Mixed Transition Metal Oxide Supported on Nitrogen Doped Carbon Nanotubes – a Simple Bifunctional Electrocatalyst Studied with Scanning Electrochemical Microscopy

Abstract

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...

Local probe investigation of electrocatalytic activity

“…185 Nevertheless, many advancements have been made in this direction by utilizing SECM. For instance, Seiffarth et al 186 12, and the collection efficiency was assumed to be 100%, considering that the variation of the tip-to-sample distance in a range of 10-15-35 mm did not signicantly affect the ORR and H 2 O 2 currents. 53 The current densities of ORR and H 2 O 2 oxidation are presented in Fig.…”

“…This difference in values exists supposedly because the measurement of current in SECM takes place at the UME, while in linear sweep voltammetry (LSV) measurement it takes place at the catalyst layer. 186 By adopting a similar methodology to SECM, and hence sequential pulses for obtaining H 2 O 2 and ORR currents and SG/ TC mode for OER current, Chen et al 56 have studied nickel and cobalt-based oxides (Ni x O y and Co x O y respectively) embedded in nitrogen doped carbon (NC) for their bifunctional electroactivity. Normalized SECM images were obtained to illustrate oxygen reduction, oxygen evolution and hydrogen peroxide production.…”

“…Nevertheless, many advancements have been made in this direction by utilizing SECM. For instance, Seiffarth et al 186 have synthesized a catalyst by combining an oxide Ni 0.9 Co 0.1 Fe 2 O 4 with nitrogen doped carbon nanotubes (NCNTs) and investigated its electrocatalytic activity by utilizing a loading of only 20 μg cm −2 . The applied pulse profile introduced in 2007 ( ref.…”

“…They all show roughly a four-electron transfer, while the slight decrease in the activity of the heated combination was associated with the reduction of FeNiCo oxides. 186 …”

Stable and Efficient Nitrogen‐Containing Carbon‐Based Electrocatalysts for Reactions in Energy‐Conversion Systems

“…Recently,e merging investigationso ns ynthesis strategies for Ncontaining carbon-based electrocatalysts decorated with TM components have been reported, including electrocatalysts based on N-containing carbon nanoribbons, [51b, 100,166] nanosheets, [60,66,72,98,106,[125][126][127]167] aerogels, [77,168] spheres, [73, 134f,i, 169] and other structures. [134l, 141, 170] Wang and co-workers use carbon black to construct ac omposite structure to support PtRu alloys.…”