Theory of electrocatalysis: hydrogen evolution and more

Density functional theory (DFT) by itself is insufficient to model electrochemical reactions, because the interface is too large, and there is no satisfactory way to incorporate the electrode potential. In our group we have developed a theory of electrocatalysis, which combines DFT with our model for electrochemical electron transfer, and thereby avoids these difficulties. Our theory explains how a metal d band situated near the Fermi level can lower the energy of activation for a charge transfer reaction. An explicit application to the hydrogen evolution reaction gives results that agree very well with experimental data obtained both on plain and on nanostructured electrodes. Finally, we outline how our method can be extended to other reactions and present first results for the adsorption of OH on Pt(111).     

Crystal growth of two new photoluminescent oxides: Sr3Li6Nb2O11 and Sr3Li6Ta2O11

Single crystals of Sr3Li6M2O11 (M = Nb, Ta) were grown out of a high-temperature Sr(OH)2/LiOH/KOH flux. The single crystal X-ray diffraction data were indexed to the orthorhombic Pmma system, with a = 10.5834(15) A, b = 8.3103(13) A, c = 5.8277(8) A, V = 512.55(13) A(3), and Z = 2 for Sr3Li6Nb2O11 and a = 10.5936(6) A, b = 8.3452(5) A, c = 5.8271(4) A, V = 515.15(6) A(3), and Z = 2 for Sr3Li6Ta2O11. The crystal structure consists of sheets of interconnected SrO8 polyhedra that are separated by M-O layers and an intervening LiO(x) polyhedral framework, representing a new structural type. The M-O layers exhibit a rare occurrence of both five- and six-coordinated M(5+) ions in the same structure. The oxides, upon excitation at 250 nm, exhibit violet emission at room temperature.     

Transition from hydrogen atom to hydride abstraction by Mn4O4(O2PPh2)6 versus [Mn4O4(O2PPh2)6]+: O-H bond dissociation energies and the formation of Mn4O3(OH)(O2PPh2)6

Synthesis, characterization, and reactions of the novel manganese-oxo cubane complex [Mn(4)O(4)(O(2)PPh(2))(6)](ClO(4)), 1+ (ClO(4)(-)), are described. Cation 1+ is composed of the [Mn(4)O(4)](7+) core surrounded by six bidentate phosphinate ligands. The proton-coupled electron transfer (pcet) reactions of phenothiazine (pzH), the cation radical (pzH(.+)(ClO(4)(-)), and the neutral pz* radical with 1+ are reported and compared to Mn(4)O(4)(O(2)PPh(2))(6) (1). Compound 1+ (ClO(4)(-)) reacts with excess pzH via four sequential reduction steps that transfer a total of five electrons and four protons to 1+. This reaction forms the doubly dehydrated manganese cluster Mn(4)O(2)(O(2)PPh(2))(6) (2) and two water molecules derived from the corner oxygen atoms. The first pcet step forms the novel complex Mn(4)O(3)(OH)(O(2)PPh(2))(6) (1H) and 1 equiv of the pz+ cation by net hydride transfer from pzH. Spectroscopic characterization of isolated 1H is reported. Reduction of 1 by pzH or a series of para-substituted phenols also produces 1H via net H atom transfer. A lower limit to the homolytic bond dissociation energy (BDE) (1H --> 1 + H) was estimated to be >94 kcal/mol using solution phase BDEs for pzH and para-substituted phenols. The heterolytic BDE was estimated for the hydride transfer reaction 1H --> 1+ + H(-) (BDE approximately 127 kcal/mol). These comparisons reveal the O-H bond in 1H to be among the strongest of any Mn-hydroxo complex measured thus far. In three successive H atom transfer steps, 1H abstracts three hydrogen atoms from three pzH molecules to form complex 2. Complex 2 is shown to be identical to the "pinned butterfly" cluster produced by the reaction of 1 with pzH (Ruettinger, W. F.; Dismukes, G. C. Inorg. Chem. 2000, 39, 1021-1027). The Mn oxidation states in 2 are formally Mn(4)(2II,2III), and no further reduction occurs in excess pzH. By contrast, outer-sphere electron-only reductants such as cobaltacene reduce both 1+ and 1 to the all Mn(II) oxidation level and cause cluster fragmentation. The reaction of pzH(.+) with 1+ produces 1H and the pz+ cation by net hydrogen atom transfer, and terminates at 1 equiv of pzH(.+) with no further reaction at excess. By contrast, pz* does not react with 1+ at all, indicating that reduction of 1+ by electron transfer to form pz+ does not occur without a proton (pcet to 1+ is thermodynamically required). Experimental free energy changes are shown to account for these pcet reactions and the absence of electron transfer for any of the phenothiazine series. Hydrogen atom abstraction from substrates by 1 versus hydride abstraction by 1(+ )()illustrates the transition to two-electron one-proton pcet chemistry in the [Mn(4)O(4)](7+) core that is understood on the basis of free energy consideration. This transition provides a concrete example of the predicted lowest-energy pathway for the oxidation of two water molecules to H(2)O(2) as an intermediate within the photosynthetic water-oxidizing enzyme (vs sequential one-electron/proton steps). The implications for the mechanism of photosynthetic water splitting are discussed.     

