NiFe and CoFe (MFe) layered double hydroxides (LDHs) are among the most active electrocatalysts for the alkaline oxygen evolution reaction (OER). Herein, we combine electrochemical measurements, ...operando X-ray scattering and absorption spectroscopy, and density functional theory (DFT) calculations to elucidate the catalytically active phase, reaction center and the OER mechanism. We provide the first direct atomic-scale evidence that, under applied anodic potentials, MFe LDHs oxidize from as-prepared α-phases to activated γ-phases. The OER-active γ-phases are characterized by about 8% contraction of the lattice spacing and switching of the intercalated ions. DFT calculations reveal that the OER proceeds via a Mars van Krevelen mechanism. The flexible electronic structure of the surface Fe sites, and their synergy with nearest-neighbor M sites through formation of O-bridged Fe-M reaction centers, stabilize OER intermediates that are unfavorable on pure M-M centers and single Fe sites, fundamentally accounting for the high catalytic activity of MFe LDHs.
Manganese based precious metal-free electrocatalysts for the oxygen evolution reaction (OER) are promising materials for energy storage systems based on dark or photo-coupled water electrolysis, ...because they are active, inexpensive and of low toxicity. In this work, atomic scale structure-activity relationships of two different nano-structured manganese oxides, MnO sub(x), are established using a combination of X-ray absorption, diffraction and electrochemistry. Prepared by chemical symproportionation (s-MnO sub(x)) and impregnation (i-MnO sub(x)), the s-MnO sub(x) catalyst consisted of a layered structure similar to delta -MnO sub(2) while the i-MnO sub(x) catalyst displayed a mixture of tunnelled, 3D cross-linked beta - and defective gamma -MnO sub(2) structures. During electrocatalytic oxygen evolution the structural motifs of both MnO sub(x) remain largely unchanged, but the oxidation state of Mn increases from 3.5 to 3.9-4. Kinetic parameters of the electrocatalytic oxygen evolution reaction were extracted using Tafel slope analysis and pH titration experiment, and the role of the protons abstracted was analyzed. The study reveals fundamental differences of general importance in the catalytic activity between layered and cross-linked structures. The exclusive presence of di- mu -oxo-bridged Mn ions in the layered structure is coupled to a pronounced redox and charge capacity behaviour. This ensured efficient use of surface and bulk active sites, and resulted in a relatively large Tafel slope. Consequently, the intrinsic OER activity is especially high in s-MnO sub(x). In contrast, 3D cross-linked structures with both mono- and di- mu -oxo-bridged Mn ions resulted in lower intrinsic activity but smaller Tafel slope, and thus favourable activity at technological water-splitting rates. The insights from this comparative study will provide guidance in the structural design and optimization of other non precious metal oxide OER catalysts.
Electrochemical reduction of carbon dioxide (CO2RR) is an attractive route to close the carbon cycle and potentially turn CO2 into valuable chemicals and fuels. However, the highly selective ...generation of multicarbon products remains a challenge, suffering from poor mechanistic understanding. Herein, we used operando Raman spectroscopy to track the potential-dependent reduction of Cu2O nanocubes and the surface coverage of reaction intermediates. In particular, we discovered that the potential-dependent intensity ratio of the Cu–CO stretching band to the CO rotation band follows a volcano trend similar to the CO2RR Faradaic efficiency for multicarbon products. By combining operando spectroscopic insights with Density Functional Theory, we proved that this ratio is determined by the CO coverage and that a direct correlation exists between the potential-dependent CO coverage, the preferred C–C coupling configuration, and the selectivity to C2+ products. Thus, operando Raman spectroscopy can serve as an effective method to quantify the coverage of surface intermediates during an electrocatalytic reaction.
The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels
.... Electrocatalysts accelerate the reaction by facilitating the required electron transfer
, as well as the formation and rupture of chemical bonds
. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential
. Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler-Volmer theory, which focuses on electron transfer
, enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium
or steady-state assumptions
. However, the charging of catalyst surfaces under bias also affects bond formation and rupture
, the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis
and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance.
Water splitting catalysed by earth-abundant materials is pivotal for global-scale production of non-fossil fuels, yet our understanding of the active catalyst structure and reactivity is still ...insufficient. Here we report on the structurally reversible evolution of crystalline Co3O4 electrocatalysts during oxygen evolution reaction identified using advanced in situ X-ray techniques. At electrode potentials facilitating oxygen evolution, a sub-nanometre shell of the Co3O4 is transformed into an X-ray amorphous CoOx(OH)y which comprises di-μ-oxo-bridged Co(3+/4+) ions. Unlike irreversible amorphizations, here, the formation of the catalytically-active layer is reversed by re-crystallization upon return to non-catalytic electrode conditions. The Co3O4 material thus combines the stability advantages of a controlled, stable crystalline material with high catalytic activity, thanks to the structural flexibility of its active amorphous oxides. We propose that crystalline oxides may be tailored for generating reactive amorphous surface layers at catalytic potentials, just to return to their stable crystalline state under rest conditions.
