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      Multicomposite Nanostructured Hematite–Titania Photoanodes with Improved Oxygen Evolution: The Role of the Oxygen Evolution Catalyst

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          Abstract

          We present a sol–gel processed hematite–titania-based photoanode, which exhibits a photocurrent of up to 2.5 mA/cm 2 at 1.23 V RHE under simulated AM 1.5 G illumination (100 mW/cm 2) thanks to the addition of an amorphous cocatalyst with the nominal composition Fe 20Cr 40Ni 40O x . To unveil the role of the cocatalyst interconnected to the photoanode, we performed impedance measurements. According to the one order of magnitude higher value for the capacitance associated with surface states ( C SS) compared to the bare photoanode, the function of the catalyst−photoanode interface resembles that of a p−n-like junction. In addition, the charge transfer resistance associated with charge transfer processes from surface states ( R ct,ss) was unchanged at potentials between 0.8 and 1.1 V RHE after adding the cocatalyst, indicating that the catalyst has a negligible effect on the hole transport to the electrolyte. The understanding of the role of oxygen evolution catalysts (OECs) in conjunction with the photoanodes is particularly important for water splitting because most OECs are studied separately at considerably higher potentials compared to the potentials at which photoanode materials are operated.

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          Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting.

          Frank E Osterloh (2013)
          The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe(2)O(3), but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.
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            Photochemical route for accessing amorphous metal oxide materials for water oxidation catalysis.

            Rodney D L Smith, Mathieu S. Prévot, Randal Fagan … (2013)
            Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe(100-y-z)Co(y)Ni(z)O(x) are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.
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              Oxygen Evolution Reaction Dynamics, Faradaic Charge Efficiency, and the Active Metal Redox States of Ni-Fe Oxide Water Splitting Electrocatalysts.

              Peter Strasser, Holger Dau, Ralph Krähnert … (2016)
              Mixed Ni-Fe oxides are attractive anode catalysts for efficient water splitting in solar fuels reactors. Because of conflicting past reports, the catalytically active metal redox state of the catalyst has remained under debate. Here, we report an in operando quantitative deconvolution of the charge injected into the nanostructured Ni-Fe oxyhydroxide OER catalysts or into reaction product molecules. To achieve this, we explore the oxygen evolution reaction dynamics and the individual faradaic charge efficiencies using operando differential electrochemical mass spectrometry (DEMS). We further use X-ray absorption spectroscopy (XAS) under OER conditions at the Ni and Fe K-edges of the electrocatalysts to evaluate oxidation states and local atomic structure motifs. DEMS and XAS data consistently reveal that up to 75% of the Ni centers increase their oxidation state from +2 to +3, while up to 25% arrive in the +4 state for the NiOOH catalyst under OER catalysis. The Fe centers consistently remain in the +3 state, regardless of potential and composition. For mixed Ni100-xFex catalysts, where x exceeds 9 atomic %, the faradaic efficiency of O2 sharply increases from ∼30% to 90%, suggesting that Ni atoms largely remain in the oxidation state +2 under catalytic conditions. To reconcile the apparent low level of oxidized Ni in mixed Ni-Fe catalysts, we hypothesize that a kinetic competition between the (i) metal oxidation process and the (ii) metal reduction step during O2 release may account for an insignificant accumulation of detectable high-valent metal states if the reaction rate of process (ii) outweighs that of (i). We conclude that a discussion of the superior catalytic OER activity of Ni-FeOOH electrocatalysts in terms of surface catalysis and redox-inactive metal sites likely represents an oversimplification that fails to capture essential aspects of the synergisms at highly active Ni-Fe sites.
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                Author and article information

                Journal
                Journal ID (nlm-ta): ACS Omega
                Journal ID (iso-abbrev): ACS Omega
                Journal ID (publisher-id): ao
                Journal ID (coden): acsodf
                Title: ACS Omega
                Publisher: American Chemical Society
                ISSN (Electronic): 2470-1343
                Publication date (Electronic): 15 August 2017
                Publication date Collection: 31 August 2017
                Volume: 2
                Issue: 8
                Pages: 4531-4539
                Affiliations
                [† ]Laboratory for Multifunctional Materials, Department of Materials, ETH Zurich , Vladimir-Prelog-Weg 5, 8093 Zurich, Switzerland
                [‡ ]Centre of New Technologies (CeNT), University of Warsaw , Żwirki i Wigury 93, 02-089 Warsaw, Poland
                [§ ]Laboratory for Mechanics of Materials and Nanostructures, Swiss Federal Laboratories for Materials Science and Technology (EMPA) , Feuerwerkerstrasse 39, 3602 Thun, Switzerland
                Author notes
                Article
                DOI: 10.1021/acsomega.7b00696
                PMC ID: 6641740
                SO-VID: 0eb52f46-62a1-4670-b02e-ef515751c9bc
                Copyright © Copyright © 2017 American Chemical Society
                License:

                This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.

                History
                Date received : 29 May 2017
                Date accepted : 25 July 2017
                Categories
                Subject: Article
                Custom metadata
                document-id-old-9 ao7b00696
                document-id-new-14 ao-2017-00696f
                ccc-price

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