Query: Activated Manganese Oxide Acid Oxygen Evolution Catalyst

title: A functionally stable manganese oxide oxygen evolution catalyst in acid.
abstract: First row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ~60 mV/decade Tafel slope and inverse first order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi infinite Tafel slope and zeroth order dependence on proton concentration. The results reflect two competing mechanisms: a one electron one proton PCET pathway that is dominant under alkaline conditions and a Mn(3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non noble metal oxide OER catalyst may be operated in acid by exploiting a self healing process.

title: A promising amendment for water splitters: Boosted oxygen evolution at a platinum, titanium oxide and manganese oxide hybrid catalyst
abstract: A hybrid catalyst composed of a platinum thin layer and modified with manganese oxide (MnOx) is recommended for the oxygen evolution reaction (OER) The Pt layer of the catalyst was physically sputtered onto a TiOx coated Si substrate (this TiOx layer was sputtered inbetween the Si substrate and Pt layer to improve their adhesion and prevent their mutual diffusion) On top of the Pt layer, another thin TiOx layer ~60 nm) was spun before the electrochemical deposition of MnOx. The investigation focused primarily to evaluate the impact of the catalyst's annealing in oxygen atmosphere on its catalytic activity toward OER. Interestingly, before the modification with MnOx, a large catalytic enhancement both in activity ~228 mV negative shift at 20 mA cm 2 if compared to conventional bare Pt catalysts) and stability was achieved at the catalyst annealed at 600 degC toward OER in 0.5 M KOH. Surprisingly, the addition of MnOx to the catalyst synergized a boosted activity amplifying the negative shift to 470 mV at the same current density. Bunch of materials and electrochemical techniques were combined to reveal important remarks about the catalyst's morphology, structure, composition and intrinsic activity which was attributed to electronic rather than geometric factors.

title: Elucidation of Structure Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L Edge X ray Absorption Spectroscopy and 2p3d Resonant Inelastic X ray Scattering
abstract: Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene supported Ni3MnO4 catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L edge X ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X ray scattering (RIXS) These data are compared to reference measurements on NiO, b Ni(OH)2, b NiOOH, and g NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH) the oxides of the Ni3MnO4 catalyst are converted to hydroxides. At the onset of catalysis (1.47 V) the b Ni(OH)2 like phase is oxidized and converted to a dominantly g NiOOH phase. The present study thus challenges the notion that the b NiOOH phase is the active phase in OER and provides further evidence that the g NiOOH phase is catalytically active. The ability to use Ni L edge XAS and 2p3d RIXS to provide a rational basis for structure activity c.

title: Nature of Activated Manganese Oxide for Oxygen Evolution.
abstract: Electrodeposited manganese oxide films (MnOx) are promising stable oxygen evolution catalysts. They are able to catalyze the oxygen evolution reaction in acidic solutions but with only modest activity when prepared by constant anodic potential deposition. We now show that the performance of these catalysts is improved when they are "activated" by potential cycling protocols, as measured by Tafel analysis (where lower slope is better) upon activation the Tafel slope decreases from ~120 to ~70 mV/decade in neutral conditions and from ~650 to ~90 mV/decade in acidic solutions. Electrochemical, spectroscopic, and structural methods were employed to study the activation process and support a mechanism where the original birnessite like MnOx (d MnO2) undergoes a phase change, induced by comproportionation with cathodically generated Mn(OH)2, to a hausmannite like intermediate (a Mn3O4) Subsequent anodic conditioning from voltage cycling or water oxidation produces a disordered birnessite like phase, which is highly active for oxygen evolution. At pH 2.5, the current density of activated MnOx (at an overpotential of 600 mV) is 2 orders of magnitude higher than that of the original MnOx and begins to approach that of Ru and Ir oxides in acid.

title: Molybdenum Doped Manganese Oxide as a Highly Efficient and Economical Water Oxidation Catalyst
abstract: The development of efficient and noble metal free electrocatalysts for the challenging oxygen evolution reaction (OER) is crucial for sustainable energy solutions. In this work, a facile co precipi.

title: Conversion of inert cryptomelane type manganese oxide into a highly efficient oxygen evolution catalyst via limited Ir doping
abstract: The oxygen evolution reaction (OER) is a critical half reaction for energy storage techniques and is regarded as a major challenge due to its sluggish kinetics and complex reaction mechanism. The traditional OER catalysts, such as IrO2, RuO2 and their binary or ternary oxides, have finite large scale commercial applications due to their significant cost and rareness. Here, we hydrothermally synthesized cry Ir by doping Ir into non OER active cryptomelane type manganese oxide to significantly reduce the Ir mass ratio by 60.3% from 85.7% in IrO2 to 34% in the developed catalyst, along with higher OER performance with a lower onset potential and 10 times higher specific mass activity. The special tunnel structure of cryptomelane plays an important role in promoting its OER activity through facilitating water molecular insertion into the tunnel. We combined Raman, XPS and TEM mapping to confirm that no IrO2 composite is present on the cry Ir surface. The XPS and XAS spectra indicate substitution of Ir4+ on the Mn3+ site and the presence of more 5d states in the Ir site compared to IrO2. The differences in VBS spectra between cry Ir, IrO2 and cry Mn indicate that the electronic structure of Ir sites is modified when Ir substitutes Mn3+ sites. Thus, this special tunnel structure and modified Ir electronic structure in cry Ir are responsible for the outstanding OER performance. Our studies provide an approach for designing effective Ir based OER catalysts whilst significantly reducing the consumption of precious elements.

