GREEN HYDROGEN: TECHNOLOGY EVALUATION CRITERIA FOR ELECTROLYSER SELECTION

1 Introduction

Green Hydrogen is one of the core elements of the global strategy to shift to low Carbon, sustainable sources of energy. Hydrogen is unique among fuels, as it produces only Water upon combustion. Further, the Water in exhaust gases from Hydrogen combustion can be recycled by condensation in most industrial and domestic settings. Green Hydrogen is produced by the electrolysis of Water using electricity from renewable  sources. The entire energy production cycle using Green Hydrogen, is therefore cyclic, sustainable and has no Carbon Dioxide emissions. This concept is illustrated in Figure 1.

The momentum of investment in green Hydrogen projects has accelerated in the past few years. In the year 2020, the global installed capacity of Water electrolysis for Hydrogen production was around 300 MW and increased to 500 MW by the end of 2021. The IEA has projected that by the end of this decade, the installed green Hydrogen capacity could  be 720 GW[1]

Given the significance of Green Hydrogen in the future energy mix, this articles provides an overview of the current state of art and provides guidance on choosing the right electrolyser for different types of applications.

2 Water Electrolysis Technology

An electrolyser splits Water into Hydrogen and Oxygen, using electricity. The history of Water Electrolysis may be traced to the early nineteenth century. Michael Faraday, a British Scientist, enunciated the Laws of Electrolysis in 1833 and provided the foundation for future developments in this field. The first industrial model of a Water  Electrolyser is attributed to In the, Dimitry Lachinov, a Russian Physicist who, in the year 1888, invented the concept of bipolar electrodes, using parchment separators in an Iron tank. In the year 1899, at Zurich, Dr O.Schmidt, presented  the first industrial bipolar electrolyser, stacked up similar to a filter press. This design was commercialised by Oerlikon [2]. By the year 1902, more than 400 industrial water electrolysis units were in operation and in 1939, the first large Water electrolysis plant with a capacity of 10,000 Nm3 H2/h went into operation [3].

Most of the Water present on Planet Earth is Sea Water. Due to the presence of Chlorides in Sea Water, large quantities of Chlorine gas are produced during electrolysis. Hence large scale industrial utilization of Water electrolysis initially occurred in the manufacture of Chlorine by Saline Water electrolysis. Hydrogen was seen only as a by-product, which could be used to manufacture products like Hydrochloric acid.

When the intent is to produce only Hydrogen, then present-day electrolysis technology requires the use of demineralized Water. However, this is most economical only if  demineralized Water is produced from freshwater. Major developments in green Hydrogen production were therefore pioneered in areas where freshwater was abundant. Hence Norway, which had abundant freshwater, established a 135 MW Water electrolyser operating on Hydel power in 1927, at Rjukan. It produced 30000 Nm3/h of Hydrogen and was operational till the 1970s. A similar 135 MW installation was in operation at Glom fjord, Norway, from 1953 to 1991 [4]. Since then, however, Hydel power went out of fashion due to ecological concerns about big dams. Currently Steam reforming of Natural gas is the dominant Hydrogen production technology displacing electrolysis. In recent times, global warming concerns have brought back Water electrolysis into consideration, though project economics are hampered by the high costs of electrical power and of electrolysers.

2.1 Electrolyser Working principle

Water is a stable compound and energy must be externally supplied to break intramolecular bonds. Figure 2 is a representation of the ‘energy hill’ that separates molecular Hydrogen (H2) and Oxygen (O2) from their reaction product, Water (H2O). This indicates that energy needs to be supplied to split Water and form molecular H2 and O2. Energy in the form of direct current electric supply at high voltage is used to split Water into Hydrogen and Oxygen. In an electrolysis cell, the electrodes which carry electricity also function as catalysts, The presence of a catalyst facilitates the reaction by reducing the energy requirement.

2.2 Construction

The components of a Water electrolysis cell are illustrated in Figure 3 and described below:

Containment Shell: All the working components of an electrolyser are held within pressure retaining containment shell. The shell is designed to withstand maximum operating pressures and temperatures. It also provides impact protection during transportation and operation.

