Seawater Direct Electrolysis Hydrogen Production Technology

Seawater Direct Electrolysis Hydrogen Production Technology – Cheersonic

The seawater electrolysis reaction includes two half-reactions, the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER).

The theoretical minimum voltage to simultaneously drive OER and HER is 1.23 V, however, additional potential is required to deactivate and overcome the original reaction energy barrier, i.e., the overpotential (η), during practical electrolysis.

Therefore, reducing the overpotential and energy consumption of water electrolysis as much as possible is the key to the development of electrolytic hydrogen production, and adding catalysts can reduce the overpotential and improve the reaction rate.

In the process of seawater electrolysis for hydrogen production, for HER, various dissolved cations (Na⁺, Mg²⁺, Ca²⁺, etc.), bacteria/microbes, and small particles and other impurities are present in natural seawater.

These impurities may produce Mg(OH)₂, Ca(OH)₂ precipitates covering the active sites of the catalyst with the seawater electrolysis process, thereby poisoning the catalyst and inactivating it.

For anodes, OER is a complex four-electron proton transfer reaction with slow reaction kinetics and higher overpotentials.

The chlorine evolution reaction (ClER) and the formation of hypochlorite caused by the high concentration of chloride ions in seawater are both two-electron reactions. Compared with the OER reaction, the reaction kinetics are faster, so it will interfere with and compete with the OER. , thereby reducing the conversion efficiency.

Therefore, developing seawater electrolysis catalysts with high activity and selectivity is crucial to avoid the influence of ions and impurities in seawater. In terms of hydrogen production from seawater electrolysis at home and abroad, the current research mainly focuses on HER catalysts, OER catalysts, bifunctional catalysts and electrolysis systems.

1. Cathodic reaction

For cathodic hydrogen evolution reaction (HER), the most challenging problem in direct seawater splitting is the presence of various dissolved cations (Na⁺, Mg²⁺, Ca²⁺, etc.), bacteria/microbes and impurities such as small particles in natural seawater.

These impurities can block the electrodes as the seawater electrolysis process progresses, thereby poisoning or accelerating the aging of the electrodes/catalysts in the electrolysis system, resulting in poor durability.

Specifically, with the increase of the electrolytic current density, the local pH of the electrode surface changes sharply, which may lead to the formation of Ca(OH)₂ and Mg(OH)₂ precipitates and block the cathode active sites.

To solve this problem, current seawater electrolysis systems require a buffer solution or additives to stabilize pH fluctuations. In addition to this, other strategies such as designing suitable electrolyzers and separators have the potential to overcome this challenge.

Furthermore, depending on the applied electrolysis potential, competing reactions involving metal ions (such as Na⁺, Cu²⁺, Pb²⁺) at the cathode may also occur during seawater electrolysis. Therefore, suppressing these electrochemical processes is crucial for the design of HER electrocatalysts in seawater.

In this regard, the use of suitable membranes to separate catalysts from metal ions in seawater, the development of catalysts with corrosion resistance or selectivity, or the use of permselective barriers such as those attached to the catalysts, etc. Potential solutions for long-term stability of catalysts in seawater.

Platinum group metals are considered as benchmark electrocatalysts for HER, showing the best performance under acidic, basic and neutral conditions. However, during seawater electrolysis, its HER performance is far from that in freshwater electrolytes.

In addition, the scarcity and high cost of precious metals greatly hinder their large-scale applications. Therefore, in practical applications, it is crucial to reduce the use of platinum while maintaining high activity.

Yang Fengning et al. prepared a Pt/Ni-Mo hydrogen evolution catalyst by a two-step method, simulating seawater and industrial conditions at an overpotential of 113mV, and it can run stably in alkaline solution for more than 140 hours in brine (1M KOH+0.5 M NaCl) Reaching a current density of 2000 mA/cm², the best performance to date, enables large-area fabrication of 700 cm².

In addition to noble metal catalysts, exploring cheap, efficient and stable electrocatalytic materials is an important direction for seawater electrolysis to produce hydrogen. The catalytic activity of transition metals is considered to be second only to Pt group metals and inexpensive, among which Ni is considered to be one of the most promising catalysts.

Some researchers have prepared Ni-based alloy catalysts Ti/NiM (M=Co, Cu, Au, Pt), which show remarkable activity in HER, but the new Ni-based catalysts also suffer from insufficient stability, which is the potential barriers to adoption.

In addition, non-noble metal HER catalysts also include transition metal oxides and hydroxides, transition metal nitrides (TMNs), transition metal phosphides (TMPs), transition metal chalcogenides, transition metal carbides, transition metal hybrids, etc.

TMPs are used for seawater HER due to their abundant content, high activity and good stability. Lv Qingliang et al. reported a porous PF-NiCoP/NF hydrogen evolution catalyst with high activity and persistence in natural seawater and a current density of 10 mA/cm² at 287mV overpotential, superior to commercial Pt /C (20 wt%), its research believes that the three-dimensional morphology, hole structure and conductive substrate improve the specific surface area, electron transfer and active sites, which are conducive to H₂ release.

