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Electrochemical synthesis of ammonia is a major challenge aimed at making production of ammonia sustainable. Currently ruthenium is the transition metal of choice for catalyzing the industrial Haber-Bosch process. However, electrochemical ammonia synthesis on ruthenium suffers from high overpotential and the competing hydrogen evolution reaction. Recently layered transition metals carbides and nitrides (MXenes) have been identified as a potential material class for ammonia synthesis. MXenes are particularly interesting owing to the high degree of tunability in surface chemistry due to the transition metal choice, interlayer distance, number of layers in the material, and surface termination. These choices affect the electron density of the surface and hence the binding strength of MXenes with key intermediates. In this work, we use density functional theory (DFT) to compute adsorption free energies of relevant intermediates to identify MXenes that are promising for ammonia synthesis. Using uncertainty quantification capabilities within the Bayesian error estimation functional (BEEF), we also compute the probability density functions for catalytic activity predictions. We obtain free energy diagrams and scaling relations and finally report prediction confidence values on the limiting potential and insights into the prospects of using MXenes for nitrogen fixation.