Electrodes: These constitute the  interface through which electrons from the external current source enter and leave the Water. The positive terminal, called the Anode, receives negatively charged ions from the Water and strips out their electrons. The Cathode is the negative terminal, which receives electrons from the external circuit and pumps them into the Water.

Electrolyte: Alkaline or acidic Water is the electrolyte as well as the raw material. An electrolyte is a material that allows ions (such as H+ an OH-) to flow but not electrons. The addition of acid or alkali into Water increases the number of ionic charge carriers. Hydrogen ions are converted to Hydrogen molecules by the addition of 2 electrons to 2 Hydrogen ions in solutions. The ion-traffic therefore influences the production rate.

Separator: Electrical short-circuiting between the Anode and Cathode is prevented by the separator. This is designed to permit ion movements, while preventing physical contact between electrodes.

DC power supply: To sustain the movement of electrons and ions for continuous Hydrogen and Oxygen production, the minimum theoretical voltage under standard conditions is 1.23 V. In practical applications, various electrical resistances at electrode interface and electrolyte necessitate a higher applied voltage, ranging around 1.8 V-2.0 V.

An electrolyser cell system typically includes several individual cells arranged in one or more stacks with reactant Water flowing through the cells via input and output conduits formed within the stack structure. Apart from the Stack, the overall system requires many other equipment, termed Balance of System (BOS) or Balance of Plant (BOP). The building blocks of an electrolyser installation are shown in Figure 4.

2.3 Types of Electrolysers

At this point in time, only two types of Electrolysers are being utilised for the majority of  industrial scale projects. They are Proton Exchange Membrane (also called Polymer Electrolyte Membrane), abbreviated as PEM and Alkaline Water Electrolyser abbreviated as AWE. Two more promising technologies on threshold of active commercialisation are Solid Oxide Electrolysis (SOEC) and Anion Exchange membrane (AEM). A description of each of these is provided in the following paragraphs.

2.3.1. Alkaline Water Electrolysis

Hydrogen production by alkaline water electrolysis was first introduced by Troostwijk and Diemann in 1789 [3], so it is by now a well-established technology. Figure 5 is sketch of an Alkaline Water Electrolyser (AWE).

These electrolysers use an alkaline solution, comprising 20%-30% solution of Potassium Hydroxide in pure Water as the electrolyte. So essentially, the electrolyte solution is a sea of H+ and OH- ions, that participate in the charge transfer process. The Anode and Cathode are kept apart by a porous diaphragm, across which the transport of electrical charge happens via OH- ions migrating towards the Anode.

Electrons from the external DC circuit enter the cell at the Cathode, where two molecules of Water are reduced to one molecule of Hydrogen (H2), and two Hydroxyl ions (OH-) are produced .

Cathode: 2 H2O + 2 e− → H2 + 2 OH−

The produced Hydrogen is collected from the Cathode surface and piped out, while the Hydroxyl ions transfer through the porous diaphragm to the Anode, driven by the difference in voltage between the electrodes. At the Anode,  two Water molecules are oxidized, forming one diatomic oxygen (O2) molecule and four Hydrogen atoms.

Anode: 2 OH− → H2O + 1/2 O2 + 2 e−

Hence the Overall electrochemical equation, by  adding both half-equations is:

H2O → H2 + 1/ 2 O2

Alkaline Water Electrolysers can be  configured as unipolar or bipolar (filter press) designs. In the unipolar design, the electrodes are submerged in the alkaline electrolyte, inside a tank. The electrolyte is a 20%-30% solution of Potassium Hydroxide in pure Water. Each cell is connected in parallel. Figure 6 illustrates these two configurations.

AWE electrodes are made of Nickel. In the earliest designs, Asbestos was used as the diaphragm material, but modern designs use advanced composite materials like ZIRFON Perl diaphragm (Zirconium Dioxide and hydrophilic polymer, developed by Agfa Europe).

2.3.2  Proton-Exchange Membrane Electrolysis

This type is also known as a Polymer Electrolyte Membrane Electrolysers. This is because it uses solid specialty plastic material as electrolyte instead of an alkaline (KOH) electrolyte.

