ORE Open Research Exeter TITLE Electrodeposited lead dioxide coatings AUTHORS Li, Xiaohong; Walsh, Frank C.; Pletcher, Derek JOURNAL Chemical Society Reviews DEPOSITED IN ORE 02 December 2015 This version available at http://hdl.handle.net/10871/18850 COPYRIGHT AND REUSE Open Research Exeter makes this work available in accordance with publisher policies. A NOTE ON VERSIONS The version presented here may differ from the published version. If citing, you are advised to consult the published version for pagination, volume/issue and date of publication Chem Soc Rev Dynamic Article Links 1 1 Cite this: DOI: 10.1039/c0cs00213e www.rsc.org/csr CRITICAL REVIEW 5 5 Q1 Electrodeposited lead dioxide coatings a b a Q2 Xiaohong Li, Derek Pletcher and Frank C. Walsh 10 10 Received 15th December 2010 DOI: 10.1039/c0cs00213e Lead dioxide coatings on inert substrates such as titanium and carbon now offer new 15 15 opportunities for a material known for 150 years. It is now recognised that electrodeposition allows the preparation of stable coatings with different phase structures and a wide range of surface morphologies. In addition, substantial modification to the physical properties and catalytic activities of the coatings are possible through doping and the fabrication of 20 nanostructured deposits or composites. In addition to applications as a cheap anode material in 20 electrochemical technology, lead dioxide coatings provide unique possibilities for probing the dependence of catalytic activity on layer composition and structure (256 references). 25 1. Introduction suffered from continuous corrosion of the underlying lead 25 substrate. It is only much more recently that the preparation Electrode coatings are now extensively employed to improve by electrodeposition of stable lead dioxide layers on inert 1 the performance of modern electrochemical technology. Lead substrates (typically titanium or carbon) has re-ignited interest dioxide (PbO2) is perhaps the grandfather of such coatings in lead dioxide as an anode material. The reasons for this first investigated more than 150 years ago.2 The earlier studies 30 interest are clear. Lead dioxide coatings have a low cost 30 and applications were, however, limited to coatings formed compared to those based on precious metals, high electrical in situ on lead or lead alloys and anodes of this type always conductivity (comparable to metals), high oxygen over- potential and good stability in controlled conditions leading a Electrochemical Engineering Laboratory, School of Engineering to possible application in new processes. In addition, from an Sciences, University of Southampton, Highfield, academic viewpoint lead dioxide is a fascinating material. 35 Southampton SO17 1BJ, UK 35 b School of Chemistry, University of Southampton, Highfield, Lead dioxide coatings can be prepared with different phase Southampton SO17 1BJ, UK structures, a wide range of surface morphologies and as both 40 Dr Xiaohong Li received a Derek Pletcher obtained his 40 BSc in Organic Chemistry BSc and PhD in Chemistry (Lanzhou University), a from the University of MSc in Analytical Chemistry Sheffield and has been a (Lanzhou University) and a member of the Electrochemistry PhD in Physical Chemistry Group in Southampton since 45 (Lanzhou University & Peking 1967 (presently, a Professor 45 University). After experience Emeritus). His research as an engineer/project involving >200 co-workers manager for PetroChina Ltd, has sought to build bridges since 2003, Xiaohong Li has between fundamental electro- 50 worked in the University of chemistry and electrochemical 50 Southampton and is now a technology. These activities Senior Consulting Engineer in have resulted in >330 papers Xiaohong Li the Research Institute for Derek Pletcher and >30 reviews. Derek Industry in the School of Pletcher has been a consultant Engineering Sciences. Her research interests include flow to companies in Europe and North America, an organiser/ 55 batteries, water electrolysers, and nanoscale materials (for presenter of short courses for academic and industrial audiences 55 electrocatalysis, optoelectronics, thermoelectric devices and and an author including ‘‘A First Course in Electrode sensors). Xiaohong Li is the author of >40 papers and is a Processes’’ (RSC, 2009). He is a Fellow of US Electrochemical member of both the International Society of Electrochemistry Society and has been awarded prizes including the Vittorio de and the US Electrochemical Society. Nora Prize (US Electrochem. Soc., 2010). This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, ]]],1–16 | 1 1 doped and undoped materials giving rise to significantly also consider potential applications of such PbO2 anodes. We 1 different catalytic activities. In the long term, lead dioxide are not aware of any substantial reviews of lead dioxide can contribute substantially to a fundamental understanding electrodes in recent years. The history and