About PEM Fuel Cells

Nedstack focuses on the production and development of Proton Exchange Membrane (PEM) fuel cells. PEM fuel cells are widely considered to be the most versatile of currently available technologies. Please find below answers to frequently asked questions about PEM fuel cells.

PEM Fuel Cells in a nutshell

Proton Exchange Membrane or PEM Fuel Cells are considered to be the most versatile type of fuel cells currently in production. They produce the most power for a given weight or volume of fuel cell. Because they are lightweight, have such high power density, and cold start capability, they qualify for many applications, such as stationary power, transport, portable power and application in space. Nedstack is the stack provider of choice for integrators who deliver energy systems for telecom and utilities applications, material handling and city transportation.

Furthermore, PEM fuel cells offer the most efficient and hassle-free way of recovering energy from hydrogen produced as a by-product in chlor-alkali plants.

What is at the core of a PEM fuel cell?

Several different types of fuel cell exist, but all are based on the same principle. Two electrodes are separated by an electrolyte. The anode breaks the hydrogen atom down to its positive component, the proton, and its negative component, the electron. The electrolyte carries the proton between the electrodes, while the negatively charged electrons are carried through an external circuit, thus creating an electrical current. Combustion is no part of the process.

Nedstack focuses on the production and development of Proton Exchange Membrane (PEM) fuel cells, the most versatile of currently available technologies. Essential parts of PEM fuel cells are the Membrane Electrolyte Assembly or MEA, and the bipolar plates to separate the MEAs.

What happens in the Membrane Electrolyte Assembly (MEA)?

A key component of a PEM fuel cell is the Membrane Electrolyte Assembly or MEA. The MEA consists of two electrodes, the anode and the cathode. These are porous carbon electrodes, which are each coated on one side with a low amount of platinum catalyst and separated by a proton exchange membrane (PEM). The PEM is the electrolyte in this assembly. It is a thin sheet that is only permeable for protons and water. It must allow hydrogen protons to pass through but prohibit the passage of electrons and gases.

In a fuel cell, hydrogen gas flows to the anode. There, with the help of the catalyst, the molecules are broken down into protons (hydrogen ions) and electrons. The positively charged protons go through the porous membrane and migrate toward the cathode. The membrane blocks the electrons, which flow from the anode to the cathode of the adjacent cell. On a stack level, this flow can be used to power electric applications.

At the cathode, the hydrogen protons, the electrons from the external circuit and oxygen from the air combine to form water. This reaction is also accelerated by a catalyst. The reaction is exothermic, which means that it generates heat.

PEM fuel cells do not require corrosive fluids like some fuel cells. They only need hydrogen, oxygen from the air, and water to operate.

What are bipolar plates and what do they do?

Bipolar plates, also known as flow-field plates, are positioned on either side of a MEA. They help distribute gases and serve as current collectors.

The bipolar plates contain a fine mesh of gas channels. Through these channels, hydrogen gas is directed to the anode. Air flows through the channels to the cathode. At the cathode, the oxygen in the air forms water with the protons that come through the membrane and the electrons coming from the external circuit. The air flow removes this water. The design of the bipolar plate is critical for a correct operation of the fuel cells.

When multiple fuel cells are combined, the bipolar plates collect the current generated in the individual cells. Therefore, the bipolar plates need to be electrically conductive.

What is a fuel cell stack?

A single fuel cell consists of the membrane electrode assembly and two flow-field plates. A single fuel cell delivers typically a voltage between 0.5 and 1V. This is too low for most applications. Just like batteries, individual cells are stacked to achieve a higher voltage and power. This assembly is called a fuel cell stack, or just a stack.

The power output of a given fuel cell stack will depend on its size. Increasing the number of cells in a stack increases the voltage, while increasing the surface area of the cells increases the current. Our 10kW full stack measures 190 × 270 x max 520 mm, and weighs max 35 kg. A stack is finished with end plates and connections for ease of further use.

Nedstack produces stacks that can easily be integrated into a system to provide an operational power system tailored for a specific application or market. Nedstack has developed this system integration know-how and provides support to system integrators who want to build systems using our stacks.

How do you fuel a PEM fuel cell?

Like a car needs petrol, fuel cells need fuel to operate. PEM fuel cells need hydrogen. Pure hydrogen, which is delivered to the stack from a commercial-grade hydrogen supply, is the most elegant solution. It is also possible to use so-called reformate, a hydrogen-rich gas generated from a hydrogen carrier, like methanol, ethanol, ammonia, etc. This requires a reformer, which is integrated into the fuel cell system. The emissions from reforming these hydrocarbon fuels are still cleaner than those from a combustion process. It is also possible to obtain hydrogen by separating water by electrolysis.

Nedstack is primarily producing and selling fuel cell stacks working on hydrogen. Although Nedstack stacks are suitable for operation on reformate as well, its output, efficiency and lifetime are lower than when operating on hydrogen.

The fuel cell keeps operating as long as a fuel is supplied.

How is the output put to use?

Fuel cell stacks generate unregulated direct current (DC) energy. This energy is passed to a DC/DC converter, to provide the high-quality regulated dc electricity to serve the system. If alternating current (AC) is required, the DC output of the fuel cell must be routed through a conversion device called an inverter.

Fuel cells produce ultra-pure water vapour as a by-product. This is removed from the stack by the airflow through the bipolar plates. This water can be reused in the cooling loop and/or to humidify the incoming air and hydrogen.

As the electrochemical process is exothermic, it generates heat. With PEM cells this heat is of limited use as the cooling water exit temperature is in the range of 60°C, however, the heat can be used for preheating feed streams within or outside the fuel cell system.

What is the lifetime of a PEM fuel cell?

The lifetime of our fuel cells depends on the application. For backup applications, our stacks are designed to run for 4,000hrs. For continuous operation in a PEM Power Plant, we deploy stacks with an expected lifetime of over 20,000hrs. At our Delfzijl PEM Power Plant our latest type of fuel cells have been operating for more than 25,000hrs without interventions. The degradation that is observed thus far indicates these units have a lifetime approaching 40,000hrs.

How can you tell that a PEM fuel cell is at the end of its lifespan?

Both batteries and PEM fuel cells at some point need to be replaced. There are two important differences. First, a Nedstack PEM fuel cell stack has a much longer lifespan than batteries. Secondly, with a fuel cell stack you can actually tell when it is time to replace them. Batteries give a steady voltage output, but they loose the capacity to store energy, this capacity is very difficult to measure, let alone predict. Therefore, there is no easy way to measure when batteries are due for renewal. As a result, many companies err on the safe side and replace them before their time, adding to the total cost of ownership of the battery application. Fuel cells on the other hand show a slight decrease in their voltage output over time. This decrease is very small but measurable and impacts the fuel efficiency. Extrapolating from this voltage degradation, it is possible to predict how many more hours of life it has left. The user decides when the voltage output has dropped to a point where the fuel efficiency requires a replacement of the stack (trading fuel costs versus maintenance cost).

What happens with PEM fuel cell stacks at the end of their lifetime?

Our stacks are developed with both cost levels and sustainability in mind. This is why most materials used are easy to recycle and allow reuse of the major components. At the end of its lifetime, the product should be returned to Nedstack for recycling. This ensures sustainable practices.