Chemically Modifying Electrodes in 3D
Authors: Long Jeffrey W. , Sassin Megan B. , Chervin Christopher N. , Rolison Debra R. 
Published: February 2020

Carbon aerogels, nanofoams, nanofoam-filled carbon papers, and related ultraporous nanoarchitectures are attractive electrode materials for electrochemical sensing, catalysis, and energy storage/conversion applications because of their conductive carbon framework and inherent characteristics of high surface area and through-connected networks of porosity with tunable pore size.
[1]
These conductive nanoarchitectures serve as effective three-dimensional electrochemical interfaces, and enhance the performance of rate-critical devices, such as electrochemical capacitors and fuel cells. The principal limitation of such carbon nanoarchitectures is their lack of the electrochemical functionalities that are beneficial for faradaic charge-storage and electrocatalysis. To address this issue, we modify carbon nanoarchitectures to incorporate such functionalities as electroactive and ion-conducting polymers,
[2]

,
[3]

mixed-conducting metal oxides,
[4]

,
[5]

and specifically adsorbed noble-metal nanoparticles.
[6]

Unlike specialized nanostructured carbons (fullerenes, graphene, nanotubes), which although exist as a unit building block, all too often agglomerates when fabricated into an electrode structure, carbon nanoarchitectures are pre-assembled objects with in-built, co-continuous pore volume and solid scaffolding. Modifying the solid “walls” in such architectures introduces the complication of “painting” blind, i.e., without direct sight lines for precursor ingress. The critical step in generating controlled, non-line-of-sight, conformal polymer or oxide coatings within ultraporous, macroscale nanoarchitectures is the use of conditions where growth self-limits. A corollary benefit arises with conformal, self-limiting growth in that the starting 3D open pore structure remains through-connected.
When starting with a conductive nanoarchitecture, electrochemistry offers remarkably effective modification protocols that satisfy the stringent conditions imposed by non-line-of-sight self-limiting growth. We rely on direct electrodeposition, electroless deposition, and specific adsorption to uniformly modify the interiors of carbon nanofoam papers even when they are as thick as 0.3 mm.
Simply soaking carbon nanofoam papers in a solution containing a transition-metal oxidant (RuO4, MnO4
–, and FeO4
2–) yields conformal, nanometric oxide coatings that extend throughout the carbon-paper-supported nanofoam structure.
[7]
The redox reactions of the charge-storing oxide paint boost the energy density of the resulting hybrid nanoarchitectures for electrochemical capacitor applications,
[8]

,
[9]
,

[10]
catalyze oxygen reduction in alkaline media, such as when used as the air-breathing cathode in metal–air battery cells
[11]
or alkaline fuel cells, and offer a 3D platform to build all-solid-state 3D batteries.4
,
[12]

Electrocatalytic nanoarchitectures are also achieved by modifying carbon aerogels/nanofoams with thiophene functionalities that readily adsorb pre-formed metal nanoparticles (e.g., Pt, Pd, Ag, Au), resulting in an ultraporous electrode structure for fuel-cell applications.

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