NOVELTY - A solution for forming a solid comprises a solid dissolved in at least one solvent and carbon nanotubes dispersed in the solvent. At least 1 (preferably at least 10, at least 50 or all) wt.% of nanotubes has unobstructed inner passages.    USE - Used for forming a solid that supports a mass transfer; in catalyst ink and layers, which is used in membrane electrode assemblies and in fuel cells (claimed) useful for communicating gas and/or electric charges, and also in electrochemical cells.    ADVANTAGE - The carbon nanotubes in catalyst inks and catalyst layer substantially improve the efficiency of the inks and layers over the prior art and act as paths for electron conduction and for efficient gas communication, thus the catalyst ink and layers can be formed using less catalyst than in some inks and layers of the prior art without a corresponding loss in efficiency. Electrochemical membrane exchange assemblies and fuel cell that incorporate catalyst layers can be constructed at reduced cost and increased efficiency.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (a) a solid (S1) that supports mass transfer comprising a solid material; and carbon nanotubes distributed in the solid material. At least 1 (preferably at least 5, 10, 50 or all) wt.% of the nanotubes has unobstructed inner passages and operative to communicate gas within the solid material;    (b) a membrane electrode assembly (A1) comprising a solid electrolyte having two opposing surfaces, and a catalyst layer on each of the two opposing surfaces of the solid electrolyte. The catalyst layer has a thickness of not more than 500 microns and comprises a proton-conducting material, precious metal catalyst particles dispersed in the proton-conducting material and carbon nanotubes dispersed in the proton-conducting material. At least 10 wt.% of the carbon nanotubes has unobstructed inner passages with a diameter of 6-20 nm; and    (c) the preparation of the solution useful to form a solid for supporting mass transfer involving: dissolving the solid material into the solvent; and dispersing the carbon nanotubes into the solvent to form the solution such that at least about 1 (preferably 10 or 50)% of the carbon nanotubes having an unobstructed inner passage.