Dopant-vacancy activated tetragonal transition metal selenide for hydrogen evolution electrocatalysis

Hydrogen production from water electrolysis using renewable electricity is a highly promising route to solve the energy crisis of human society. The tetragonal 3d-transition metal selenide with metallic feature has been discovered to efficiently catalyze the hydrogen evolution electrocatalysis; however, its performance is still unsatisfactory and further improvement is necessary. Herein, the hydrogen evolution reaction of the functional tetragonal 3d-transition metal selenide with the heteroatom-dopant as well as cationic vacancy is fully investigated by means of density functional theory calculations. Our results identify 53 promising candidates endowed with good activity due to the absolute free energy of hydrogen adsorption |∆G_H| ≤ 0.30 eV wherein 15 candidates with |∆G_H| ≤ 0.09 eV possess compelling performance in comparison with the benchmark Pt material. Interestingly, the functional CuSe systems account for 29 out of 53 candidates, being high attractive for experimental synthesis. According to the analysis of electronic structure, the enhanced performance stems from the upshift of the sp orbitals, which benefits for the improved affinity toward hydrogen capture. This work provides new direction and guidance for the design of novel electrocatalysts.     

Activity volcanoes for the electrocatalysis of homolytic and heterolytic hydrogen evolution

This paper derives a simple kinetic model for the electrochemical current generated by a heterolytic model for hydrogen evolution, as it would apply to the reaction taking place on molecular catalysts, and compares the activity trends to the classical Volmer-Heyrovsky-Tafel models. It is demonstrated that in the heterolytic mechanism, pH plays a crucial role in optimizing the overall activity.     

Nickel surface anodic oxidation and electrocatalysis of oxygen evolution

The processes of nickel surface anodic oxidation taking place within the range of potentials preceding oxygen evolution reaction (OER) in the solutions of 1 M KOH, 0.5 M K₂SO₄, and 0.5 M H₂SO₄ have been analyzed in the present paper. Metallic nickel, thermally oxidized nickel, and black nickel coating were used as Ni electrodes. The methods of cyclic voltammetry and X-ray photoelectron spectroscopy were employed. The study was undertaken with a view to find the evidence of peroxide-type nickel surface compounds formation in the course of OER on the Ni electrode surface. On the basis of experimental results and literature data, it has been suggested that in alkaline solution at E ≈ 1.5 V (RHE) reversible electrochemical formation of Ni(IV) peroxide takes place according to the reaction as follows: This reaction accounts for both the underpotential (with respect to ) formation of O₂ from NiOO₂ peroxide and also small experimental values of dE/dlgi slope (<60 mV) at low anodic current densities, which are characteristic for the two-electron transfer process. It has been inferred that the composition of the γ-NiOOH phase, indicated in the Bode and revised Pourbaix diagrams, should be ∼5/6 NiOOH + ∼1/6 NiOO₂. The schemes demonstrating potential-dependent transitions between Ni surface oxygen compounds are presented, and the electrocatalytic mechanisms of OER in alkaline, acid, and neutral medium have been proposed.     

Ultrasmall Pt2Sr alloy nanoparticles as efficient bifunctional electrocatalysts for oxygen reduction and hydrogen evolution in acidic media

Electrocatalytic oxygen reduction reaction(ORR) and hydrogen evolution reaction(HER) in acidic media are vital for the applications of renewable energy electrolyzers.However,the low mass activity of noble Pt urgently needs to be improved due to the strong binding energetics of oxygen species(*O) with Pt sites.Here we report fine PtxSr alloy(～2 nm) supported on N-doped carbon(NC) pyrolyzing from ZIF-8 as bifunctional electrocatalysts toward ORR and HER in acidic media.The representative Pt_2<...     

