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Double perovskite oxides are one of the most promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) due to their adjustable electronic structures via doping with different metal cations or engineering of defects. Herein, we report a systematic study on the tuning of the electronic structure of La2-xSrxNiMnO6 with 0 ≤ x ≤ 1.0 to promote the bifunctional OER/ORR activity. The bifunctional index (ΔE) is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La1.4Sr0.6NiMnO6, exceeding that of widely studied LaNiO3. Our study on electronic structures reveals that the enhancement of the ORR and OER activities strongly correlates with the appearance of Ni3+ oxidation states and the upshift of the O 2p-band center promoted by Sr doping. Furthermore, an increase of hole states, derived from Ni3+ states, reduces the energy barrier for the electron transfer from 0.44 to 0.12 eV and hence improves the intrinsic OER activities. The tuning of the electronic structure that leads to higher OER and ORR activities in La2-xSrxNiMnO6 can be extended to other materials for the design of active bifunctional electrocatalysts.

Perovskite transition metal oxides are promising non-precious metal electrocatalysts for promoting the oxygen evolution reaction (OER) in many electrochemical energy conversion devices. This work reports a systematic study of the relation between the electronic structure and OER performance of perovskite La1−xSrxFeO3 (0 ≤ x ≤ 1). The partial substitution of La for Sr in LaFeO3 results in the oxidation of Fe3+ to Fe4+ and substantially enhances OER activity. A comprehensive X-ray spectroscopic study reveals a strong correlation of the enhanced OER activity with these Fe4+ states. The presence of Fe4+ leads to a stronger Fe–O bond due to stronger Fe 3d–O 2p orbital hybridization and shifts the energy position of the valence band (VB) to EF. Such an electronic modulation optimizes the surface adsorption energetics of *OH intermediates, contributing to faster OER kinetics. Furthermore, a new unoccupied state is created at ∼0.9 eV below the conduction band. This hole state reduces the energy barrier for electron transfer from 1.34 eV to 0.44 eV, thereby facilitating charge transfer at the interface. The Fe4+-induced electronic states responsible for higher OER activity have implications for the understanding of structure–activity relationships in other Fe-based electrocatalysts such as highly active Fe-doped NiOOH.

This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3-xO4 (x = 0-1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm-2 BET for pure Co3O4 to 0.32 mA cm-2 BET for x = 1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in the +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ∼1 eV above the EF. Such electronic features favor the adsorption of OH surface intermediates on NixCo3-xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3-xO4 can be extended to other transition metal oxides for rational design of highly active catalysts.

A new phase of cobalt(II) hydroxyl phosphate γ-Co2(PO4)(OH) has been successfully synthesized through a conventional hydrothermal method. This compound crystallizes in the trigonal system of space group P31m, showing a warping two-legs spin ladder chain structure arranged in an unusual triangular network, in which a new spin-lattice of (3, 6, 4, 6)(4, 62) is built by magnetic Co2+ ions. Magnetic measurements indicate that γ-Co2(PO4)(OH) may show a spin-glass behavior at low temperature.

A novel compound Cu3(CH3COO)4(OH)2·5H2O was obtained by a hydrothermal method with a special synthesis process. The crystal structure is found to exhibit a unique one-dimensional diamond chain along the c-axis. Magnetic measurements confirmed that this compound exhibits unusual magnetic behaviors of a possible valence-bond condensation state at low temperature, where the two-thirds of all spins are condensed into spin singlets and one-third of the spins still remain in the paramagnetic state.

This work reports a fundamental study on the relationship of the electronic structure, catalytic activity and surface reconstruction process of Fe doped NiS2 (FexNi1-xS2) for the oxygen evolution reaction (OER). A combined photoemission and X-ray absorption spectroscopic study reveals that Fe doping introduces more occupied Fe 3d6 states at the top of the valence band and thereby induces a metallic phase. Meanwhile, Fe doping also significantly increases the OER activity and results in much better stability with the optimum found for Fe0.1Ni0.9S2. More importantly, we performed detailed characterization to track the evolution of the structure and composition of the catalysts after different cycles of OER testing. Our results further confirmed that the catalysts gradually transform into amorphous (oxy)hydroxides which are the actual active species for the OER. However, a fast phase transformation in NiS2 is accompanied by a decrease of OER activity, because of the formation of a thick insulating NiOOH layer limiting electron transfer. On the other hand, Fe doping retards the process of transformation, because of a shorter Fe-S bond length (2.259 Å) than Ni-S (2.400 Å), explaining the better electrochemical stability of Fe0.1Ni0.9S2. These results suggest that the formation of a thin surface layer of NiFe (oxy)hydroxide as an active OER catalyst and the remaining Fe0.1Ni0.9S2 as a conductive core for fast electron transfer is the base for the high OER activity of FexNi1-xS2. Our work provides important insight and design principle for metal chalcogenides as highly active OER catalysts.

