Part of a membrane-bound complex that couples electron transfer with translocation of ions across the membrane. Catalyzes Na(+) transport, most probably coupled to electron transfer from reduced ferredoxin to methanophenazine and heterodisulfide reductase. Involved in heterodisulfide reduction during methanogenesis from acetate. The Rnf complex is probably composed of eight subunits, including RnfA, RnfB, RnfC, RnfD, RnfE and RnfG. Deletion of the rnf operon abolishes growth on acetate and ferredoxin:heterodisulfide oxidoreductase-coupled Na(+) transport. Belongs to the NqrB/RnfD family.