Recent trends in hydrogen and oxygen electrocatalysis for anion exchange membrane technologies

This review aims at presenting recent findings in the understanding of oxygen and hydrogen electrocatalysis in alkaline electrolytes that are key processes for the emergence of sustainable energy storage and conversion devices such as anion exchange membrane fuel cells and electrolyzers. In these systems, the exchange of electrons through electrochemical reactions provides a unique pathway to reversibly convert the electricity vector into chemical one: hydrogen. A concise and critical review of advances made during the last past years in the design of catalysts is provided. Challenges and opportunities for the development of the next catalyst generation are also addressed.     

The carbides Gd3Mn2C6 and Tb3Mn2C6

Crystals of the title compounds were obtained by arc-melting cold-pressed pellets of the elemental components, followed by annealing the reaction products in an argon atmosphere slightly below the melting point. The crystal structures of these isotypic, hexagonal carbides (P6₃/m, Z=2) were determined from single-crystal X-ray data; Gd₃Mn₂C₆: a=815.0(2) pm, c=504.93(9) pm, R=0.012 for 526 structure factors and 18 variable parameters; Tb₃Mn₂C₆: a=810.5(2) pm, c=500.5(2) pm, R=0.025 (225 F′s, 18 variables). The carbon atoms form pairs with C—C bond distances corresponding to double bonds. The three-dimensional, polyanionic managanese carbon network contains relatively large trigonal-bipyramidal voids formed by three lanthanoid and two manganese atoms. The rationalization of chemical bonding on the basis of the 18-electron rule suggests that these voids are filled by nonbonding electrons of the adjacent manganese atoms.     

Visible-light driven Gd2Ti2O7/GdCrO3 composite for hydrogen evolution

A series of Gd(2)Ti(2)O(7)/GdCrO(3) composites are prepared by solid state combustion method using Gd(NO(3))(3), TiO(2), Cr(2)O(3) as metal source and urea as a fuel. The composites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-visible diffuse reflectance spectra (DRUV-vis), Brunauer-Emmett-Teller (BET) surface area measurements, photoluminescence spectra (PL), X-ray photoelectron spectroscopic (XPS) studies, photocurrent measurements etc. The photocatalytic activity of the composites is examined towards hydrogen production without using any co-catalyst under visible light illumination. The rate of formation of hydrogen is measured by the photocatalytic activity measurement device and gas chromatography (GC). The highest efficiency is observed over the composite GTC (Cr:Gd:Ti = 1:1:1). On the basis of photocurrent measurements and PL, a mechanism for the enhanced photocatalytic activity has been discussed.     

Synthesis and Properties of Rb6Mn2O6

Rb₆Mn₂O₆ was prepared via the azide/nitrate route. Stoichiometric mixtures of the precursors (Mn₃O₄, RbN₃ and RbNO₃) were heated in a special regime up to 500 °C and annealed at this temperature for 75 h in silver crucibles. Single crystals have been grown by annealing a mixture with a slight excess of rubidium components at 450 °C for 500 h. According to the single crystal structure analysis, Rb₆Mn₂O₆ is isotypic to K₆Mn₂O₆, and crystallizes in the monoclinic space group P2₁/c with a = 6.924(1) Å, b = 11.765(2) Å, c = 7.066(1) Å, β = 99.21(3)°, 2296 independent reflections, R₁ = 5.23 % (all data). Manganese is tetrahedrally coordinated and two tetrahedra are linked by sharing a common edge, forming a dimer [Mn₂O₆]⁶⁻. The magnetic behavior has been investigated.     

Preparation of Sr7Mn4O13F2 by the topotactic reduction and subsequent fluorination of Sr7Mn4O15

The topotactic reduction and subsequent fluorination of Sr7Mn4O15 yields a phase of composition Sr7Mn4O13F2. Characterization of this phase utilizing powder neutron diffraction and 19F NMR shows that the fluoride ions are located on a single anion site, the same crystallographic site that is vacant in the reduced intermediate Sr7Mn4O13.     

