PhD Defence Shri Kannan Chandrasekar | Structured porous silicon gas diffusion layers for electrochemical CO2 reduction

Structured porous silicon gas diffusion layers for electrochemical CO2 reduction

Shri Kannan Chandrasekar is a PhD student in the Department of Photocatalytic Synthesis. (Co)Promotors are prof.dr. G. Mul and dr. B.T. Mei from the Faculty of Science & Technology.

This thesis focuses on fabricating structurally well-defined gas diffusion layers (GDLs) and exploring their role in electrochemical CO₂ reduction. It also establishes strategies for characterizing and operating CO₂ electrolyzers using these structured gas diffusion electrodes (GDEs) while investigating the relationship between their morphology and transport properties. The fabricated porous Si membranes serve as model GDLs to study how pore size and porosity affect performance.

The main result of the research is presented in the following four chapters.

• Chapter 02 deals with the micro-fabrication procedure for fabricating structurally well-defined GDLs in the cleanroom. The fabrication process is outlined in three phases. Phase 01 defines the process flow for fabricating porous silicon and highlights the challenges encountered during the micro-fabrication process. Phases 02 and 03 describe the solutions to the problems identified during Phase 01 to fabricate porous Si successfully. At the end of this chapter, the challenges with the direct functionalization of Ag CL on the porous Si membrane and the issues with CO2 utilization are discussed.
• Chapter 03 is divided into two parts. The first part investigates the challenges of CO2 utilization identified in Chapter 01 by using pressure control and optical imaging of the gas-liquid interface. The end of the first part discusses the stability of the established interface and the challenges in performing electrolysis and gas analysis. In part 02, the difficulties with forming and maintaining the gas-liquid interface were eliminated by moving to a zero-gap electrolyzer using the porous Si membrane. Further, the effect of porosity on CO2 electrolysis is presented with a discussion of the limitations caused by the mechanical stability of porous Si and the need for developing a stable and robust porous Si.
• Chapter 04 concludes the study on the morphological properties of GDEs with structured porous Si. This chapter uses robust porous Si to present the effect of porosity and how it compares to the porosity of commercial carbon GDL substrates. Then, the electrochemical performance of porous Si with two different pore sizes at different current densities is compared, and a hypothesis on water and gas transport is presented.
• Chapter 05 turns to conventional GDEs on the cathodic side in a zero-gap configuration and focuses on the stability of the CO2 electrolyzer influenced by the choice of the catalyst material on the anodic side. The baseline failure mechanisms were established with the widely used stable but expensive and rare Ir anode. Then, the cheaper and more abundant metallic Ni, a common anode material used in alkaline water electrolysis, was presented as an alternative, along with the failure mode and the studies undertaken to stabilize Ni.