A new tellurate compound Na4CuTeO6 was synthesized by a conventional high-temperature solid state reaction. The titled compound crystallizes in the monoclinic system of a space group C2/c and exhibits a unique spin-chain structure built by planar CuO4 squares and TeO6 octahedra along the [101] direction. Magnetic measurements confirm that Na4CuTeO6 possesses an antiferromagnetic ordering at TN = ∼2.5 K. The intrachain and interchain interactions are estimated by a spin-chain model, showing a less pronounced 1D character in the system.

A new two-dimensional (2D) fluorophosphate compound Na3Cu5(PO4)4F·4H2O with a Cu5 cluster has been synthesized using a conventional hydrothermal method. The compound crystallizes in the orthorhombic crystal system with space group Pnma. The 2D layered structure is formed by cap-like {Cu5(PO4)4F} building units consisting of a Cu4O12F cluster plus a residual cap Cu2+ ion. Magnetic susceptibility exhibits a broad maximum at T2 = 19.2 K due to low-dimensional character followed by a long-range antiferromagnetic ordering at T1 = 11.5 K, which is further confirmed by the specific heat data. High-field magnetization measurement demonstrates a 2/5 quantum magnetization plateau above 40 T. The ESR data indicate the presence of magnetic anisotropy, in accordance with the 2D structure of the system.

Novel mixed-anionic-group compounds M 2 (SeO 3 )(Te 3 O 7 ) (M = Co 2+ , Ni 2+ ) are successfully synthesized by a conventional hydrothermal method. Two compounds are isostructural which crystallize in the orthorhombic system of a space group Pnma . The framework shows a (MO 6 ) ∞ chain structure along the b -axis, while SeO 3 groups are isolated and Te 3 O 7 groups form the (Te 3 O 7 ) ∞ chains running along the a -axis. This is the first time to realize transition-metal compounds coexisting with Se 4+ and Te 4+ anionic groups. Magnetic measurements show that Co 2 (SeO 3 )(Te 3 O 7 ) possesses an antiferromagnetic ordering at ∼11 K, while Ni 2 (SeO 3 )(Te 3 O 7 ) exhibits a similar antiferromagnetic ordering at ∼31 K. Also, the fitting of magnetic susceptibility using a spin-chain model gives the exchange coupling J /k B = −1.515 K for Co 2 (SeO 3 )(Te 3 O 7 ) and J /k B = −8.963 K for Ni 2 (SeO 3 )(Te 3 O 7 ), respectively.

Three new molybdate-tellurites, K2M9(TeO3)4(MoO4)4(OH)4 (M = NiII and CoII) and (NH4)2Cu9II(TeO3)4(MoO4)4(OH)4, have been successfully synthesized by conventional hydrothermal method. Due to the influence of the Jahn-Teller effect, these compounds crystallize in different space groups of rhombohedral R3m, monoclinic P21/c and triclinic P1 for Ni-, Co-, and Cu-analogues, respectively. The topological arrangements of magnetic ions in these compounds show that deficient capped-kagome spin-lattices with kagome positions exhibit different 1/6 depletion. Magnetic and specific heat measurements confirm that K2Ni9(TeO3)4(MoO4)4(OH)4 exhibits a spin-glass behavior at low temperature, while (NH4)2Cu9(TeO3)4(MoO4)4(OH)4 possesses a long-range canted antiferromagnetic ordering with TN ∼ 10.8 K and further shows a 3/5 magnetization plateau in the magnetization curve at low temperature.

Two new compounds Sr2Cu5(PO4)4X2·8H2O (X = Cl and Br) are synthesized by a conventional hydrothermal method. Sr2Cu5(PO4)4Cl2·8H2O crystallizes in the tetragonal system with a space group of P4212, while Sr2Cu5(PO4)4Br2·8H2O crystallizes in the space group P4/nmm, which are found to have a similar framework of layered structure, in which the crown-like {Cu5(PO4)4X2} building units connect to each other forming a 2D corrugated sheet with vacancies, while the Sr2+ cations are located along the vacancies. The spin lattice of two compounds built by Cu2+ ions shows a new type of corrugated square. Magnetic measurements confirmed that both Sr2Cu5(PO4)4X2·8H2O (X = Cl and Br) exhibit antiferromagnetic ordering at low temperatures. A fit of theoretical model shows exchange interaction J = −25.62 K for the Cl-analogue and J/kB = −26.47 K for the Br-analogue.

