Nickel sulfamate electrolytes have been traditionally used for electroforming of nickel,which refers to the fabrication of free-standing nickel structures grown by electrodeposition on a mandrel. Despite its early commercialisation in 1939, there remains considerable confusion about the role of boric acid, a constituent added to the sulfamate electrolyte to obtain current efficiencies of 100% and prevent the formation of OH– at the cathode surface resulting from the parasitic hydrogen evolution reaction. The debatable role of boric acid during nickel electroforming is in part due to a lack of adequate systematic data on deposition kinetics and current efficiency under important bath conditions such as boric acid concentration, temperature, and agitation. On the other hand, a variety of nickel materials are routinely employed as anodes in sulfamate electrolytes. Modern nickel anodes come in different shapes and compositions, some of which contain a reasonable amount of sulfur inclusions (>0.015 wt% S) calledsulfur-depolarised anodes and others which contain negligible amounts, referred to aslow-sulfur nickel anodes. The performance of these modern anodes produced by theMond process have not been determined in sulfamate-based electrolytes. Therefore,it is unclear if these anodes have a propensity to passivate and breakdown the electrolyte. The purpose of this project is to deliver a critical understanding of the influenceof boric acid and anode materials on nickel deposition and dissolution, respectively,during a sulfamate-based electroforming process. This study uses electrochemical potentiodynamic studies and galvanostatic deposition in a bespoke electrochemical cell alongside Electrochemical Quartz Crystal Microbalance (EQCM) measurements to interrogate and resolve some unanswered questions about the role of boric acid. The current efficiency and kinetics of nickel electrodeposition (and hydrogen evolution) at a stainless-steel electrode at different boric acid concentrations (0 to 0.81 M), solution temperatures (40-55 oC), and electrode rotation speeds (0-1600 rpm) have been determined. The results of EQCM studies were used to verify the current efficiency and rate of nickel deposition at selected boric acid concentrations and temperatures. It was found that addition of boric acid produced deposits with current efficiencies close to 100%, high cathodic Tafel slopes ranging from 120 to 251 mV/decade, and high deposition rates under any condition in unagitated systems. In contrast to the belief that boric acid provides buffering action, it is proposed that boric acid molecules adsorb on the electrode surface and form complexes with NiX+ (where X = OH or Cl) from solution to facilitate nickel ion discharge. Cathodic potentiodynamic measurements were collected in a blank electrolytewithout Ni2+ to investigate the interaction of boric acid with different substrate materials(Ni, Au, Cu, and 304 SS). In the absence of boric acid, the cathodic Tafel slopes weremeasured at 243, 213, 189, and 199 mV/decade for 304 SS, Au, Cu, and Ni electrodes,respectively, indicating the presence of surface oxides/hydroxides most probably resulting from OH– generation via water reduction which likely promote the oxidation of the electrode surface. Simultaneous measurements of charge and mass at an EQCMshowed that a significant amount of hydroxides/oxides was formed in the early stagesof water reduction. The mass change observed in the presence of boric acid showedthat the electrode surface got covered by 4-5 monolayers of boric acid molecules at potentials more negative than -0.9 V vs SCE, which later desorbed. Boric acid adsorbedto different extents on the investigated substrates in the following order: 304SS > Ni> Au > Cu. In a second series of experiments, the kinetics of interfacial reactions wasstudied in the presence of a small amount of Ni2+ (17.85 mM) in the electrolyte. In thiscase, the surface hydroxides/oxides grew at a faster rate due to the electroprecipitation of Ni(OH)2 or NiO. In the presence of boric acid, nickel electrodeposition occurredsimultaneously with hydrogen evolution via proton reduction only. Furthermore, the formation of nickel oxides/hydroxides in the presence of boric acid and Ni2+ was blocked.The high sensitivity of the EQCM (± 1.3 ng) made it a useful tool for the investigation ofadsorbed species which could not be done using a rotating disc electrode. The findingsin this thesis affirm the adsorption of boric acid as a plausible mechanism for suppressing the adsorption of water and facilitating nickel ion discharge. This is attributed to theability of nickel ions to form Ni-O bonds with adsorbed boric acid molecules which arestronger than hydrogen bonds formed with water. Not only do such nickel borate complexes contribute to an increased stability of nickel ions on the electrode surface, butthey also allow for dense packing of nickel ions to form an effective barrier against thediffusion of water from solution to the electrode surface. In other investigations, it was revealed that low sulfur nickel anodes often required high anode potentials in the transpassive dissolution region to support current densities employed for electroforming. This led to anode current efficiencies of 50- 100%. On the other hand, sulfur-activated nickel could support current densities over 100 mA cm–2 at low potentials in the active region and dissolved with current efficiencies close to 100%. The generation of sulfamate ion oxidation products, which give rise to a distinct UV band at 245 nm, was investigated using UV-Vis spectroscopy. It was found that these products were not generated at a sulfur-activated anode but were present at low-sulfur and Pt anodes in quantities that were dependent on the anode current density employed; a lower current density led to a lower rate of generation of sulfamate products. Azodisulfonate was determined to be the principal oxidation product. Indication of the presence of sulfite ions could also be gleaned from the UV-Vis spectra of solutions electrolysed with a low-sulfur nickel anode. The sulfamate oxidation products generated at the low-sulfur nickel anode were found to depolarise the cathode but had a minimal influence on the cathode current efficiency. Earlier investigations have indicated that, while the presence of azodisulfonate and sulfite in electroforming solutions led to the generation of nickel deposits with lower internal stressand increased hardness, they also increased the sulfur content in nickel parts makinglow-sulfur nickel anodes unemployable in applications where sulfur embrittlement is anissue. Therefore, sulfur-activated anodes which do not decompose sulfamate ions arerecommended for such applications.
Date of Award | 18 Apr 2023 |
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Original language | English |
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Awarding Institution | - University Of Strathclyde
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Sponsors | University of Strathclyde |
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Supervisor | Sudipta Roy (Supervisor) & Todd Green (Supervisor) |
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