Additive for carbon dioxide reduction catalysts, catalyst layer, cathode, ion exchange membrane-electrode assembly and solid electrolyte electrolysis device

An additive for a carbon dioxide reduction catalyst, comprising:

• a carrier comprising carbon,
• wherein the additive has a ratio of a water vapor adsorption amount at 25° C. and a water vapor pressure of 2.2 kPa with respect to a water vapor adsorption amount at the same temperature and a water vapor pressure of 3.1 kPa of less than 0.5.

A catalyst layer, comprising:

• an additive comprising a carrier having an aryl group on a surface thereof and comprising carbon;
• and
• a catalyst comprising a carrier comprising carbon, inorganic fine particles or a metal complex being supported on the carrier.