Enhancement of the OER Kinetics of the Less-Explored α-MnO2 via Nickel Doping Approaches in Alkaline Medium

Enhance Water Electrolysis for Green Hydrogen Production with Material Engineering: A Review

Water electrolysis, a traditional and highly technology, is gaining significant attention due to the growing demand for renewable energy resources. It stands as a promising solution for energy conversion, offer substantial benefits in environmental protection and sustainable development efforts. The aim of this research is to provide a concise review of the current state-of-the-art in the field of water electrolysis, focusing on the principles of water splitting fundamental, recent advancements in catalytic materials, various advanced characterization methods and emerging electrolysis technology improvements. Moreover, the paper delves into the development trends of catalysts engineering for water electrolysis, providing insight on how to enhance the catalytic performance. With the advancement of technology and the reduction of costs, hydrogen production through water electrolysis is expected to assume a more significant role in future energy ecosystem. This paper not only synthesizes existing knowledge but also highlights emerging opportunities and potential advancements in this field, offering a clear roadmap for further research and innovation.

One-Step Hydrothermal Method Realizing Oxygen Vacancy Construction and P Doping of MnO2 to Optimize Its Oxygen Evolution Performance

MnO2 is considered one of the most potential catalysts for the oxygen evolution reaction, but its activity, which is determined by its electronic structure, crystal phase, and morphology, needs to be improved further. However, it is difficult to realize these multiscale structural regulations of MnO2 simultaneously during the preparation. In this study, α-MnO2 nanomaterial with a lot of oxygen vacancies (OVs) and Mn3+ is prepared by a one-step hydrothermal method, during which the protons of HCl can take the oxygen atom away from MnO2 to form a lot of OVs. The introduction of NH4H2PO4 can realize P doping in MnO2 to stabilize the product in the α-phase. In addition, the OV and P can increase the content of Mn3+ and regulate the Mn-O bond length synergistically to optimize the reaction kinetics. As a result, the product shows obviously enhanced catalytic activity. This study provides a one-step method for multiscale structural regulation of MnO2, which can easily create oxygen vacancies and achieve nonmetallic doping to optimize the oxygen evolution reaction (OER) performance of MnO2.

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.

Ru0.1Mn0.9Ox Electrocatalyst for Durable Oxygen Evolution in Acid Seawater

Currently, direct electrolysis of seawater for green hydrogen production is primarily focused on neutral and alkaline systems. However, the precipitation of calcium and magnesium ions restricts the advancement of this technology. An acidic system can effectively address this issue. Given that Ru/Ir-based catalysts with high oxygen evolution reaction (OER) activity also exhibit high chlorine evolution reaction (CER) activity, acid seawater splitting requires anodes with higher selectivity and stability compared to the other two systems. In this study, we propose a non-precious Ru0.1Mn0.9Ox as the active anode for direct acid seawater electrolysis, which exhibits a high OER selectivity and remarkable stability for more than 1200 hours. Different from the Cl--free system, *Cl occupied on Ru sites could shift the OER active center to Mn on Ru0.1Mn0.9Ox, which prevents the lattice oxygen consumed on Ru and hinders the metal site dissolution. As the CER-insensitive catalytic center, Mn activated by the introduction of Ru can adsorb a substantial amount of *OH, creating an OER-favored local environment that inhibits CER. We introduce Cl--assisted transfer of OER active sites to CER-insensitive Mn as a fundamental strategy for achieving highly selective and durable oxygen evolution in acidic seawater.