title: Alkaline manganese electrochemistry studied by in situ and operando spectroscopic methods metal dissolution, oxide formation and oxygen evolution.
abstract: Manganese based systems are considered as candidate electrocatalysts for the electrochemical oxygen evolution reaction (OER) because of their abundance in biochemical oxygen producing catalyst systems. In this work, the surface of metallic manganese was investigated in situ and operando in potentiodynamic cyclic voltammetry (CV) experiments and potentiostatic chronoamperometry (CA) experiments in NaOH. In both cases, the surfaces were initially reduced. At corresponding potentials, no oxide species can be detected by Raman spectroscopy, though electrochemical data and the absence of dissolution above the reversible potential for reactions of type Mn MnII indicate that the material is passive. The CV shows anodic peaks at potentials in line with expectations on the basis of thermodynamic data for the oxidation to Mn3O4 and Mn2O3; the thickness of the surface layer increases by a few nm during these peaks, as evidenced by spectroscopic ellipsometry. Dissolution of Mn as evidenced by downstream electrolyte analysis by inductively coupled plasma mass spectrometry in a scanning flow cell (SFC ICP MS) of the electrolyte is negligible in the range of electrode potential vs. Ag|AgCl|3 M KCl, EAg|AgCl, up to 0.3 V. Remarkably, Raman spectra already show the occurrence of a MnO2 at EAg|AgCl 0.25 V, which is ca. 0.5 V below the potential at which oxidation to MnO2 is expected. This observation is attributed to disproportionation above a certain level of MnIII. For EAg|AgCl 0.4 V, dissolution sets in, at a constant layer thickness. Above the onset potential of the OER, at EAg|AgCl 0.6 V, SFC ICP MS analysis shows fast dissolution, and the oxide layer thickness is constant or increases. CA experiments during the OER show strong dissolution, and the re formation of a strongly disordered, b MnO2 like oxide, which exists in a quasi stationary state at the interface. Several CV cycles increase the dissolution per cycle and the fraction of a MnO2 on the surface which cannot be reduced. The high dissolution currents show that metallic Mn is hardly suitable as an OER catalyst, however, at least the MnIV oxides remain stationarily present in the system.

title: Electrochemical synthesis of a nanohybrid film consisting of stacked graphene sheets and manganese oxide as oxygen evolution reaction catalyst
abstract: A new electrochemical process yielded a thin film consisting of stacked graphene (GR) sheets and manganese oxide. The process involved anodic deposition of manganese oxide nuclei on an electrode substrate and on the surface of GR sheets through electrolysis of aqueous Mn2+ ions in the presence of cationic GR colloids modified with poly(diallydimethylammonium) (PDDA+ followed by extraction of the PDDA. The catalytic activity of Mn oxide for the oxygen evolution reaction in an alkaline electrolyte was dramatically enhanced by hybridizing it with GR; an onset overpotential of 335 mV and a Tafel slope of 58 mV per decade were observed with excellent stability. These results can be attributed to a synergistic effect of the catalytic activity of nanosized Mn oxide deposits and the ability of GR to transfer electrons from the catalysts.

title: Nucleation and growth mechanisms of an electrodeposited manganese oxide oxygen evolution catalyst
abstract: We investigate the mechanisms of nucleation and steady state growth of a manganese oxide catalyst (MnOx) electrodeposited from Mn2+ solutions in a weakly basic electrolyte. Early catalyst growth was probed through chronoamperometry transients, which were fit to reveal a progressive nucleation mechanism for initial catalyst formation. Time dependent atomic force microscopy snapshots of the electrode surface reveal a rapid increase in nucleus size together with a sluggish rise in coverage, which is also characteristic of progressive nucleation. Electrochemical kinetic studies of the catalyst growth yield a Tafel slope of approximately 2.3 x RT/2F and a rate law consisting of a second order and inverse fourth order dependence on [Mn2+ and proton activity, respectively. These results are consistent with a deposition mechanism involving rate limiting disproportionation of aqueous Mn3+ resolving a longstanding ambiguity surrounding the deposition of manganese oxides under nonacidic conditions.

title: In situ synthesis of mesoporous manganese oxide/sulfur doped graphitized carbon as a bifunctional catalyst for oxygen evolution/reduction reactions
abstract: cost materials with high electrochemical catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are in urgent demand for energy storage and conversion devices. In this work, we propose a novel and environmentally friendly approach to synthesize in situ a meso porous MnO x /S doped graphitized carbon (MnO x /S GC) hybrid with a specific surface area of 502.6 m 2 g 1 The synthesis involves the thermal treatment of polyvinyl chloride (PVC) together with KMnO 4 and (NH 4 2 Fe(SO 4 2 It is the first time a MnO x /S GC hybrid is fabricated directly from a carbon source without any intermediate products such as graphene oxide or additional chemical reagents. In addition, since PVC is the major component of plastics, this method has provided a new approach for the utilization of waste. Most importantly, the as prepared hybrid showed a high catalytic activity towards both ORR and OER. The current density reached 3 mA cm 2 at 0.81 V and 10 mA cm 2 at 1.62 V vs. reversible hydrogen electrode respectively.