The first PEM Electrolyser was conceptualized by Grubb in the early 1950s, and later developed by General Electric Co in 1966, who leveraged the invention of perfluorinated ion-exchange membranes Nafion ^TM by DuPont.

The  name Proton Exchange Membrane is attributed to the fact that Protons (H+) are the charge carriers. These Protons are produced by the electrolysis of Water at the Anode and migrate across the membrane electrolyte, to the Cathode:

   Anode:  2H2O → 4H+ + 4e- + O2

At the Cathode the protons combine with Electrons that are pushed in by the DC electric source, to form Hydrogen gas:

Cathode: 4 H+ + 4 e- → 2 H2

The main barrier for the mass production of PEM electrolysers is the high cost of the materials needed. Expensive materials like Platinum, Palladium, Iridium and Rhodium are needed for the catalyst, due to the corrosive nature of the Electrolyte. Further, Deionized Water with a high degree of purity is needed to avoid damage to the electrodes. The membrane and noble metals for the electrocatalyst, make the PEM electrolyser more expensive than other kinds. Further, PEM electrolysers will experience some degradation in performance. An increase in the voltage is expected because of equilibration of water content in the membrane and oxidation of catalyst and other metallic components. There are also potential safety issues with PEM electrolysers operating under intermittent power conditions. As Hydrogen and Oxygen production is proportional to the current density, at very low loads, the production of the gases may be lower than the permeation rate through the electrolyte, mixing with each other and thus generating dangerous combustible conditions inside the electrolyser. Another problem is related to the operating temperature of the electrolyser. It takes some time for the electrolyser to reach its nominal temperature, so intermittent operation could keep the electrolyser from reaching this temperature and working at its highest efficiency.

2.3.3. Solid Oxide Electrolysis

Donitz and Erdle were the first to develop Solid Oxide Electrolysis (SOEL), in the 1980s. The unique feature of this method is that it uses Water in the form of Steam and operates at high temperatures and pressures. This opens up many potential applications where energy in waste heat can be recovered and stored in the form of Hydrogen. The process is also more efficient than either AWE or PEM.

SOEL’s operating principle is very similar to AWE. However , the charge carriers in this case are  Oxygen ions. The corresponding half reaction equations are:

Cathode: H2O + 2 e− → H2 + O 2−

Anode: O2− → 1 /2 O2 + 2 e−

Using Steam instead of liquid Water enables comparatively higher energy efficiency, since most of  the energy needed for the electrolysis process is added as enthalpy of Steam. In addition, the high temperature accelerates the reaction kinetics, reducing the energy loss due to electrode polarization, thus increasing the overall system efficiency. The high temperature system employs Oxygen ion conducting ceramics as the electrolyte (ZrO2 stabilised by Y2O3, MgO or CaO). The fluid to be dissociated is 200^oC Steam which after being further heated to 800-1000^oC enters on the Cathode side. After the Steam is split to Hydrogen gas and O2- ions at the Cathode, the Oxygen ions are transported through the ceramic material to the Anode, where they discharge and form Oxygen gas. Despite this high efficiency with respect to electricity, the high temperature system still produces Hydrogen at about four times the cost of conventional Natural Gas based Steam Reformer Hydrogen.

2.3.4 Anion Exchange Membrane Water Electrolysers

Anion exchange membrane water electrolysis (AEMWE) is a potentially low-cost and sustainable technology for Hydrogen production that combines the advantages of a solid electrolyte as in PEM water electrolysis and the cost effective electrode materials used in AWE systems.

In the AEMWE, the charge carriers are Hydroxide ions (OH-) which migrate through a dense polymeric Anion Exchange membrane (AEM).The Anion Exchange membrane is a semipermeable membrane. It allows Anions to permeate, while Oxygen or Hydrogen are blocked. This exchange process can produce high purity Hydrogen and reduces the risk of  Hydrogen crossover. The electrochemical reactions occurring within an AEMWE are the same as those occurring inside an AWE.

The membrane technology is evolving. In recent years, different polymeric backbones like polysulfone, polystyrene, poly(phenylene oxide) have been studied for AEM preparation. The electrode catalysts are coated onto these membranes to for bipolar Membrane  Electrode Assemblies (MEA).