Effect of Mn2+ concentration on the growth of δ-MnO2 crystals under acidic conditions

δ-MnO2 is an important component of environmental minerals and is among the strongest sorbents and oxidants. The crystalline morphology of δ-MnO2 is one of the key factors affecting its reactivity. In this work, δ-MnO2 was initially synthesized and placed in an acidic environment to react with Mn2+ and undergo a crystalline transformation. During the transformation of crystalline δ-MnO2, kinetic sampling was conducted, followed by analyses of the structures and morphologies of the samples. The results showed that at pH 2.5 and 4, δ-MnO2 nanoflakes spontaneously self-assembled into nanoribbons via edge-to-edge assembly in the initial stage. Subsequently, these nanoribbons attached to each other to form primary nanorods through a face-to-face assembly along the c-axis. These primary nanorods then assembled along the (001) planes and lateral surfaces, achieving further growth and thickening. Since a lower pH is more favorable for the formation of vacancies in δ-MnO2, δ-MnO2 can rapidly adsorb Mn2+ directly onto the vacancies to form tunnel walls. At the same time, the rapid formation of the tunnel walls leads to a quick establishment of hydrogen bonding between adjacent nanoribbons, enabling the assembly of these nanoribbons into primary nanorods. Therefore, in a solution with the same concentration of Mn2+, the structure transformation and morphology evolution of δ-MnO2 to α-MnO2 occur faster at pH 2.5 than at pH 4. These findings provide insights into the mechanism for crystal growth from layer-based to tunnel-based nanorods and methods for efficient and controlled syntheses of nanomaterials.

Adjusted MnO oxygen vacancy for highly selective ORR production of H2O2

The oxygen reduction reaction (ORR) via the two-electron pathway is an important method of hydrogen peroxide (H2O2) production. This study demonstrates that MnO with different oxygen vacancies possesses great 2e- ORR activity. The H2O2 selectivity increased from 10% to 93% with increasing oxygen vacancy concentration by adjusting the reaction temperature and time. Moreover, the H2O2 yield of the optimal MnO reached 544.1 mmol g-1 h-1, and it showed extraordinary stability over a long period of time (10 000 circles CV), surpassing most of the reported transition metal catalysts. This provides a new strategy for efficient and low-cost electrochemical production of H2O2.

High-value utilization of Cr-containing sludge: Eco-friendly and ultra-low-cost electrocatalyst for efficient OER in alkaline media from Cr-containing sludge

Economically sustainable development requires more viable waste recycling solutions. In this context, we address the problem of utilizing chromium-containing sludge, a prevalent and environmentally hazardous waste. Meanwhile, sustainable energy development must develop ecology-friendly and low-cost electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. Herein, we report an ultra-low-cost electrocatalyst from chromium-containing sludge. The optimum preparation conditions are determined by optimizing the calcination temperature and the loading of nickel acetylacetonate. The optimized catalyst delivers excellent stability and outstanding OER activity with overpotentials of 320 mV at 10 mA cm-2 in alkaline media. Density functional theory calculations reveal that the energy barrier of OER is decreased because of the catalyst's heterogeneous structure arrangement and confirm the influence of chromium on performance improvement. The concept of "turning waste into treasure" stimulates the search for methods to process Cr-containing waste and produce low-cost, high-performance electrocatalysts.

Ni-Doped MnO2 Nanosheet Arrays for Efficient Urea Oxidation

Urea oxidation reaction (UOR), with a low thermodynamic potential, offers great promise for replacing anodic oxygen evolution reaction of electrolysis systems such as water splitting, carbon dioxide reduction, etc., thus reducing the overall energy consumption. To promote the sluggish kinetics of UOR, highly efficient electrocatalysts are required, and Ni-based materials have been widely investigated. However, most of these reported Ni-based catalysts suffer from large overpotentials, as they generally undergo self-oxidation to form NiOOH species at high potentials, which act as catalytically active sites for UOR. Herein, Ni-doped MnO₂ (Ni-MnO₂) nanosheet arrays were successfully prepared on nickel foam. The as-fabricated Ni-MnO₂ shows distinct UOR behavior with most of the previously reported Ni-based catalysts, as urea oxidation on Ni-MnO₂ proceeds before the formation of NiOOH. Notably, a low potential of 1.388 V vs reversible hydrogen electrode was required to achieve a high current density of 100 mA cm^-2 on Ni-MnO₂. It is suggested that both Ni doping and nanosheet array configuration are responsible for the high UOR activities on Ni-MnO₂. The introduction of Ni modifies the electronic structure of Mn atoms, and more Mn³⁺ species are generated in Ni-MnO₂, contributing to its outstanding UOR performance.

