Electrocatalytic water oxidation with manganese phosphates

Employing microalga Chlorella sorokiniana in the biosynthesis of paramagnetic and catalytically functional manganese cluster

Finding vehicles for biosynthesis of metal clusters with advantageous magnetic and catalytic properties is an important industrial and environmental task. We have found previously that green microalga Chlorella sorokiniana produces a multivalent Mn-O cluster with structure that is similar to photosynthetic oxygen-evolving complex (OEC). Here we reported magnetic and redox properties and the site of accumulation of this cluster, and we proposed the mechanisms of biosynthesis and the protocol for extraction. The cluster was paramagnetic even at room temperature, with an antiferromagnetic transition at ∼13 K. The separation between ground and excited state of ΔE ≈ 15.0 cm-1 matched the separation energy of OEC in S2 state. Nano X-ray fluorescence microscopy and 31P NMR showed that the cluster is accumulated in acidocalcisomes, a lysosome-type organelles rich in polyphosphates. The conditions in these organelles resemble the settings of chemical synthesis of OEC mimics, including mildly acidic pH and the availability of Ca2+ ions. Polyphosphates are likely to play a role of stabilizing ligands and modulators of redox properties of Mn2+ in the cluster synthesis. The cluster shares redox potentials with OEC and showed catalase-like activity. However, we could not confirm OEC-like performance because the cluster was prone to degradation by oxidizing agents in the presence of organic residue in the extract. The biosynthesis showed an overall yield of ∼25 % and appears to be cost-competitive with chemical synthesis. This study shows that metabolic trades of selected microalgae can be employed in the green synthesis of catalytically functional clusters.

Jahn-Teller-Driven Electronic Modulation of Bio-Heterojunction for Wound Regeneration after Postoperative Tumor Resection

Abundant ·OH, 1O2, and ·O2- provide an efficient methodology for rapid tumor and bacteria killing, whereas a limitation focuses on the catalytic efficiency. Thus, Jahn-Teller-driven electronic modulation of a bioheterojunction (bioHJ) platform is developed for the remedy in diabetic infectious wound regeneration after postoperative tumor resection. The bioHJ is composed of MoTe2/MnO2 and glucose oxidase (GOx). GOx depletes glucose to H2O2, which intercepts their glucose metabolism. The H2O2 can be further converted into highly lethal ·OH owing to peroxidase-mimetic activity via the Jahn-Teller effect, while GSH can be consumed due to its GPx-mimetic activity. Both of which can be further amplified upon NIR irradiation as NIR-activatable enzyme-mimetic activities. In vivo studies in a subcutaneous tumor model and infectious model authenticate the ability to kill tumor, defeat bacterial infection, and accelerate wound regeneration. This work enlightens a powerful platform for postoperative infectious wound regeneration of tumor resection using an engineered bioHJ.

Lewis Acid Sites in Hollow Cobalt Phytate Micropolyhedra Promote the Electrocatalytic Water Oxidation

The acid-base microenvironment of the metal center is crucial for constructing advanced oxygen evolution reaction (OER) electrocatalysts. However, the correlation between acidic site and OER performance remains unclear for cobalt-based catalysts. Herein, Lewis acid sites in hollow cobalt phytate micropolyhedra (M-CoPA, M = Cu, Sr) were synthesized by a cation-exchange strategy, and their OER performances were studied systematically. Experimentally, Lewis acid Cu2+ sites with stronger Lewis acidity exhibited superior intrinsic activity and long-term stability in alkaline electrolytes. The spectroscopic and electrochemical studies show Lewis acid sites in hollow cobalt phytate micropolyhedra can modulate the electronic distribution of the adjacent cobalt center and further optimize the adsorption strength of oxygenated species. This study figures out the effect of Lewis acid sites on the OER kinetics and provides an effective way to develop high-efficiency electrocatalysts for energy conversion systems.

High-Spin Manganese(V) in an Active Center Analogue of the Oxygen-Evolving Complex

In a comprehensive investigation of the dinuclear [Mn2O3]+ cluster, the smallest dimanganese entity with two μ-oxo bridges and a terminal oxo ligand, and a simplified structural model of the active center in the oxygen-evolving complex, we identify antiferromagnetically coupled high-spin manganese centers in very different oxidation states of +2 and +5, but rule out the presence of a manganese(IV)-oxyl species by experimental X-ray absorption and X-ray magnetic circular dichroism spectroscopy combined with multireference calculations. This first identification of a high-spin manganese(V) center in any polynuclear oxidomanganese complex underscores the need for multireference computational methods to describe high-valent oxidomanganese species.

