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Ammonia (NH₃) has received much attention as a hydrogen carrier because it can be easily liquefied with a high hydrogen storage density and emits no greenhouse gas during the dihydrogen evolution process. The ammonia oxidation reaction (AOR) in an electrochemical system has an important merit in which a very high-purity dihydrogen gas can be obtained without an additional separation process that is typically needed for thermochemical decomposition processes. Herein, the electrochemical AOR was carried out in a solid acid electrolysis cell (SAEC) at an intermediate temperature around 250 °C, in which a solid composite of CsH₂PO₄ mixed with SiP₂O₇ was used as an electrolyte and Pt/C-based electrocatalysts were employed as the electrode materials of both anode and cathode. The Pt/C electrode material was modified with the CsH₂PO₄/SiP₂O₇ electrolyte in order to enhance the electrocatalytic activity for the AOR with an improved H₂ production rate. Over the SAEC system reported here, a high AOR performance was obtained with a current density of 67.1 mA/cm² and Faradaic efficiency (FE) of 98.2%. This study can suggest the significant potential of SAEC for the carbon-free H₂ production from the selective electrochemical oxidation of NH₃.