Delito Informático – Incidente DELINF0998.

The performance loss mechanisms are split into the four main components of a PEMFC; Mem- brane, Catalyst Layer, GDL, and BIP of which they effect. This is to help to understand which failure modes occur in which of the main sub-layers of a PEMFC single cell.

3.3.1

Membrane

The membrane forms the heart of the PEMFC stack and is the electrolyte that blocks the flow of electrons, however it allows the passage of H+protons through from anode to cathode side. It can suffer from 8 separate basic failure modes, that can lead on to localised and system effects.

• Incorrect BIP torque can increase the membrane’s resistance consistent with the vis- coelastic compression of the membrane.

• Polymer ’creep’ causes membrane thinning and can lead to mechanical damage of the membrane.

• OH and OOH radicals & H2O2can contaminate the PTFE material of the membrane

through end group unzipping.

• The presence of foreign cationic ions can adsorb onto the membrane, and they have a stronger affinity with H+ ions in the membrane. This can lead to extensive drying of the membrane and attenuated water flux/protonic conductivity.

• Ice formation can seriously impair the mechanical strength of the membrane by rear- ranging ionomer at a molecular level through freeze cycling. Additionally, frozen water reduces the conductivity of the membrane and the impermeability of the membrane. • Fatigue from relative humidity and temperature cycling can cause a weakness

and eventual mechanical breach of the membrane.

• Excessive heat in a PEMFC can cause Sulphur Dioxide OH radical formation, and the glass transition state of Per-FluoroSulfonic Acid (PSFA) polymers. Additionally, excessive heat can dry out the membrane, causing a drop in protonic conductivity.

• Flooding swells the membrane which increases pressure build-up, and actively blocks pores, reducing protonic conductivity, and risks freezing if operating temperatures are low.

3.3.2

Catalyst Layer

The Catalyst Layer is sandwiched between the membrane and GDL, and it facilitates the electrochemical reaction kinetics of the Hydrogen Oxidation Reaction (HOR) and the Oxygen Reduction Reaction (ORR). This component can suffer from 6 main failure modes, leading to a reduction in cell performance.

• Pt agglomeration and particle growth is the electrochemical phenomenon often re- ferred to as ’Otswald Ripening’. This is where small nano-particles of Pt tend to want to group together, and form larger particles. This reduces the surface area of the Pt catalyst, slowing the reaction kinetics.

• Pt elemental loss is where Pt particles can separate from the catalyst layers binding, and moving out of the cell without predisposition, reducing the speed of the reaction. • Pt migration is where the Pt particles move from their original position to other areas

of the cell, such as the membrane.

• Pt can be contaminated by impurities in the gas feed, air feed or system born contaminants, such as silicone from gaskets or metals from BIP. These poison the catalyst layer.

• Flooding blocks the porous pathways, leading to reactant mal-distribution which can further lead to either; delaminanation of the membrane, cut-off of electron pathway and Pt agglomoration/dissolution.

• Ice Formation as with flooding.

GDL

The GDL provides electrical conductivity from the reaction sites to the external circuit. It also diffuses the gas feeds to the reaction sites on the catalyst layer. This component can experience 3 main failure modes, however the first listed can effect two main areas of the GDL.

• OH radicals can degrade the PTFE material used to make the GDL hydrophobic as with the membrane failure mode.

• The OH radicals can also contaminate the Carbon material decreasing GDL conductivity and hydrophobicity.

• Water can flood the GDL which blocks the passage of gasses to the reaction sites through the GDL pore.

• Water can also freeze in the GDL, which also blocks gas flow through the pores of the GDL.

3.3.3

BIP

The BIP encapsulates a single cell of the PEMFC. It separates fuel, oxidant gas and coolant. It also homogeneously distributes reactant gasses to the GDL, whilst collecting current from the FC reaction. This component suffers from 2 failure modes.

• An oxide film can develop that slowly builds up in thickness. This increases the resistance of the BIP and reduces the current collected by the BIP.

• The BIP can corrode leading to the release of Fe, Ni (Nickel) and Cr (Chromium) atoms into the PEMFC.

3.3.4

Differences from the literature

The previous work in [30] identified 22 failure modes attributable to reduction in performance or catastrophic cell failure. Certain failure modes identified in [30] were omitted from this work. Namely:

• ‘Membrane short circuit’ - This is not necessary as this failure would be noticed during the pre-commissioning checks by the manufacturer, rather than a developing fault. • ‘Gas leak from seals’ - This is not considered as part of this work as seal degradation

has been singled out as a negligible failure mode when PEMFC construction is quality controlled.

“Only in a couple of long-term experiments was seal degradation observed, and this might have been the consequence of an inappropriate materials

selection.” pp.18 [40]

Additionally, gasket seals that do suffer from degradation are the liquid applied sealant types used in the past. Modern systems use a solid type gasket that doesn’t suffer the same degradation.

• ‘BPP warping of polymer matrix’ - As the boundaries for this work state a steel BIP, a polymer BIP failure is not necessary.

• ‘BPP cracking’ - This has been omitted as the steel plates do not suffer from cracking, only the polymer and graphite BIPs suffer from this.

• ‘Injection-moulded BPP low electrical conductivity’ - Polymer material BIPs are not considered in this work.

• ‘Coated stainless-steel BPP loss of surface electrical conductivity’ - As above, only plain stainless steel BIPs are considered in this work.

The work in [30] considered multiple construction materials for the BIP, however for the sake of an accurate end result pertaining to a single construction type PEMFC that would be manufactured with only one type of BIP, this work only considers one construction type BIP (stainless-steel). Hence the four failure modes related to the BIP are all omitted due to the material considerations. Six failure modes were removed when compared to other works, however five new failure modes were identified and added to advance this area of work. This work is therefore more specific and detailed in comparison to the existing works.

Aside from the above omissions from previous work, this work shares some similarities with previous examples, however developments to logic and basic event definitions have been made.