Citation: Barua, S.; Balˇ ci ¯ unait ˙ e, A.; Vaiˇ ci ¯ unien ˙ e, J.; Tamašauskait˙ e-Tamaši ¯ unait ˙ e, L.; Norkus, E. Bimetallic 3D Nickel-Manganese/Titanium Bifunctional Electrocatalysts for Efficient Hydrogen and Oxygen Evolution Reaction in Alkaline and Acidic Media. Coatings 2023, 13, 1102. https://doi.org/10.3390/ coatings13061102 Academic Editor: Rosalba Passalacqua Received: 15 May 2023 Revised: 9 June 2023 Accepted: 13 June 2023 Published: 15 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). coatings Article Bimetallic 3D Nickel-Manganese/Titanium Bifunctional Electrocatalysts for Efficient Hydrogen and Oxygen Evolution Reaction in Alkaline and Acidic Media Sukomol Barua , Aldona Balˇ ci ¯ unait˙ e *, J ¯ urate Vaiˇ ci ¯ unien˙ e, Loreta Tamašauskait˙ e-Tamaši ¯ unait˙ e and Eugenijus Norkus Department of Catalysis, Center for Physical Sciences and Technology (FTMC), LT-10257 Vilnius, Lithuania; sukomol.barua@ftmc.lt (S.B.); jurate.vaiciuniene@ftmc.lt (J.V.); loreta.tamasauskaite@ftmc.lt (L.T.-T.); eugenijus.norkus@ftmc.lt (E.N.) * Correspondence: aldona.balciunaite@ftmc.lt Abstract: In this work, 3D nickel-manganese (NiMn) bimetallic coatings have been studied as electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline (1.0 M KOH) media and the HER in acidic (0.5 M H 2 SO 4 ) media. The catalysts have been deposited on a titanium substrate (1 × 1 cm 2 ) using low-cost and facile electrochemical deposition method through a dynamic hydrogen bubble template technique. The electrocatalytic performance of these fabricated catalysts was investigated by using Linear Sweep Voltammetry (LSV) for HER and OER at different temperatures ranging from 25 up to 75 ◦ C and also was characterized by scanning electron microscopy (SEM) and inductively coupled plasma optical emission spectroscopy (ICP-OES). It was found that fabricated NiMn/Ti-5 electrocatalyst with Ni 2+ /Mn 2+ molar ratio of 1:5 exhibits excellent HER activity in alkaline media with overpotential of 127.1 mV to reach current density of 10 mA cm −2 . On the contrary, NiMn/Ti-1 electrocatalyst that fabricated with Ni 2+ /Mn 2+ molar proportion of 1:1 and lowest Mn-loading of 13.43 μgcm −2 demonstrates exceptional OER activity with minimum overpotential of 356.3 mV to reach current density of 10 mA cm −2 . The current densities increase ca. 1.8–2.2 times with an increase in temperature from 25 ◦ C to 75 ◦ C for both HER and OER investigation. Both catalysts also have exhibited excellent long-term stability for 10 h at constant potentials as well as constant current density of 10 mA cm −2 that assure their robustness and higher durability regarding alkaline water splitting. Keywords: nickel; manganese; bifunctional electrocatalyst; electrodeposition; hydrogen evolution reaction; oxygen evolution reaction 1. Introduction Over the last decade, a substantial research focusing on uninterrupted supply of re- newable and clean energy has become a key issue due to alarming environmental threat and rapid depletion of fossil fuels [1–4]. In order to find potential substitutes, hydro- gen is considered the most promising alternative to fossil fuels because of its advantages of zero carbon emissions, high gravimetric energy density (140 MJ·Kg −1 ), and high effi- ciency [5–10]. Comparing with major methods for industrial hydrogen production e.g., coal gasification and steam methane reforming, the electrocatalytic water splitting in large-scale can also be considered as the most prospecting method [11]. This is not only due to the low conversion efficiency of methane and coal steam into H 2 and CO 2 , and their consequences of carbon-emissions and global climate warming but also the advantageous feature of high purity industrial-level H 2 production from abundant natural resource with free-carbon emission and sustainability. This promising method of green H 2 production can also be the most convenient way to store the intermittent renewable energy like solar and wind energy by converting the electricity into H 2 fuels [12]. Coatings 2023, 13, 1102. https://doi.org/10.3390/coatings13061102 https://www.mdpi.com/journal/coatings