Mo-Doped α-MnO2 for Enhanced Electrocatalytic Water Oxidation

Manganese is a key metal involved in the catalysis of natural photosynthesis. Thus, the investigation of Mn-based electrocatalysts for water oxidation is of high importance. This work reports the doping of Mo into α-MnO2 nanorods to improve the water oxidation performance. The doping of Mo can transform the microstructure of α-MnO2 from nanorods into nanosphere superstructures. As a dopant, Mo expands the α-MnO2 lattice to result in a decrease in the average oxidation state of Mn and the generation of oxygen vacancies, which are beneficial to water oxidation catalysis. Under optimized doping, the overpotential of 2.34 wt.% Mo/α-MnO2 is reduced by 80 mV (at 10 mA/cm2) compared with pure α-MnO2.

Interfacial creation of positively charged sites in LaPO4/Fe3(PO4)2 heterojunctions for high-current-density oxygen evolution

The creation of positively charged active sites is crucial for enhancing the adsorption of negatively charged hydroxide ions, thereby improving the sluggish oxygen evolution reaction (OER). We engineered LaPO4/Fe3(PO4)2 heterojunctions with a low flat-band potential, facilitating interfacial charge transfer and generation of positively charged holes, resulting in a highly active and stable OER catalyst at 500 mA cm-2.

Interface engineering for photoelectrochemical oxygen evolution reaction

Photoelectrochemical (PEC) water splitting provides a promising approach for solving sustainable energy challenges and achieving carbon neutrality goals. The oxygen evolution reaction (OER), a key bottleneck in the PEC water-splitting system occurring at the photoanode/electrolyte interface, plays a fundamental role in sustainable solar fuel production. Proper surface or interface engineering strategies have been proven to be necessary to achieve efficient and stable PEC water oxidation. This review summarizes the recent advances in interface engineering, including junction formation, surface doping, surface passivation or protection, surface sensitization, and OER cocatalyst modification, while highlighting the remarkable research achievements in the field of PEC water splitting. The benefits of each interface engineering strategy and how it enhances the device performance are critically analyzed and compared. Finally, the outlook for the development of interface engineering for efficient PEC water splitting is briefly discussed. This review illustrates the importance of employing rational interface engineering in realizing efficient and stable PEC water splitting devices.

Highly dispersed Ir nanoparticles on Ti3C2Tx MXene nanosheets for efficient oxygen evolution in acidic media

The industrialization of hydrogen production technology through polymer electrolyte membrane water splitting faces challenges due to high iridium (Ir) loading on the anode catalyst layer. While rational design of oxygen evolution reaction (OER) electrocatalysts aimed at effective iridium utilization is promising, it remains a challenging task. Herein, we present exfoliated Ti3C2Tx MXene as a highly conductive and corrosion-resistant support for acidic OER. We develop an alcohol reduction method to achieve uniform and dense loading of ultrafine Ir nanoparticles on the MXene surface. The IrO2/TiOx heterointerface is formed in situ on the Ir@Ti3C2Tx MXene surface, acting as a catalytically active phase for OER during electrocatalysis. The electron interactions at the IrO2/TiOx heterointerface create electron-rich Ir sites, which reduce the adsorption properties of oxygen intermediates and enhance intrinsic OER activity. Consequently, the prepared Ir@Ti3C2Tx exhibits a mass activity that is 7 times greater than that of the benchmark IrO2 catalyst for OER in acidic media. In addition, the /Ti3C2Tx MXene support can stabilize the Ir nanoparticles, so that the stability number of Ir@Ti3C2Tx MXene is about 2.4 times higher than that of the IrO2 catalyst.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Constructing Co4(SO4)4 Clusters within Metal-Organic Frameworks for Efficient Oxygen Electrocatalysis

Multinuclear metal clusters are ideal candidates to catalyze small molecule activation reactions involving the transfer of multiple electrons. However, synthesizing active metal clusters is a big challenge. Herein, on constructing an unparalleled Co4(SO4)4 cluster within porphyrin-based metal-organic frameworks (MOFs) and the electrocatalytic features of such Co4(SO4)4 clusters for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. The reaction of CoII sulfate and metal complexes of tetrakis(4-pyridyl)porphyrin under solvothermal conditions afforded Co4-M-MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co4-M-MOFs have the same framework structure, having the Co4(SO4)4 clusters connected by metalloporphyrin units through Co─Npyridyl bonds. In the Co4(SO4)4 cluster, the four CoII ions are chemically and symmetrically equivalent and are each coordinated with four sulfate O atoms to give a distorted cube-like structure. Electrocatalytic studies showed that these Co4-M-MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co4(SO4)4 cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co4-Co-MOF displays the highest electrocatalytic activity in this series of MOFs by showing a 10 mA cm-2 OER current density at 357 mV overpotential and an ORR half-wave potential at 0.83 V versus reversible hydrogen electrode (RHE). Theoretical studies revealed the synergistic effect of two proximal Co atoms in the Co4(SO4)4 cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co4(SO4)4 clusters in framework structures for oxygen electrocatalysis and to demonstrate the cooperation between two proximal Co atoms in such clusters during the O─O bond formation process.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.