2. Anode reaction

For the anode, the presence of a large amount of electrochemically active anions (such as Cl-) in seawater can interfere with and compete with the anode OER.

Therefore, the development of electrocatalysts with high selectivity for OER is crucial to avoid the formation of ClER and hypochlorite during the direct electrolysis of seawater.

Several other inefficiencies exist in existing methods for producing hydrogen from brine. In particular, standard electrolysis in an unbuffered solution results in the production of oxygen (below ~2.25V) in saline.

However, chlorine is produced above ~2.26V. Any chlorine produced at the anode is immediately hydrolyzed, which also produces H⁺. As the acidity of the anode increases, chlorine compounds are preferentially oxidized at the anode to form chlorine gas, Cl₂, which is a corrosive substance. Cl₂ also reacts with water to form hypochlorous acid (HOCl).

During this process, the increased acidity of the solution can corrode the electrode material, require it to be replaced, and render the solution toxic, necessitating the disposal of hazardous chemicals.

For a long time, the electrocatalysts with high oxygen evolution activity are usually noble metal catalysts such as IrO₂ and RuO₂. However, the rarity of these two elements determines the necessity of developing high-activity transition-family OER catalysts with abundant reserves.

Since the complex four-electron transfer process of OER is characterized by slow reaction kinetics, in order to cope with the challenge of competition between ClER and OER, three main strategies have been proposed for the selective seawater electrolysis of OER, namely basic design principle, OER selectivity Site catalyst and Cl-barrier.

The basic design principle is mainly based on thermodynamic and kinetic considerations, which can maximize the thermodynamic potential difference between OER and ClER, thereby ensuring high selectivity to OER. Transition metal oxides and hydroxides have effective active sites in alkaline water due to the introduction of oxygen vacancies, thus exhibiting good electrocatalytic activity for OER.

In addition, the selectivity of OER can be improved by doping Mo, Co, Fe, Ni, Mn or increasing active sites. Yun Kuang et al. grew nickel sulfide (NiSx) on nickel foam, and then electrodeposited a layer of NiFe-LDH layered double metal hydroxide on nickel sulfide to form a multi-layer electrode structure.

Among them, nickel foam acts as a conductor, NiFe-LDH acts as a catalyst, and the intermediate nickel sulfide will evolve into a negative charge layer, which repels chloride ions in seawater due to electrostatic repulsion, thereby protecting the anode.

Because of this multilayer design, the anode can operate for over 1000 hours at industrial electrolysis current densities (0.4-1A/cm²). However, there are still many engineering details to be studied in this research, and scale-up experiments are needed to achieve scale and industrialization.

3. Dual function catalyst

Designing bifunctional electrolysis catalysts for HER and OER with high activity and durability remains challenging. Although different types of bifunctional water electrolysis catalysts exist in alkaline media, such as metal chalcogenides, nitrides, oxides and phosphides that can make necessary changes in electronic properties and morphology, among them direct electrolysis in seawater still very few.

In 2020, Wu Libo et al. prepared the bimetallic heterophosphide Ni₂P-Fe₂P by the three-step synthesis method of “in situ growth-ion exchange-phosphating”, which is a kind of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). ) bifunctional catalyst, which realizes the efficient and stable hydrogen production of seawater, the current density of the total water splitting system can reach 500mA/cm² at a voltage of 2.004V, and it can run stably for more than 38 hours.

4. Electrolysis system

From an application perspective, in addition to developing stable and efficient catalysts, it is also necessary to design suitable high-performance, low-cost seawater electrolyzers.

At present, two low temperature (<100℃) electrolyzers, alkaline water electrolyzer (AWE) and proton exchange membrane water electrolyzer (PEMWE), are relatively mature in the commercial market;

In addition, there are two emerging technologies, low temperature anion exchange membrane water electrolysis cell (AEMWE) and high temperature water electrolysis cell (HTWE). °C).

When these electrolyzers are used directly to electrolyze seawater, the complex natural composition of seawater affects the electrolysis. Among them, the main problems are physical or chemical clogging of ion exchange membranes and corrosion of metal components, such as Na⁺, Mg²⁺ and Ca²⁺ ions in seawater will reduce the performance of HTWE and PEMWE proton exchange membranes;

Anions such as Cl-, Br-, SO4²- in turn adversely affect the membrane properties of AEMWE, AWE and HTWE. Therefore, developing stable membranes is an important challenge for the direct electrolysis of seawater.

The study believes that the simple filtration of natural seawater by ultrafiltration and microfiltration can largely solve the physical blockage caused by solid impurities, sediments and microorganisms.

LiuZhao et al. tried seawater electrolysis to produce hydrogen at high temperature based on solid oxide electrolysis technology. Without using noble metal catalysts, long-term constant current electrolysis was carried out at a current density of 200 mA/cm² for 420 h, and the hydrogen production rate was 183 mL. /min.

On the premise of not recovering high-temperature exhaust gas, its energy conversion efficiency can be as high as 72.47%. In addition, since the seawater is heated and evaporated first, most of the impurities in the seawater are not in contact with the electrolytic cell, so it is difficult to cause damage to the electrolytic cell, so it has a good application prospect.

Article source: Energy Planet

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.