In ammonia-based solar thermochemical energy storage systems (TCES), solar energy is stored via endothermic ammonia dissociation reaction and released when the ammonia synthesis reaction is utilized to heat the working fluid for a power block. It has been shown that a working fluid i.e., supercritical steam, can be heated in an ammonia synthesis reactor to 650 degrees C. In this paper, steam and supercritical carbon dioxide (sCO(2)) are proposed to be heated to a higher temperature, i.e., 800 degrees C, utilizing the ammonia synthesis in the context of ammonia-based TCES. Steam at 800 degrees C has the potential to be used for high-temperature electrolysis with a relatively high electrical-to-hydrogen efficiency. Supercritical carbon dioxide (sCO(2)) at 800 degrees C can be used for a high temperature Brayton cycle, which has the potential to achieve a more compact power cycle with higher efficiency than steam Rankine cycle. Two different synthesis systems, i.e., System I and System II, are designed in this paper for achieving heating the working fluid i.e., steam or sCO(2). System I consists of a heat recovery reactor to heat the working fluid with ammonia synthesis, a preconditioning system and an autothermal synthesis reactor to preheat the synthesis gas. System II is similar to System I except the heat recovery reactor is replaced by a heat exchanger, which eliminates the need for another catalyst-filled reactor. A two-dimensional pseudo-homogeneous model is developed to simulate heating of the working fluid in reactors of the systems. Another two-dimensional model is developed to simulate the heat transfer process in heat exchangers. The results demonstrated the feasibility of heating steam or sCO(2) to 800 degrees C in the two systems. But the autothermal synthesis reactors for preheating the synthesis gas are required to be relatively large for both systems. The autothermal synthesis reactor in System II is required to be even larger than that in System I since the synthesis gas without reaction in a heat exchanger need be preheated to a higher temperature. The result shows both the required catalyst volume and reactor wall volume of System I are lower than System II. This study provides a baseline for further design refinement and economic analysis.