*3.4. Detection of H2O<sup>2</sup>*

In a typical experiment, 1 mL hydrogen peroxide (50–600 µM) was added to the 1mLcolourimetric reagent, i.e., 10 mg phenol, 10 mg of 4-aminoantipyrine, 50 mg of RuO<sup>2</sup> nanoparticles, dissolved in 20 mL of 100 mM acetic acid buffer (pH 5.6) [49]. The test tubes containing different concentrations of H2O<sup>2</sup> and blank, i.e., without H2O<sup>2</sup> were incubated in a water-bath for 10 min at 30 ◦C, and the progress of the reaction was monitored by spectrophotometer at 505 nm.

#### **4. Conclusions**

We have successfully synthesized the ultrafine ruthenium oxide nanoparticles via a simple co-precipitation method at 300 ◦C. The application of synthesized nanoparticles was successfully studied, and it was concluded that RuO<sup>2</sup> nanoparticles are an effective bifunctional and stable material for OER and ORR reactions in the air, N<sup>2</sup> and O<sup>2</sup> atmosphere. Also, RuO<sup>2</sup> nanoparticles were used as sensors for the detection of H2O<sup>2</sup> in a solution. The RuO<sup>2</sup> nanoparticles have a comparable limit of detection and linear dynamic values ranging from 600 to 10 µM of H2O2. The economically viable

as-synthesized nanoparticles could be used as an active nonenzymatic electrochemical sensor for the selective detection of H2O2. Further, these nanoparticles showed efficient electrocatalytic activity with low energy loss. Tafel slopes were found to be very low and the electrode material was stable as established by CA studies. Therefore, the ruthenium oxide nanoparticles consumed less energy during the water redox reaction (OER and ORR) and proved to be a better electrode material for OER/ORR reactions, showing its excellent potential for further applications in the future.

**Author Contributions:** R.P. is responsible for the experimental reactions, basic characterization and first draft of the manuscript, M.P. did the detection of H2O<sup>2</sup> measurements, J.A. did the electrochemical measurement and its discussion, M.S. was the co-supervisor of first author and wrote the discussion part for the detection of H2O<sup>2</sup> measurements, S.M.A. helps in the editing and electrochemical measurements, N.A. designed the electrochemical set for the water redox reaction and did the primary measurements, M.A.M.K.helps in the discussion and measurements electrochemical studies and T.A. is responsible for supervision, creation ideas, infrastructure, editing and final draft of the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the scheme (SPARC/2018-2019/P843/SL) of MHRD, Government of India and King Saud University Research Project (RSP-2020/29).

**Acknowledgments:** T.A. thanks the MHRD-SPARC scheme of the Government of India for financial support. R.P. especially thanks to UGC, New Delhi, for the Senior Research Fellowship. Authors also acknowledge the measurement support provided through the DST PURSE program at CIF, Jamia Millia Islamia and AIIMS, New Delhi for electron microscopic studies. The authors extend their sincere appreciation to the Researchers Supporting Project at King Saud University for funding this Research.

**Conflicts of Interest:** The authors declare that there is no conflict of interest.
