Developing clean energy alternatives to meet civilization’s needs is one of the critical challenges facing humankind in the twenty first century. And, overcoming the limitations of current state-of-the-art energy conversion and storage systems is the central issue to solving the energy challenge. To advance the state-of-the-art, a complete understanding of the chemical reaction mechanisms and fundamental properties of the active materials in energy conversion and storage systems is required. Operando synchrotron X-ray techniques are ideally suited to characterize the structure of active materials under operating conditions on length scales ranging from atomic to macroscopic. This thesis consists of four projects in which two operando synchrotron X-ray techniques are applied to energy systems, revealing novel properties of the active materials under operating conditions. The two techniques are crystal truncation rod (CTR) analysis and X-ray imaging. The background of the techniques is introduced in Chapter 1. In Chapters 2 and 3, I applied operando CTR measurements to study the surface and bulk structures of the catalysts for photoelectrochemical cells (SrTiO3) and fuel cells (Pb2Ru2O6.5), respectively, under catalytically relevant conditions. In Chapters 4 and 5, operando X-ray imaging was applied to investigate the microstructure evolution in the two essential components in the “beyond lithium ion” batteries: sulfur cathode and lithium metal electrode. In Chapter 6, I summarize my work and provide a future outlook. This thesis presents the following original contributions to scholarship:

1. Measure the atomic-scale surface structure of SrTiO3 in an irreversible surface reconstruction driven by the applied potential. The reconstructed surface exhibits a three-fold increase in photocatalytic activity.
2. Develop a Fourier space fitting method to calculate the real space structure from CTR. Characterize the structure of Pb2Ru2O6.5 thin film catalysts and the reversible surface reconstruction at applied potentials associated with the oxygen reduction reaction (ORR).
3. Observe the morphological evolution of sulfur particles in a sulfur electrode in Li-S batteries and characterize the growth and dissolution of lithium dendrites in a lithium symmetric cell during battery operation, and find the critical operating conditions (current density, electrolyte and temperature) that control the microstructure of the active material.