I've been looking through textbooks, school worksheets and tutoring notes, and their cathode half equations are slightly different to one another. They have something like these:
2MnO2(s) + 2NH4+(aq) + 2e- --> Mn2O3(s) + H2O(l) +2NH3(aq)
MnO2(s) + NH4(aq) + H2O(l) + e- --> Mn(OH)3(s) +...