Abstract
Electrochemical oxidation is an effective wastewater treatment method. Metal oxide-coated substrates are commonly used as anodes in this process. This article compiles the developments in the fabrication, application, and performance of metal oxide anodes in wastewater treatment. It summarizes the preparative methods and mechanism of oxidation of organics on the metal oxide anodes. The discussion is focused on the application of SnO2, PbO2, IrO2, and RuO2 metal oxide anodes and their effectiveness in wastewater treatment process.
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Abbreviations
- AO7:
-
Azo dye acid orange 7
- AOX:
-
Adsorbable organohalogens
- ALD:
-
Atomic layer deposition
- BDD:
-
Boron-doped diamond
- COD:
-
Chemical oxygen demand
- DCP:
-
Dichlorophenol
- DSA:
-
Dimensionally stable anode
- OA:
-
Oxalic acid
- OER:
-
Oxygen evolution reaction
- PFBA:
-
Perfluorobutanoic acid
- PFCA:
-
Perfluorocarboxylic acid
- PFDA:
-
Perfluorodecanoic acid
- PFHpA:
-
Perfluoroheptanoic acid
- PFHxA:
-
Perfluorohexanoic acid
- PFPA:
-
Perfluoropentanoic acid
- PFNA:
-
Perfluorononanoic acid
- PFOA:
-
Perfluorooctanoic acid
- RB-4:
-
Reactive Blue 4
- RO-16:
-
Reactive Orange 16
- SCE:
-
Standard calomel electrode
- SEM:
-
Scanning electron microscopy
- TOC:
-
Total organic content
- XPS:
-
X-ray photoelectron spectroscopy
References
Abaci S, Tamer U, Pekmez K, Yildiz A (2005) Performance of different crystal structures of PbO2 on electrochemical degradation of phenol in aqueous solution. Appl Surf Sci 240:112–119
Amjoud M’B, Maury F, Soukane S, Duverneuil P (1998) Making of specific electrodes by CVD. Surf Coat Technol 100–101:169–172
Andrade LS, Tasso TT, da Silva DL, Rocha-Filho RC, Bocchi N, Biaggio SR (2009) On the performances of lead dioxide and boron-doped diamond electrodes in the anodic oxidation of simulated wastewater containing the Reactive Orange 16 dye. Electrochim Acta 54:2024–2030
Aquino JM, Rocha-Filho RC, Bocchi N, Biaggio SR (2010) Electrochemical degradation of the Acid Blue 62 dye on a b-PbO2 anode assessed by the response surface methodology. J Appl Electrochem 40:1751–1757
Attia SM, Wang J, Wu G, Shen J, Ma J (2002) Review on sol–gel derived coatings: process, techniques and optical applications. J Mater Sci Technol 18(3):211–214
Beer HB (1980) The invention and industrial development of metal. Reviews and news. J Electrochem Soc 127(8):303C–307C
Brillas E, Sires I, Oturan MA (2009) Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry. Chem Rev 109:6570–6631
Brinker CJ, Frye GC, Hurd AJ, Ashley CS (1991) Fundamentals of sol–gel dip coating. Thin Solid Films 201:97–108
Brungs A, Haddadi-Asl V, Skyllas-Kazacos M (1996) Preparation and evaluation of electrocatalytic oxide coatings on conductive carbon-polymer composite substrates for use as dimensionally stable anode. J Appl Electrochem 26:1117–1123
Carvalho DA, Rocha JHB, Fernandes NS, da Silva DR, Martínez-Huitle CA (2011) Application of electrochemical oxidation as alternative for removing methyl green dye from aqueous solutions. Latin Am Appl Res 41:127–133
Chandler GK, Genders JD, Pletcher D (1997) Electrodes based on noble metals—essential components for electrochemical technology. Platinum Met Rev 41(2):54–63
Chatzisymeon E, Dimou A, Mantzavinos D, Katsaounis A (2009) Electrochemical oxidation of model compounds and olive mill wastewater over DSA® electrodes: 1. The case of Ti/IrO2 anode. J Hazard Mater 167:268–274
Chen X, Chen G, Yue PL (2001) Stable Ti/IrOx-Sb2O5-SnO2 anode for O2 evolution with low Ir content. J Phys Chem B 105:4623–4628
Chen G, Chen X, Yue PL (2002) Electrochemical behavior of novel Ti/IrOx-Sb2O5-SnO2 anodes. J Phys Che B 106:4364–4369
