Noble Metal Coated Porous Transport Layers for Polymer Electrolyte Membrane Water Electrolysis

Polymer electrolyte membrane (PEM) water electrolyzers are electrochemical energyconversion devices that split water into its constituent elements of hydrogen and oxygen. Titaniumbasedporous transport layers (PTL) are widely used due to their good bulk conductivity, highcorrosion resistivity, and excellent mechanical strength. However, titanium-based PTLs situatedat the anode side of PEM electrolyzers are subjected to harsh oxidizing conditions such as highanode overpotential, low pH and oxygen evolution. Under these conditions, titanium (Ti0) changesits oxidation state over time, which induces the formation of a thin but continuously growing layerof passivated titanium (TiOx). Consequently, the contact resistance of titanium PTLs is adverselyaffected, critically decreasing cell performance and durability.In this thesis, a very simple and scalable method is used to protect the titanium-based PTL frompassivation by sputtering very thin layers of noble metal coatings such as Ir, Pt, or Au onto thePTLs. The 20 to 50 nm thick noble metal coatings on the titanium fibers significantly decreasedinterfacial contact resistance between the PTL and catalyst layer, and improved cell performance.The single cells assembled with Ir- or Pt-coated PTLs delivered higher cell performance than cellswith Au-coated PTLs, and nearly identical cell performance as carbon paper, which is prone tocarbon corrosion under these operating conditions.The high cost of using noble metal coatings can be decreased by reducing the loading of thenoble metals. The loading of Ir as a protective layer on the PTL has an impact on the cellperformance. The amount of iridium on one side of the PTL was reduced to 0.025 mgIr∙cm-2 andshowed identical cell performance as Ir-coated PTLs with higher iridium loading, whicheffectively reduced the cost of the Ir. The total amount of iridium is 40 times less compared towhat is usually used in an anode catalyst layer, and 20 times less than Au or Pt typically used asprotective layers in contemporary and commercial electrolyzers.The critical passivation of the bare titanium-based PTL is also one significant factor thatrestricts the durability of a PEM water electrolyzer. In order to investigate the durability of noblemetal coatings (Ir, Pt, Au) on the PTLs, a series of long-term measurements were performed under2 V and 80 °C on the single cells assembled with Ir-coated, Pt-coated and Au-coated PTLs,respectively. Compared to the cell without the coatings, the cell assembled with iridium andplatinum coatings showed degradation rates close to zero, while the identical cell performance wasobserved after 4000 hours with a cell voltage of 2 V. These results demonstrate that iridium andplatinum coatings on titanium-based PTLs are highly effective at protecting the PTL againstpassivation, ultimately improving cell performance and durability