Insulating Polymer‐Hydrogel Nanocomposite Thin Film ‐ Based Catalytic Electrode for Efficient Oxygen Evolution Reaction

Abstract

The counter-intuitive choice of an insulating polymer for embedding electrocatalysts is shown to facilitate a simple and general strategy to fabricate catalytic electrodes for efficient oxygen evolution reaction (OER) during water splitting. The hydrogel characteristics and appreciable swelling of the polymer in aqueous medium are the key enabling factors; electrolyte absorbed in the polymer matrix is likely to be involved in the electrocatalytic process. Nanocomposite thin films of chitosan spin-cast on common conducting substrates, with an optimal content of NiO and [Ni,Fe]O nanoplates generated through a facile and simple in situ protocol, are shown to effect OER with excellent overpotentials (down to 240 mV at 10 mA/ cm 2 ), low Tafel slope, high Faradaic efficiency, appreciable turnover frequency, and extended stability with high current density. Preliminary investigations with a range of catalystpolymer combinations illustrate the general applicability of the approach.The generation of ever more efficient electrocatalysts for the fundamentally important oxygen and hydrogen evolution reactions (OER and HER) has been the prime focus in the water splitting and related energy research front. Equally important and challenging, but relatively less focused on, is the development of simple and general protocols for the fabrication of robust electrodes bearing the catalyst. Methods like electrodeposition are commonly used to fabricate catalytic electrodes, but their temporal stability under electrolysis and gas-forming conditions can be limited; prominent issues include catalyst dissolution requiring compensation by feeding the relevant ions in the electrolyte, [1] and adverse composition changes of mixed metal oxide catalysts over extended electrolytic runs. [2] The critical problem of fixing powder catalysts has been highlighted in recent studies. Mills and coworkers have discussed the relevance of 'powder-to-electrode' fabrication, in particular the utility of a mechanical, solvent-free approach. [3][4][5] The problem has also been addressed by replacing common binders like Nafion having relatively low electrical conductivity, by carbon material generated from polymer precursors; [6] the latter step requires calcination at 450°C. Enhancement of electrode surface wettability using electrochemically inert but super-hydrophilic carbon nitride that enables also wider exposure of the catalyst sites, is another interesting approach, [7] but again involves high temperature (550°C) calcination and multiple fabrication steps. New strategies to fabricate stable catalyst electrodes, avoiding multiple steps and lengthy processing are desirable. We envisioned that hydrogel polymers, even if electrically insulating, would be widely adaptable and versatile matrices for embedding electrocatalysts that carry out efficient water splitting. Swelling of the polymer in aqueous medium makes this unlikely choice of material highly relevant, and an optimal polymer-catalyst combination can ensure a robust and efficient electrode system; ...

In Situ-Fabricated Hydrogel Polymer—Semiconductor Nanocomposite Thin Film as a Highly Efficient and Robust Electrocatalyst for Hydrogen Evolution Reaction

“…18 The evidence listed above show that this could be achieved at a significantly lower temperature in the PHEMA film; similar observations have been made with other metal oxide−polymer combinations. 13 It is likely that the ligation of Ru 3+ by the ester moiety in PHEMA, as revealed by the notable shifts in the relevant peaks in the Fourier transform infrared (FTIR) and Raman spectra (Figures S7 and S8), leads to effective solubilization of the precursor salt in the polymer matrix and easier oxidation. Another interesting observation is the changes and shifts in specific FTIR spectral peaks (Figure S7 and Table S1) that indicate cross-linking of the polymer occurring during the thermal annealing, concomitantly with the formation of RuO 2 ; this possibly lends extended stability to the nanocomposite polymer thin film against dissolution and peeling off during the electrocatalytic process.…”

“…To demonstrate the promise of polymer-semiconductor nanocomposite thin-film electrodes in the total water splitting process, we have assembled a cell with RuO 2 -PHEMA on Ni foam as the cathode and [Ni,Fe]O-CS on Ni foam as the anode; the high efficiency of the latter (fabricated with the biopolymer, chitosan (CS)) for OER has been demonstrated earlier. 13 Electrolysis of water was carried out at pH = 13.6. As shown in Figure 7a, the cell provides significantly higher current density compared with a cell with the standard electrode Pt/C and [Ni,Fe]O-CS at different operating voltages; an appreciable current is obtained at as low a potential as 1.5 V. The stability is revealed by the chronoamperometry plots in Figure 7b,c.…”

“…12 We have demonstrated the wider scope of deploying insulating but hydrogel polymers as versatile matrices for encapsulating the electrocatalyst, which, rather than impeding, actually enhances the kinetics of OER. 13 The polymer can perform multiple roles in the fabrication and action of the catalytic electrode: it can facilitate the in situ generation of semiconductor nanoparticles within the solid thin film under mild conditions, promote efficient electrocatalytic processes by localizing the electrolyte molecules/ions around the nanocatalyst and the conducting substrate, and effectively circumvent the dissolution and decay of the catalyst over extended catalytic runs. We demonstrate this desirable scenario by the facile in situ production of RuO 2 nanoplates inside a poly(hydroxyethyl methacrylate) (PHEMA) thin film on a conducting substrate and the use of the resulting polymer-semiconductor nanocomposite thin film to achieve highly efficient HER in an alkaline medium; an overpotential of 30 mV at a current density of 10 mA cm −2 and a turnover frequency (TOF) of 24.2 s −1 at an overpotential of 100 mV are demonstrated.…”

“…In the context of catalytic electrodes, the problem highlighted as one of “powder to electrode” transformation has traditionally relied on the singular choice of a Nafion binder . We have demonstrated the wider scope of deploying insulating but hydrogel polymers as versatile matrices for encapsulating the electrocatalyst, which, rather than impeding, actually enhances the kinetics of OER . The polymer can perform multiple roles in the fabrication and action of the catalytic electrode: it can facilitate the in situ generation of semiconductor nanoparticles within the solid thin film under mild conditions, promote efficient electrocatalytic processes by localizing the electrolyte molecules/ions around the nanocatalyst and the conducting substrate, and effectively circumvent the dissolution and decay of the catalyst over extended catalytic runs.…”

Metal-Immobilized Micellar Aggregates of a Block Copolymer from a Mixed Solvent for a SERS-Active Sensing Substrate and Versatile Dip Catalysis

“…Metals such as Ag and Pd nanoparticles are efficient catalysts for a number of chemical reactions, but their reuse and easy separation from the reaction mixture are real challenges. It has been reported that polymer thin films where metal nanoparticles are embedded could be an alternative class of efficient, reusable, and easily fabricated catalysts. ,, As discussed earlier, an amphiphilic block copolymer layer containing different micelle structures could be used as a template to fabricate nanostructured surfaces of metal and can be used for reusable, easily separable, and cost-effective catalysts. There are several reports of metal nanoparticle synthesis using block copolymer micelles as templates for catalysis, but in most of cases, the catalyst is in the solution phase. − …”