Alkaline Water Electrolysis Technologies

 Alkaline water electrolysis is a mature industrial hydrogen production technology with commercial applications reaching the multi-megawatt range worldwide. The phenomenon of alkaline electrolysis of water was first proposed by Troostwijk and Diemann in 1789. After several developments, in 1939, the first industrial large-scale alkaline water electrolyzer plant was put into operation with a capacity of 10,000 Nm3 H2/h (already reaching a scale of approximately 50MW).

In the late 19th century, more than 400 industrial alkaline electrolyzer units were successfully installed and operating for industrial applications. However, alkaline water electrolyzers operate at lower temperatures (30-80◦C) with concentrated alkaline solutions (5M KOH/NaOH). Furthermore, in alkaline water electrolysis cells, nickel (Ni)-coated stainless steel electrodes and asbestos/PPS/composite ZrO2-based separators are used as separators. The ionic charge carrier is hydroxide ion (OH -), KOH/NaOH and water permeate through the porous structure of the separator, providing the function of electrochemical reaction. Alkaline water electrolysis is a system design suitable for large-scale applications. At present, the investment cost of foreign alkaline water electrolysis is 500-1,000 US dollars/kW, and the system life is 90,000 hours. However, the main challenge associated with alkaline electrolysis is the limited current density (0.1-0.5 A/cm2) due to modest OH-mobility and the use of corrosive (KOH) electrolytes. Due to the high sensitivity of the KOH electrolyte to surrounding CO2 and the subsequent formation of K2CO3 salts, the number of hydroxyl ions and ionic conductivity decrease. In addition, the salt of K2CO3 closes the pores of the anode gas diffusion layer, thereby reducing the transferability of ions through the separator, thereby reducing the hydrogen production.

In addition to this, alkaline water electrolysis produces low purity (99.9%) gases (hydrogen and oxygen) because existing separators cannot completely prevent the crossover of gases from one half-cell to the other.

1. Working principle of ALK

Alkaline water electrolysis is an electrochemical water splitting technology under the action of electricity. Electrochemical water splitting consists of two independent half-cell reactions, namely the cathodic hydrogen evolution reaction (HER) and the anode oxygen evolution reaction (OER). In the alkaline electrolysis process, 2Mol of alkaline solution is first reduced on the cathode side to generate 1Mol of hydrogen (H2) and 2Mol of hydroxyl ions (OH-). The generated H2 can be separated from the cathode surface, and the remaining hydroxyl ions (OH-) are It is transferred to the anode side through a porous separator under the influence of the circuit between anode and cathode. At the anode, hydroxyl ions (OH-) are released, generating 1/2 molecule of oxygen (O2) and 1 molecule of water (H2O), as shown in Figure 1 below.

                                Figure 1: Schematic diagram of alkaline electrolyzed water

2. Components of ALK electrolyzed water

The main components of the alkaline electrolytic cell are the diaphragm/separator, current collector (gas diffusion layer), separation plate (bipolar plate) and end plate. Generally speaking, asbestos/zirconium/nickel coated porous stainless steel separators are used as separators and separators for alkaline electrolyzed water. Nickel mesh/nickel foam is used as the gas diffusion layer, and stainless steel/nickel-plated stainless steel separators are used as bipolar plates and end plates respectively. See Figure 2 below:

                                                    Figure 2: Schematic diagram of Cell structural components

3. Research and development of ALK

Alkaline water electrolysis is a mature and well-established technology that can reach multi-megawatt levels. It has been successfully deployed and used in industrial applications by many manufacturers around the world. There are so many commercial alkaline water electrolyzers and their manufacturers that they are too numerous to mention. However, the technology still needs improvements, such as increasing current density and reducing gas crossover. To achieve these challenges, new electrode materials and separators need to be developed.

In addition, alkaline water electrolyzers can be combined with renewable energy sources (solar, wind), which is more beneficial for reducing capital expenditure costs. In this direction, some research institutions/organizations are still actively working on improving efficiency and reducing hydrogen production costs.

For example, [email protected] heterojunction hybrid structure has been reported as a low-cost, noble metal-free bifunctional electrocatalyst for OER and HER in alkaline media. The developed [email protected] electrocatalyst has a significantly reduced HER overpotential of 104 mV at 10 mA/cm2, and an OER overpotential of 182 mV at 10 mA/cm2, which has the highest HER and OER activities. In addition, under the condition of continuous operation for 15 hours, the battery voltage of the whole water splitting was low, only 1.86 V, and showed excellent stability. This improvement is mainly due to the strong integration between ultrathin MoS2 nanosheets and non-chemically measured Ni0.96S nanocrystals, exposing more active sites, as well as the presence of abundant heterojunctions in [email protected] interface. Others (2021) developed an efficient bifunctional nanoheterostructure electrocatalyst NiCo-NiCoO2@Cu2O@CF for overall water splitting and studied its performance in 1M KOH solution. The prepared NiCo-NiCoO2@Cu2O@CF electrocatalyst showed good electrochemical performance for HER and OER in alkaline medium, with overpotentials of 133 and 327 mV, respectively, and current density of 10 mA/cm2, for HER and OER. Tafel slopes are 119 and 118 mV/dec respectively.

