Alkaline water electrolysis

Alkaline water electrolyzers aren’t new; they’re one of the oldest types of electrolyzers still in use. However, their role in producing green hydrogen (H₂) has thrust them into the spotlight as a cornerstone technology for a sustainable future. Their reliability, well-understood technology, and the ability to operate at large scales make them a practical choice for kickstarting the hydrogen economy.

How alkaline water electrolysis produces hydrogen

An alkaline water electrolyzer consists of two electrodes—an anode and a cathode—that are submerged in the electrolyte, which is typically potassium hydroxide (KOH) or sodium hydroxide (NaOH). These electrodes are separated by a permeable diaphragm or separator. The separator allows the passage of ions and prevents the mixing of hydrogen and oxygen produced.

How alkaline water electrolysis produces hydrogen
Alkaline water electrolysis produces hydrogen.

When a direct current (DC) is applied across the electrodes, water at the cathode side is reduced to hydrogen gas and hydroxide ions (OH⁻) vis the half reaction:

2H₂O + 2e⁻ → H₂ + 2OH⁻

The hydroxide ions then migrate through the electrolyte to the anode side, where they are oxidized to produce oxygen gas (O₂) and water, according to the half reaction:

4OH⁻ → O₂ + 2H₂O + 4e⁻

The overall reaction facilitates the separation of hydrogen and oxygen from water:

2H₂O → 2H₂ + O₂

Alkaline electrolyzers typically operate at temperatures between 70-100 ºC. The temperature is mainly limited by the structural stability of the diaphragm. Therefore, the alkaline electrolyzer is generally perceived as a low-temperature electrolysis technology.

The operating pressure is usually less than 40 bar. The operation current density ranges from 0.2 to 1.2 A/cm², which influences the rate of hydrogen production. At a lower current density, the alkaline water electrolyzer has a higher efficiency.

Alkaline water electrolysis cell components

The diagram below illustrates the components of an alkaline water electrolyzer.

The components of alkaline water electrolyzer cell
The components of alkaline water electrolyzer cell.
The components of an alkaline water electrolyzer cell (3D view).
The components of an alkaline water electrolyzer cell (3D view).
  • Flow field plate

Flow field plates have patterned channels which help distribute gasses and liquids. They are typically made from materials like coated stainless steel.

  • Electrolyte:

The electrolyte is a liquid solution containing 25-40 wt% KOH or NaOH. It facilitates the movement of ions between the electrodes during electrolysis.

  • Electrodes:

The electrodes are typically made from a porous, conductive gas diffusion layer (GDL) coated with nickel (Ni), cobalt (Co) or other non-noble metals. Catalysts increase the efficiency of the hydrogen and oxygen evolution reactions at the cathode and anode, respectively. The alkaline environment is less corrosive than acidic environments. This enables the use of cheaper materials in alkaline water electrolyzers compared to those required for PEM electrolyzers.

  • Diaphragm/separator:

A thin porous material, often made from materials like asbestos, polyphenylene sulfide, or Zirfon, separates the electrodes. It prevents the mixing of hydrogen and oxygen gasses while allowing the passage of hydroxide ions.

The diagram below depicts the working mechanism of an alkaline water electrolyzer cell.

The working mechanism of an alkaline water electrolyzer cell
The working mechanism of an alkaline water electrolyzer cell.

Alkaline solution is fed to the electrolyzer cell. When a direct current is applied to the cell, the alkaline electrolyte is first reduced on the cathode side. This process produces 1 mol of hydrogen gas and 2 mol of hydroxyl ions.

The generated hydrogen gas is separated from the surface of the cathode. The remaining hydroxyl ions are transported to the anode side through a porous separator under the influence of the circuit between the two electrodes.

At the anode, hydroxyl ions are oxidized. This process yields 0.5 mol of oxygen gas and 1 mol of water.

Alkaline water electrolysis tack

An industrial alkaline water electrolyzer is a cell stack. The cell stack is composed of multiple individual cells arranged in either a monopolar or bipolar configuration. Each cell contains two electrodes (an anode and a cathode) separated by a diaphragm or separator that allows the passage of hydroxide ions while preventing the mixing of the produced gasses. Channels or pipes that distribute the electrolyte to each cell and collect gasses.

  • Monopolar Configuration

In a monopolar alkaline electrolysis stack, each cell is connected in parallel. This means that the voltage across each pair of electrodes is equal to the total cell voltage, and the sum of the cell currents equals the total stack current.

In this setup, the same electrochemical reaction occurs on both sides of each electrode, which can be either the hydrogen evolution reaction or the oxygen evolution reaction, depending on the polarity of the electrodes.

  • Bipolar Configuration

In contrast, a bipolar stack has each cell connected in series. This configuration allows for a higher voltage across the stack, as the voltages of individual cells add up. Each cell in a bipolar stack is connected to an inlet and outlet manifold, which can be done through pipes or internal channels for the distribution of the electrolyte.

Alkaline water electrolyzer with bipolar configuration
Alkaline water electrolyzer with bipolar configuration.

The stacks come in various sizes and configurations, with the number of cells ranging from small stacks with a single cell to large stacks with up to 60 cells or more, tailored to meet specific hydrogen production needs.

