Nickel(II) hydroxide is a well-known material for the oxygen evolution reaction (OER) in alkaline media, particularly when iron is incorporated into its lattice. Moderate heat treatment of nickel(II) hydroxide (≤700 °C) leads to the formation of nickel(II) oxide (nano)particles, which exhibit reduced OER activity the higher the heat treatment temperature was. In this work, we report that heat treatment of nickel(II) hydroxide in air at even higher temperatures (60 min at 900 °C) results in an oxide material with high OER activity superior to that of the nickel(II) hydroxide. Similarly, the stability of the nickel(II) oxide under electrochemical conditions is increased compared to nickel(II) hydroxide. Electrochemical in situ Raman measurements show the formation of surface nickel oxy-hydroxides (NiOOH) at positive potentials and are significantly affected by the initial heat treatment. From XPS, Raman, and XRD results, it is concluded that a Ni 3+ -enriched phase, possibly a higher-valent mixed nickel oxide, is present at the surface of the nickel(II) oxide sample treated at 900 °C resulting in an increased OER activity compared to NiOOH. This basic understanding of high-temperature-treated nickel oxide may contribute to resolving the present stability issues of OER electrocatalysts and may help to leverage alkaline electrolysis as important key technology for a renewable energy supply.

We present a spectroelectrochemical setup, in which Raman microscopy is combined with scanning electrochemical microscopy (SECM) in order to provide both spectroscopic and electrochemical information on the very same location of an electrode at the same time. The setup is applied to a subject of high academic and practical interest, namely, the oxygen evolution reaction at Ni and Ni/Fe electrodes. It comprises a transparent substrate electrode, onto which Ni and Ni/Fe thin films are deposited. An ultramicroelectrode (UME) is placed closely above the substrate to obtain electrochemical information, while a Raman microscope probes the same sample spot from below. To obtain information on oxygen evolution activity and structural changes, increasingly positive potentials from 0.1 up to 0.7 V vs Hg|HgO|1 M KOH were applied to the Ni/Fe-electrodes in 0.1 M KOH solution. Evolved oxygen is detected by reduction at a Pt UME, allowing for the determination of onset potentials, while the substrate current, which is recorded in parallel, is due to both overlapping oxygen evolution and the oxidation of Ni(OH) to NiOOH. An optimum of 15% Fe in Ni/Fe films with respect to oxygen evolution activity was determined. At the same time, the potential-dependent formation of γ-NiOOH characterized by the Raman double band at 475 and 557 cm allows for the conclusion that a certain amount of disorder introduced by Fe atoms is necessary to obtain high oxygen evolution reaction (OER) activity.

Dedicated to Prof.D r. Wolfgang Schuhmann on the occasiono fh is 60th birthday. 1IntroductionThei ncreasing availability of fluctuating renewable energy like wind or solar power calls for efficient means of energy storage in times of excess power combined with the ability to provide the stored energy in times of shortage.E lectrochemical energy conversion, i.e.t he conversion of electrical energy into storable chemical energy and vice versa,e merges as am ajor contribution to solve this challenge with water splitting into hydrogen and oxygen by electrolysis and theirr econversion into water and electricale nergy in fuel cells as ap romising technique.C ombining both electrolysis and fuel cell in one single device,aso called reversible fuel cell, would be highly desirable.F rom an electrochemical point of view, major challenges are both the oxygen evolution reaction (OER)a nd the oxygen reduction reaction (ORR). Similar to metal-air-batteries [1],i ti sac onsiderablec hallenge to find materials whichc atalyze both reactionse fficiently, which however would be aprerequisite for high-efficiency reversible fuel cells.Among the various materials investigated towards this application,s pinel transition metalo xides were shown to be promising catalysts for both OER and ORR [2][3][4][5].Because of their large variety,s pinel type materials offer opportunities for synthesis and optimization towards active electrocatalysts.Another material extensively investigated towards electrocatalytic applications are carbon nanotubes (CNTs), whichi s, amongsto therr easons,d ue to their high surface area and high electrical conductivity.I t has frequently been shown that nitrogen dopingo ft he CNTs to formN CNTse nhances their activity towards the ORR, which is attributed to alterationsi nt he electronic structure and enhancedb asicity [6][7][8].N CNTsa re com-Abstract:Afacile way to prepare ab ifunctional electrocatalyst, which is active for both the oxygen evolution reaction (OER) and the oxygen reduction reaction( ORR) in alkaline media, is presented.ANi 0.9 Co 0.1 Fe 2 O 4 mixed oxide,w hich is synthesized usingapolyol mediateds ynthesis route,i sc ombinedw ith nitrogen doped carbon nanotubes (NCNT) known as electrically conducting material with high activity for the ORR. Thec ombined bifunctional catalystw as investigated towards its structural properties using TEM, XRD,X PS and Raman spectroscopy and studied withr elevante lectrochemical characterization techniques,n amely rotating disk electrode measurements for ORR and linears weepv oltammetry for OER as well as with scanning electrochemical microscopy (SECM). Thed ifference between onsetp otentials for OER and for ORR was similar to that of other published bifunctional catalysts,i ndicatingt hat the simple synthetic approachy ields materials well suited for bifunctional catalysis applications.F or ORR investigations the redox competition mode of SECM indicated high selectivity of the materials towards 4-electron reduction of oxygen to water. Them ixed material showed an enhan...

No abstract