Cobalt-Boride Nanostructured Thin Films with High Performance and Stability for Alkaline Water Oxidation Suraj Gupta,* ,† Harshada Jadhav, ‡ Sucharita Sinha, ‡ Antonio Miotello, § Maulik K. Patel, † Arindam Sarkar, ∥ and Nainesh Patel* ,⊥ † School of Engineering, University of Liverpool, Harrison Hughes Building, Brownlow Hill, Liverpool L69 3GH, U.K. ‡ Laser & Plasma Surface Processing Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India § Department of Physics, Università degli Studi di Trento, Via Sommarive, 14, Povo, Trento 38123, Italy ∥ Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ⊥ Department of Physics, University of Mumbai, Tilak Bhavan, Vidyanagari, Santacruz (East), Mumbai 400098, India * S Supporting Information ABSTRACT: Nanocrystalline cobalt boride (Co-B) thin films prepared by pulsed laser deposition were used as an anode catalyst to study the water oxidation reaction in alkaline medium. Elemental depth profiling revealed the bulk of the film to be metallic, which helps in improving conduction of charges, while the surface of the film was rich in CoOOH-type species to facilitate the oxygen evolution reaction (OER). Comparison of OER performance with boron-free samples suggests that inclusion of B helps in improving the OER rate by preventing the conversion of surface Co to stable oxides. The Co-B film achieved a current density of 10 mA/cm 2 at merely 280 mV, with potentiostatic stability for 45 h in alkaline medium, highlighting its superior performance than the powder catalyst. This work not only establishes the advantage of developing thin-film catalysts but also presents a new approach to understand the OER mechanism in metal borides. KEYWORDS: cobalt boride, oxygen evolution reaction, thin film, alkaline water-splitting, XPS depth profile ■ INTRODUCTION In the present era of renewable energy technologies, electrocatalytic water-splitting is the most viable scheme for producing clean hydrogen (H 2 ). 1,2 However, wide-scale implementation of this technology is inhibited by the use of expensive and earth-scarce metals like Pt, Ru, Ir, and so forth. 2 Amongst the numerous low-cost alternatives, metal borides have earned reputation as robust materials in alkaline medium. 3, 4 Especially, for the energy-intensive oxygen evolution reaction (OER), 5,6 metal borides have shown extremely high performances in alkaline medium, surpassing that of standard RuO 2 and IrO 2 catalysts. 3,7 Majority of the inexpensive catalysts, including metal borides, are often synthesized in the powder form, consisting of different nanostructures. 3,4,7,8 However, industrial application demands feasibility of coating these materials on suitable substrates. 9,10 Chemical routes 11,12 to prepare catalyst coatings have been employed by many researchers, but the desired precision in controlling the film adhesion and morphology is difficult to achieve with these methods. The most commonly used method to prepare catalyst coatings is by dispersing the powder catalyst in suitable solvents, along with conducting binders (usually Nafion) to prepare ink, which is then coated on test surfaces. However, in this method, the catalyst particles agglomerate, thereby losing their nanosize properties, and also the inclusion of conducting binders adds to the overall resistance of the coating. 13 In wake of these issues, develop- ment of catalyst coatings by physical deposition techniques like pulsed laser deposition (PLD) remains unexplored. PLD is a versatile technique to fabricate catalyst thin films with a wide range of morphologies obtained by varying the various deposition parameters, which also offers a higher degree of freedom to control the film properties (thickness, nano- structuring, adhesion, etc.). 14,15 In case of metal borides, Co-B is one of the most popular catalyst that has been explored in the form of powders and chemical coatings for water-splitting applications. 3,16−19 Our past work showed that nanostructured Co-B thin films prepared by PLD can produce H 2 with similar rates as that of the noble Pt/C catalyst by dissociation of chemical hydrides. 20 However, surprisingly, there has been no such report for electrocatalytic water-splitting. Thus, this article Received: July 11, 2019 Revised: August 24, 2019 Published: September 17, 2019 Research Article pubs.acs.org/journal/ascecg Cite This: ACS Sustainable Chem. Eng. 2019, 7, 16651-16658 © 2019 American Chemical Society 16651 DOI: 10.1021/acssuschemeng.9b03995 ACS Sustainable Chem. Eng. 2019, 7, 16651−16658 Downloaded via UNIV OF CALIFORNIA SAN FRANCISCO on October 21, 2019 at 20:19:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.