Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt

Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt Zhuangzhuang Liu, Xiaobo Yang, Shengting Li, Guimin Lu, Ze Sun This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9195389/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 11 You are reading this latest preprint version Abstract Iron impurities severely impair the magnesium electrolysis process by inducing cathode passivation, which inevitably reduces current efficiency and causes irreversible magnesium loss. This study systematically investigates the electrochemical reduction of Fe(III) and its induced cathode passivation mechanism in an anhydrous carnallite (MgCl₂-KCl with a 1:1 molar ratio) melt at 973 K. Through cyclic voltammetry, square wave voltammetry, chronoamperometry, and chronopotentiometry alongside scanning electron microscopy and energy-dispersive X-ray spectroscopy, the results reveal that the reduction of Fe(III) is a single-step, diffusion-controlled quasi-reversible process. Current reversal Chronopotentiometry measurements indicated that the current efficiency substantially decreased from an initial 92.5% to 79.1% after 10 min of electrolysis. Comprehensive morphological and efficiency analyses clearly illuminate the underlying microscopic passivation mechanism. During the initial electrolysis stages, Fe(III) is preferentially reduced and deposits on the cathode surface as a highly porous, sponge-like structure. This newly formed iron matrix readily adsorbs MgO impurities from the molten salt, ultimately generating an insulating Fe-MgO composite film on the electrode. Consequently, this passivation film significantly deteriorates the wettability of the cathode surface to liquid magnesium. It heavily hinders the spreading and agglomeration of magnesium droplets, forcing them to precipitate as fine, discrete, caviar-like particles. Macroscopically, this phenomenon manifests as drastically reduced current efficiency. By elucidating the fundamental electrochemical behavior of Fe(III) and the microscopic mechanics of the Fe-MgO film formation, this study not only enriches essential electrochemical data but also provides a solid theoretical foundation for mitigating cathode passivation in modern industrial magnesium electrolysis processes from a mechanistic perspective. Anhydrous carnallite molten salt system Cathode passivation Fe-MgO composite film Caviar-like Full Text Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 11 May, 2026 Reviews received at journal 09 May, 2026 Reviewers agreed at journal 01 May, 2026 Reviews received at journal 30 Apr, 2026 Reviews received at journal 15 Apr, 2026 Reviewers agreed at journal 15 Apr, 2026 Reviewers agreed at journal 13 Apr, 2026 Reviewers invited by journal 01 Apr, 2026 Editor assigned by journal 25 Mar, 2026 Submission checks completed at journal 25 Mar, 2026 First submitted to journal 23 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9195389","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":616184200,"identity":"031e3b81-2848-4256-85e0-4ff64ba834b2","order_by":0,"name":"Zhuangzhuang Liu","email":"","orcid":"","institution":"East China University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhuangzhuang","middleName":"","lastName":"Liu","suffix":""},{"id":616184212,"identity":"71bc46c7-66c7-4a33-8591-abfabb6636b1","order_by":1,"name":"Xiaobo Yang","email":"","orcid":"","institution":"Qinghai Minzu University","correspondingAuthor":false,"prefix":"","firstName":"Xiaobo","middleName":"","lastName":"Yang","suffix":""},{"id":616184225,"identity":"fb6accbf-c569-4190-b4ca-03911dd291af","order_by":2,"name":"Shengting Li","email":"","orcid":"","institution":"Qinghai Yanhu Industry Co.,Ltd.","correspondingAuthor":false,"prefix":"","firstName":"Shengting","middleName":"","lastName":"Li","suffix":""},{"id":616184236,"identity":"d42d816f-9f2e-4857-b2d3-d4739fe27b3a","order_by":3,"name":"Guimin Lu","email":"","orcid":"","institution":"East China University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Guimin","middleName":"","lastName":"Lu","suffix":""},{"id":616184246,"identity":"ad17450b-74ea-4650-9eaa-09446ce2e97d","order_by":4,"name":"Ze Sun","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwUlEQVRIiWNgGAWjYLCCDzAGD7E6GGeASDZStDDzkKRFvv+M2WObCrvE+fMbGB+8bWOQNyfsqBxz45wzyYkbjjEwG85tYzDc2UDIURI8ZtK5bQcSN7AxsEnztjEkGBwgoIWN/4yZtOW/A4nz2xjYfxOlhYchx0yaseFAYsMxBjZmorRISKSVSfYcSzbecCyxWXLOOQnDDYS0yPcf3ibxo8ZOdn7z4YMf3pTZyBO0BQkwNoBsJV79KBgFo2AUjALcAACRhDe9R6xQIwAAAABJRU5ErkJggg==","orcid":"","institution":"East China University of Science and Technology","correspondingAuthor":true,"prefix":"","firstName":"Ze","middleName":"","lastName":"Sun","suffix":""}],"badges":[],"createdAt":"2026-03-23 