# AEM Technology

# Hydrogen in General

# What is hydrogen?

Hydrogen is the first chemical element of the periodic system. Hydrogen is the most abundant element in the universe. It is the lightest and simplest element we know, one proton and one electron, yet it is high in energy. Hydrogen is an energy carrier and a great multi-talent: it can be transformed into electricity, used as a fuel for transport, used for heating and cooling purposes, as well as various other industrial applications.

# Why do we talk about hydrogen?

We believe that hydrogen will play a central role for the design of modern energy systems to allow for complete green energy independence and security. A burgeoning global industry is taking shape around hydrogen’s potential as a storable fuel or energy carrier. The many advantages it has over battery-electric technology result in hydrogen gaining traction with industry, environmentalists and leading governments. With an abundance of variable renewable energy resources coming on-line, green hydrogen is the solution to power the green energy system of the future.

# Where is hydrogen currently used?

Hydrogen is an energy carrier and as such, a true multi talent. Today, hydrogen is directly used mainly in industrial processes of many kinds, such as ammonia fertilizer production, food processing purposes, the float glass industry, cooling for power plants, semiconductor and electronics industry, and many more.

Hydrogen also finds application as a fuel in transport often with water as the only by-product/emission. Vehicles with fuel cells on-board (cars, buses, trains, drones, planes) use hydrogen as the fuel to power their electric propulsion systems. But fuel cells are increasingly important in the power sector also. They can supply power to residential homes, commercial and industrial buildings, and remote locations. They can provide 24/7 power or serve as a backup power device. Hydrogen offers much greater energy storage density and longer autonomy than batteries.

# Why green hydrogen production?

The vast majority, around 99%, of hydrogen used globally is still produced from fossil fuels. Most of that is done by steam methane reforming of natural gas, a process which emits large amounts of greenhouse gases. We speak about green hydrogen when renewable energy sources are used in an electrolyser to make hydrogen from water. Hydrogen is the bridge between renewable power generation and other types of energy vectors and allows us to clean up more than just the electricity sector with fossil-free fuels.

# Why does it make sense to couple hydrogen with intermittent renewable energy sources?

The world has reached a turning point in our understanding of energy. Solar and wind are the two fastest growing energy sources. While governments and industry increasingly understand that fossil fuels are a thing of the past, the challenge remains to make solar and wind usable when we need them. Variable renewable are competitive, and customers are increasingly demanding reliable, secure and independent energy supply from sustainable sources. On site green hydrogen production allows for complete green energy independence and security. A burgeoning global industry is taking shape around hydrogen’s potential as a storable fuel or energy carrier and many advantages over battery-electric technology result in hydrogen gaining traction with industry, environmentalists and leading governments.

# What are the losses over time through leakage when stored in a tank? Does hydrogen have an “expire date”?

When properly stored, there are no losses. Unlike diesel for example, hydrogen does not have an expiry date and can be stored for years.

# Is hydrogen safe?

Hydrogen is a flammable gas and like with any other gas, appropriate safety measures when handling it must be ensured at all time. Hydrogen’s properties make it safer to handle than commonly used fuels. It is non-toxic, and it is an element lighter than air, so, it will quickly disperse in case of a leak. When planning a hydrogen system installation, it is important to implement appropriate safety measures, such as ventilation and leak detection.

# What is the energy content of hydrogen?

The energy content of hydrogen is described by its (lower and higher) heating value. The lower heating value of hydrogen can be expressed as 33.33 kWh/kg or 2.78 kWh/Nm³. The higher heating value is 39.41 kWh/kg or 3.28 kWh/Nm³. A practical medium value to keep in mind is roughly 3 kWh/Nm³. The energy content of 1 Nm³ hydrogen gas is equivalent to 0,34 L gasoline, 1 L liquid hydrogen is equivalent to 0,27 L gasoline and 1 kg hydrogen is equivalent to 2.75 kg gasoline (based on the lower heating value).

# How much does hydrogen weigh?

The weight of hydrogen is 0.08988 g/L.

# How much hydrogen can be produced by Enapter’s electrolyser and how long does it take to fill a 500 L tank?

Enapter’s electrolyser produce 0.5 Nm³/h (500 NL/h) or 0.04494 kg/h. One electrolyser module produces 12 Nm³ of hydrogen gas in 24 hours, weighting >1kg (1.0785kg). At the normal output pressure of the electrolyser with 35 bar, 1 kg of hydrogen occupies a volume of 0.343m3 (343 L).

A full tank of hydrogen for a passenger vehicle contains about 5 kg of hydrogen gas (stored at 700 bar) and can drive for over 500 km.

Filling up a 500 L tank with one EL running at 100% and at 35 bar takes about 500 L * 35 / 500 NL/h = 35h until it is full.