Cheng-chun J, Jia-fa Z (2007) Progress and prospect in electro-Fenton process for wastewater treatment. J Zhejiang Univ Sci A 8(7):1118–1125
Choi H, Stathatos E, Dionysiou DD (2007) Photocatalytic TiO2 films and membranes for the development of efficient wastewater treatment and reuse systems. Desalination 202:199–206
Chu YY, Wang WJ, Wang M (2010) Anodic oxidation process for the degradation of 2,4-dichlorophenol in aqueous solution and the enhancement of biodegradability. J Hazard Mater 180:247–252
Comninellis C (1994) Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochem Acta 39:1857–1862
Comninellis Ch, Chen G (2010) Electrochemistry for the environment. Springer, Berlin. doi:10.1007/978-0-387-68318-8
Comninellis C, Vercesi GP (1991) Characterization of DSA®-type oxygen evolving electrodes: choice of a coating. J Appl Electrochem 21:335–345
Correa-Lozano B, Comninellis C, de Battisti A (1996a) Preparation of SnO2–Sb2O5 films by the spray pyrolysis technique. J Appl Electrochem 26:83–89
Correa-Lozano B, Comninellis C, de Battisti A (1996b) Electrochemical properties of Ti/SnO2-Sb2O5 electrodes prepared by the spray pyrolysis technique. J Appl Electrochem 26(7):683–688
Correa-Lozano B, Comninellis C, De Battisti A (1997) Service life of Ti/SnO2-Sb2O5 anodes. J Appl Electrochem 27:970–974
Costa CR, Montilla F, Morallón E, Olivi P (2010) Electrochemical oxidation of synthetic tannery wastewater in chloride-free aqueous media. J Hazard Mater 180:429–435
da Silva RG, Neto SA, De Andrade AR (2011) Electrochemical degradation of reactive dyes at different DSA® compositions. J Braz Chem Soc 22(1):126–133
del Río AI, Molina J, Bonastre J, Cases F (2009) Study of the electrochemical oxidation and reduction of C.I. Reactive Orange 4 in sodium sulfate alkaline solutions. J Hazard Mater 172:187–195
Devilliers D, Dinh Thi MT, Mahé E, Le Xuan Q (2003) Cr(III) oxidation with lead dioxide-based anodes. Electrochim Acta 48:4301–4309
Duby P (1993) The history of progress in dimensionally stable anodes. JOM 45(3):41–43
Duverneuil P, Maury F, Pebere N, Senocq F, Vergnes H (2002) Chemical vapor deposition of SnO2 coatings on Ti plates for the preparation of electrocatalytic anodes. Surf Coat Technol 151 –152:9–13
Fachinotti E, Guerrini E, Tavares AC, Trasatti S (2007) Electrocatalysis of H2 evolution by thermally prepared ruthenium oxide effect of precursors: nitrate vs. chloride. J Electroanal Chem 600:103–112
Feng Y, Cui Y, Logan B, Liu Z (2008) Performance of Gd-doped Ti-based Sb-SnO2 anodes for electrochemical destruction of phenol. Chemosphere 70:1629–1636
Fenga YJ, Li XY (2003) Electro-catalytic oxidation of phenol on several metal-oxide electrodes in aqueous solution. Water Res 37:2399–2407
Ferro S, Martínez-Huitle CA, De Battist A (2010) Electroxidation of oxalic acid at different electrode materials. J Appl Electrochem 40:1779–1787
Fierro S, Comninellis C (2010) Kinetic study of formic acid oxidation on Ti/IrO2 electrodes prepared using the spin coating deposition technique. Electrochim Acta 55:7067–7073
Fornazari ALT, Malpass GRP, Miwa DW, Motheo AJ (2012) Application of electrochemical degradation of wastewater composed of mixtures of phenol–formaldehyde. Water Air Soil Pollut 223:4895–4904
Gaber M, Ghalwa NA, Khedr AM, Salem MF (2012) Electrochemical degradation of reactive yellow160 dye in real wastewater using C/PbO2-, Pb + Sn/PbO2 + SnO2-, and Pb/PbO2 modified electrodes. J Chem 2013:1–9
Guinea E, Brillas E, Centellas F, Cañizares P, Rodrigo MA, Sáez C (2009) Oxidation of enrofloxacin with conductive-diamond electrochemical oxidation, ozonation and Fenton oxidation: a comparison. Water Res 43:2131–2138
He D, Mho S (2004) Electrocatalytic reactions of phenolic compounds at ferric ion co-doped SnO2:Sb5+ electrodes [J]. J Electroana Chem 568:19–27
