Elsevier

Materials Letters

Volume 258, 1 January 2020, 126759
Materials Letters

The influence of thermal treatment on electrocatalytic properties of Mn-Co nanofilms on nickel foam toward oxygen evolution reaction activity

https://doi.org/10.1016/j.matlet.2019.126759Get rights and content

Highlights

  • Mn-Co nanofilm is electrodeposited in one-step process on nickel foam.

  • The influence of thermal post-treatment of the electrode toward OER is studied.

  • The highest OER activity is obtained for as-deposited Mn-Co with LDH structure.

  • Post-annealing of the Mn-Co based catalyst results in a lower catalytic activity.

  • The lower activity is associated with the both catalyst and substrate degradation.

Abstract

This work evaluates electrodeposited and differently treated Mn-Co catalysts for their oxygen evolution reaction activity. Catalysts are evaluated in the as-deposited and heat treated state: after 350 °C and 600 °C. Results show that the highest electrochemical activity is obtained for the as-deposited Mn-Co oxyhydroxide, which possibly possess a layered double hydroxide structure. After the heat treatment process, especially after 600 °C, the electrochemical performance decreases considerably.

Introduction

Recently, manganese and/or cobalt based catalysts have become a promising, eco-friendly and low cost electrode materials for oxygen evolution reaction (OER) in water splitting process [1], [2], [3], [4].

Yan et al. fabricated MnO2 on carbon cloth with overpotential of 424 mV at 10 mA·cm−2 [5]. A. Ramirez et al. [3] investigated OER activity of electrochemically deposited manganese oxides such as Mn2O3, Mn3O4 and MnOx on F:SnO2/glass. Lower OER overpotential compared to bare substrates was obtained for different cobalt oxide synthesized on platinum, nickel and iron [6], [7], [8]. The spinels such as ZnxCo3−xO4 on gold [9], ZnCo2O4 on platinum [10] Zn0.3Co2.7O4 on glassy carbon [11] were also developed for OER. In most of the above mentioned studied cases catalysts were thermally post-treated at 400–650 °C in the air in order to obtain subsequent transformation of the catalyst into the crystalline spinel oxide phase [2].

Because in the literature, the post-treatment of Mn-Co based electrodes for OER is still intensively performed, in this work, the influence of thermal treatment on structural and catalytic properties of Mn-Co nanofilms has been studied. The work clearly shows that annealing the sample at higher temperatures results in much lower OER activity compared to as-deposited film.

Section snippets

Experimental

Mn-Co based films were electrochemically synthesized at −1.1 V vs. Ag/AgCl in aqueous (DI, >15 MΩ) solution of 2 mM Mn(NO3)2·4H2O and 8 mM Co(NO3)2·6H2O for 200 mC at 25 °C (reagents purchased from Sigma Aldrich). Some of the as-deposited samples were additionally annealed at 350 °C or 600 °C for 2 h in a tube furnace.

For microstructure analysis, scanning (SEM) and transmission (TEM) electron microscopes were used. FEI Quanta FEG 250 and FEI Titan G2-300 were used, respectively. X-ray powder

Results and discussion

XRD spectra of the evaluated catalysts are presented in Fig. 1. At room temperature, after exposure to KOH, the crystalline phase Co(OH)2 (card JCPDS-ICDD #74-1057) with a possible Mn addition (as determined by TEM/EDS) is detected (characteristic peaks at ~19.1° and 38.2°), which vanishes after higher temperature exposure, where the spinel (Mn,Co)3O4 structure forms. Peaks, though with relatively low intensity, fit well to the postulated formation of the cubic spinel structure. The intensity

Conclusions

Electrodeposited Mn-Co layers with an LDH/(Mn,Co)(OH)2 structure have high surface area and show high electrochemical performance: for j = 10 mA·cm−2 low overpotential of 335 mV was obtained.

The results show that post-annealing of the Mn-Co based catalyst at temperatures higher than 350 °C results in a lower OER catalytic activity compared to the as-deposited material, which could be seen as a higher overpotential, onset potential or Tafel slope for OER. The film consisted of mixed Mn/Co LDH

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the “Nanocrystalline ceramic materials for efficient electrochemical energy conversion” project carried out within the First TEAM programme of the Foundation for Polish Science (grant agreement POIR.04.04.00-00-42E9/17-00). K.C. acknowledges the Foundation for Polish Science START stipend.

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