2.3.5 Technology Readiness Level

According to an IEA analysis [1], AWE and PEM electrolysers are considered to be at the same technology readiness level (TRL9) for the purpose of Hydrogen production projects. Of course, Alkaline Electrolysers have been around in the Chlor-Alkali industry for several decades. Considering the projects under development, currently AWE is dominating the commercial space.

SOEC electrolysis is at demonstration stage, with a 2.6 MW SOEC electrolyser expected to become operational at a Neste Refinery in the Netherlands at the end of 2022. Anion Exchange Membrane (AEM) electrolysers were considered by IEA  to be at TRL6 (full prototype at scale). Figure 8 maps out the current technology readiness levels of all the four technologies.

3 Technology Evaluation Criteria

Though currently only Alkaline and PEMC types of Electrolysers are being commercially deployed, there are numerous new technologies that are expected in the Green Hydrogen marketplace in the near future. When multiple technologies have to be compared, it is necessary to have common evaluation criteria that can be applied to all current and emerging technologies. This is quite complex, since Green Hydrogen projects target diverse markets. Some of the emerging applications are in industrial heating, domestic heating, road transportation, rail transportations, Green Ammonia, and many other niche applications in chemical and metallurgical industries.

Regardless of the technology, there are common aspects that need consideration in all cases. Based on field experience with Green Hydrogen projects, the key factors that need consideration for electrolyser technology selection are listed below:

Key Factors for Electrolyser Technology Selection:

• Availability and stability of renewable power
• Operating Pressure (barg)
• Operating Temperature (⁰C)
• Current Density (A/cm2)
• Voltage (V)
• Specific Energy consumption (kwh/kg H2)
• Efficiency
• Start-up time
• Electrodes
• Membrane or Diaphragm
• Dimensions and weight of stack
• Replacement Interval
• Catalyst poisoning
• Technology Maturity Level

Why are these technology evaluation criteria considered important? The significance of each criterion is discussed more detail in the following sections:

3.1 Availability and Stability of Renewable Power

Renewable energy for electrolysis may be sourced from Wind, Solar PV, Geothermal or Hydel sources or a combination of these. The type and quality of renewable electricity influences electrolysis technology selection, in cases where there is no other source of power supply. Hydel or geothermal energy are not widespread, and  inherently suited only for vary large scale electrolyser projects. The majority of projects reported to be under implementation or planned in the near future are in the small to medium range and utilise Wind turbines and Solar PV as green electricity sources.

Both solar and wind are characterised by variable intensity and intermittency and cannot supply continuous supply on their own to electrolysers. Due to fluctuations, the electrolyser efficiency is affected. Prolonged cycling also leads to permanent degradation of the electrolyser components. Different types of electrolysers vary in their ability to withstand these variations and in their speed of response to fluctuations. For example, PEM electrolysers are faster in their response to changes in power supply than AWE electrolysers, but more vulnerable to damage due to fluctuations. This is therefore an important criterion to be considered in technology evaluation.

Current projects in some countries have been able to mitigate this problem by permitting electrolysers to draw power from the existing power grids. To obtain Green certification, it is necessary to have a power purchase agreement with a green electricity supplier who will supply the equivalent amount of green electricity into the grid. This basically transfers the electrolyser project’s problem to the grid managers and is an administrative rather than  technical solution to the problem of renewable energy instability. Not all countries have grids that are robust enough to handle this kind of an arrangement.

Further, Electrolysers that draw their power supply entirely from Wind or Solar PV must be adequately oversized to meet their daily Hydrogen production targets within the energy availability window of a few hours. Hydrogen is stored and supplied to consumers throughput the day. Green Hydrogen Projects therefore require additional capital investment when compared to grid operated electrolysers of the same capacity.

3.2 Operating Pressure

Commercially available electrolysers generally operate from near atmospheric pressure to about 30 barg. From an electrochemical perspective, there is no barrier to developing pressure within the electrolyser since the reaction rate is not influenced by pressure. However, possibility of Hydrogen crossover from the Hydrogen sections to the Oxygen sections poses a safety hazard at higher pressures, necessitating design modifications. Additionally, high pressure operation requires heavier mechanical design, thicker diaphragms/membranes, and special design of  polymeric and elastomeric components such as gaskets that can degrade at higher Hydrogen pressures. Hydrogen embrittlement of Steel is another concern at higher pressures.

