Resistance brazing is not spot welding so the electrode face geometry does not have to concentrate the heat into a small area. Frequently the electrode face is machined to conform to the surface that it is in contact with. Frequently the electrode will contact the entire work piece surface. In some cases the electrode faces is milled out so the work piece can be nested into the face of the electrode in order to insure proper part placement and alignment. A small vacuum is sometimes pulled through the center of the electrode to hold the part in place before the electrode closes. This is very common for small part brazing.
The electrode material is frequently RWMA Class 2. It could be copper and in some cases carbon electrodes are used.
When resistance brazing contact materials, refractory faced electrodes RWMA Class 11, 13 and 14 as well as Class 2 are common used.
Water cooling is essential with faced electrodes to insure their own braze joints do not reach the melting point and fail.
References: CMW Inc. Product Resistance Welding Products Catalog
Tuffaloy Products Catalog
RWMA Resistance Welding Manual, Section 1, Chapter 6