Solid Oxide Fuel Cell /Electrolyser FAQs


SOFC stands for Solid Oxide Fuel Cell. The “Solid Oxide” part of the name refers to the type of electrolyte that is used. Popular electrolyte materials include yttria-stabilized zirconia (YSZ), scandia stabilized zirconia (ScSZ) and gadolinium doped ceria (GDC).

A solid oxide fuel cell (SOFC) produces electricity and heat from a fuel source such as methane, biogas or hydrogen. A solid oxide electrolyser (SOE) or Solid Oxide Electrolysis Cell (SOEC) converts water in the form of steam into hydrogen and oxygen. They both operate at high temperatures (depending on the design anywhere from 650°C to 900°C) and they use the same underlying electro-chemical mechanism. You can think of them as being two-sides of the same coin. Some manufacturers have truly reversible systems that can switch from fuel cell mode to electrolysis mode depending on whether there is a requirement to produce power or to produce hydrogen. In most cases though, manufacturers tend to focus on optimising their units for a single mode of operation but will utilise the same underlying architecture for both.

There is not a clear winner between these two technologies. Despite both devices electrochemically convert the energy of a fuel into electricity, they have some unique properties that make them more suitable for certain applications. For example, PEMFC operate at low temperature, have fast response time and are more compact, hence making them suitable for automotive applications. Differently, the high operating temperature of SOFC allows the use of methane or other hydrocarbons and the production of good quality waste heat, thus making them suitable for combined heat and power systems and uninterruptible power systems.

When talking about costs, it is important to distinguish between the stack, that is the core of the technology, and the complete system, which includes the control system, power electronic and the balance of plants. Today, the production cost of a SOFC stack is around 4000 EUR/kWe and it is expected to decrease below 800 EUR/kWe by 2030. At system level, the CAPEX is approximately 10000 EUR/kWe and it will reach 2000 – 3500 EUR/kWe by 2030 for small (<5 kWe) and large (51-500 kWe) systems, respectively 1.

In a fuel cell, the energy of a fuel is converted directly into electricity by electrochemically reacting it with oxygen. The fuel and the oxygen are electrically connected via an external circuit so that they can exchange electrons, but they are kept separated by a membrane that allows only the passage of ions. The fuel, for example hydrogen, is fed to the anode where it forms hydrogen ions thanks to the presence of a catalyst; the electrons released in the process travel through the external circuit to reach the oxygen present at the cathode and reacting with it to form oxygen ions. In a SOFC, the membrane allows the formed oxygen ions to migrate from the cathode to the anode, where they meet the fuel ions and react producing steam and/or CO2, depending on the fuel used. The passage of electrons through the external circuit is the electricity that we can harvest for our use.

SOFC technology provides some unique benefits compared to other fuel cell technologies. All fuel cell systems are sensitive to certain chemicals however unlike a PEM fuel cell that needs very pure hydrogen to operate, an SOFC can utilise a wider range of fuels such as methane, biogas or hydrogen. Many systems are what are known as “fuel flexible” which means they can use different fuels or blends of say methane and hydrogen which makes them very attractive for customers who are concerned about the future proofing of their investments.

SOFC are very efficient and today the efficiency of a SOFC system is today around 47-55% depending on the fuel used, whether hydrogen or methane, respectively. This means that approximately half of the energy contained in the fuel is converted into net usable electricity while the rest is mostly by-product heat. However, the high operating temperatures of SOFC allow the recovery of this good quality heat, thus the total efficiency, that is the ratio between useful output energy and the input fuel energy can reach 85-90% 1.

Traditionally, SOFC fuel cells were only considered for stationary applications such as distributed power and microCHP (combined heat and power) systems. This was because of their high operating temperatures and the belief that movement and vibration would be damaging to performance. Whilst it is fair to say that stationary applications such as data centres, offices, and residential applications are the most common, system manufacturers are also offering units for battery electric vehicle range-extenders (BEVRE), aerospace and drone applications and rucksack sized mobile units for military and humanitarian aid applications. Other companies are investigating the opportunities for larger systems in the megawatt (MW) range to power larger factories.