Two fluorophosphates, BaMPO4F (M = Cu, Co), have been synthesized by a conventional hydrothermal method, and they crystallize in the orthorhombic system with the space group P212121, exhibiting a tunnel structure built by distorted MO4F square-pyramids and PO4 tetrahedra. The magnetic behaviors of the two compounds are investigated by means of magnetic susceptibility, magnetization, and heat capacity measurements. The results indicate that BaCuPO4F exhibits a paramagnetic behavior down to 2 K, while BaCoPO4F undergoes long-range antiferromagnetic ordering at 11.3 K and short-range magnetic ordering at ∼22 K.

A new vanadate compound NaKV4O9·2H2O is successfully synthesized by a conventional hydrothermal method. This compound crystallizes in the monoclinic system with the space group C2/c, showing a typical 2D layered structure built from VO5 pyramids, in which the layers are separated by Na(+), K(+), and H2O. The topology structure of magnetic V(4+) ions shows a quite interesting 1/5-depleted square lattice, which is quite similar to that of a famous low-dimensional quantum spin system CaV4O9. A structural and magnetic comparison confirmed that the title compound may exhibit a more pronounced 2D character with a large spin gap.

A selenite chloride, Na2Cu7(SeO3)4O2Cl4 (), was prepared via a conventional hydrothermal method. Na2Cu7(SeO3)4O2Cl4 crystallizes in the triclinic space group P1[combining macron] and features an isolated reverse triangular dipyramid Cu7, which is assembled with two corner-shared Cu4 tetrahedral units. Magnetic measurements suggest that shows the spin-frustration effect with antiferromagnetic ordering at ∼5 K, while an unusual magnetization plateau is observed at an applied field of4 T.

The family of Archimedean lattices has eleven numbers, yet several of these lattices have not been reported for transition metal oxides. Here, the last number of Archimedean lattice was firstly realized in a layered phosphate.

A brackebuschite-type compound Pb2Mn(VO4)2(OH) was synthesized by a hydrothermal method. Single crystal X-ray diffraction reveals that Pb2Mn(VO4)2(OH) crystallizes in the space group P21/m, with the structure built of single [010] chains of edge-shared MnO6 octahedra bridged by VO4-PbO6 groups along a and c axes, as a result forming a very large inter-chain distance of about 7.67 Å. The magnetic and optical measurements indicate that Pb2Mn(VO4)2(OH) is an S = 2 quasi-one-dimensional ferromagnetic semiconductor, with a very small specific value of intra- and inter-chain exchange coupling Jintra/Jinter = 10(-3), and a band gap of 1.72 eV. The electronic structure calculations indicate that a 90° Mn-O-Mn dpσ-dpσ correlation superexchange dominates the intra-chain ferromagnetic coupling in Pb2Mn(VO4)2(OH).

Two barium copper phosphates, BaCu2(PO4)2(H2O) (1) and Ba2Cu(HPO4)(PO4)(OH) (2), were synthesized under mild hydrothermal conditions. The Cu cation in 1 adopts a CuO4(H2O) square pyramidal coordination configuration, forming alternating chains along the b axis through alternative corner and edge sharing, while the geometry of the Cu center in 2 is a CuO4(OH)2 octahedron which further connects each other by edge sharing to constitute uniform chains along the b axis. Magnetic behaviors of both compounds were analyzed by susceptibility, magnetization and heat capacity measurements. The dominant intrachain couplings are antiferromagnetic in 1 with a long-range ordering at 14 K and ferromagnetic in 2 without long-range ordering above 2 K. The first principles calculations indicate that the intrachain ferromagnetic couplings in 2 originate from Cu(1)-O(7)H-Cu(1) dpσ correlation superexchanges. The susceptibility data of compounds 1 and 2 are fitted by using suitable antiferromagnetic chain and ferromagnetic chain models, respectively. In addition, we report the results of the infrared and thermal measurements of both the compounds.

Two new tellurite–sulfates A2Cu5(TeO3)(SO4)3(OH)4 (A = Na, K) have been synthesized by a conventional hydrothermal method. Both compounds feature 1D kagome strip structure built by distorted CuO6 octahedra, which can be regarded as the dimensional reduction of kagome lattice. Magnetic measurements confirmed that the titled compounds possess antiferromagnetic ordering at low temperature, while a field-induced magnetic transition can be observed at critical field. To the best of our knowledge, this is the first time to obtain distorted kagome strip compounds.