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nanomaterials Article The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins Bogdan-Ovidiu Taranu 1,* and Eugenia Fagadar-Cosma 2,* 1 2 * Correspondence: b.taranu84@gmail.com (B.-O.T.); efagadar@yahoo.com or efagadarcosma@acad-icht.tm.edu.ro (E.F.-C.) Abstract: Hydrogen, considered to be an alternative fuel to traditional fossil fuels, can be generated by splitting water molecules into hydrogen and oxygen via the use of electrical energy, in a process whose efficiency depends directly on the employed catalytic material. The current study takes part in the relentless search for suitable and low-cost catalysts relevant to the water-splitting field by investi- gating the electrocatalytic properties of the O2 and H2 evolution reactions (OER and HER) of two metalloporphyrins: Zn(II) 5,10,15,20-tetrakis(4-pyridyl)-porphyrin and Co(II) 5,10,15,20-tetrakis(3- hydroxyphenyl)-porphyrin. The TEM/STEM characterisation of the porphyrin samples obtained using different organic solvents revealed several types of self-assembled aggregates. The HER and OER experiments performed on porphyrin-modified graphite electrodes in media with different pH values revealed the most electrocatalytically active specimens. For the OER, this specimen was the electrode manufactured with one layer of Co-porphyrin applied from dimethylsulfoxide, exhibiting an overpotential of 0.51 V at i = 10 mA/cm2 and a Tafel slope of 0.27 V/dec. For the HER, it was the sample obtained by drop casting one layer of Zn-porphyrin from N,N-dimethylformamide that displayed a HER overpotential of 0.52 V at i = −10 mA/cm2 and a Tafel slope of 0.15 V/dec. Keywords: metalloporphyrins; aggregates; electrocatalysis; water splitting; electron microscopy 1. Introduction The continuous and rapid increase in the human population worldwide is directly related to the escalating global energy demand. This demand is currently being addressed by using renewable and non-renewable energy sources, with the caveat that non-renewable fuels contribute substantially to the total energy production [1] despite their negative impact on the environment [2]. Instead, the focus should be on the development of eco- friendly technologies for fuels that could be used to avoid a future energy crisis. Hydrogen is regarded as an alternative to the traditional sources that are environmentally harmful due to their combustion products [3]. While it can be produced with technologies that rely on fossil fuels (such as coal gasification, steam reforming, and catalytic partial oxidation), it can also be obtained by employing solar, wind, wave, and tidal energy [1,4]. The specified eco-friendly energy sources are employed to procure hydrogen by powering the water electrolysis process [5], which is responsible for the decomposition of water molecules into hydrogen and oxygen via an electric current [6]. Once generated, hydrogen can be stored, moved, and either reconverted to electrical energy via fuel cells or used to obtain different fuels [7]. Important progress has been made in the water-splitting domain [8–10], and one noticeable aspect of this progress concerns the materials utilized to catalyse the two half-cell reactions involved in water electrolysis, namely, the hydrogen evolution reaction (HER), during which H2 is produced at the cathode as a result of water National Institute for Research and Development in Electrochemistry and Condensed Matter, Dr. A. Paunescu Podeanu Street No. 144, 300569 Timisoara, Romania Institute of Chemistry “Coriolan Dragulescu”, Mihai Viteazu Ave. 24, 300223 Timisoara, Romania Citation: Taranu, B.-O.; Fagadar-Cosma, E. The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins. Nanomaterials 2022, 12, 3788. https://doi.org/10.3390/ nano12213788 Academic Editor: Genqiang Zhang Received: 5 October 2022 Accepted: 24 October 2022 Published: 27 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Nanomaterials 2022, 12, 3788. https://doi.org/10.3390/nano12213788 https://www.mdpi.com/journal/nanomaterialsPDF Image | pH Water-Splitting Electrocatalytic Graphite Electrodes
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