Metal-Functionalized Hydrogels as Efficient Oxygen Evolution Electrocatalysts

3D-printed gelled electrolytes for electroanalytical applications

In this work, several gelators were employed to formulate a conducive gel phase (ionic conductivity) compatible with direct ink writing/bioprinting/robocasting (different names in the literature describe the same printing technology). The main goal of this work was to evaluate gelled phases being a mixture of background electrolyte (NaCl), redox probe (Fe(CN)63-/4-), and gel precursor (guar gum, gelatine, agarose, and agar-agar). The studied concentration of gelators ranged from 0.1 to 4% depending on the employed system. Each gelator required a customized formulation protocol. We have found that guar gum exhibits the best printing properties (lack of aggregates blocking the printing nozzle) while giving the least reproducible electrochemical results (when a glassy carbon electrode was employed as the working electrode). The study of two other gelators (agarose and gelatin) indicated significant changes in the electrochemical properties of the investigated surface as their concentration and number of voltammetric scans were varied. The best electrochemical performance was obtained for agar-agar however, this was also a gelator causing the most problems during 3D printing. Finally, we have employed six screen-printed electrodes displaying approximate properties, that were further covered with a 3D-printed conductive gelled cube (direct printing over the electrode surface). We have found that such a system allowed for a surprisingly good electroanalytical response when the model redox probe (Fe(CN)63-/4-) was considered. This work is a prelude to 3D-printed gel-based detection devices we are currently developing in our team.

Conductive Polymer Hydrogel-Derived 3D Nanostructures for Energy and Environmental Electrocatalysis

Renewable energy and advanced water treatment technologies hold profound significance for driving sustainable development in modern society. Given the environmental friendliness and high efficiency of electrocatalysis processes, great expectations are placed on their applications in energy and water-related fields. However, the electrocatalysis is limited by the selectivity, activity, and durability of the electrocatalytic reactions. Hydrogels, with their hierarchical porous structure, compositional and structural tunability, and ease of functionalization, are bringing surprising advances in advanced energy and environment. Hydrogel catalysts, inheriting the advantages of hydrogel materials, hold promise for achieving significant breakthroughs in electrochemical performance. Here, the latest advancements in energy and environmental electrocatalytic fields are summarized based on the 3D nanostructured hydrogel catalysts. In addition, future potentials and challenges of continuing research on hydrogel materials for energy and environment are discussed.

Mesoporous Single Atom-Cluster Fe-N/C Oxygen Evolution Electrocatalysts Synthesized with Bottlebrush Block Copolymer-Templated Rapid Thermal Annealing

Current electrocatalysts for oxygen evolution reaction (OER) are either expensive (such as IrO2, RuO2) or/and exhibit high overpotential as well as sluggish kinetics. This article reports mesoporous earth-abundant iron (Fe)-nitrogen (N) doped carbon electrocatalysts with iron clusters and closely surrounding Fe-N4 active sites. Unique to this work is that the mechanically stable mesoporous carbon-matrix structure (79 nm in pore size) with well-dispersed nitrogen-coordinated Fe single atom-cluster is synthesized via rapid thermal annealing (RTA) within only minutes using a self-assembled bottlebrush block copolymer (BBCP) melamine-formaldehyde resin composite template. The resulting porous structure and domain size can be tuned with the degree of polymerization of the BBCP backbone, which increases the electrochemically active surface area and improves electron transfer and mass transport for an effective OER process. The optimized electrocatalyst shows a required potential of 1.48 V (versus RHE) to obtain the current density of 10 mA/cm2 in 1 M KOH aqueous electrolyte and a small Tafel slope of 55 mV/decade at a given overpotential of 250 mV, which is significantly lower than recently reported earth-abundant electrocatalysts. Importantly, the Fe single-atom nitrogen coordination environment facilitates the surface reconstruction into a highly active oxyhydroxide under OER conditions, as revealed by X-ray photoelectron spectroscopy and in situ Raman spectroscopy, while the atomic clusters boost the single atoms reactive sites to prevent demetalation during the OER process. Density functional theory (DFT) calculations support that the iron nitrogen environment and reconstructed oxyhydroxides are electrocatalytically active sites as the kinetics barrier is largely reduced. This work has opened a new avenue for simple, rapid synthesis of inexpensive, earth-abundant, tailorable, mechanically stable, mesoporous carbon-coordinated single-atom electrocatalysts that can be used for renewable energy production.