The operation conditions of an electrolyzer stack depends on the stack size, for example:

  1. Current and voltage Ranges

The electrolyzer stack operates within a specific current range (e.g., 60-100 A for a 20-cell stack and 6-10 A for a 10-cell stack) and voltage range (e.g., 32-40 V for a 20-cell stack and 17-20 V for a 10-cell stack), which are essential parameters for its operation.

  1. Operating temperature range

These stacks are designed to operate within a certain temperature range, typically between 10-80 ºC for a 20-cell stack and 15-70 ºC for a 10-cell stack, to ensure optimal performance and longevity.

  1. Hydrogen and oxygen flow rates

The stacks have specified flow rate ranges for hydrogen (e.g., 0-14 L/min for a 20-cell stack and 700 mL/min for a 10-cell stack) and oxygen (e.g., 0-7 L/min for a 20-cell stack and 350 mL/min for a 10-cell stack), which indicate the amount of gas produced.

  1. Lifetime

The estimated operational lifetime for these stacks is around 6,000 hours, with nickel-based electrodes having a longer life cycle of over 10,000 hours.

Alkaline water electrolysis operates with renewable energy sources

An inherent limitation of alkaline water electrolyzers is not imposed by the reaction kinetics of water splitting under alkaline conditions when it comes to managing intermittent energy sources. Instead, their responsiveness to rapid load changes is largely determined by the process and power control systems. As a result, contemporary alkaline water electrolyzers are capable of functioning with intermittent energy sources by virtue of the strategic control of the process, the routing of gas and liquid streams, and the suitable choice of power electronics.

However, when alkaline water electrolyzers operate at low current densities, their ability to handle the fluctuations of renewable electricity is challenging.

In an alkaline water electrolyzer, oxygen and hydrogen cross over when the pressure of the hydrogen in the cathode chamber is higher than the pressure of the oxygen in the anode chamber. When the amount of hydrogen in the oxygen stream (or oxygen in the hydrogen stream) goes over 4 vol%, the mixtures are explosive. So, a water electrolyzer needs to be shut down safely when the amount of hydrogen in the oxygen stream (or the other way around) goes over 4 vol%.

When alkaline water electrolyzers operate at low current density, the efficiency of water electrolysis is high. In other words, more gasses are made. The gas diffusive flux through the separator, on the other hand, doesn’t depend much on the current density. Because of this, the amount of hydrogen in the anode chamber and the amount of oxygen in the cathode chamber rises when the current density drops. This leads to safety problems and a safety shutdown.

How alkaline water electrolysis produces hydrogen at $1/kg H2

Technical objectives have been established by the U.S. Department of Energy (DOE) for alkaline water electrolysis. In order to accomplish the goal of reducing hydrogen production costs to $1/kg H2 by 2031, the alkaline water electrolyzer stack performance is designed to achieve values of 2.0 A/cm² at 1.7 V/cell and 74% electrical efficiency (lower heating value, LHV). The attainment of these objectives would significantly augment the capacity of forthcoming alkaline water electrolyzer stacks to generate hydrogen.

Technical targets for alkaline water electrolyzer stacks
Technical targets for alkaline water electrolyzer stacks (from DOE).

To improve the performance of alkaline water electrolyzers and reduce the cost of hydrogen production, several strategies can be employed:

  • Enhancing efficiency and consistency

Addressing inefficiencies and inconsistencies at low loads is crucial, especially when AWEs are powered by fluctuating renewable energy sources (RESs). A multi-mode self-optimization electrolysis converting strategy has been proposed to double the maximum efficiency compared to conventional DC power supply setups, increasing efficiency from 29.27% to 53.21% under certain conditions.

  • Optimizing operating parameters

The performance of AWEs can be influenced by temperature, pressure, and diaphragm thickness. Adjusting these parameters can lead to improved electrolyzer performance.

  • Advanced materials and design

The development of novel electrocatalysts, improved diaphragm materials, and cell designs can enhance the efficiency of AWEs. For example, the use of nickel-based electrodes and innovative diaphragm materials can improve the overall efficiency and durability of the system.

  • Thermal management

Proper thermal management can reduce the energy required for electrolysis. For instance, increasing the temperature of the electrolyzer can lower the voltage required for water electrolysis, thus reducing energy consumption.

  • Electrolyte concentration

The concentration of the electrolyte, such as potassium hydroxide, affects the electrical conductivity and the overall performance of the electrolyzer. Optimizing the concentration can lead to better performance and efficiency.

  • Flow dynamics

Understanding and optimizing gas–liquid flow dynamics within the electrolyzer can lead to more uniform flow patterns and improved efficiency.

  • Cost reduction strategies

Policies and strategies to drive innovation and cut costs for electrolyzers are essential. This includes scaling up manufacturing to achieve economies of scale, which can significantly reduce the capital expenditure (CAPEX) and operational expenditure (OPEX) of AWEs.

  • Techno economic analysis

Performing a techno economic analysis to estimate the cost of hydrogen production, including CAPEX, OPEX, and the potential for cost reduction through improved technology and increased scale, is important for guiding research and development efforts.

  • Prognostics and health management

Implementing strategies for predicting and mitigating potential failures can enhance the durability and operational reliability of AWEs, reducing maintenance costs and downtime.

By focusing on these areas, the performance of alkaline water electrolyzers can be significantly improved, leading to a reduction in the cost of hydrogen production and making green hydrogen a more competitive energy carrier.

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