Single-atom catalysts (SACs) exhibit tremendous potential in electrocatalysis because of their high intrinsic activity and remarkable selectivity arising from their tunable electronic structures and maximal atom utilization. A high density of SACs is fundamental for enhancing the activity and durability during electrochemical reactions. In this review, we first summarize the leading strategies for the synthesis of metal single-atom electrocatalysts and the use of machine learning in the design and screening of SACs, with a focus on maximizing the metal loading through deliberate temperature control, followed by the application of such high-loading SACs to a range of important reactions in electrochemical energy technologies, such as the oxygen reduction reaction (ORR), H2O2 electrosynthesis, the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), the carbon dioxide reduction reaction (CO2RR), the nitrate reduction reaction (NO3RR), and the reactions in lithium-sulfur batteries. The review concludes with a perspective highlighting the key challenges and future research directions in the development and application of high-density SACs.