Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

Highlights This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure–activity relationship. Abstract Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H2 production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Solution combustion synthesis of Ni-based hybrid metal oxides for oxygen evolution reaction in alkaline medium

Ni-based mixed transition metal oxides (MTMO) (NixM1−xOy) were synthesized using the solution combustion synthesis (SCS), and investigated as electrocatalysts towards oxygen evolution reaction (OER) in alkaline medium.

Evaluation of mixed transition metal (Co, Mn, and Cu) oxide electrocatalysts anchored on different carbon supports for robust oxygen reduction reaction in neutral media

Mixed transition metal (Co, Mn, and Cu) oxide electrocatalysts anchored on different carbon supports for oxygen reduction reaction.

Mixing ReaxFF parameters for transition metal oxides using force-matching method

We present the development of the method for the refitting the ReaxFF parameters for a system consisting of the mixed transition metal oxides. We have tested several methods allowing to calculate the differences between the vectors of the forces acting on atoms obtained from the reference DFT simulation and the parameters-dependent ReaxFF. We conclude that the footrule method yields the best parameters among the investigated options. We then validate the parameters using the system consisting of the hematite supported (TiO2)n clusters. The results indicate the refitted parameters allow to obtain acceptable geometries of the clusters upon MD simulation on the ReaxFF level, and despite the short timescale lead to the stable structures.

Construction of NiCoZnS materials with controllable morphology for high-performance supercapacitors

A facile two-step hydrothermal approach with post-sulfurization treatment was put forward to construct the mixed transition metal sulfide (NiCoZnS) with a high electrochemical performance. The different morphologies of NiCoZnS materials were successfully fabricated by adjusted the Ni/Co molar ratio of the NiCoZn(OH)F precursor. Moreover, the in-situ phase transformation from the NiCoZn(OH)F phase to Zn0.76Co0.24S and NiCo2S4 phases and lattice defects via the S2− ion-exchange were determined by XRD, TEM and XPS techniques, which improved electric conductivity and interfacial active sites of the NiCoZnS, and so promoted the reaction kinetics. Significantly, the urchin-like NiCoZnS1/1 prepared at the Ni/Co molar ratio of 1.0 exhibited promising electrochemical performances with high capacitance and excellent cycling stability. Furthermore, the asymmetric device (NiCoZnS//AC) using NiCoZnS1/1 as the positive electrode had excellent supercapacitor performances with an energy density of 57.8 Wh·kg–1 at a power density of 750 W·kg–1 as well as a long cycle life (79.2% capacity retention after 10000 cycles), indicating the potential application in high-performance supercapacitors.

Mixing ReaxFF Parameters for Transition Metal Oxides Using Force-Matching Method.

We present the development of the method for the refitting the ReaxFF parameters for a system consisting of the mixed transition metal oxides. We have tested several methods allowing to calculate the differences between the vectors of the forces acting on atoms obtained from the reference DFT simulation and the parameters-dependent ReaxFF. We conclude that the footrule method yields the best parameters among the investigated options. We then validate the parameters using the system consisting of the hematite supported (TiO2)n clusters. The results indicate the refitted parameters allow to obtain much better geometries of the clusters upon MD simulation on the ReaxFF level, and despite the short timescale lead to the stable structures.

Enhancement of microbial fuel cell performance by anodic communities adaptation and cathodic mixed nickel copper oxides (NiO-CuO) on graphene electrocatalyst

Mixed transition metal (Ni & Cu) oxides supported on graphene (NiO-CuO/G) electrocatalyst was fabricated and tested as an efficient and cost-effective cathode for oxygen reduction reaction (ORR) in microbial fuel cells (MFCs). The electrocatalytic activity and selectivity of the NiO-CuO/G for ORR were examined using linear sweep voltammetry measurements (LSV) on a rotating disc electrode (RDE) in pH-neutral electrolyte. In comparison with a benchmark platinum cathode, the NiO-CuO/G showed high selectivity towards the ORR. The analysis of Koutecky-Levich relationship suggests that the electrocatalyst follows the four-electron ORR pathway. NiO-CuO/G cathode in an air-cathode MFC exhibited a slightly lower power density 21.25 mWm− 2 compared to 50.4 mW m− 2 for Pt/C. Both scanning and transmission electron microscope analyses of anodic biofilm showed that a thick biofilm was successfully developed with a rod-like shape. Biochemical characterization of the communities showed that four genera named Escherichia coli (E-coli), Shewanella putrefaciens, Bacillus cereus and Bacillus Thuringiensis/mycoides, which belonging to GammaProteobacteria and Firmicutesphylathatwerethe most abundant bacteria in the anodic biofilm. Our results revealed that NiO-CuO/G cathode demonstrates an enhanced electrocatalytic activity toward ORR in a pH-neutral solution; thus, the newly developed mixed transition metal oxides electrocatalyst can replace other expensive Pt-based catalysts for MFC application.