Tremendous attention is currently being paid to renewable sources of energy. Transition-metal dichalcogenides (TMDs) have been intensively studied for their promising catalytic activities in the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR). In this fundamental work, we explored the catalytic properties of TMD family members 2H TaS2 and 1T TaS2 . Our findings reveal that both polytypes exhibit poor HER performance, which is even more pronounced after electrochemical reduction/oxidation. Our experimental data show that 1T TaS2 has a lower overpotential at a current density of -10 mA cm-1 , despite theoretical DFT calculations that indicated that the more favorable free energy of hydrogen adsorption should make "perfect" 2H TaS2 a better HER catalyst. Thorough characterization showed that the higher conductivity of 1T TaS2 and a slightly higher surface oxidation of 2H TaS2 explains this discrepancy. Moreover, changes in the catalytic activity after electrochemical treatment are addressed here. For the ORR in an alkaline environment, the electrochemical treatment led to an improvement in catalytic properties. With onset potentials similar to that of Pt/C catalysts, TaS2 was found to be an efficient catalyst for the ORR, rather than for proton reduction, in contrast to the behavior of Group 6 layered TMDs.

Department of Control Engineering Faculty of Electrical Engineering Czech Technical University Prague Technicka 2 Prague 6 166 27 Czech Republic

Department of Inorganic Chemistry University of Chemistry and Technology Prague Technická 5 166 28 Prague 6 Czech Republic

Department of Physical Chemistry Regional Centre of Advanced Technologies and Materials Palacký university Olomouc tř 17 Listopadu 1192 12 771 46 Olomouc Czech Republic

Division of Chemistry and Biological Chemistry School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 Singapore

Engineering Science Faculty of Engineering and the Environment University of Southampton Southampton SO17 1BJ UK

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