The oxygen evolution reaction (OER), the first step of the two-step electrolysis process used to split water into hydrogen and oxygen, is plagued by poor kinetics and requires catalysts to limit energy loss and improve efficiency. Precious metals and their alloys (Au, Pt, Ir) exhibit excellent catalytic behavior, but high material costs and concerns about the environmental impact caused by their production have led to intense investigation of period four transition metal oxides (manganese, iron, cobalt, and nickel oxides) as catalyst for OER.^[1] In addition to their natural abundance and high electrochemical activity, manganese oxides represent a wide library of chemical compositions and morphologies that are stable in aqueous solutions allowing them to be utilized as OER catalysis. Tunnel manganese oxides are a family of manganese oxides composed of MnO₆ octahedra that form tunnels around stabilizing ions and water molecules. α-MnO₂ nanowires with square structural tunnels formed by two MnO₆ octahedra on each side and potassium ions inside the tunnels were shown to have superior performance among various structurally diverse manganese oxide. ^[2]Additionally, high aspect ratio nanowire morphologies with diameters less than 100 nm and lengths on the order of microns, exhibiting high surface area, are attractive for catalytic applications.

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 150^oC 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= 80-100^oC, Figure 1.a). Electrochemical activity of Co-doped α-MnO₂ nanowires (α-Co_xMnO₂ 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-Co_0.02MnO₂, AL-α-Co_0.02MnO₂) 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-Co_0.05MnO₂, which is a 125% increase as compared to the activity of pristine α-MnO₂ (Figure 1.b). The defect rich sample (AL-α-Co_0.02MnO₂) 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 ^[5]. 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.

1. Yi Cheng et al. Progress in Natural Science: Materials International. 25, 6. 2015 545-553.
2. Frey, Carolin E et al. Chemistry – A European Journal21 (42) 2015 14958-68.
3. Yuan, YF, et al. NATURE COMMUNICATIONS, vol. 7, 2016, 13374
4. Lee, Jin-Hyon, et al. Energy & Environmental Science, 5, 11, 2012, 9558-9565
5. S.Y. Lee MS thesis. University of Freiburg, 2015.