Furthermore, their performance in the single-cell electrolysis process was experimentally studied using NiCo-NiCoO2@Cu2O@CF as a bifunctional electrocatalyst for HER and OER. The results show that the electrochemical performance of this catalyst is superior to other non-noble metal electrocatalysts, with the cell voltage reaching 1.69 V and the current density reaching ≥10 mA/cm2. In addition, due to the strong mechanical adhesion between NiCo-NiCoO2 nanoparticles, the as-prepared electrocatalyst remains stable when continuously electrolyzed in a strong alkaline solution for 12 h. Since the fine NiCo-NiCoO2 nanoheterostructure is evenly distributed on the oxidized surface of the copper foam, it maximizes the use of active sites in the electrochemical reaction, thereby achieving better performance. An efficient bifunctional electrocatalyst was also developed for the overall water splitting of two-dimensional metal-organic framework (MOF)-derived NiCoP nanosheets (NiCo(nf)-P) in alkaline media. The performance of the prepared electrocatalyst in 1 M KOH electrolyte solution at a potential cycling rate of 2 mV/s was studied by linear sweep voltammetry (LSV). At a current density of 100 mV/cm2, NiCo(nf)-P showed good electrocatalytic activity for HER and OER, with initial potentials of 37 mV and 1.435 V, which were lower than commercial 5% Pt/C (43 mV) and IrO2 (1.504 V).

In addition, the overpotential of NiCo(nf)-P is 119 mV and 315 mV respectively, which are lower than commercial 5% Pt/C (291 mV) and IrO2 (400mV), and reaches a current density of 100 mV/cm2. The Tafel slopes of NiCo(nf)-P (112 mV/dec) and 66 mV/dec) may also be lower than commercial 5% Pt/C (164 mV/dec) and IrO2 (88 mV/dec), suggesting that NiCo( Rapid charge transfer of nf)-P at the electrocatalytic interface.

In addition, the stability of the NiCo(nf)-P electrocatalyst for HER and OER at current densities of 100, 500, and 1000 mA/cm2 was measured using time potential method (V-t). The results obtained show that the electrochemical performance is stable when running continuously for 30 hours, as shown in Figure 3 below:

      Figure 3: Graphical illustration of electrocatalytic activity of synthesized LDH(NS), NiCo(NS)-P, MOF(nf), NiCo(nf)-P, NiCo(NR)-P and 5% Pt/C and IrO2 electrocatalysts: (a) HER LSV curve; (b) HER overpotential at different current densities of 100, 500 and 1000 mA/cm2; (c) HER Tafel slope; (d) OER LSV curves at different current densities of 100, 500 and 1000 mA/cm2; (e) OER overpotential; (f) OER Tafel slope; (g, h) HER and OER stability study of NiCo(nf)-P electrocatalyst at different current densities of 100, 500 and 1000 mA/cm2 (V-t curve); (i) LSV curves of NiCo(nf)-P and NiCo(nf)-P; commercial IrO2 and 5% Pt/C for bulk water splitting. The inset is the V-t curve of NiCo(nf)-P at current densities of 100, 500 and 1000 mA cm2.

In addition, the HER and OER performance of the developed NiCo(nf)P electrocatalyst was also studied in an alkaline electrolyzer. At an operating current density of 1 A/cm2, its cell voltage was 1.94 V. It showed excellent performance electrocatalytic activity.

Another study synthesized n-doped carbon supported Ni-Mo-O/Ni4Mo nanointerface electrocatalyst (Ni-MoO/Ni4Mo@NC) through electrodeposition-calcination-electrodeposition technology, and studied its performance in alkaline solution ( Electrocatalytic activity for hydrogen evolution reaction in 1M KOH).

At a geometric current density of 10 mA/cm2, the synthesized Ni-Mo-O/Ni4Mo@NC electrocatalyst has high electrocatalytic activity, with an overpotential of 61 mV, which is 50% lower than Ni-Mo-O (120 mV). %, which is due to the introduction of N-doped carbon layer. In addition, the study of Ni-Mo-O/Ni4Mo@NC under neutral conditions (1M PBS solution) showed that the overpotential was < 60 mV, which is lower than the overpotential of Ni-Mo-O (~ 100 mV). The Tafel slope of Ni-Mo-O/Ni4Mo@NC is 99 mV/dec, which is relatively lower than 135 mV/dec of Ni-Mo-O. There are also literatures introducing the multi-heterostructure interface and three-dimensional porous structure of Co2P/N@Ti3C2Tx@NF as efficient HER electrocatalysts in alkaline media. First, CPN@TC with multiple heterostructure interfaces was prepared on the surface of MXene (Ti3C2Tx) modified NF using a two-step electrodeposition method and subsequent nitriding process. The performance of the prepared Co2P/N@Ti3C2Tx@NF electrocatalyst in 1M KOH solution was studied. The results showed excellent HER performance, with an overpotential of only 15 mV at 10 mA/cm2 and a small Tafel slope of 30 mV/dec, as shown in Figure 4 below:

                                   

Furthermore, it showed excellent stability in a CV test of 3000 cycles and could move only 20 mV at a current density of 100 mA/cm2. In addition, density functional theory (DFT) was used to calculate the water dissociation energy (ΔG*H2O) and hydrogen adsorption energy (ΔG*H) of different materials. According to DFT calculations, the prepared CPN@TC has the strongest binding energy of - 0.822 eV, which is beneficial to the HER reaction. The prepared CPN@TC has an H* adsorption energy closest to zero and is generally considered to be the best electrocatalyst.