In this work, we investigated post synthesis treatment approaches for improving catalytic activity of α-MnO2 nanowires in OER. More specifically, acid leaching and cobalt doping were applied to modify α-MnO2 nanowires synthesized by hydrothermally treating a KMnO4 and NH3Cl solution at 150oC for 50 hours. During the acid leaching process, samples were dispersed in concentrated nitric acid for 72 hours under vigorous mixing. Such treatment is believed to produce vacancies in potassium, manganese, and oxygen crystallographic sites (Figure 1.a).[3,4] Cobalt ions were introduced into the crystal structure via hydrothermal treatment of 20 mg of α-MnO2 nanowires in 5 ml of Co(NO3)2 melt at elevated temperatures (T= 80-100oC, Figure 1.a). Electrochemical activity of Co-doped α-MnO2 nanowires (α-CoxMnO2 x=0.02;0.03;0.05), was evaluated in comparison with the electrochemical activity of pristine material (Figure 1.b). In addition, the effect of acid leaching and cobalt doping in conjunction was examined for α-MnO2 nanowires containing 0.02 mol % of cobalt (α-Co0.02MnO2, AL-α-Co0.02MnO2) and compared to that of α-MnO2 and AL-α-MnO2 nanowires (Figure 1.c).
Synthesized materials were tested as OER catalysts using a Pine Research Instrumentation modulated speed rotator (MSP). Tests were performed at 1600 RPM in 0.1 M KOH electrolyte over a voltage window of 1 to 1.9 V vs RHE. Active electrodes were fabricated via ink casting of a 1:1 active material: carbon black ink prepared in ethanol with a Nafion® binding agent and mass loaded to have 0.05 mg of active material. Acid leaching for 72 hours resulted in the increase of electrocatalytic activity up to 26.6 mA/cm2 compared to that of pristine α-MnO2 nanowires (21.6 mA/cm2, Figure 1.c). For Co-doped samples, catalytic performance improved linearly with the increase of the cobalt content. The highest OER performance of 48.7 mA/cm2 was exhibited by α-Co0.05MnO2, which is a 125% increase as compared to the activity of pristine α-MnO2 (Figure 1.b). The defect rich sample (AL-α-Co0.02MnO2) that underwent both post synthesis treatments showed an increased activity when compared to materials individually modified by acid leaching or Co-doping (Figure 1.c). For example, cobalt doped 72-hour acid leached samples exhibited current densities of 35.0 mA/cm2, 20% higher than doped samples of the same chemical composition (29.4 mA/cm2).
The increased catalytic activity of the acid-leached α-MnO2 was attributed to newly formed oxygen vacancies and a change in oxidation state of manganese. In manganese oxides, an average manganese oxidation state of +3.5 has been shown to result in the highest catalytic activity . Acid leaching is believed to reduce the average oxidation state of manganese in α-MnO2. Co-doping potentially could further favorably affect the oxidation state of manganese while simultaneously creating additional active site for the oxygen evolution reaction leading to more efficient electrochemical water splitting. Acid leaching and cobalt doping both separately and in tandem were observed to have positive impacts on the OER catalytic activity of environmentally friendly and cost-efficient α-MnO2 catalysts while also suggesting that they could be applied to other catalytically active tunnel manganese oxides.
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- S.Y. Lee MS thesis. University of Freiburg, 2015.