The conversion of CO₂ is an attractive strategy towards the mitigation of environmental pollution, and the production of bulk chemicals and fuels by renewables. In the field of CO₂ electroreduction, copper constitutes the most versatile catalyst, being able to generate CO, C₂H₄, CH₄, as well as alcohols and acids from CO₂. Lately, research on Cu-based CO₂ electroreduction has focused on two main topics: firstly, the operation under elevated current densities through the development of stable and selective gas diffusion electrodes (GDEs), and electrolyzers; secondly, the tuning of the selectivity of Cu-catalysts towards a specific product. Regarding the latter, common methods, include doping with other metals and heteroatoms, alloying, as well as creating electrode architectures consisting often of Cu/Ag and Cu/Zn structures.

Contrary to Cu, only a few heterogeneous electrocatalysts have shown the ability to generate more complex molecules beyond CO and HCOOH, such as hydrocarbons and alcohols. Among these catalysts, the chemically robust and synthetically scalable, boron phosphide (BP) has shown the ability to favor the production of CH₃OH from CO₂ under low current densities of ca. 2 mA cm^-2.

Aiming to achieve a synergistic effect between Cu and BP, we herein investigate the role of electrode characteristics versus material properties in gas diffusion electrodes under industrially relevant current densities > 100 mA cm^-2. Starting from GDE-properties, we report on the influence of the layer arrangement of Cu and BP on the surface of the GDL, their mixing ratio as well as the deposition method and cell type on the CO₂ electroreduction. Furthermore, in order to elucidate the role of material properties towards deciding the product spectrum, we compared the activity of B‑compounds with different structural characteristics, possessing cubic as well as hexagonal structures. Concretely, cubic boron nitride (c-BN), and phosphide (BP) as well as hexagonal boron carbon nitride (h-BCN), and hexagonal BN (h-BN) are evaluated in a series of Cu-based GDEs. Depending on the selected layer arrangement, material combination and mixing ratio we are able to steer the main CO₂RR product to either CO, C₂H₄ or CH₄ at 200 mA cm^‑2. Beyond Cu‑BX GDEs, our results demonstrate that in the case of synergistic effects, material/electrocatalyst development and electrode/electrolyzer architecture must go hand-in-hand, providing important strategies towards the optimization of composite electrocatalysts.