3.1. Sample Description: morphology, texture, and structure A set of porous alloys were prepared by the method of De Marco et al. . The key steps of the synthesis are summarized as follows: metal chloride precursors are mixed with PMMA beads. The resulting solution is then spray dried and then annealed under Ar. During annealing, the polymer serves both as a pore-generating and as a reducing agent, leading porous spherical metallic particles. One high entropy alloy containing five elements was synthesized, comprising Ru, Pt, Pd, Ir, and Rh in equal mass proportions of 20 wt.%. This sample is denoted "HEA Mix". In addition, a series of less complex samples was prepared: one pure Ru, four binary, one ternary and one quaternary alloy were prepared. The binary alloys were composed of 80 wt.% Ru and 20 wt.% of a second metal: Pt, Pd, Ir, or Rh. The ternary alloy was created with a mass ratio of 60 wt.% Ru, 20 wt.% Pt, and 20 wt.% Pd. The quaternary alloy was formulated with Ru, Pt, Pd, and Ir in a mass ratio of 40-20-20-20. For all these 8 alloys, only the composition was varied, all other parameters (the proportion of PMMA, annealing and spraying parameters) were kept constant. HEA Mix was reported to show good mixing, having showed no sign of phase segregation in Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX) and as abundantly discussed by De Marco et al. . Finaly, one HEA with the same composition as HEA Mix was synthetized using the same protocol, except that no PMMA was introduced and a 10% H2 was add to the Ar flux during the annealing process. This sample is denoted as "HEA Seg", as phase segregation was found in this case, as clearly evidenced by SEM EDX (Fig S1 ). A list of each sample and their composition is provided in Table . When examining the ternary, quaternary, and quinary alloys, the situation is slightly different.