# Enapter's AEM Technology

# What is Enapter's AEM technology, and how does it work?

Enapter's core product is the standardized and stackable anion exchange membrane (AEM) electrolyser. Electrolysers use electricity to split water (H₂O) into hydrogen (H₂) and oxygen (O₂) through an electrochemical reaction. The stack is the electrolyser's heart and comprises multiple cells connected in series in a bipolar design. Enapter's unique technology is the design and operation of these cells, consisting of a membrane electrode assemble (MEA), made from a polymeric AEM and specially designed low-cost electrodes. The anodic half-cell is filled with dilute KOH (alkaline) electrolyte solution; the cathodic half-cell has no liquid and produces hydrogen from water permeating the membrane from the anodic half-cell. Oxygen is evolved from the anodic side and transported out from the stack through the circulating electrolyte. The hydrogen is produced under pressure (typically 35 bar) and already extremely dry and pure (about 99.9%). Using Enapter's ancillary dryer module, hydrogen is delivered at 99.999% purity.

# What is the difference between the Proton Exchange Membrane (PEM) technology and the Anion Exchange Membrane (AEM) technology, and what are the advantages of AEM?

Proton exchange membrane electrolysers (PEM) use a semipermeable membrane made from a solid polymer and designed to conduct protons. While PEM electrolysers provide flexibility, fast response time, and high current density, the widespread commercialization remains a challenge primarily due to the cost of the materials required to achieve long lifetimes and performance. Specifically, the highly acidic and corrosive operating environment of the PEM electrolyser cells calls for expensive noble metal catalyst materials (iridium, platinum) and large amounts of costly titanium. This poses a challenge to the scalability of PEM electrolysers.

The anion exchange membrane electrolysers use a semipermeable membrane designed to conduct anions. They are a viable alternative to PEM with all the same strengths and several key advantages that lead to lower cost. Due to the less corrosive nature of the environment, steel can be used instead of titanium for the bipolar plates. Furthermore, AEM electrolysers can tolerate a lower degree of water purity, which reduces the input water system's complexity and allows filtered rain and tap water.

# What is the difference between the traditional alkaline and AEM technology, and what are the advantages of AEM?

Traditional liquid alkaline electrolysers have been on the market for quite a while and are relatively cheap. However, they are comparatively slow at responding to a fluctuating power supply, so it is difficult and costly to pair them with renewable energy sources efficiently. Traditional liquid alkaline electrolysers operate with highly concentrated electrolyte solutions and at low pressure. They require additional purification and compression steps to produce high-quality gas at a higher output pressure. This is only cost-effective for centralized and monolithic multi-MW projects.

The AEM electrolyser builds on advantages from traditional alkaline electrolysers, but avoids its weaknesses:

  1. AEM electrolysis works in a highly diluted alkaline environment and is therefore much safer to handle.

  2. The AEM electrolyser can use similarly cost-effective materials while making much purer hydrogen at higher efficiency.

  3. The AEM electrolyser is fully scalable and is ideal for linking up with variable renewable energy sources.

# The electrolyser in general

# Where are the electrolysers manufactured? Where is Enapter producing its electrolysers?

Currently, all production takes place in Crespina, Italy, close to Pisa. Enapter is currently preparing a mass production site in Saerbeck, Germany.

# What is the lowest production rate? How much hydrogen is produced on the lowest production rate and how is the efficiency changing at partial load? How does the polarization curve look like for the stack?

The lowest production rate of the AEM stack is 60% of the 500 NL/h, meaning 300 NL/h. The lowest production limit was set to 60% to ensure the devices' safety. The amount of hydrogen in the vent line is then still well below the flammable limits. The energy consumption decreases roughly linearly with the production rate set point. The power consumption at a given production rate can be seen in the graph below.

With the electrolyser control system algorithm, ramping up the production rate by 10% takes about 21sec. Ramping down by 10% takes less than 1sec.


# What is the duration of starting the electrolyser until it is fully functional? How long is the warm-up/ramp-up time?

The ramp up time of the electrolyser depends on the electrolyte temperature (the ramp-up is slower at cooler temperatures and quicker at warm temperatures). In most cases, the system will start with a hydration period of 60 seconds, and then ramp up to the nominal production rate with the following values:

· Warm-up time (time taken for the electrolyser to heat up): The electrolyte working temperature in the electrolyser is 55°C. The electrolyser reaches a heating ratio of about 1 °C/min and reaches maximum efficiency at 55°C. That means, if the machine is started with an electrolyte temperature of 25°C, it will take about 30 min to be fully operational and perform at its maximum efficiency.

· Ramp up time (time to reach nominal production rate): Usually, the 500 NL/h production rate is reached after about 2/3 of the total warm-up time (the warm-up time is 30 min, so if starting at 25°C, it will need 20 min to reach the maximum production rate).

· Build pressure time: When the system starts, and the electrolyser starts to heat up, the hydrogen production begins immediately, and the maximum production rate is reached later. With standard setpoints, the pressure is built up completely in 1/6 of the total warm-up time (if you start at 25 °C, then the warm-up time is 30, so 5 min to build up pressure are needed)

# Do frequent start/stop cycles and ramping affect the electrolyser's longevity or performance?

As with all electrochemical devices, our AEM electrolyser stack’s lifetime is shortened with frequent start/stops. With increasing experience in the field and operational data, we can now recommend our customers to limit the electrolyser’s operative cycles to a maximum of five on/off cycles per day, and one on/off cycle per hour. This helps to ensure the longevity of the electrolyser.

