This Review addresses the technical challenges, scientific basis, recent progress, and outlook with respect to the stability and degradation of catalysts for the oxygen evolution reaction (OER) operating at electrolyzer anodes in acidic environments with an emphasis on ion exchange membrane applications. First, the term “catalyst stability” is clarified, as well as current performance targets, major catalyst degradation mechanisms, and their mitigation strategies. Suitable in situ experimental methods are then evaluated to give insight into catalyst degradation and possible pathways to tune OER catalyst stability. Finally, the importance of identifying universal figures of merit for stability is highlighted, leading to a comprehensive accelerated lifetime test that could yield comparable performance data across different laboratories and catalyst types. The aim of this Review is to help disseminate and stress the important relationships between structure, composition, and stability of OER catalysts under different operating conditions.

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The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation.

Synthesis, structure, properties and applications of MXenes: Current status and perspectives

Recent Progress in Ruthenium Oxide-Based Composites for Supercapacitor Applications

The family of MAX phases and their derivative MXenes are continuously growing in terms of both crystalline and composition varieties. In the last couple of years, several breakthroughs have been achieved that boosted the synthesis of novel MAX phases with ordered double transition metals and, consequently, the synthesis of novel MXenes with a higher chemical diversity and structural complexity, rarely seen in other families of two-dimensional (2D) materials. Considering the various elemental composition possibilities, surface functional tunability, various magnetic orders, and large spin–orbit coupling, MXenes can truly be considered as multifunctional materials that can be used to realize highly correlated phenomena. In addition, owing to their large surface area, hydrophilicity, adsorption ability, and high surface reactivity, MXenes have attracted attention for many applications, e.g., catalysts, ion batteries, gas storage media, and sensors. Given the fast progress of MXene-based science and technology, it is timely to update our current knowledge on various properties and possible applications. Since many theoretical predictions remain to be experimentally proven, here we mainly emphasize the physics and chemistry that can be observed in MXenes and discuss how these properties can be tuned or used for different applications.

Recent advances in MXenes: From fundamentals to applications

Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate from first-principles calculations that monolayer Ti2C and Ti2N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00 UB per formula unit, respectively, while monolayer V2C is a metal with instable antiferromagnetism, and monolayer V2N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti2C. Monolayer Ti2N is still a nearly half-metal under a proper biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V2C and V2N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti2C and Ti2N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

Monolayer MXenes: promising half-metals and spin gapless semiconductors

A TiO2–RuO2–IrO2–Ta2O5 anode coating, with a particle size of about 7 nm, on titanium was prepared by thermal decomposition at 500°C. The phase structure of the quaternary oxide coating was analyzed using X-ray diffraction. The microstructure of the coating was investigated using scanning electronic microscopy and high-resolution transmission electronic microscopy. The crystallite phases of the coating are rutile-type solid solutions, including the TiO2-based solid solution, (Ti, Ru, Ir)O2, as the major phase and the IrO2-based solid solution, (Ir, Ti, Ru)O2, as the minor phase. In addition, tantalum remains amorphous in the coating. The coating contains primary cracks and secondary cracks. Large amount of isometric crystallites exist in the coating. There are also some columnar crystallites of (Ir, Ti, Ru)O2 at the bottom of primary cracks.

Phase Structure and Microstructure of a Nanoscale TiO2–RuO2–IrO2–Ta2O5 Anode Coating on Titanium

Energy storage properties of selectively functionalized Cr-group MXenes

Phase stability of (Ru1−x,Tix)O2 solid solution was investigated via materials computation and X-ray diffraction phase analyses. The unit cell volume of (Ru1−x,Tix)O2 was found not increasing monotonously with the fraction of TiO2, making the volume–composition relationship no longer suitable for the quantitative phase analysis. A monotonous nonlinear c/a–x relation obtained from the ab initio density functional theory (DFT) computation was proved critical to achieving accurate phase analysis results. Gibbs free energy of mixing and its derivatives were determined for (Ru1−x,Tix)O2 by a combination of DFT and thermodynamic calculations, where approximation of the temperature dependence of the interaction parameter (Ω) played a critical role. It was found that the conventional approximation method of arbitrarily choosing a value & [10,25] kJ (mole of atoms)−1 for Ω near a critical temperature of a solid solution structure failed to predict the correct symmetry and span of the miscibility gap for (Ru1−x,Tix)O2. In comparison, Ω calculated from disordered (Ru1−x,Tix)O2 was found a good approximation at the critical temperature. The calculated miscibility gap for (Ru1−x,Tix)O2, therefore, showed good agreement with the accurate quantitative phase analysis results for the samples prepared by a sol–gel method.

Phase Stability and Phase Structure of Ru–Ti–O Complex Oxide Electrocatalyst

A phase stability diagram of ruthenium–zirconium oxide (Ru–Sn–O) was constructed by a combination of ab initio density functional theory and thermodynamic calculations. Results suggest that the phase separation/segregation that has been reported in the literature for the RuO2–SnO2 system is through a typical spinodal decomposition mechanism. Ru0.45Sn0.55O2 films were prepared by thermal co-decomposition of precursors at 500 °C for varied duration. Quantitative phase analyses of the prepared films based on X-ray diffraction and high-resolution transmission electron microscopy confirmed the spinodal nature of the phase separation. The present fundamental study provides a theoretical guideline for the phase and microstructure design of Ru–Sn–O based mixed oxides for electrocatalysis applications.

The nature of phase separation in a Ru–Sn–O ternary oxide electrocatalyst

Equilibrium phase diagram of ruthenium–zirconium oxide (Ru–Zr–O) was calculated by a combination of ab initio density functional theory and thermodynamic calculations. The phase diagrams suggest that the solubility between ruthenium oxide and zirconium oxide is very low under normal experimental conditions, in good agreement with the prior reports in literature. Also provided is the theoretical support for the reported phenomenon that doping or amorphization of Ru–Zr–O might suppress phase separation of the oxide. To study the spinodal decomposition of Ru–Zr–O, we successfully prepared an amorphous Ru0.48Zr0.52O2 film of a solid solution phase by a thermal decomposition at a low temperature (563 K). In situ transmission electron microscopy was utilized to witness the spinodal decomposition of the amorphous Ru0.48Zr0.52O2 solid solution by electron–beam annealing. The present fundamental study of the phase behavior of Ru–Zr–O provides a guideline for the phase and microstructure design of Ru-based mixed ox...

Xin Wang

Papers

Phase Structure and Microstructure of a Nanoscale TiO2–RuO2–IrO2–Ta2O5 Anode Coating on Titanium

Energy storage properties of selectively functionalized Cr-group MXenes

Phase Stability and Phase Structure of Ru–Ti–O Complex Oxide Electrocatalyst

The nature of phase separation in a Ru–Sn–O ternary oxide electrocatalyst

Stability of Solid-Solution Phase and the Nature of Phase Separation in Ru–Zr–O Ternary Oxide