The basic design of a fuel sell requires three basic components – an anode, a cathode and an electrolyte. These components are assembled into a thin sandwich; 10’s or 100’s of these layers are placed on top of one another with metal interconnects in between to form a “stack”. Because of the high operating temperatures of a solid oxide fuel cell the metallic materials have to be able to withstand these extreme conditions. Some manufacturers have designed stacks that operate at more moderate temperatures around 650-700°C which means that more readily available metals such as stainless steel can be used which helps to reduce costs and makes mass manufacture easier.

1. Sealing a solid oxide fuel cell (or electrolyser) is not simple. There are two main ways to create a seal – one by using a compound that melts and fuses the repeating layers together at high temperature. Alternatively, a compression seal can be used. One of the limitations of glass seals are that they are prone to cracking and cannot handle thermal cycling situations without careful thermal control. If a seal is compromised in any part of the stack it can lead to a gas leak which can diminish the performance or lifespan of the unit or in the worst-case cause failure of the stack requiring an immediate shutdown.

2. The compression seal inside a fuel cell or electrolyser is not simply a gasket, but an engineered component that performs a number of critical functions within the stack:

  1. Provide a gas tight seal at very low loads
  2. Work from ambient up to operating temperature
  3. Cope with many thermal cycles over an extended life of 40,000 hrs or more
  4. Electrically isolate the layers to facilitate the current collection and avoid a short circuit
  5. Accurately control the gap thickness between the interconnect layers to aid with effective gas flow and proper electrical contact
  6. To be thermally, chemically & physically stable
  7. Not poison the chemistry of the cell
  8. To be compatible with both reducing & oxidising atmospheres
  9. Be able to be formed into complex shapes
  10. and to be cost effective

Due to the design of an SOFC, it is not usually feasible to replace individual seals within the stack.

The beauty of Solid Oxide Fuel Cells is that the stack is fully reversible: the same stack that is used to convert hydrogen and oxygen into electricity and water vapour, can be used to split water into hydrogen and oxygen using electricity. In a SOE, water vapour is fed to the cathode side where, thanks to the electricity input (more precisely, to the external voltage applied), it is split into hydrogen and oxygen ions. The oxygen ions can move through the membrane and reach the anode where they form oxygen and release the electrons that can travel through the external electrician circuit connecting anode and cathode.

Yes, an SOFC can use 100% hydrogen. SOFC’s however can use other fuels including natural gas, methane, propane, biogas, coal gas and methanol.

Hydrogen, natural gas, methane, propane, biogas, coal gas and methanol. System are typically “Fuel flexible” or “Fuel Agnostic” which means different fuels could be used in a single unit. And in some cases manufacturers advise that they units can take a variable blend of hydrogen and methane for example without any impact to performance.

See how does it work.

A SOFC or SOE system is composed of several components in addition to the stack, such as fans, pre-reformers, heat-exchangers, and gas processing units. Some of these components also operate at high temperature and are hence enclosed together with the stack in the so-called Hot Box. To connect the components, a mix of flanged, screwed, cone & tread fittings (metal to metal) connections can be found in a system. In addition to SOFC and SOE stack sealing solutions, Flexitallic offers the best products for flanged connections inside and outside the hot box.

Although often referred to as the least mature fuel cell and electrolyser technology, there are already operating pilot and demonstration systems based on solid oxide technologies. At the moment, the landscape is quite varied, with some companies already having systems in the market, others in the pre-industrialization phase, and others that are developing new concepts with R&D projects.

A fuel cell converts a fuel (e.g. hydrogen) and oxygen from the air into electricity. The fuel is fed to the anode and the air is fed to the cathode. In a solid oxide fuel cell, the catalyst at the cathode separates the oxygen molecules into oxygen ions. The oxygen ions pass through a solid electrolyte layer separating the anode and cathode and react with the fuel on the other side forming steam.  The electrons released in this process are transported through an electrical circuit to produce an electrical output.


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