Atomic-Scale CoOx Species in Metal-Organic Frameworks for Oxygen Evolution Reaction
Summary (2 min read)
1. Introduction
- The electrochemical oxygen evolution reaction (OER) has been regarded as the core process in metal-air batteries and water splitting devices.[1-2].
- Fortunately, the formation of Co oxide species are beneficial for electrocatalyzing OER.
- The O2 plasma treatment on ZIF-67 leads to the on-site formation of atomic-scale CoOx species in MOFs with high surface area.
- For the O2 plasma treatment, the authors applied the RF power of 200 W, and the pressure was controlled at 120 Pa, and the treating time was 1 h.
- X-ray photoelectron spectroscopic (XPS) measurements were carried out on an AXIS ULTRA (Kratos Analytical).
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- The Faradaic efficiency (ε) was determined by collecting the ring current when fixing the disk current at 200 μA and ring potential at 0.4 V vs. RHE in N2-saturated 1 M KOH solution.
- Id is the disk current, Ir is the ring current, and N is the current collection efficiency (0.21 in this study) which was determined by IrO2 catalyst thin film electrode.
- The electrochemical surface area (ECSA) was evaluated by measuring the double layer capacitance method via CVs at different scan rate from 20 to 100 mV s-1 in the range of no Faradaic processes occurred.
- The ECSA could be calculated from the double layer capacitance according to: ECSA= Cdl Cs Where.
- The turnover frequency (TOF) was evaluated by the following equation [15]: TOF= J×A 4×m×F.
3. Results and Discussion
- As illustrated in Figure 1a, O2-plasma was applied to treat ZIF-67.
- As shown in Figure S3, the absorption band at 425 and 1580 cm-1 are assigned to the stretching vibration of Co-N and C=N, respectively.
- Notably, the intensity of this small feature is higher in ZIF-67 than CoOx-ZIF which indicates the higher oxidized states of Co in ZIF-67 than in CoOx-ZIF.
- Thus, the lower intensity observed in CoOx-ZIF indicates more symmetrical atomic structures around Co after plasma treatment.
- It is well known that the poor conductivity of ZIF-67 hinders their electrocatalytic applications.
OER.
- The Tafel plots were also collected to investigate the OER kinetics in Figure 4c.
- The CoOx-ZIF displays a smaller Tafel slope (70.3 mV dec -1) than pristine ZIF-67 (108.8 mV dec1), which demonstrates the intrinsic reason for CoOx-ZIF owning better OER activity than pristine ZIF-67.
- To study the reaction mechanism of OER, the authors used rotating ring-disk electrode (RRDE) and collected the ring current by fixing the ring potential at 1.5 V vs. RHE in 1 M KOH solution at 1600 rpm.
- The presence of Co3O4 provides active sites to catalyze OER, thus shows better OER performance than the pristine.
4. Conclusion
- In summary, the authors have successfully obtained the atomic-scale CoOx species in the MOFs through a simple but efficient plasma treatment.
- The atomic-scale CoOx species provide rich active sites for OER, demonstrating highly efficient electrocatalytic activity, which is even better than RuO2.
- The unique atomic-scale dispersed structure of MOFs provides excellent precursors for the on-site formation of atomic-scale catalyst species for OER.
- The abundant pores in the ZIF-67 provide channels for O2 plasma to activate the atomic Co ions in MOFs to on-site produce atomic-scale CoOx species.
- Furthermore, the remained large surface area and etched surface of ZIFs ensures excellent mass transport during OER.
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Frequently Asked Questions (15)
Q2. What is the role of O2 plasma in the electrocatalytic process?
O2 plasma is a powerful tool to modify materials for its etching effect, and the metal atom exposed in the O2 atmosphere would be inevitable oxidized.
Q3. What is the role of the pores in ZIF-67?
The abundant pores in ZIF-67 provide channels for O2 plasma to activate the Co ions in MOFs to on-site produce atomic-scale CoOx species, which act as the active sites to catalyze the oxygen evolution reaction with an even better activity than RuO2.
Q4. What was the effect of the plasma treatment on the CoOx species?
During the plasma treatment, the Co-N coordination bonds in the ZIFs were partially broken and the suspended Co species could beeasily reacted with O2 present in the system to obtain CoOx species locally.
Q5. What is the reason why the ZIF-67 was overtreated?
While the RF power reaches up to 300 W, poor OER performance obtained probably due to that the ZIF-67 wasovertreated and the porous structure of ZIF-67 was broken down.
Q6. What is the effect of plasma etching on the Co-N bond?
The destructive effect of plasma etching leads to the break of the Co-N coordination bond and the suspended Co species was rapidly oxidized in the presence of O2 to produce CoOx.
Q7. Why did the surface area of ZIF-67 decrease after plasma treatment?
Thedecrease of surface area after plasma treatment is because the plasma treatment partiallydestroyed the porous structure of ZIF-67 by the etching effect.
Q8. What was the method used for the electrochemical measurement of CoOx-ZIF?
Electrochemical measurement4 mg of CoOx-ZIF was dispersed in 2 mL ethanol followed by ultrasonication for 30 min, 100 μL 5 % Nafion solution was added to the dispersion and ultrasonication for another 30min to obtain the catalytic ink.
Q9. How did the authors fix the ring current?
To study the reaction mechanism of OER, the authors used rotating ring-disk electrode (RRDE) andcollected the ring current by fixing the ring potential at 1.5 V vs. RHE in 1 M KOH solutionat 1600 rpm.
Q10. What is the main absorption peak of CoOx-ZIF?
The main absorption peak (indicated by thevertical bar) of CoOx-ZIF in Figure 3b is enhanced, indicating CoOx-ZIF loses some charges at Co site and thus increases the oxidation state, shifting the absorption peak as well as theabsorption edge to higher energy.
Q11. What is the effect of the O2 plasma treatment on CoOx-ZIF?
After O2 plasma treatment, the Co 2p 3/2 peak of CoOx-ZIF shows a slight broadening and shifting to lower binding energy (Figure S4b).
Q12. What is the effect of oxygen on the co-Ox-ZIF?
Co sites more significantly than nitrogen and therefore giverise to higher unoccupied orbitals at Co sites in CoOx-ZIF (Figure 3b).
Q13. What is the ohm resistance of CoOx-ZIF?
The fitted electrochemical impedance spectroscopy (EIS) indicates that the ohm resistanceof CoOx-ZIF (6.31 Ω) is smaller than that of ZIF-67 (7.60 Ω), which is consistent with the EPR results, indicating the improved conductivity of CoOx-ZIF.
Q14. What are the main problems of OER electrocatalysts?
To solve this problem, many studies have been carried out to develop highly efficient andlow-cost OER electrocatalysts, such as developing transition metal compound and even metal-free materials[4].
Q15. What is the effect of O2 plasma on CoOx-ZIF?
These results confirm that Co-N coordination bonds in ZIF-67 were partially broken and the Co species was oxidized to form CoOx species by O2 plasma.