Ce-Modified Flowerlike NiFe-MOF Nanostructure Based on Ion Competitive Coordination for Enhancing the Oxygen Evolution Reaction

Metal-organic framework (MOF) has become a popular electrocatalyst for the oxygen evolution reaction (OER) because of its large specific surface area and adjustable porosity. Nevertheless, the electrochemical performance of MOFs has been greatly limited by poor intrinsic conductivity and catalytic activity. Herein, we report a Ce-doped nanoflower-like MOF material Ce@NiFe-MOF-5 via a facile ion competitive coordination effect and doping method. Benefiting from the nanoflower structure formed by the stacking of nanosheets, a large number of active sites can be exposed, which favors electron/mass transfer during water oxidation. The coordination substitution of Ce ions not only promoted an increase in the number of active sites on the surface of the nanosheets but also optimized the electronic structure of pristine NiFe-MOF. The well-designed Ce@NiFe-MOF-5 catalysts exhibited superior OER performance under basic conditions, which only required an overpotential of 258 mV at a current density of 10 mA cm^-2 and a Tafel slope of 54.44 mV dec^-1. Moreover, when Ce@NiFe-MOF-5 served as an anode and Pt/C as a cathode, the two-electrode system only needed 1.56 V to drive overall water splitting at 10 mA cm^-2.

Hybrid Carbon Sphere Chain-MnO2 Nanorods as Bifunctional Oxygen Electrocatalysts for Rechargeable Zinc-Air Batteries

It is now recognized that the development of self-supported and efficient bifunctional air cathodes via the direct growth of earth-abundant catalysts onto the surface of the conductive collector would be a cutting-edge strategy to reduce interfacial resistance, enhance the mechanical tenure, and reduce the final weight and cost of manufacturing of rechargeable Zn-air batteries (ZABs). This work reports an innovative self-supported precious metal-free electrode, comprising carbon sphere chains (CSCs) directly grown onto a carbon paper (CP) substrate, wherein the CSCs have a functionalized surface bearing carbon nanobud defects, oxygen functional groups, and high-density MnO₂ hierarchical nanorods (NRs), uniformly coating the surface of CSCs. Not only is the metal-free functionalized CSC catalyst functional for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) but its combination with MnO₂ NRs impressively enhances the ORR/OER activities. A homemade ZAB assembled with functionalized CSC/MnO₂ air cathode can successfully power a timer for a period of 17 days with no voltage loss, whereas two series-connected ZABs can light up 39 red light-emitting diode (LED) bulbs. The self-supported and earth-abundant-based CSC/MnO₂ materials open up an opportunity for lightweight and cost-effective ZABs and metal-air batteries in general.

Anchoring Highly Dispersed Pt Electrocatalysts on TiOx with Strong Metal-Support Interactions via an Oxygen Vacancy-Assisted Strategy as Durable Catalysts for the Oxygen Reduction Reaction

Pt electrocatalysts with high activity and durability have still crucial issues for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). In this study, a novel catalyst consisting of Pt nanoparticles (NPs) on TiO_x/C composites (TiO_x-V_o-H/C) with abundant oxygen vacancies (V_o) is proposed, which is abbreviated as PTO-V_o-H/C. The introduction of V_o helps anchor highly dispersed Pt NPs with low loading and strengthen the strong metal-support interaction (SMSI), which benefits to the enhanced ORR catalytic activity. Moreover, the accelerated durability test (ADT) demonstrates the higher retention of ORR activity for PTO-V_o-H/C. Experimental and theoretical analyses reveal that electronic interactions between Pt NPs and TiO_x/C composite support give rise to an electron-rich Pt NPs and strong SMSI effect, which is favorable for the electron transfer and stabilization of Pt NPs. More importantly, the assembled PEMFC with PTO-V_o-H/C shows only 6.9% of decay on maximum power density after 3000 ADT cycles while the performance of Pt/C sharply decreased. This work provides a new insight into the unique vacancy regulation of dispersive Pt on metal oxides for superior ORR performance.