04:23:34","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9195389/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9195389/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106287155,"identity":"591678d0-c584-4620-8d7a-1f738cfbf77f","added_by":"auto","created_at":"2026-04-07 07:13:58","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1168346,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9195389/v1_covered_3820ba24-e99d-456f-8762-b55c7fb3e3b1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt","fulltext":[],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":false,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":true,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":true,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Anhydrous carnallite molten salt system, Cathode passivation, Fe-MgO composite film, Caviar-like","lastPublishedDoi":"10.21203/rs.3.rs-9195389/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9195389/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIron impurities severely impair the magnesium electrolysis process by inducing cathode passivation, which inevitably reduces current efficiency and causes irreversible magnesium loss. This study systematically investigates the electrochemical reduction of Fe(III) and its induced cathode passivation mechanism in an anhydrous carnallite (MgCl₂-KCl with a 1:1 molar ratio) melt at 973 K. Through cyclic voltammetry, square wave voltammetry, chronoamperometry, and chronopotentiometry alongside scanning electron microscopy and energy-dispersive X-ray spectroscopy, the results reveal that the reduction of Fe(III) is a single-step, diffusion-controlled quasi-reversible process. Current reversal Chronopotentiometry measurements indicated that the current efficiency substantially decreased from an initial 92.5% to 79.1% after 10 min of electrolysis. Comprehensive morphological and efficiency analyses clearly illuminate the underlying microscopic passivation mechanism. During the initial electrolysis stages, Fe(III) is preferentially reduced and deposits on the cathode surface as a highly porous, sponge-like structure. This newly formed iron matrix readily adsorbs MgO impurities from the molten salt, ultimately generating an insulating Fe-MgO composite film on the electrode. Consequently, this passivation film significantly deteriorates the wettability of the cathode surface to liquid magnesium. It heavily hinders the spreading and agglomeration of magnesium droplets, forcing them to precipitate as fine, discrete, caviar-like particles. Macroscopically, this phenomenon manifests as drastically reduced current efficiency. By elucidating the fundamental electrochemical behavior of Fe(III) and the microscopic mechanics of the Fe-MgO film formation, this study not only enriches essential electrochemical data but also provides a solid theoretical foundation for mitigating cathode passivation in modern industrial magnesium electrolysis processes from a mechanistic perspective.\u003c/p\u003e","manuscriptTitle":"Electrochemical behavior of Fe(III) and mechanism of cathode passivation induced by Fe(III) in MgCl₂-KCl molten salt","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-07 07:12:44","doi":"10.21203/rs.3.rs-9195389/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-05-11T15:56:24+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-09T16:40:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"86158201088472705196246820440530562976","date":"2026-05-01T14:40:21+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-30T05:10:33+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-16T03:59:18+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"234152445236885979668153088807441304719","date":"2026-04-15T17:08:17+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"87526857940363672639525024943409178968","date":"2026-04-13T16:22:57+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-01T19:15:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-25T23:47:42+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-25T23:47:36+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2026-03-23T04:11:13+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"14f0ca4c-01c8-4737-a848-f56b9e465b1f","owner":[],"postedDate":"April 7th, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-05-11T15:56:24+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-09T16:40:15+00:00","index":44,"fulltext":""},{"type":"reviewerAgreed","content":"86158201088472705196246820440530562976","date":"2026-05-01T14:40:21+00:00","index":42,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-30T05:10:33+00:00","index":41,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-05-11T16:11:40+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-07 07:12:44","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9195389","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9195389","identity":"rs-9195389","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}