosystem II. The Mn-compounds MnO₂, αMn₂O₃, Mn₃O₄, CaMn₂O₄, and Mn₃(PO₄)₂ were tested and compared to MnCl₂. In general, addition of the Mn-compounds resulted in a decrease in the amount of P⁺ while the reduced quinone was still present, demonstrating that the Mn-compounds can serve as secondary electron donors. The extent of P⁺ reduction for the Mn-oxides was largest for αMn₂O₃ and CaMn₂O₄ and smallest for Mn₃O₄ and MnO₂. The addition of Mn₃(PO₄)₂ resulted in nearly complete P⁺ reduction, similar to MnCl₂. Overall, the activity was correlated with the initial oxidation state of the Mn-compound. Transient optical measurements showed a fast kinetic component, assigned to reduction of P⁺ by the Mn-oxide, in addition to a slow component due to charge recombination. The results support the conjecture that the incorporation of Mn-oxides by ancient anoxygenic phototrophs was a step in the evolution of oxygenic photosynthesis.

Water-Oxidation Electrocatalysis by Manganese Oxides: Syntheses, Electrode Preparations, Electrolytes and Two Fundamental Questions

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this reaction, scientists around the globe have investigated the possibility to use manganese oxides (“MnOx”) for this task. This perspective article will look at selected examples from the last about 10 years of research in this field. At first, three aspects are addressed in detail which have emerged as crucial for the development of efficient electrocatalysts for the anodic oxygen evolution reaction (OER): (1) the structure and composition of the “MnOx” is of central importance for catalytic performance and it seems that amorphous, MnIII/IV oxides with layered or tunnelled structures are especially good choices; (2) the type of support material (e.g. conducting oxides or nanostructured carbon) as well as the methods used to immobilize the MnOx catalysts on them greatly influence OER overpotentials, current densities and long-term stabilities of the electrodes and (3) when operating MnOx-based water-oxidizing anodes in electrolyzers, it has often been observed that the electrocatalytic performance is also largely dependent on the electrolyte’s composition and pH and that a number of equilibria accompany the catalytic process, resulting in “adaptive changes” of the MnOx material over time. Overall, it thus has become clear over the last years that efficient and stable water-oxidation electrolysis by manganese oxides can only be achieved if at least four parameters are optimized in combination: the oxide catalyst itself, the immobilization method, the catalyst support and last but not least the composition of the electrolyte. Furthermore, these parameters are not only important for the electrode optimization process alone but must also be considered if different electrode types are to be compared with each other or with literature values from literature. Because, as without their consideration it is almost impossible to draw the right scientific conclusions. On the other hand, it currently seems unlikely that even carefully optimized MnOx anodes will ever reach the superb OER rates observed for iridium, ruthenium or nickel-iron oxide anodes in acidic or alkaline solutions, respectively. So at the end of the article, two fundamental questions will be addressed: (1) are there technical applications where MnOx materials could actually be the first choice as OER electrocatalysts? and (2) do the results from the last decade of intensive research in this field help to solve a puzzle already formulated in 2008: “Why did nature choose manganese to make oxygen?”.

Recent Advances in the Development of Molecular Catalyst-Based Anodes for Water Oxidation toward Artificial Photosynthesis

Catalytic water oxidation represents a bottleneck for developing artificial photosynthetic systems that store solar energy as renewable fuels. A variety of molecular water oxidation catalysts (WOCs) have been reported over the last two decades. In view of their applications in artificial photosynthesis devices, it is essential to immobilize molecular catalysts onto the surfaces of conducting/semiconducting supports for fabricating efficient and stable water oxidation anodes/photoanodes. Molecular WOC-based anodes are essential for developing photovoltaic artificial photosynthesis devices and, moreover, the performance of molecular WOC on anodes will provide important insight into designing extended molecular WOC-based photoanodes for photoelectrochemical (PEC) water oxidation. This Review concerns recent progress in the development of molecular WOC-based anodes over the last two decades and looks at the prospects for using such anodes in artificial photosynthesis.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Electrocatalytic Water Oxidation Promoted by 3 D Nanoarchitectured Turbostratic δ-MnOx on Carbon Nanotubes

The development of manganese-based water oxidation electrocatalysts is desirable for the production of solar fuels, as manganese is earth-abundant, inexpensive, non-toxic, and has been employed by the Photosystem II in nature for a billion years. Herein, we directly constructed a 3 D nanoarchitectured turbostratic δ-MnO_x on carbon nanotube-modified nickel foam (MnO_x /CNT/NF) by electrodeposition and a subsequent annealing process. The MnO_x /CNT/NF electrode gives a benchmark catalytic current density (10 mA cm^-2 ) at an overpotential (η) of 270 mV under alkaline conditions. A steady current density of 19 mA cm^-2 is obtained during electrolysis at 1.53 V for 1.0 h. To the best of our knowledge, this work represents the most efficient manganese-oxide-based water oxidation electrode and demonstrates that manganese oxides, as a structural and functional model of oxygen-evolving complex (OEC) in Photosystem II, can also become comparable to those of most Ni- and Co-based catalysts.

A new preparation of a bifunctional crystalline heterogeneous copper electrocatalyst by electrodeposition using a Robson-type macrocyclic dinuclear copper complex for efficient hydrogen and oxygen evolution from water

Electro-deposited Cu(OH)₂/Cu₂O-based thin film on FTO with the macrocyclic dicopper complex shows excellent water splitting activity in excellent Faradaic efficiency.