Magdalena
Warczak
a,
Maciej
Gryszel
a,
Marie
Jakešová
a,
Vedran
Đerek
ab and
Eric Daniel
Głowacki
*a
aLaboratory of Organic Electronics, ITN Campus Norrköping, Linköpings Universitet, Bredgatan 34, 60221, Norrköping, Sweden. E-mail: eric.glowacki@liu.se
bCenter of Excellence for Advanced Materials and Sensing Devices, Ruđer Bošković Institute, 10000 Zagreb, Croatia
First published on 11th January 2018
Hydrogen peroxide is one of the most important industrial chemicals and there is great demand for the production of H2O2 using more sustainable and environmentally benign methods. We show electrochemical production of H2O2 by the reduction of O2, enabled by an organic semiconductor catalyst, N,N′-dimethyl perylenetetracarboxylic diimide (PTCDI). We make PTCDI cathodes that are capable of stable and reusable operation in aqueous electrolytes in a pH range of 1–13 with a catalytic figure of merit as high as 26 kg H2O2 per g catalyst per h. These performance and stability open new avenues for organic small molecule semiconductors as electrocatalysts.
We fabricated semiconductor electrodes for electrocatalytic oxygen reduction using a metallic thin film covered with a layer of the semiconducting small molecule PTCDI (Fig. 1a), which was sublimed under high vacuum to produce a nanocrystalline morphology (Fig. 1b and c; ESI,† Fig. S1). Scanning electron micrographs reveal continuous layers of rounded crystallites of average diameter 57 ± 7 nm when evaporated on Au. Various metallic films were evaluated as back electrodes, such as titanium, indium tin oxide, fluorine-doped tin oxide, and finally gold. Mechanical delamination of the PTCDI layer was found to occur in all cases, except Cr/Au, where adhesion of PTCDI was ideal. It became immediately apparent that 50 nm gold films on top of a 2 nm chromium sticking layer afforded a combination of high currents and excellent stability, and were thus used throughout this study as a back contact material. 100 nm-thick PTCDI films were used throughout, as we found no strong dependence of the cathodic current on the thickness (ESI,† Fig. S2). We estimated via cyclic voltammetry in a nonaqueous electrolyte the valence and conduction band levels of the PTCDI (ESI,† Fig. S3), calculating ECB of −3.9 eV versus vacuum, or −0.6 V vs. NHE, which are amply reductive with respect to the standard potential for O2 reduction to H2O2, +0.7 V. The PTCDI electrodes were then characterized under aqueous conditions using cyclic voltammetry and galvanostatic electrolysis experiments with separated cathode and anode chambers, as shown in Fig. 1d. Cathodic currents are visible with an onset around 0 V vs. Ag/AgCl at pH 1, while at higher pH 7–13 the onset shifts to −0.15 V (Fig. 2a–c). Purging the cathode chamber with argon leads to a substantial decrease of these cathodic currents, while saturation with oxygen enhances them, unequivocally implicating ORR (Fig. 2). The remaining cathodic current under argon purged conditions is attributed to the hydrogen evolution reaction (HER). Further cathodic polarization beyond −0.4 V leads to dominance of HER, where current densities as high as 6 mA cm−2 can be achieved; however, this is beyond the scope of the present work (ESI,† Fig. S4). Using the horseradish peroxidase/tetramethylbenzidine assay,14 we determined that galvanostatic cathodic electrolysis with O2-bubbled electrolytes gives H2O2 as the dominant product of oxygen reduction. We found that H2O2 production occurs over the entire pH range, with higher Faraday efficiency for the 2e−/2H+ acidic process. At pH 1, we observed the two-regime Tafel slope characteristic of ORR, with 98 and 82 mV dec−1, and an exchange current density of 1 μA cm−2.
Having established peroxide as the product of ORR, we focused on the deployment of PTCDI electrodes for practical electrosynthesis of H2O2. Galvanostatic measurements were conducted with the cathode and anode chambers separated by an agarose bridge, with a constant flow of oxygen into the catholyte (150 mL min−1). The concentration of H2O2 and the corresponding Faraday efficiency over time of electrolysis at different pH and current density values are shown in Fig. 3a and b. H2O2 was accumulated in the catholyte, with the Faraday efficiency, generation rate, and total concentration highest at low pH. Na2SO4, with H2SO4 and NaOH added to adjust pH, was used in all cases to provide a stable electrolyte without undesired side reactions. In particular, chloride containing electrolytes were avoided to prevent problems with chloride oxidation and also etching of the gold layers. The most optimal conditions found for H2O2 generation were pH 1 and 0.5 mA cm−2 (Fig. 3c). Under these conditions a stable cell voltage of −2.5 V was maintained for 48 hours, with the agarose separator being the limiting factor for obtaining lower voltages. Under all conditions of different current densities and pH values, the Faraday efficiency decreased over time. Two explanations for this observation are possible: either the electrocatalyst suffers from degradation or the increased H2O2 concentration begins to favor the subsequent reduction of peroxide to water. We establish that the second hypothesis is correct, as the same cathode was reused for up to six rounds of electrolysis, with the Faraday efficiency value returning to the initial higher values at the beginning of the electrolysis experiment (Fig. 3d). In total, samples were used for up to 344 hours in this study without degradation in performance. SEM imaging and optical reflectivity measurements of electrodes before and after extended electrolysis revealed no apparent morphological changes or changes in optical absorbance, respectively (ESI,† Fig. S1 and S5). Secondly, the further reduction of H2O2 to H2O can be clearly evidenced by measuring the cathodic current upon the addition of peroxide to the electrolyte (ESI,† Fig. S4). Therefore the problem of why higher equilibrium H2O2 concentrations are not possible is because once [H2O2] reaches similar values as [O2] (≈1 mM under 1 atm of pure O2), the further reduction of H2O2 to H2O becomes competitive with ORR. Future electrocatalyst improvement should focus on obtaining molecular semiconductors with higher overpotential for H2O2 reduction. Overall, the performance benchmarks of PTCDI cathodes for hydrogen peroxide production are competitive, with best results at initial rates for pH 1: 26 kg H2O2 are produced per one gram of catalyst per hour. Considering the initial rates, this process consumes 4.8 kW h per kg of H2O2, corresponding to a 21% thermodynamic electrical energy-to-peroxide conversion efficiency. Higher peroxide yields and an eventual industrial application could be enabled by incorporating PTCDI electrodes into flow reactors, as has been suggested before for upscaled peroxide production.4
Organic molecular semiconductors comprise a rich field of applied research; however, deployment of organic semiconductors as electrocatalysts represents an untapped direction. Herein we have shown that the archetypical carbonyl pigment semiconductor PTCDI can be a stable n-type electrocatalyst which can facilitate the oxygen reduction reaction to produce H2O2. Our finding that an intrinsic (undoped) molecular semiconductor can support such high catalytic electron currents is an unexpected result which should stimulate research into deploying such materials as electrocatalysts. On the other hand, our results make electrosynthesis of H2O2 closer to being an approach competitive with the incumbent AO process of production for this key industrial chemical. Future research should focus on molecular structures which suppress the reduction of H2O2 to H2O and on exploring other possibilities of organic semiconductors as catalytic materials in general.
We acknowledge funding from the Wallenberg Center for Molecular Medicine at Linköping University for support of this work.
Footnote |
† Electronic supplementary information (ESI) available: SEM and reflectance of PTCDI electrodes, frontier energy level calculations, and PTCDI electrochemistry in oxygen-free water. See DOI: 10.1039/c7cc08471d |
This journal is © The Royal Society of Chemistry 2018 |