fundamental of the relationship between the coating structure and catalytic chemistry of PbO2 anodes is covered in 1972 and 1986 reviews 5 activity, important to all fields of catalysis. by Hampson,5,11 a 1979 book by Kuhn,8 a 1990 Chemical 5 This review focuses on lead dioxide coatings, typically with Review of electrode materials12 and a more recent chapter by a thickness of 5–20 mm, formed ex situ by electrodeposition Salkind et al.10 The period since the last review has seen major onto inert substrates such as titanium or carbon from aqueous developments in lead dioxide coatings. electrolytes containing Pb(II). Those formed in situ on lead 10 metal or lead alloy anodes in media such as sulfuric acid where 2. Basic electrochemistry 10 an insoluble lead(II) compound is an intermediate in the formation of the lead dioxide layer are only considered where 2.1 Thermodynamics knowledge of their chemistry is helpful to the story of lead Lead dioxide may be electrodeposited onto inert substrates dioxide layers on inert substrates. Discussion of the lead such as titanium or carbon from a number of media where the 15 dioxide electrode as met in the positive plate in the lead-acid Pb(II) is soluble, e.g. nitric acid, perchloric acid, methane- 15 battery will be completely excluded; in this context, the 2+ sulfonic acid, acetate buffer when the Pb(II) is present as Pb electrode consists of a thick layer of PbSO /PbO paste, quite 4 2 or sodium/potassium hydroxide when the Pb(II) is present as different from the anode coatings considered here. Even so, it plumbite. In the acidic media the overall electrode reaction should be recognised that the lead-acid battery is to be found may be written 20 in all automobiles and is, perhaps, the battery most familiar to 20 2+ - + À the layman—it still represents some 40–45% of the sales value Pb +2H2O PbO2 +4H +2e (1) of all batteries worth some $15 billion to their manufacturers. and in the alkaline media The interested reader is referred to a number of texts that 3–10 À À À cover lead-acid battery technology. HPbO2 +OH - PbO2 +H2O+2e (2) 25 Much of the recent literature addresses the twin goals of 25 At 298 K, the equilibrium potential for the PbO /Pb(II) couple developing lead dioxide coatings that allow high current 2 is given by13 densities and efficiencies for particular electrode reactions and enhanced stability of the coating. Long term stability is Ee(1) = 1.455 À 0.029 log aPb2+ À 0.118 pH (3) always a critical issue when lead dioxide anodes are considered 30 because corrosion of the coating must lead to contamination Fig. 1 shows the equilibrium potential plotted as a function 30 of pH for two concentrations of Pb(II) and the equilibrium of the product/effluent with toxic lead(II). This review will highlight the extensive literature of the past twenty years and potential for the O2/H2O reaction as a function of pH. Three cover the chemistry and electrochemistry of lead dioxide layers conclusions are evident: formed by electrodeposition, the influence of deposition con- (i) In acid solution and on open circuit, lead dioxide is only metastable since its dissolution by reaction with water to give 35 ditions on the structure and properties of the PbO2 layers 35 including doped, composites and nanostructured layers. It will oxygen and Pb(II) is always thermodynamically favourable. In media where the only Pb(II) in solution arises from corrosion of the PbO2 coating (as, for example, in any application of a PbO2 anode), lead dioxide will be metastable even in neutral 40 Frank Walsh received a 40 BSc in Applied Chemistry (University of Portsmouth), a MSc in Materials Protec- tion (UMIST) and a PhD in Electrochemical Engineering 45 (University of Loughborough). 45 Following periods in industry and academic appointments in Portsmouth, Strathclyde and Bath, he is now a Professor 50 in the School of Engineering 50 Sciences at the University of Southampton, leading the Frank C. Walsh Electrochemical Engineering Group. Frank Walsh is the author of >300 papers and 4 books, a willing participant in 55 short courses and an active consultant. He has been an Invited 55 Lecturer at International Conferences throughout the world and received a number of awards, most recently election as a Fellow Fig. 1 The equilibrium potential plotted as a function of pH for two of NACE International (the International Society focused on concentrations of Pb(II) and the equilibrium potential for the O2/H2O corrosion). reaction as a function of pH. 2 | Chem.Soc.Rev., 2011, ]]],1–16 This journal is c The Royal Society of Chemistry 2011 1 solutions. Only in alkaline solutions will lead dioxide be 1 thermodynamically stable. When employed in a practical electrolysis when the electrolyte may contain organics, halides or other species, the rate of corrosion will also need to consider 5 oxidation of these species by lead dioxide (lead dioxide is, in 5 fact, used as an oxidant in organic synthesis).