Low pressure electrolysers obviously cost less than high pressure electrolysers of the same size. However, they are not the most cost effective for emerging business segments such as Green Ammonia and Hydrogen fuelled vehicles. Figure 9 illustrates the pressure ranges required for various commercial application of Green Hydrogen.

For vehicle refuelling applications, it has been estimated that if electrolyser capital cost is about USD 900 per KW, then Hydrogen  compressor capital cost is around USD 3800 per KW. There will be additional operating cost for the Hydrogen compressor and associated cooling systems as well. It is clear that for all the high pressure applications, the  compression of Hydrogen will be a significant element of capital and operating costs. From this perspective, high pressure electrolysis, in which the pressure is allowed to rise within the electrolyser will reduce Hydrogen compression requirements. Designs for high pressure electrolysis are rapidly approaching commercial maturity for both Alkaline and PEMC electrolysers. Figure 10 summarizes the work done by Avalence LLC on Alkaline electrolysers working upto 6500 psig [7].

PEM Electrolysers are also being developed to operate at High pressure. However, no major large-scale manufacturer has  commercially  launched  high pressure PEM electrolysers. Some of the issues being faced with high pressure PEM electrolysers are summarized below:

Mechanical Integrity/ Membrane creep

• Loss of Electrolyser Stack Sealing efficiency.
• Membrane extrudes into fluid ports due to pressure.
• Leakage (internal & external).
• There is a need to improved strength of the membrane while preserving

Durability

• Membrane degrades faster at higher operating pressure.

Efficiency

• Thin membranes have low resistance, allowing more current density. However there high back diffusion on account of the thinner membrane.

Apart from issues with the electrolyser cells at high pressures, there are challenges to be overcome in the balance of plant components. These include design of Hydrogen driers, gas-liquid separators, polymeric and elastomeric components, and overall safety.

3.3 Operating Temperature

Temperature influences the rate of electrochemical reactions as well as the electrical resistance, For an electrolyser, higher temperature leads to higher  efficiency. As can be seen from Figure 11. The disadvantage of higher temperature is that it adversely impacts the  longevity of an electrolyser. At higher operating temperatures, the service life is reduced. The objective therefore is to obtain a balance between high efficiency and service life. Both AWE and PEM electrolysers are operated in the 40^oC to 90^oC range. In contrast, SOECs work in the 650 ^oC to 1000 ^oC range. Temperature control is crucial. As cell electrical resistivity increases, higher cell voltage is needed which increases the heat production. The stack cooling systems must be evaluated for their capability to handle the maximum cooling loads during the electrolyser life.

3.4 Current Density

Current density is directly proportional to the rate of Hydrogen generation. This is because higher current density implies more electrons participating in the electrochemical reaction. The area of electrodes in contact with electrolyte and reactants determines the current density.

Interestingly, the current versus voltage characteristic is such that higher current density also means a higher voltage drop, reducing the electrolyser Voltage efficiency. Low current density will result in a lesser rate of production of Hydrogen, unless the electrode contact surface area is increased, which means higher costs. However, the electrolyser voltage efficiency will be higher. So, there is a trade-off to be considered for the same production rate, between, higher CAPEX/higher efficiency at lower current density versus higher OPEX/lower efficiency at higher current density. PEM electrolysers operate at current densities of 0.6 to 2 Amps/cm2 which is higher than corresponding AWE current densities of 0.2 to 0.4 Amps/cm2, within the same voltage range. SOECs have been observed to give higher current densities at lower voltage ranges, so as the technology matures, this will be a clear area of advantage for SOECs.

3.5 Voltage

The thermodynamically ideal reversible cell voltage that must be applied, as a minimum, to split water into its components of H2 and O2 is 1.23 V. It is computed from the Nernst equation, which relates voltage to temperature, and partial pressures of all reactants and products.