Catalytic water splitting to hydrogen and oxygen is considered as one of the convenient routes for the sustainable energy conversion. Bifunctional catalysts for the electrocatalytic oxygen reduction ...reaction (ORR) and the oxygen evolution reaction (OER) are pivotal for the energy conversion and storage, and alternatively, the photochemical water oxidation in biomimetic fashion is also considered as the most useful way to convert solar energy into chemical energy. Here we present a facile solvothermal route to control the synthesis of amorphous and crystalline cobalt iron oxides by controlling the crystallinity of the materials with changing solvent and reaction time and further utilize these materials as multifunctional catalysts for the unification of photochemical and electrochemical water oxidation as well as for the oxygen reduction reaction. Notably, the amorphous cobalt iron oxide produces superior catalytic activity over the crystalline one under photochemical and electrochemical water oxidation and oxygen reduction conditions.
Mixed bimetallic oxides offer great opportunities for a systematic tuning of electrocatalytic activity and stability. Here, we demonstrate the power of this strategy using well-defined thermally ...prepared Ir–Ni mixed oxide thin film catalysts for the electrochemical oxygen evolution reaction (OER) under highly corrosive conditions such as in acidic proton exchange membrane (PEM) electrolyzers and photoelectrochemical cells (PEC). Variation of the Ir to Ni ratio resulted in a volcano type OER activity curve with an unprecedented 20-fold improvement in Ir mass-based activity over pure Ir oxide. In situ spectroscopic probing of metal dissolution indicated that, against common views, activity and stability are not directly anticorrelated. To uncover activity and stability controlling parameters, the Ir–Ni mixed thin oxide film catalysts were characterized by a wide array of spectroscopic, microscopic, scattering, and electrochemical techniques in conjunction with DFT theoretical computations. By means of an intuitive model for the formation of the catalytically active state of the bimetallic Ir–Ni oxide surface, we identify the coverage of reactive surface hydroxyl groups as a suitable descriptor for the OER activity and relate it to controllable synthetic parameters. Overall, our study highlights a novel, highly active oxygen evolution catalyst; moreover, it provides novel important insights into the structure and performance of bimetallic oxide OER electrocatalysts in corrosive acidic environments.
Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO
to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the ...metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu
Zn
nanoparticles supported on ZnO/Al
O
, γ-Al
O
and SiO
to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al
O
and CuZn/SiO
, but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO
. Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO
. In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity.
Abstract
Water electrolysis to produce ‘green H
2
’ with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode ...limits the efficiency. Co
3
O
4
with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co
3
O
4
(111), CoFe
2
O
4
(111), and Fe
3
O
4
(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe
2
O
4
(111) is found to be up to about four times more active than Co
3
O
4
(111) and about nine times more than Fe
3
O
4
(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe
2
O
4
(111), the oxyhydroxide’s Fe/Co concentration ratio is stabilized by partial iron dissolution.
The fabrication and design of earth-abundant and high-performance catalysts for the oxygen evolution reaction (OER) are very crucial for the development and commercialization of sustainable energy ...conversion technologies. Although spinel catalysts have been widely explored for the electrochemical oxygen evolution reaction (OER), the role of two geometrical sites that influence their activities has not been well established so far. Here, we present more effective cobalt–zinc oxide catalysts for the OER than ‘classical’ Co 3 O 4 . Interestingly, the significantly higher catalytic activity of ZnCo 2 O 4 than that of Co 3 O 4 is somewhat surprising since both crystallize in the spinel-type structure. The reasons for the latter remarkable difference of ZnCo 2 O 4 and Co 3 O 4 could be deduced from structure–activity relationships of the bulk and near-surface of the catalysts using comprehensive electrochemical, microscopic and spectroscopic techniques with a special emphasis on the different roles of the coordination environment of metal ions (octahedral vs. tetrahedral sites) in the spinel lattice. The vital factors influencing the catalytic activity of ZnCo 2 O 4 over Co 3 O 4 could be directly attributed to the higher amount of accessible octahedral Co 3+ sites induced by the preferential loss of zinc ions from the surface of the ZnCo 2 O 4 catalyst. The enhanced catalytic activity is accompanied by a larger density of metal vacancies, defective sites and hydroxylation. The results obtained here clearly demonstrate how a surface structural modification and generation of defects of catalysts can enhance their OER performance.
PDF
