Evaluation of electrodeposited α-Mn 2 O 3 as a catalyst for the oxygen evolution reaction

TLDR: In this paper, Mechanically stable α-Mn2O3 electrodes are prepared by annealing of galvanostatically deposited MnOOHx layers on F:SnO2-coated glass.

About: This article is published in Catalysis Today.The article was published on 2017-07-15 and is currently open access. It has received 63 citations till now. The article focuses on the topics: Overpotential & Electrocatalyst.

Figures

Fig. 4. a) Polarization curves of annealed and pre-oxidized (E = 1.45 V vs. RHE for 30 min) -Mn2O3 films using different amounts of deposited charge Q. All measurements were performed with a scan rate of 5 mVs-1 in 1 M KOH. The current densities are corrected for capacitive charging currents and iR losses. b) Corresponding Tafel plots of the polarization curves shown in a).

Fig. 2. Differential capacitance Cd of the annealed films as a function of charge Q used to deposit the MnOOHx films. The upper axis shows the estimated mass of the α-Mn2O3-electrodes (see text). The right axis depicts the estimated electrochemically active surface area (ECSA).

Fig. 6. J/(ECSA) of the of the -Mn2O3 layers vs. the deposited charge Q using the specific capacitance as determined in this work (Cs = 0.19 ± 0.08 mFcm-2).

Fig. 5. Current density J [mAcm-2] vs. differential capacitance Cd [mFcm-2] of the -Mn2O3 layers: Behavior of the electrodes at an overpotential = 280 mV deposited a) with charges Q 50 mCcm-2 and b) with charges 400 mCcm-2. Corresponding curves for an overpotential = 350 mV are shown in c) and d). The dashed lines are straight lines to guide the eye.

Fig. 1. Scanning electron micrographs of a) pristine FTO-glass substrate. b) and c) are showing α-Mn2O3 particles grown on top of the F:SnO2 crystallites (edge length up to 50nm). In the micrographs d) – f) porous αMn2O3 films consisting of small particle sizes are shown. g) and h) Cross section micrographs of a layer deposited after a charge flow of 400 mCcm-2. This highly porous layer of ~ 1 µm thickness consists of particles of about 20 nm in diameter.

Figure 7. Galvanostatic stability measurements of rotated Mn2O3 electrodes deposited with different loadings and recorded at a current density of 10 mAcm-2 in 1 M KOH. Best stabilities were obtained with layers deposited with Q ≥ 40 mCcm-2.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Evaluation of electrodeposited α-mn2o3 as a catalyst for the oxygen evolution reaction" ?

In this paper, annealing of galvanostatically deposited MnOOHx layers on F: SnO2-coated glass was used to achieve a current density of 10 mAcm-2 at an overpotential of 360 mV in 1 M KOH. 

Q2. What is the promising approach to store renewable energies in large quantities?

The generation of hydrogen by solar-driven electrochemical water splitting is a promising approach to store renewable energies as a non-fossil fuel in large quantities. 

Q3. What is the morphology of the annealed films?

As soon as the deposition charge Q exceeds 10 mCcm-2, the structure of the annealed films changes and shows homogeneous and highly porous layers now fully covering the substrate. 

Q4. How can these catalysts be used in advanced electrolyzers?

These catalysts can be used in advanced electrolyzers, for example photoelectrochemical water splitting devices, by being deposited as co-catalysts on the surface or at the back contact of suitable photoelectrodes. 

Q5. What is the average oxidation state of MnOOHx?

Since the average oxidation state of manganese in the asdeposited amorphous MnOOHx-films is not known at this stage, a direct conversion of the consumed charge into the deposited mass of Mn2O3 is not possible. 

Q6. How is the average oxidation state of the MnOOHx?

Based on the mass deposited and the charge consumed during electrodeposition, the average oxidation state in the as-deposited amorphous MnOOHx is estimated to be about 3.7. 

Q7. How was the differential capacitance of the -Mn2O3 films determined?

The differential capacitance Cd of the α-Mn2O3 films was determined by fast potential sweep curves in a potential range from 1.2 – 1.3 V vs. RHE, where the Faradaic currents are negligible. 

Q8. What is the reason for the deviation from the extrapolated line?

At very low electrode loadings (< 0.3 mFcm-2) also a deviation from the extrapolated line is evident as the current density increases only very moderately with capacitance (Fig 5a and 5c). 

Q9. How much is needed to generate a current density of 10 mAcm2?

Despite the modest intrinsic OER activity of -Mn2O3, a surprisingly low overpotential of 340 mV is needed to generate a current density of 10 mAcm2. 

Q10. What is the specific capacitance of -Mn2O3?

Combined with the measured capacitance Cd = (0.37 ± 0.03) mFcm-2, the specific capacitance Cs of α-Mn2O3 is calculated to be CS(α-Mn2O3) = (0.19 ± 0.08) mFcm-2. 

Q11. How can the authors calculate the differential capacitance of Mn oxide films?

The differential capacitance can be calculated from the slope of the current density vs. scan rate (Fig. S4b) using the following equation [21,22]:= ∙ Eq. (1)Fig. 2 (left y-axis) shows the differential capacitance Cd of all annealed films, determined at a potential of 1.25 V vs. RHE, as a function of the charge Q used to deposit the films. 

Q12. Why is the shape of the curve similar to the one of the specific activity?

Due to the nearly linearrelationship of the mass and the ECSA of the films, the shape of the curve is similar to the one of the specific activity (see Fig. 5). 

Q13. How was the stability of the catalyst evaluated?

The stability of the catalysts was evaluated by monitoring the change in overpotential needed to keep the current density constant at 10 mA cm-2 after two hours of operation. 

Q14. What can be done to determine the ECSA of different catalysts?

turnover frequencies and specific activities can be calculated from the estimated electrochemical active surface area (ECSA) ofthe electrodes enabling a meaningful comparison of different catalysts. 

Q15. What is the ohmic drop in the deposited layer?

If this were not the case, the internal ohmic drop within the deposited layer would lead to a continuously increasing deposition potential in order to maintain the fixed current density of 0.25 mAcm-2. 

Q16. How many Mn2O3 electrodes were weighed after annealing?

nine Mn2O3electrodes with a deposited charge of 400 mCcm-2 were weighed after annealing and the mass of the FTO/glass substrate was subtracted. 

Q17. How are the Tafel plots of different thicknesses determined?

In order to compare them, Tafel slopes of the electrodes of different thicknesses were determined in the current density range 0.1 - 1 mAcm-2, since in this range the Tafel plots are almost perfectly linear. 

Q18. What was the overpotential of the -Mn2O3 films?

The overpotential η for the OER was determined using the equation = − 1.23 .Structural and Morphological Characterization: XRD patterns of the α-Mn2O3 films were obtained with a Bruker D8 Advance diffractometer with a CuKα (λ = 1.5406 Å) x-ray tube in Bragg−Brentano geometry and a Sol-X energy dispersive detector. 

Q19. What is the possible explanation for the behavior of the particles after annealing?

A possible explanation for this behavior is the change of the morphology after annealing as a function of the deposited amount of MnOOHx (see Figs. 1a-f).