Applications of ALD MnO to electrochemical water splitting.

Atomic layer deposition (ALD) is an attractive method to deposit uniform catalytic films onto high surface area electrodes. One interesting material for ALD synthesis is MnOx, a promising earth-abundant catalyst for the oxygen evolution reaction (OER). It has previously been shown that catalysts beginning as MnO synthesized using ALD on smooth glassy carbon (s-GC) electrodes and Mn2O3 obtained upon annealing MnO on s-GC are active OER catalysts. Here, we use ALD to deposit MnO on high surface area GC (HSA-GC) substrates, forming an active catalyst on a geometric surface area basis. We then characterize three types of catalysts, HSA-GC MnO, s-GC MnO, and annealed MnO (Mn2O3), using cyclic voltammetry (CV), scanning electron microscopy (SEM), and ex situ X-ray absorption spectroscopy (XAS). We show that under OER conditions, all three catalysts oxidize to similar surface states with a mixture of Mn(3+)/Mn(4+) and that MnOx surface area effects can account for the observed differences in the catalytic activity. We also demonstrate the need for a high surface area support for high OER activity on a geometric basis.

Simultaneous detection of electronic structure changes from two elements of a bifunctional catalyst using wavelength-dispersive X-ray emission spectroscopy and in situ electrochemistry.

Multielectron catalytic reactions, such as water oxidation, nitrogen reduction, or hydrogen production in enzymes and inorganic catalysts often involve multimetallic clusters. In these systems, the reaction takes place between metals or metals and ligands to facilitate charge transfer, bond formation/breaking, substrate binding, and release of products. In this study, we present a method to detect X-ray emission signals from multiple elements simultaneously, which allows for the study of charge transfer and the sequential chemistry occurring between elements. Kß X-ray emission spectroscopy (XES) probes charge and spin states of metals as well as their ligand environment. A wavelength-dispersive spectrometer based on the von Hamos geometry was used to disperse Kß signals of multiple elements onto a position detector, enabling an XES spectrum to be measured in a single-shot mode. This overcomes the scanning needs of the scanning spectrometers, providing data free from temporal and normalization errors and therefore ideal to follow sequential chemistry at multiple sites. We have applied this method to study MnOx-based bifunctional electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). In particular, we investigated the effects of adding a secondary element, Ni, to form MnNiOx and its impact on the chemical states and catalytic activity, by tracking the redox characteristics of each element upon sweeping the electrode potential. The detection scheme we describe here is general and can be applied to time-resolved studies of materials consisting of multiple elements, to follow the dynamics of catalytic and electron transfer reactions.

Understanding interactions between manganese oxide and gold that lead to enhanced activity for electrocatalytic water oxidation.

To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnO(x), a promising OER catalyst. We conclusively demonstrate that adding Au to MnO(x) significantly enhances OER activity relative to MnO(x) in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnO(x) catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnO(x) that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnO(x).

In situ X-ray absorption spectroscopy investigation of a bifunctional manganese oxide catalyst with high activity for electrochemical water oxidation and oxygen reduction.

In situ X-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems, with the ability to elucidate the chemical nature of electrocatalysts under reaction conditions. In this study, we perform in situ XAS measurements on a bifunctional manganese oxide (MnOx) catalyst with high electrochemical activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we find that exposure to an ORR-relevant potential of 0.7 V vs RHE produces a disordered Mn3(II,III,III)O4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs RHE, relevant to the OER, we observe an oxidation of approximately 80% of the catalytic thin film to form a mixed Mn(III,IV) oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged Mn3(II,III,III)O4. XAS and electrochemical characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidation states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. This result suggests that the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, we hypothesize that the Mn(III,IV) oxide, rather than Mn3(II,III,III)O4, is the phase pertinent to the observed OER activity.

Meso-structured platinum thin films: active and stable electrocatalysts for the oxygen reduction reaction.

