Intrinsically stretchable bioelectronic devices based on soft and conducting organic materials have been regarded as the ideal interface for seamless and biocompatible integration with the human body. A remaining challenge is to combine high mechanical robustness with good electrical conduction, especially when patterned at small feature sizes. We develop a molecular engineering strategy based on a topological supramolecular network, which allows for the decoupling of competing effects from multiple molecular building blocks to meet complex requirements. We obtained simultaneously high conductivity and crack-onset strain in a physiological environment, with direct photopatternability down to the cellular scale. We further collected stable electromyography signals on soft and malleable octopus and performed localized neuromodulation down to single-nucleus precision for controlling organ-specific activities through the delicate brainstem.
The Gibbs free energies of key elementary steps for the electrocatalytic oxygen reduction reaction (ORR) are calculated with B3LYP type of density functional theory: O2 + M + H+ + e- (0 eV) --> HOO-M (deltaG1), HOO-M + M --> HO-M + O-M (deltaG2), O2 + 2M + H+ + e- (0 eV) --> O-M + HO-M (deltaG3), and HO-M + O-M + 3H+ + 3e- (0 eV) --> 2H2O + 2M (deltaG4), where H+ is modeled as H3(+)O(H2O)3 and M stands for the adsorption site of a metal catalyst modeled by a single metal atom as well as by an M3 cluster. Taking Pt as a reference, deltaG4 is plotted against deltaG1 for 17 metals from groups V to XII. It is found that no single metal has both deltaG1 and deltaG4 more negative than Pt, although some of them have either more negative deltaG1 or more negative deltaG4. This enables us to explain thermodynamically why no other single metal catalyzes the ORR as effectively as Pt does. Moreover, a thermodynamic analysis reveals that the signs of delta deltaG (the difference between deltaG of other metals and deltaG of Pt) strongly correlate with the valence electronic structure of metals, i.e., delta deltaG1 < 0 and delta deltaG4 > 0 for metals M with vacant valence d orbitals, whereas delta deltaG1 > 0 and delta deltaG4 < 0 for metals M' with fully occupied valence d orbitals. Thus, a simple thermodynamic rule for the design of bimetallic catalysts for the ORR is proposed: couple a metal M (delta deltaG1 < 0) with a second metal M' (delta deltaG4 < 0) to form an alloy catalyst MM'3. The rationale behind this selection is based on M being more efficient for the rate-determining step, i.e., for the formation of the adsorbed species M-OOH, while M' can enhance the reductions of O and OH in the last three electron-transfer steps.
Car-Parrinello molecular dynamics simulations have been performed to investigate the oxygen reduction reaction (ORR) on a Pt(111) surface at 350 K. By progressive loading of (H3O)(+)(H2O)(2,3) + e- into a simulation cell containing a Pt slab and O2 for the first reduction step, and either products or intermediate species for the subsequent reduction steps, the detailed mechanisms of the ORR are well illustrated via monitoring MD trajectories and analyzing Kohn-Sham electronic energies. A proton transfer is found to be involved in the first reduction step; depending on the initial proton-oxygen distance, on the degree of proton hydration, and on the surface charge, such transfer may take place either earlier or later than the O2 chemisorption, in all cases forming an adsorbed end-on complex H-O-O*. Decomposition of H-O-O* takes place with a rather small barrier, after a short lifetime of approximately 0.15 ps, yielding coadsorbed oxygen and hydroxyl (O + HO*). Formation of the one-end adsorbed hydrogen peroxide, HOO*H, is observed via the reduction of H-O-O*, which suggests that the ORR may also proceed via HOO*H, i.e., a series pathway. However, HOO*H readily dissociates homolytically into two coadsorbed hydroxyls (HO* + HO*) rather than forming a dual adsorbed HOOH. Along the direct pathway, the reduction of H-O* + O* yields two possible products, O* + H2O* and HO* + HO*. Of the three intermediates from the second electron-transfer step, HOO*H from the series pathway has the highest energy, followed by O* + H2O* and HO* + HO* from the direct pathway. It is therefore theoretically validated that the O2 reduction on a Pt surface may proceed via a parallel pathway, the direct and series occurring simultaneously, with the direct as the dominant step.
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