Hou Y, Qu J, Zhao X, Lei P, Wan D, Huang CP (2009) Electro-photocatalytic degradation of acid orange II using a novel TiO2/ACF photoanode. Sci Total Environ 407:2431–2439
Jiancheng L, Jie Y, Weishan L, Qiming H, Hongkang X (2012) Electrochemical degradation of Reactive Brilliant Red K-2BP on Ti/RuTiIrSnMn oxide anode in a batch cell. J Electrochem Sci Eng 2:171–183. doi:10.5599/jese.2012.0022
Jiang-tao K, Shao-yuan S, Xiu-ping Z, Jin-ren N (2007) Effect of Sb dopant amount on the structure and electrocatalytic capability of Ti/Sb-SnO2 electrodes in the oxidation of 4-chlorophenol. J Environ Sci 19:1380–1386
Jüttner K, Galla U, Schmieder H (2000) Electrochemical approaches to environmental problems in the process industry. Electrochim Acta 45:2575–2594
Kapałka A, Fóti G, Comninellis C (2008) Kinetic modelling of the electrochemical mineralization of organic pollutants for wastewater treatment. J Appl Electrochem 38:7–16
Klamklang S, Vergnes H, Pruksathorn K, Damronglerd S (2012) Electrochemical incineration of organic pollutants for wastewater treatment: Past, present and prospect. In: Puzyn T, Mostrag-Szlichtyng A (eds) Organic pollutants ten years after the Stockholm convention – Environmental and Analytical update. doi:10.5772/1381. Available from: http://www.intechopen.com/books/organic-pollutants-ten-years-after-the-stockholm-convention-environmental-and-analytical-update/electrochemical-incineration-of-organic-pollutants-for-wastewater-treatment-past-present-and-prospec. Accessed Aug 2013
Kong H, Lu H, Zhang W, Lin H, Huang W (2012) Performance characterization of Ti substrate lead dioxide electrode with different solid solution interlayers. J Mater Sci 47:6709–6715
Koparal AS, Yavuz Y, Gürel C, Öğütveren UB (2007) Electrochemical degradation and toxicity reduction of C.I. Basic Red 29 solution and textile wastewater by using diamond anode. J Hazard Mater 145:100–108
Kusmierek E, Chrzescijanska E, Szadkowska-Nicze M, Kaluzna-Czaplinska J (2011) Electrochemical discolouration and degradation of reactive dichlorotriazine dyes: reaction pathways. J Appl Electrochem 41:51–62
Li M, Feng C, Hu W, Zhang Z, Sugiura N (2009) Electrochemical degradation of phenol using electrodes of Ti/RuO2–Pt and Ti/IrO2–Pt. J Hazard Mater 162:455–462
Li X, Pletcher D, Walsh FC (2011) Critical review: electrodeposited lead dioxide coatings. Chem Soc Rev 40:3879–3894
Lin H, Niu J, Xu J, Huang H, Li D, Yue Z, Feng C (2013a) Highly efficient and mild electrochemical mineralization of long-chain perfluorocarboxylic acids (C9 C10) by Ti/SnO2-Sb-Ce, Ti/SnO2-Sb/Ce-PbO2 and Ti/BDD electrodes. Environ Sci Technol. doi:10.1021/es4034414
Lin H, Niu J, Xu J, Li Y, Pan Y (2013b) Electrochemical mineralization of sulfamethoxazole by Ti/SnO2-Sb/Ce-PbO2 anode: kinetics, reaction pathways, and energy cost evolution. Electrochim Acta 97:167–174
Lipp L, Pletcher D (1997) The preparation and characterization of tin dioxide coated titanium electrodes. Electrochim Acta 42(7):1091–1099
Liu Y, Liu H (2008) Comparative studies on the electrocatalytic properties of modified PbO2 anodes. Electrochim Acta 53:5077–5083
Liu Y, Liu H, Ma J, Wang X (2009) Comparison of degradation mechanism of electrochemical oxidation of di- and tri-nitrophenols on Bi-doped lead dioxide electrode: effect of the molecular structure. Appl Catal B 91:284–299
Liu H, Vajpayee A, Vecitis CD (2013) Bismuth-doped tin oxide-coated carbon nanotube network: improved anode stability and efficiency for flow-through organic electrooxidation. Appl Mater Interfaces. doi:10.1021/am402621v
Lu PJ, Chien CW, Chen TS, Chern JM (2010) Azo dye degradation kinetics in TiO2 film-coated photoreactor. Chem Eng J 163:28–34