However, in reality, certain irreversible losses will occur due to various reasons including activation energy losses and ohmic losses. A voltage known as Thermoneutral Voltage equal to 1.48 V, is considered to be the voltage at which a real electrolysis cell would function at 100% efficiency. Voltage efficiency is the ratio of the thermoneutral voltage to the actual voltage required by the cell. The electrolyser produces heat at potentials above 1.48 V and absorbs heat in at potentials below this value, at constant cell temperature. The distance between the two electrodes affects the voltage drop. If the electrode are Brough closer as in bipolar filter press arrangements, the charge carriers have to traverse less distance between the Anode and the Cathode. However, bringing the oppositely charge electrodes closer also increases the risk of short circuit and mixing of gases. Since AWEs use porous diaphragms to separate electrodes , the spacing if electrodes in usually more than in PEM electrolysers, making AWE cells larger. Both AWE and PEM cells generally operate in the 1.8 V to 2.2 V range.

3.6 Specific energy consumption

The  electricity consumed (kwh ) by an electrolyser stack to produce one kg of Hydrogen is called specific energy consumption. It is expressed  kwh/kg H2. This is the most widely used parameter in comparing electrolyser technologies. The specific energy consumption for AWEs is about 55 kwh/kg H2 and for PEM electrolyser it is about 60 kwh/kg H2. Note that since the density of Hydrogen is 0.0899 Kg/Sm3 (at Standard temperature and pressure), the energy consumption per unit volume of Hydrogen is 4.94 kwh/Sm3 H2 for AWE and 5.39 kwh /Sm3 H2 for PEM electrolysers.

3.7 Efficiency

The efficiency of the electrolysis systems is crucial for cost effective production of electrolytic Hydrogen. This is because operating costs of electrolysers are dominated by power costs. Completive costs can be achieved only by increasing overall efficiencies.

Electrolyser energy efficiency can be defined as the energy in the Hydrogen produced, divided by the energy used to produce it. The input and output energies are usually expressed in terms of heating value (HHV) which facilitates comparison with fossil fuel based Hydrogen production.

The overall efficiency of the electrolyser installation is the combined effect of:

• Stack efficiency
• System efficiency
• Plant efficiency

The boundaries for energy efficiency calculation will depend upon which type of efficiency is being calculated. Hence based on the scope, losses may be included from AC/DC conversion and transformer losses, energy used for BOP items such as water treatment, cooling systems, buildings and auxiliary packages, compression and Hydrogen purification.

Example: The HHV of hydrogen is 142 MJ/kg, which is equal to 39.4 kWh/kg. So, an electrolyser that consumes 50 kWh of electricity to produce one kilogram of Hydrogen has an efficiency of 39.4 kWh/kg divided by 50 kWh/kg, which is 79%.

As discussed earlier, a electrolyser can be operated very efficiently, if the current density is kept low. From an energy efficient  design perspective, the biggest overall efficiency gains can usually be achieved by optimizing the design of the Balance of Plant components.

3.8 Start-up time

Due to the intermittent nature of renewable energy supplies to a green Hydrogen electrolysis plant, the electrolyser technology must permit rapid start upon from cold conditions. Even in the case of Grid supply, there are different consumers connect to the grid, so fast response Hydrogen electrolysers are seen as a load balancing element in the grid. PEM electrolysers  are typically faster to start-up than Alkaline electrolysers. However constant cycling leads to faster performance degradation of PEM electrolysers than in the case of AWEs.

3.9 Electrodes

Due to the corrosivity of Polymer electrolytes used in PEM electrolysers, the electrodes are made from very costly materials such as like Platinum, Palladium, Iridium and Rhodium. In contrast AWEs can use low cost materials like Nickel for electrodes. Capital investment is usually the deciding factor for any project and electrodes in hundreds of cells in a typical installation, will need to be replaced after some time due to degradation. Hence, life cycle costs of electrodes are an important criterion for technology evaluation.

3.10 Membrane or Diaphragm

In the case of membrane based electrolysers such as PEM and AEM, the quality assurance of membranes is a major concern. This must be addressed with the manufacturers during the technology evaluation process. The membrane is part of a composite assembly known as the Membrane Electrode Assembly (MEA). There is a significant possibility of it getting  damaged during the manufacturing process.