Improving both the activity and the stability of the cathode catalyst in platinum-based polymer electrolyte fuel cells is a key technical challenge. Here, we synthesize a high surface area meso-structured Pt thin film that exhibits higher specific activity for the oxygen reduction reaction (ORR) than commercial carbon-supported Pt nanoparticles (Pt/C). An accelerated stability test demonstrates that the meso-structured Pt thin film also displays significantly enhanced stability as compared to the commercial Pt/C catalyst. Our study reveals the origin of the high turnover frequency (TOF), and excellent durability is attributed to the meso-structure, which yields a morphology with fewer undercoordinated Pt sites than Pt/C nanoparticles, a key difference with substantial impact to the surface chemistry. The improved catalyst activity and stability could enable the development of a high-performance gas diffusion electrode that is resistant to corrosion even under the harsh conditions of start-up, shut-down, and/or hydrogen starvation.

Identifying active surface phases for metal oxide electrocatalysts: a study of manganese oxide bi-functional catalysts for oxygen reduction and water oxidation catalysis.

Progress in the field of electrocatalysis is often hampered by the difficulty in identifying the active site on an electrode surface. Herein we combine theoretical analysis and electrochemical methods to identify the active surfaces in a manganese oxide bi-functional catalyst for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). First, we electrochemically characterize the nanostructured α-Mn(2)O(3) and find that it undergoes oxidation in two potential regions: initially, between 0.5 V and 0.8 V, a potential region relevant to the ORR and, subsequently, between 0.8 V and 1.0 V, a potential region between the ORR and the OER relevant conditions. Next, we perform density function theory (DFT) calculations to understand the changes in the MnO(x) surface as a function of potential and to elucidate reaction mechanisms that lead to high activities observed in the experiments. Using DFT, we construct surface Pourbaix and free energy diagrams of three different MnO(x) surfaces and identify 1/2 ML HO* covered Mn(2)O(3) and O* covered MnO(2), as the active surfaces for the ORR and the OER, respectively. Additionally, we find that the ORR occurs through an associative mechanism and that its overpotential is highly dependent on the stabilization of intermediates through hydrogen bonds with water molecules. We also determine that OER occurs through direct recombination mechanism and that its major source of overpotential is the scaling relationship between HOO* and HO* surface intermediates. Using a previously developed Sabatier model we show that the theoretical predictions of catalytic activities match the experimentally determined onset potentials for the ORR and the OER, both qualitatively and quantitatively. Consequently, the combination of first-principles theoretical analysis and experimental methods offers an understanding of manganese oxide oxygen electrocatalysis at the atomic level, achieving fundamental insight that can potentially be used to design and develop improved electrocatalysts for the ORR and the OER and other important reactions of technological interest.

A bifunctional nonprecious metal catalyst for oxygen reduction and water oxidation.

There is a growing interest in oxygen electrochemistry as conversions between O(2) and H(2)O play an important role in a variety of renewable energy technologies. The goal of this work is to develop active bifunctional catalyst materials for water oxidation and oxygen reduction. Drawing inspiration from a cubane-like CaMn(4)O(x), the biological catalyst found in the oxygen evolving center (OEC) in photosystem II, nanostructured manganese oxide surfaces were investigated for these reactions. Thin films of nanostructured manganese oxide were found to be active for both oxygen reduction and water oxidation, with similar overall oxygen electrode activity to the best known precious metal nanoparticle catalysts: platinum, ruthenium, and iridium. Physical and chemical characterization of the nanostructured Mn oxide bifunctional catalyst reveals an oxidation state of Mn(III), akin to one of the most commonly observed Mn oxidation states found in the OEC.

Substrate selection for fundamental studies of electrocatalysts and photoelectrodes: inert potential windows in acidic, neutral, and basic electrolyte.

The selection of an appropriate substrate is an important initial step for many studies of electrochemically active materials. In order to help researchers with the substrate selection process, we employ a consistent experimental methodology to evaluate the electrochemical reactivity and stability of seven potential substrate materials for electrocatalyst and photoelectrode evaluation. Using cyclic voltammetry with a progressively increased scan range, we characterize three transparent conducting oxides (indium tin oxide, fluorine-doped tin oxide, and aluminum-doped zinc oxide) and four opaque conductors (gold, stainless steel 304, glassy carbon, and highly oriented pyrolytic graphite) in three different electrolytes (sulfuric acid, sodium acetate, and sodium hydroxide). We determine the inert potential window for each substrate/electrolyte combination and make recommendations about which materials may be most suitable for application under different experimental conditions. Furthermore, the testing methodology provides a framework for other researchers to evaluate and report the baseline activity of other substrates of interest to the broader community.