One-step spontaneous growth of NiFe layered double hydroxide at room temperature for seawater oxygen evolution
Graphical abstract
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.
Section snippets
Chemicals
Iron (II) sulfate heptahydrate (FeSO4·7H2O, ≥99%, Sigma-Aldrich), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O, ≥98%, Alfa Aesar), nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O, ≥97%, Sigma-Aldrich), cobalt (II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥98%, Sigma-Aldrich), ammonium fluoride (NH4F, 96%, Alfa Aesar), sodium chloride (NaCl, Fisher Chemical), potassium hydroxide (KOH, 50% w/v, Alfa Aesar), urea (CO(NH2)2, Promega), ethanol (C2H5OH, Decon Labs, Inc.), and hydrochloric acid (HCl,
Results and discussion
The spontaneous growth of NiFe LDH at room temperature is schematically illustrated in Fig S1. In general, a piece of NF was immersed in a solution of 0.15 M Ni(NO3)2·6H2O and 0.15 M FeSO4·7H2O for several hours at room temperature and NiFe LDH was spontaneously grown on the NF surface based on the mechanism shown in Fig S1, in which the oxidation of Fe2+ is the main driving force for the spontaneous growth. Optical images of samples prepared with different amounts of immersion time are
Conclusions
In summary, we have developed a cost-effective way to fabricate a very efficient OER catalyst, NiFe LDH, at room temperature. The NF/NiFe LDH catalyst was found to exhibit high activity and good stability towards OER in both alkaline fresh water and alkaline natural seawater. After coupling it with a NF/NiMoN cathode, we obtained an outstanding seawater electrolyzer, NF/NiFe LDH||NF/NiMoN, which displayed excellent activity for alkaline natural seawater splitting, as well as good durability and
Credit author statement
Minghui Ning: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Writing – review & editing, Libo Wu: Formal analysis, Resources, Fanghao Zhang: Formal analysis, Resources, Dezhi Wang: Resources, Shaowei Song: Formal analysis, Tian Tong: Resources, Jiming Bao: Resources, Shuo Chen: Formal analysis, Luo Yu: Formal analysis, Resources, Writing – review & editing, Supervision, Zhifeng Ren: Writing – review & editing,
Declaration of competing interest
There are no conflicts of interest to declare.
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