Using well-established atomistic techniques, we investigate the defect chemistry, structural effects, and energetics of proton incorporation at the Sigma 5(310)/[001] and Sigma 5(210)/[001] symmetrical tilt grain boundaries of yttria-stabilized zirconia. Building upon past work, we consistently show a dramatic decrease (similar to 4-5 eV) in the proton incorporation and hydration energies in and around the grain boundary structures compared to values obtained for the bulk material and undoped ZrO2 grain boundaries. This decrease is prevalent in both Y segregated grain boundaries and grain boundaries where the distribution of Y is completely random. The results presented here strongly support the argument that proton conduction in this system is primarily interfacially driven, as reported by numerous experimental studies. Redox properties are also presented for grain boundaries structures both with and without defect segregation. The methodology and results presented here can also be applied to a wide range of proton conductors and will prove essential in any future assessment of the effects of grain boundaries on the defect chemistry of protons in these systems.