Carbon dioxide (CO2) electroreduction to valuable chemicals such as ethylene is an avenue to store renewable electricity and close the carbon cycle. Membrane electrode assembly (MEA) electrolyzers have attracted recent interest in light of their high stability and despite low productivity (a modest partial current density in CO2-to-ethylene conversion of approximately 100 mA cm-2). Here we present an adlayer functionalization catalyst design: a catalyst/tetrahydro-phenanthrolinium/ionomer (CTPI) interface in which the catalytically active copper is functionalized using a phenanthrolinium-derived film and a perfluorocarbon-based polymeric ionomer. We find, using electroanalytical tools and operando spectroscopies, that this hierarchical adlayer augments both the local CO2 availability and the adsorption of the key reaction intermediate CO on the catalyst surface. Using this CTPI catalyst, we achieve an ethylene Faradaic efficiency of 66% at a partial current density of 208 mA cm-2 - a 2-fold increase over the best prior MEA electrolyzer report - and an improved full-cell energy efficiency of 21%.