The electrolyser works most efficiently and is most durable when operating continuously. However, our modular design and the Enapter Energy Management System are perfectly suited to accommodate for changing renewable energy supply or fluctuating demand. Individual electrolysers can be ramped from 60-100%, and the combination of many electrolysers will allow you to achieve any flow rate needed. If hydrogen demand is intermittent during the day, the addition of an appropriately sized buffer tank can minimize on/off cycles of the electrolyser.

# How to shut down the system?

Shutting down the system is rather easy: either manually by pressing/clicking the stop-button or automatically when the maximum pressure set point is reached at the outlet. One thing to note is that after every shutdown, the system will release the internal working pressure and purge a small amount of hydrogen gas from the purge line.

# How is the water in the electrolyser filled up?

The AEM electrolyser has an internal tank of approximately 3.5 litres. To produce hydrogen, clean water must be provided to the electrolyser via a refilling pipe at a pressure between 1 bar and 4 bar. The electrolyser refills about 1.5 litres of water every 3 hours.

# What is the water input quality requirement for the electrolyser?

The electrolyser is highly resilient to water input and can be fed with purified rainwater or tap water. Simple and cheap reverse osmosis processes with resin filters can provide the required water quality. The water input to the electrolyser needs to be desalinated and have a low conductivity. For details, please see the datasheet. It is not possible to use saltwater in the electrolyser.

# What are the differences between the air-cooled and the liquid cooled electrolyser?

The air-cooled and liquid-cooled electrolyser are nearly identical devices. The only difference is in the heat exchanger subassembly, which has the primary function to maintain a stable electrolyte temperature for the electrolyser operation.


The air-cooled electrolysers use a fan to blow ambient air past the electrolyte in order to keep the electrolyte at the nominal operating temperature, currently 55°C. For the air-cooled unit, the customer must supply fresh air at the correct temperature to the front of the device, and the device will eject warmer air out of the back. The customer must ensure sufficient air flow can freely flow.

Pros: uses ambient air, therefore easy and fast to set up

Cons: in small rooms or containers, puts higher requirements on HVAC and installation space


The liquid-cooled electrolysers have a liquid-liquid heat exchanger and use a valve to start/stop the flow of a cooling liquid that must be supplied by the customer. The liquid-cooled version of the electrolyser only has minimal air flow requirements for safety purposes and to cool the electronics, therefore the space required for installation due to air flow requirements is reduced. It has an additional cooling liquid inlet and outlet on the front panel. The customer must supply pressurized cooling liquid at the inlet, and the device will release the cooling liquid at a slightly higher temperature from the outlet. The temperature increase depends on the supplied pressure and flow rate, but in any case, the quality of heat obtainable is limited as the operating temperature of the electrolyte is currently just 55°C.

Pros: more compact setup as air flow requirements is reduced

Pros: reduced requirements on the HVAC system for indoor installations

Cons: requires additional efforts from the integrator to provide the cooling liquid

Using waste heat

In both the air-cooled and liquid-cooled cases, the total waste heat energy from the electrolysis process is the same. This waste heat, while of relatively “low quality”, could potentially be used by integrators in some specific applications to increase overall efficiency of their energy systems. In most cases however, it is just released to the environment.

# Can CO₂ contamination negatively affect the lifetime of the electrolyser?

CO₂ contamination in the air is not a problem for the electrolyser, as the system design avoids potential interaction with the surrounding air. However, CO₂ in the electrolyte (e.g. by refilling with carbonized water reduces the pH value and requires a more frequent electrolyte exchange. When maintained regularly (exchanging the electrolyte), this is reversible and does not contribute to explicit degradation of the electrochemical system.

# Is Nitrogen used during the process?

Enapter's electrolysers do not use Nitrogen.

# What is the lifetime of Enapter’s electrolysers?

We expect a lifetime of the stacks of >30,000 hours. For more information, please see the datasheet.

# Stack specifications

# What is the surface area of the membrane?

Unfortunately, we cannot provide this information as it is part of Enapter's intellectual property.

# What is the electrolyser cell’s voltage and current range?

Unfortunately, we cannot provide this information as it is part of Enapter's intellectual property.

# How is the pressure controlled for H₂? Is it using a pressure switch and valve?

Enapter utilizes a proportional relief valve to pressurize the system and several pressure transmitters to control and monitor stack and outlet pressures. A solenoid valve opens and closes to return the system to a safe state if an error occurs.

# What is the current density?

Unfortunately, we cannot provide this information as it is part of Enapter's intellectual property.

# What is the electrolytic cell rated operating temperature in degree Celsius and degree Fahrenheit?

The rated operating temperature is 55 °C / 131 °F.