Sukomol Barua

Electrocatalytic seawater splitting is an ideal strategy for the large-scale production of green hydrogen. Compared to scarce freshwater, oceanic seawater electrolysis represents a game-changer for the hydrogen economy. Herein, we report a cost-effective one-step synthesis of binder-free, self-supported 3D nickel–manganese–cobalt (NiMnCo) coatings on titanium (Ti) substrates and evaluated their electrocatalytic performance for the hydrogen evolution reactions (HERs) in alkaline media (1.0 M KOH), simulated seawater (SSW, 1.0 M KOH + 0.5 M NaCl) and alkaline natural seawater (ASW, 1.0 M KOH + natural seawater). These ternary coatings were electrodeposited on Ti substrates using an electrochemical deposition method via a dynamic hydrogen bubble template (DHBT) technique. The optimized ternary NiMnCo/Ti-2 electrocatalyst exhibited an enhanced HER activity in both alkaline and seawater media, achieving an ultra-low overpotential of 29, 59 and 66 mV to reach the benchmark current density of 10 mA cm⁻² in SSW, ASW and 1.0 M KOH, respectively. This efficient 3D ternary NiMnCo/Ti-2 electrocatalyst demonstrated stable long-term performance at a constant potential of −0.23 V (vs. RHE) and a constant current density of 10 mA cm⁻² for 50 h without any significant degradation. Furthermore, it exhibited long-term stability in alkaline electrolyte and simulated seawater during multi-step chronopotentiometric testing at variable current densities from 20 mA cm⁻² to 100 mA cm⁻² for 18 h. This superior performance can be attributed to its unique intermetallic structure and multi-component composition, which provides good Cl⁻ resistance, electrochemical stability and synergistic effects among its constituents. Therefore, the optimized NiMnCo/Ti-2 electrocatalyst is a promising candidate for practical seawater electrolysis aiming at green hydrogen production.

The perfect strategy for the generation of green and renewable hydrogen (H₂) fuels is the direct electrocatalytic splitting of plentiful seawater rather than scarce freshwater. One of the half-reactions taking place during the electrocatalytic splitting of seawater is oxygen evolution (OER). However, the OER is affected by slow four-electron transfer kinetics as well as competitive chlorine evolution reactions (CERs) in seawater. To overcome the kinematic and competitive barriers of seawater splitting and achieve an excellent overall performance of seawater splitting, we herein report a facile, low-cost, one-step fabrication procedure of 3D structured nickel–manganese (NiMn) coatings using a dynamic hydrogen bubble template (DHBT) technique. The electrocatalytic activities of the thus synthesized catalytic materials for OER in simulated seawater (0.5 M NaCl + 1 M KOH, denoted as SSW) and alkaline natural seawater (natural seawater + 1 M KOH, denoted as ASW) were investigated using linear sweep voltammetry (LSV) at varying temperatures from 25 to 75 °C. Scanning electron microscopy (SEM) and inductively coupled plasma–optical emission spectroscopy (ICP–OES) were used to examine the surface morphology and composition of the prepared catalysts. It was found that the prepared NiMn/Ti-1 catalyst in a plating bath containing a molar ratio of 1:1 Ni²⁺:Mn²⁺ and having the lowest Mn loading of 13.43 µg cm⁻² exhibited quite reasonable activity for OER in Cl⁻ ion rich SSW and ASW. To achieve the benchmark current density of 10 mA cm⁻² in SSW and ASW, the NiMn/Ti-1 electrocatalyst requires overpotentials of 386 and 388 mV, respectively. In addition, this optimal bimetallic electrocatalyst also demonstrated superior long-run stability at 1.81 V (vs. RHE) and 10 mA cm⁻² for 24 h in both working electrolytes. Impressively, the two-electrode electrolyzer—NiMn/Ti-5₍₋₎||NiMn/Ti-1₍₊₎—needs only 1.619 V to deliver 10 mA cm⁻² current density for overall alkaline seawater electrolysis, which is even 0.075 V lower than the noble metal-based electrolyzer (Pt₍₋₎||NiMn/Ti-1₍₊₎).

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 H₂SO₄) media. The catalysts have been deposited on a titanium substrate (1 × 1 cm²) 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 Ni²⁺/Mn²⁺ 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 Ni²⁺/Mn²⁺ 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.

In this study, three-dimensional gold-nickel-molybdenum (Au(NiMo)) catalysts have been studied as catalysts for the oxygen evolution reaction (OER). The catalysts have been deposited on a titanium surface using electroplating and galvanic displacement techniques. The modification of NiMo with a low amount of Au crystallites in a range of 1.2– 5.2 μgAucm–2 results in enhanced activity for OER in the alkaline medium compared to respective binary un-modified NiMo catalysts. The current densities for OER increase ca.1.2–7.3 and 1.3–5.1 times with an increase in temperature from 25 up to 75°C using the prepared 3D binary NiMo/ Ti and Au(NiMo)/Ti catalysts, respectively.

In this study, NiMo catalysts that have different metal loadings in the range of ca. 28–106 µg cm⁻² were electrodeposited on the Ti substrate followed by their decoration with a very low amount of Au-crystallites in the range of ca. 1–5 µg cm⁻² using the galvanic displacement method. The catalytic performance for hydrogen evolution reaction (HER) was evaluated on the NiMo/Ti and Au(NiMo)/Ti catalysts in an alkaline medium. It was found that among the investigated NiMo/Ti and Au(NiMo)/Ti catalysts, the Au(NiMo)/Ti-3 catalyst with the Au loading of 5.2 µg cm⁻² gives the lowest overpotential of 252 mV for the HER to reach a current density of 10 mA·cm⁻². The current densities for HER increase ca. 1.1–2.7 and ca. 1.1–2.2 times on the NiMo/Ti and Au(NiMo)/Ti catalysts, respectively, at −0.424 V, with an increase in temperature from 25 °C to 75 °C.