We found conducting biopolymers to be excellent candidates: they consist of (p-doped) positively charged insoluble polycations functionalized with electroactive hydrogen bonds.[35, 36] They exhibit electrical conductivity greater than 0.1 S cm−1 sufficient to minimize parasitic electrode resistances and work functions ϕ deeper than -5.0 eV—ideal for HER. However, to increase ϕ further and, in parallel, to minimize the hydrogen affinity, we had to modify the hydrogen-bonded structure by embedding motifs that exhibit low Δ𝐺∗HΔGH∗ into the polymers backbone. Using density functional theory, we identify keto-indoleamine hydrogen-bonds as favorable to drive the reaction. This catalytic site possesses a low Δ𝐺∗HΔGH∗ at 220 meV, leading to favorable energetics to enhance the rate-determining Tafel recombination step. We predicted that it will produce molecular hydrogen at a high rate without requiring a metal site. We sought an experimental method to activate the keto-indoleamine motifs in the PDA structure. PDA gas phase synthesis at a high reaction temperature led to a high oxidation power and selective promotion of the keto-indoleamine motifs that significantly enhanced H2 productioтand lowered the overpotential. We find that, upon keto-indoleamine activation, PDA exhibits surface characteristics similar to those of noble metals—low
(similar to Ni) and ϕ (5.6 eV, similar to Ir). In addition it exhibited (electro)chemical robustness in aqueous acidic media. These features combine to provide a novel HER electrocatalyst that shows continuous hydrogen production at minimum losses and without notable degradation over an initial 300 h operation period.
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