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Sustainable electrocatalysis of the oxygen evolution reaction (OER) constitutes a major challenge for the realization of green fuels. Oxides based on Ni and Fe in alkaline media have been proposed to avoid using critical raw materials. However, their ill-defined structures under OER conditions make the identification of key descriptors difficult. Here, we have studied Fe−Ni−Zn spinel oxides, with a well-defined crystal structure, as a platform to obtain general understanding on the key contributions. The OER reaches maximum performance when: (i) Zn is present in the Spinel structure, (ii) very dense, equimolar 1 : 1 : 1 stoichiometry sites appear on the surface as they allow the formation of oxygen vacancies where Zn favors pushing the electronic density that is pulled by the octahedral Fe and tetrahedral Ni redox pair lowering the overpotential. Our work proves cooperative electronic effects on surface active sites as key to design optimum OER electrocatalysts.

The incorporation of the natural amino acid L-proline in the synthesis to vanadium-chromium Prussian blue derivatives results in materials exhibiting magnetic ordering including chiral magnetic centers. Although the amorphous nature of these materials makes difficult to assess the structural features of these proline-containing compounds, magnetic and spectroscopic data confirms their multifunctionality. They exhibit high-temperature magnetic ordering (Tc &lt, 255 K) and a circular dichroic signal, representing the molecule-based chiral magnets with the highest ordering temperatures reported to date. In addition, the presence of chiral L-proline (or D-proline) has additional benefits, including higher redox stability and the appearance of magnetic hysteresis. The latter was not observed in the parent compounds, the series of room temperature molecule-based magnets V[Cr(CN)6]x.

A new series of tridentate N-8-quinolyl-salicylaldi- mine ligands, Hqsal-5,7-X2 [X = Cl(1), Br(2), I(3)], halo- substituted at the 5,7 position of the aminoquinoline moiety and their corresponding complexes with FeIII were synthesized and formulated as [Fe(qsal-5,7-X2)(NCS)(MeO)]2·solv. (X = Cl (1a), Br (2a), I (3a, solv = 1/2MeOH), [Fe4(qsal-5,7-X2)4(NCS)2- (MeO)2]·solv. (X = Br (2b), I (3b; solv = 4CH2Cl2)) by single- crystal X-ray diffraction analysis. 1a and 2a are isostructural dimers where each FeIII metal ion, showing a distorted octahedral environment, is bound by a N,N,O tridentate (qsal-5,7-X2)− (X = Cl and Br) ligand, a N-coordinated SCN− anion, and a bridging methanolate anion. 2b and 3b are centrosymmetric tetramers where each FeIII is bound by three nitrogen atoms and three oxygen atoms derived from a tridentate (qsal-5,7-X2)− (X = Br and I), a SCN−, a bridging methanolate anion, and a bridging μ2- oxy moiety. In 3b, the iodine atoms dominate the packing interactions through the establishment of a halogen-bonding network. The magnetic behavior of 1a−3a dimers and 2b−3b tetramers indicate the presence of strong antiferromagnetic interactions between FeIII centers (S = 5/2), mediated by the alkoxy bridges. Experimental data can be modeled with an isotropic Hamiltonian, H = −2J(S1 · S2) for dimers (J = −15 cm−1) and H = −2J(S2 · S3) − 2J′(S1 · S2 + S3 · S4), for tetramers (J = −24 cm−1, J′ = −11 cm−1). The magnetic behavior of 1a−3a dimers indicates the presence of strong antiferromagnetic interactions between FeIII centers (S = 5/2), mediated by the alkoxy bridges. 2b and 3b show the same magnetic behavior since they contains analogous bridges between paramagnetic centers, but for a linear tetramer. Density functional theory (DFT) calculations, based on hybrid functional mPW1PW paralleled by the Def2SVP all-electron split-valence basis sets, support the experimental results, showing that monomers could possibly show a spin crossover (SCO) behavior, even though the formation of complexes with an even number of metal ions results in the strong antiferromagnetic interactions. Accordingly, broken symmetry DFT calculations carried out on 1a clearly show that the antiferromagnetic coupling of the two HS FeIII centers results in the lowest energy electron configuration of the complex.

A novel extended triazole-based ligand (PM-Tria) has been synthe- sized and an unprecedented MOF 3D has serendipitously been formed by assembling iron(II), PM-Tria ligand and fluoride anions. This MOF contains a perfectly linear one-dimensional {Fe(II)–F}n bridging chain that shows an antiferromagnetic behaviour. Further- more, the structure is compared with a 14th century mosaic found in the Alhambra Palace in Granada showing a surprising symmetry resemblance.

The magnetic behavior of the polyanion [Fe3(μ-L)6(H2O)6]6– (L2– = (1,2,4-triazol-4-yl)ethanedisulfonate) as the corresponding dimethylammonium salt shows memory effect above room temperature, with a dynamic thermal hysteresis cycle over 90 K and temperature-induced excited spin state trapping (TIESST) phenomena at the highest temperatures reported. Taking advantage of the polyanionic nature of this tri- metallic complex, we were able to substitute the dimethylammonium cations by the monovalent heavy alkali metal cesium. This methathesis yielded the salt Cs6[Fe3(μ-L)6(H2O)6], with different molecular packing that increases the number and strength of cation–anion interactions, including a more robust H-bonded network. In this phase, the spin transition still occurs above room temperature, but it is more abrupt and narrow (≈50 K wide hysteresis). Despite these differences, TIESST is observed with almost identical characteristic temperature (TTIESST = 240 K) than in the parent compound, which is an additional experimental evidence supporting the molecular origin of the TIESST behavior in these materials.

Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO2 catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm–2. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

This study investigates the activity and stability of a Prussian blue analogue (PBA) as an inexpensive anode catalyst for Polymer Electrolyte Membrane Water Electrolysis (PEMWE). While some PBAs have recently been reported to catalyze the oxygen evolution reaction (OER) in acidic electrolytes, the present study focuses on their integration in a PEMWE device. Cobalt hexacyanoferrate nanoparticles were interfaced with an electrically conductive support that withstands the PEMWE anodic conditions, namely Sb-doped SnO2. The OER activity of the composite materials was first verified in liquid electrolytes and then in PEMWE. A promising current density of 50–100 mA cm- 2 was reached at 2 V cell voltage. The PBA/Sb– SnO2 anode was stable up to 1.9 V, but showed more and more instability at higher potentials. Increasing leaching rates of Sn and Sb observed above 1.9 V suggest that the material instability above 1.9 V can mainly be assigned to Sb-doped SnO2 conductive support. These results are overall promising for the use of PBAs as catalytic sites at the anode of PEMWE. The study also identifies the need for more active PBAs in order to reach a higher current density at a cell voltage of 1.6–1.9 V, a potential range necessary for an acceptable energy efficiency of the PEMWE.