Smaller manufacturers tend to use manual processes where quality control is challenging. In a large scale electrolyser installation, the will be hundreds of cells and even a few damaged membranes can cause Hydrogen purity to drop as well as create safety problems due to Hydrogen crossover. Even with the best quality control and ensuring optimum operating conditions, degradation of membranes is inevitable.

In the case of AWE electrolysers, the Diaphragm can develop mechanical damage or corrode and pores can get blocked.

3.11 Dimensions and weight of stack

There are many potential applications such as Hydrogen production in vehicle Hydrogen  fuel stations and for decentralized or portable Hydrogen supplies, where Electrolyser footprint is an important consideration. In such cases, PEM is preferred over AWE electrolysers, which are inherently larger and heavier. In large industrial settings however, currently, AWEs are dominating the market, since they are proven at scale and equipment weight, or footprint is not a major issue.

3.12 Replacement Interval

Literature on the subject, as well as vendor feedback indicates that key components of electrolysers have to be replaced, since performance and material degradation is inevitable even under ideal operating conditions. Generally, under ideal operating conditions, the replacement interval for AWEs is 75000 hours and for PEM electrolysers is 60000 hours. When there is significant cycling in power supply as in the case of renewable power and if water quality is poor, the lifetime will be reduced. Replacement costs must be included in the project cost estimate at the feasibility stage. Performing replacement for hundreds of cells in a large scale installation is a tedious operation. Considering the rapid progress in electrolyser technology, it may be worth looking at scrapping the plant and opting for newer technologies in lieu of component replacement.

3.13 Catalyst Poisoning

The catalyst on the electrodes is crucial to effective electrolysis since it reduces the activation energy and makes the reaction possible. Water quality and KOH quality are the two factors that need to be monitored since these are the routes through which contaminants can enter. Corrosion products within the cell can also cause poisoning. In some cases, there may be side reactions caused by impurities which can poison the catalyst. Hence careful attention must be given to design and operation of the Balance of Plant units, especially Water treatment plant and to ensure that corrosion resistant materials are used in the equipment and piping.

3.14 Technology Maturity Level

As discussed in an earlier section (see Figure 8), currently AWEs are dominating the market due to the fact that industry has over a hundred years of experience in this technology in the Chlor-Alkali sector. Specifically for Hydrogen production, however, IEA considers both PERM and AWE technologies to be at the same technology readiness level of TRL9 [1]. SOEC is not far behind and may eventually be the favoured technology for some industrial applications in future.

4 References

1. Electrolysers – Analysis – IEA (tracking report, September 2022)
2. History of water electrolysis – Big Chemical Encyclopedia (chempedia.info)
3. A review on water electrolysis, by Emmanuel Zoulias et al., report under the contract number ENK-CT-2001-00536 (RES2H2).
4. NEL Hydrogen brochure.
5. Recent Developments on Hydrogen Production Technologies: State-of-the-Art Review with a Focus on Green-Electrolysis, by Leonardo Vidas and Rui Castro, Appl. Sci. 2021, 11, 11363. https://doi.org/10.3390/app112311363.
6. The case for high-pressure PEM water electrolysis by Ragnhild Hancke , Thomas Holm , Øystein Ulleberg in Energy Conversion and Management 261 (2022) 115642.
7. High-Capacity, High Pressure Electrolysis System with Renewable Power Sources Paul Dunn and Dave Mauterer, Avalence LLC DOE Merit Review, 11 May 2011.
8. Hydrogen Generation From Electrolysis(Proton Energy Systems), Final Report for DOE Award DE-FC36-04GO13030,February 28, 2009.
9. Anion exchange membrane water electrolysis using Aemion™ membranes and nickel electrodes – Journal of Materials Chemistry A (RSC Publishing).
10. High Pressure PEM Electrolysis Status, Key Issues, and Challenges, by Monjid Hamdan, Giner, Inc. Electrolytic Hydrogen Production Workshop NREL, Golden, Colorado, February 27-28th, 2014.