Influence of conductive carbon and MnCo2O4 on morphological and electrical properties of hydrogels for electrochemical energy conversion

In this work, a strategy for one-stage synthesis of polymer composites based on PNIPAAm hydrogel was presented. Both conductive particles in the form of conductive carbon black (cCB) and MnCo2O4 (MCO) spinel particles were suspended in the three-dimensional structure of the hydrogel. The MCO particles in the resulting hydrogel composite acted as an electrocatalyst in the oxygen evolution reaction. Morphological studies confirmed that the added particles were incorporated and, in the case of a higher concentration of cCB particles, also bound to the surface of the structure of the hydrogel matrix. The produced composite materials were tested in terms of their electrical properties, showing that an increase in the concentration of conductive particles in the hydrogel structure translates into a lowering of the impedance modulus and an increase in the double-layer capacitance of the electrode. This, in turn, resulted in a higher catalytic activity of the electrode in the oxygen evolution reaction. The use of a hydrogel as a matrix to suspend the catalyst particles, and thus increase their availability through the electrolyte, seems to be an interesting and promising application approach.

Electrochemical Wearable Biosensors and Bioelectronic Devices Based on Hydrogels: Mechanical Properties and Electrochemical Behavior

Hydrogel-based wearable electrochemical biosensors (HWEBs) are emerging biomedical devices that have recently received immense interest. The exceptional properties of HWEBs include excellent biocompatibility with hydrophilic nature, high porosity, tailorable permeability, the capability of reliable and accurate detection of disease biomarkers, suitable device-human interface, facile adjustability, and stimuli responsive to the nanofiller materials. Although the biomimetic three-dimensional hydrogels can immobilize bioreceptors, such as enzymes and aptamers, without any loss in their activities. However, most HWEBs suffer from low mechanical strength and electrical conductivity. Many studies have been performed on emerging electroactive nanofillers, including biomacromolecules, carbon-based materials, and inorganic and organic nanomaterials, to tackle these issues. Non-conductive hydrogels and even conductive hydrogels may be modified by nanofillers, as well as redox species. All these modifications have led to the design and development of efficient nanocomposites as electrochemical biosensors. In this review, both conductive-based and non-conductive-based hydrogels derived from natural and synthetic polymers are systematically reviewed. The main synthesis methods and characterization techniques are addressed. The mechanical properties and electrochemical behavior of HWEBs are discussed in detail. Finally, the prospects and potential applications of HWEBs in biosensing, healthcare monitoring, and clinical diagnostics are highlighted.

Electronically Conductive, Multifunctional Polymer Binder for Highly Active, Stable, and Abundant Composite Electrodes for Oxygen Evolution

The electrolysis of water to form hydrogen and oxygen is a promising method to store renewable energy. This method requires electrodes that convert water into protons, electrons, and oxygen. We report a multifunctional polymer that conducts electrons and ions and may coreact with the electrocatalyst in the oxygen evolution reaction (OER). The electrodes were prepared in two steps from off-the-shelf reagents. They operate with low loadings of abundant catalysts and are among the most active (100 mA cm^-2 at 1.43 V vs RHE (1.41 V, iR-corrected)) and stable electrodes, reported to date under harsh conditions (85 °C, 6 M KOH, 120 h (0.69% loss over the first 14.5 h and then 0.61% loss over 105.5 h)). Control experiments on glassy carbon electrodes showed that the polycarbazole system significantly outperformed a Nafion system of the same catalyst loading. This simple strategy can be applied to other types of electrodes.