One-step spontaneous growth of NiFe layered double hydroxide at room temperature for seawater oxygen evolution

Highlights

• New method for the fabrication of NiFe LDH under room temperature without assists from electrochemical equipment.

• Highly active seawater oxygen evolution reaction catalyst with extremely low investment.

• Durable seawater electrolyzer withstands high current density of 500 mA/cm² over 100 h.

Abstract

Electrochemical seawater splitting is a promising technique because it addresses two major challenges, clean energy production and seawater desalination, at the same time. Therefore, seeking out a facile and cost-effective way to synthesize highly active and stable seawater-splitting catalysts is of great interest to both the research community and industry. Here we developed an Fe²⁺-driven, one-step, and spontaneous fabrication method for a seawater-oxygen-evolution-active NiFe layered double hydroxide (LDH) at room temperature. The NiFe LDH was found to exhibit very high activity and stability toward the oxygen evolution reaction (OER) in an alkaline natural seawater electrolyte, delivering current densities of 100 and 500 mA/cm² at low overpotentials of 247 and 296 mV, respectively, and with no significant degradation observed over long-term stability testing of 96 h under a large current density of 500 mA/cm² in 1 M KOH seawater electrolyte. After coupling with a good hydrogen evolution reaction (HER) catalyst, NiMoN, the two-electrode electrolyzer was found to achieve current densities of 10, 100, and 500 mA/cm² at voltages of 1.477, 1.533, and 1.665 V, respectively, in alkaline natural seawater with good durability over 100 h at 500 mA/cm². The oxidation of Fe²⁺ is the driving force for the growth of NiFe LDH, and this mechanism is universal to the fabrication of other Fe-based hydroxides as efficient OER catalysts.

Introduction

Hydrogen has been considered a promising energy resource to replace fossil fuel because of its high burning heat, low mass density, and, most importantly, carbon-free emissions. Water electrolysis is a sustainable way to produce hydrogen, but thus far most studies of water electrolysis have focused on fresh water, which is a scarce resource, especially in many arid zones [[1], [2], [3], [4], [5], [6], [7]]. Thus, seawater splitting is a more practical method for hydrogen production given the enormous amount of seawater. In addition, seawater electrolysis can not only generate clean energy, but also boosts the seawater desalination process because the product of hydrogen consumption is high-purity water [8,9]. That is, consuming energy from seawater electrolysis will help to generate fresh water and alleviate its scarcity [[9], [10], [11]].

However, seawater electrolysis is much more challenging than fresh water electrolysis due to the following: i) the presence of Cl⁻ ions in seawater will lead to the chlorine evolution reaction (CER), which is competitive with the oxygen evolution reaction (OER) [12,13]; ii) some impurity ions such as Mg²⁺and Ca²⁺ will generate insoluble precipitates under alkaline conditions, and thus block the active sites of OER and hydrogen evolution reaction (HER) catalysts [14,15]; iii) the unavoidable poisoning effect by the complex organic and inorganic impurities in seawater [16,17]. In order to solve these challenges, great effort has been made to understand the underlying mechanism of seawater electrolysis, and some solutions corresponding to these challenges are listed below: i) Early studies showed that OER is more thermodynamically but less kinetically favorable than CER [12,18]. With a pH value higher than 7, the chemical potential between OER and CER (hypochlorite formation reaction in alkaline conditions) will be maximized, with an oxidation potential difference of about 480 mV [12,18]. That is to say, if an OER catalyst can generate meaningful current densities requiring overpotentials of less than 480 mV, the CER process will be thermodynamically suppressed. ii) The majority of insoluble precipitates can be removed by a simple filtration or centrifugation pretreatment prior to electrolysis. iii) While the poisoning effect by impurities is unavoidable in most situations, it is highly variable for different catalysts [9,11,19]. Thus, researchers can seek out impurity-resistant catalysts in order to minimize the poisoning effect.

NiFe layered double hydroxide (LDH) has been proved to be an efficient and stable OER catalyst under alkaline seawater conditions [9,10,20]. Currently, the most common methods to fabricate NiFe LDH are electrodeposition and hydrothermal techniques [[21], [22], [23], [24], [25]], which require either enormous energy input or elaborate equipment, and thus would introduce a financial burden to large-scale industrial application. Therefore, developing a rapid, facile, and low-energy-consumption method to fabricate NiFe LDH is of great significance for industrial application [26,27]. Recently, our group established an ultrafast room-temperature synthesis of highly active S-doped Ni/Fe (oxy)hydroxides based on the corrosion of nickel foam via Fe³⁺ and accelerated by the additive Na₂S₂O₃. Different from the previous work, here we further developed a one-step room-temperature spontaneous deposition of NiFe LDH nanosheets based on the oxidation of Fe²⁺ ions, which is a process that many electrodeposition researches tried to avoid [28,29]. However, in this research, we utilize the oxidation of Fe²⁺ ions to generate a highly active OER catalyst through a very facile method. Commercial nickel foam (NF) was selected as the substrate and was immersed in a Ni²⁺/Fe²⁺ aqueous solution for several hours (1–5 h). A uniform layer of NiFe LDH nanosheets was subsequently found to have been successfully grown on the NF surface. Further electrochemical measurements showed that the NF/NiFe LDH electrode fabricated by this method exhibits very good OER activity and good stability under different seawater electrolytes. The NF/NiFe LDH was then coupled with a state-of-art HER catalyst, NiMo nitride (NiMoN), to fabricate an outstanding seawater electrolyzer that delivers efficient and robust performance in alkaline seawater solutions.