Sr3ZnPtO6 and Sr3CdPtO6

The flux synthesis of single crystals of the isostructural compounds tristrontium zinc platinum hexaoxide, Sr₃ZnPtO₆, and tristrontium cadmium platinum hexaoxide, Sr₃CdPtO₆, is reported. The compounds adopt the pseudo‐one‐dimensional rhombohedral K₄CdCl₆ structure type, and feature chains of face‐shared distorted ZnO₆ or CdO₆ trigonal prisms and PtO₆ octahedra, surrounded by columns of Sr²⁺ ions. All transition metals are located on the threefold axis of symmetry, while the Sr²⁺ cations lie on twofold axes.     

Über Oxidsulfide mit Åkermanitstruktur CaLaGa3S6O,SrLaGa3S6O,La2ZnGa2S6O und Sr2ZnGe2S6O

On Oxidesulfides of Åkermanitetype-Structure CaLaGa₃S₆O, SrLaGa₃S₆O, La₂ZnGa₂S₆O, and Sr₂ZnGe₂S₆O The oxide sulfides were prepared for the first time (space group and lattice constants see “Inhaltsübersicht”). The atomic positions were refined from single crystal X-ray data for CaLaGa₃S₆O and La₂ZnGa₂S₆O. Problems concerning the metal distribution on the crystallographic positions are discussed by comparing interatomic distances. The ratio c/a of the new compounds is compared with that of isotypic compounds from literature.     

Ca3Mn2O7

The tricalcium dimanganese heptaoxide (Ca₃Mn₂O₇) member of the Ruddlesden–Popper series Ca_n+1Mn_nO_3n+1, i.e. with n = 2, was previously reported with an I‐centred tetragonal lattice [a_t = 3.68 and c_t = 19.57 Å] by Fawcett, Sunstrom, Greenblatt, Croft & Ramanujachary [Chem. Mater. (1998), 10 , 3643–3651]. It is now found to be orthorhombic, with an A‐­centred lattice [a = 5.2347 (6), b = 5.2421 (2) and c = 19.4177 (19) Å]. The structure has been refined in space group A2₁am using X‐ray single‐crystal diffraction data and assuming the existence of twin domains related by the (10) plane. A comparison with the basic perovskite structure CaMnO₃ (n = ∞) is proposed.     

A review on non-noble metal based electrocatalysis for the oxygen evolution reaction

In order to find a clean, efficient and sustainable new energy source that can replace fossil fuels, hydrogen energy is considered to be the most ideal choice. Electrocatalytic oxygen evolution plays a vital role in the development of hydrogen energy, promotes the research of new electrocatalysts, and is dedicated to find materials with high electrocatalytic efficiency. This article discusses in detail the major developments in OER electrocatalysts, including recently reported metal and non-metal based materials. Metal-based catalysts, although having the advantages of high catalytic activity, have disadvantages such as poor stability and low selectivity, which hinder the further application of such materials. Non-metallic based materials avoid such disadvantages and exhibit very substantial performance in overall water decomposition. This review provides useful knowledge of a well-designed OER electrocatalyst and a possible strategy for OER/HER dual-function catalytic performance for future development.     

Molybdenum-induced tuning 3d-orbital electron filling degree of CoSe2 for alkaline hydrogen and oxygen evolution reactions

The development of high-performance non-precious metal-based robust bifunctional electrocatalyst for both hydrogen evolution reaction(HER) and oxygen evolution reactions(OER) in alkaline media is essential for the electrochemical overall water splitting technologies. Herein, we demonstrate that the HER/OER performance of Co Se₂ can be significantly enhanced by tuning the 3d-orbital electron filling degree through Mo doping. Both density functional theory(DFT) calculations and experime...     

Magnetic properties of the layered compounds Ca2Mn3O8 and Cd2Mn3O8

Ca₂Mn₃O₈ and Cd₂Mn₃O₈, which contain Mn⁴⁺ monolayers, have been prepared and characterized. Their magnetic susceptibility and electron paramagnetic resonance (EPR) behavior have been examined in detail. The Mn⁴⁺ moments in both Ca₂Mn₃O₈ and Cd₂Mn₃O₈ order antiferromagnetically near 60 and 10°K, respectively. Although the Néel temperature in Ca₂Mn₃O₈ is in reasonable agreement with molecular-field theory, that in Cd₂Mn₃O₈ is well below its expected value. It is proposed that these results, as well as those in the calcium manganite series CaMnO₃ → Ca₂MnO₄, may reflect the chemical influence of the divalent cation in modifying the MnO covalent mixing.