Understanding the physical phenomena limiting the inkjet printed PEM water electrolyzer performance
Creators: Mandal, Manas K
Publisher: name: University of Alberta Library
Year: 2022

<p>Polymer electrolyte membrane water electrolysis (PEMWE) are a promising technology to generate hydrogen using electricity and water. The hydrogen can be used to reduce the emission of greenhouse gases in several industrial processes, such as power grid, fertilizer industry, and transportation sector. PEMWE cells however suffer from high cost and low performance and durability. The focus of this thesis is to find the physical phenomena limiting the PEMWE cell performance and propose novel electrode designs that could reduce these limitations. To achieve above goal, a catalyst layer (CL) fabrication method capable of manufacturing well controlled CL is required, as the PEMWE cell performance is highly dependent on the CL manufacturing method, composition, and microstructure.</p>

<p>A novel fabrication method, inkjet printing, was used to fabricate the CLs. Its drop-by-drop deposition allows for precision deposition of catalyst ink. An unsupported IrO2 catalyst ink suitable for jetting was developed and depositing on the membrane. Scanning electron microscopy (SEM) images showed the CL was uniform and well adhered to the membrane. Energy dispersive X-ray spectroscopy imaging showed even distribution of catalyst and ionomer in the CL. A kinetic study revealed Tafel slopes similar to those in literature, however the exchange current density was lower, showing that the catalyst might not be very active. Still, the electrochemical performance outperformed most of the previously reported results in literature. These results suggested that the inkjet printing can be used to fabricate electrolyzer electrodes with great control and good performance.</p>

<p>A novel setup utilizing a hydrogen pump technique was developed to measure the protonic conductivity of the Ir and IrOx CLs. The through-plane electronic conductivity of the anode CL was measured using a two-probe method. The conductivities were obtained with varying ionomer loading. The measured conductivities showed that the limiting charge transport parameter in the studied IrOx CLs can be either protonic or electronic depending on the ionomer loading in the CL. In the Ir CLs, protonic conductivity was found to be always limiting. The implication of the obtained results is that the oxygen evolution reaction would be more active at CL-membrane interface for the Ir CL and at CL-porous transport layer (PTL) interface for the IrOx CL with an ionomer loading above 15 wt.%.  These results suggested that there might be an optimal catalyst and ionomer loading for each catalyst material to achieve a high catalyst utilization that depends on protonic and electronic CL properties, as well as catalyst activity.</p>

<p>The porosity of the PEMWE cell anode CL is low due to the use of an unsupported catalyst (typically Ir/IrOx), possibly resulting in mass transport losses and low CL surface area. By increasing the porosity of the anode CL, it is hypothesized that mass transport losses will be alleviated, and more catalyst surface area will be exposed to the reactant. Addition of carbon to the Ir or IrOx catalyst ink was investigated to increase the porosity of the anode CL. The Ir and IrOx CL porosity increased from 58% to 71% and from 23% to 41%, respectively, even without removing carbon. Then added carbon was oxidized in-situ to create further pore spaces. SEM images revealed that by adding carbon decreased the CL cracks. With an increase in the CL porosity, the ECSA of CLs with Ir increased and CLs with IrOx, first, increased and then decreased. The decrease might be due to the detachment of the catalyst from the IrOx CL as seen from SEM images. The CLs with Ir showed an improved performance at low current density due to the increased ECSA and at high current density due to the decreased high frequency resistance (HFR). On the other hand, the CLs with IrOx catalyst showed improved cell performance at lower current density due to the increased ECSA but did not at higher current density due to increased HFR. The increased HFR might be due to the increased CL thickness with carbon content, which increased the proton transport resistance of the IrOx CL. These results demonstrated that, for Ir catalyst, the addition of carbon can improve both cell performance and CL integrity. On the other hand, CCMs with IrOx catalyst had a beneficial impact at low current density only.</p>