The ornamental design for a valvecap for a fuel cell, as shown and described.
FIG. 1 is a top perspective view of the present valve cap design;
FIG. 2 is a front view of same, the rear view being identical;
FIG. 3 is a side view of same, both sides being identical; and,
FIG. 4 is a top view of same.
The broken line showing of a portion of a valve cap for a fuel cell is included for the purpose of illustrating environmental structure and forms no part of the claimed design.