Malpass GRP, Miwa DW, Santos RL, Vieira EM, Motheo AJ (2012) Unexpected toxicity decrease during photoelectrochemical degradation of atrazine with NaCl. Environ Chem Lett 10:177–182
Mao X, Tian F, Gan F, Lin A, Zhang X (2008) Comparison of the performances of Ti/SnO2–Sb, Ti/SnO2–Sb/PbO2 and Nb/BDD anodes on electrochemical degradation of azo dye. Russ J Electrochem 44:802–811
Martínez-Huitle CA, Andrade LS (2011) Electrocatalysis in wastewater treatment: recent mechanism advances. Quim Nova 34(5):850–858
Martínez-Huitle CA, Brillas E (2009) Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review. Appl Catal B Environ 87:105–145
Martínez-Huitle CA, Ferro S (2006) Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem Soc Rev 35:1324–1340
Martínez-Huitle CA, de Battisti A, Ferro S, Reyna S, Cerro-López M, Quiro MA (2008) Removal of the pesticide methamidophos from aqueous solutions by electrooxidation using Pb/PbO2, Ti/SnO2, and Si/BDD electrodes. Environ Sci Technol 42:6929–6935
Marugán J, López-Muñoz MJ, van Grieken R, Aguado J (2007) Photocatalytic decolorization and mineralization of dyes with nanocrystalline TiO2/SiO2 materials. Ind Eng Chem Res 46:7605–7610
Nava JL, Quiroz MA, Martínez-Huitle CA (2008) Electrochemical treatment of synthetic wastewaters containing alphazurine A dye: role of electrode material in the colour and COD removal. J Mex Chem Soc 52(4):249–255
Nikolić BŽ, Panić VV, Dekanski AB (2012) Intrinsic potential-dependent performances of a sol–gel prepared electrocatalytic IrO2–TiO2 coating of dimensionally stable anodes. Ectrocatalysis 3:360–368
Niu J, Lin H, Xu J, Wu H, Li Y (2012) Electrochemical mineralization of perfluorocarboxylic acids (PFCAs) by Ce-doped modified porous nanocrystalline PbO2 film electrode. Environ Sci Technol 46:10191–10198
Ozer N, DeSouza S, Lampert CM (1995) Optical and electrochemical properties of sol–gel spin coated CeO2-TiO2 films. SPIE 2531:143
Panakoulias T, Kalatzis P, Kalderis D, Katsaounis A (2010) Electrochemical degradation of Reactive Red 120 using DSA and BDD anodes. J Appl Electrochem 40:1759–1765
Panić VV, Nikolić BŽ (2007) Sol–gel prepared active ternary oxide coating on titanium in cathodic protection. J Serb Chem Soc 72(12):1393–1402
Panić V, Vidaković T, Gojkovic S, Dekanski A, Milonjić S, Nikolić B (2003a) The properties of carbon-supported hydrous ruthenium oxide obtained from RuOxHy sol. Electrochim Acta 48:3805–3813
Panić V, Dekanski A, Wang G, Fedoroff M, Milonjić S, Nikolić B (2003b) Morphology of RuO2–TiO2 coatings and TEM characterization of oxide sols used for their preparation. J Colloid Interface Sci 263:68–73
Panić VV, Dekanski AB, Mišković-Stanković VB, Milonjić SK, Nikolić BŽ (2010) Differences in the electrochemical behavior of ruthenium and iridium oxide in electrocatalytic coatings of activated titanium anodes prepared by the sol–gel procedure. J Serb Chem Soc 75(10):1413–1420
Panizza M, Cerisola G (2008) Electrochemical degradation of methyl red using BDD and PbO2 anodes. Ind Eng Chem Res 47:6816–6820
Panizza M, Cerisola G (2009) Direct and mediated anodic oxidation of organic pollutants. Chem Rev 109:6541–6569
Panizza M, Cerisola G (2010) Applicability of electrochemical methods to carwash wastewaters for reuse. Part 1: anodic oxidation with diamond and lead dioxide anodes. J Electroanal Chem 638:28–32
Panizza M, Bocca C, Cerisola G (2000) Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res 34(9):2601–2605
Papastefanakis N, Mantzavinos D, Katsaounis A (2010) DSA electrochemical treatment of olive mill wastewater on Ti/RuO2 anode. J Appl Electrochem 40:729–737
Pavlov D (1992) The lead-acid battery lead dioxide active mass: a gel-crystal system with proton and electron conductivity. J Electrochem Soc 139:3075–3080
Perednis D, Gauckler LJ (2005) Thin film deposition using spray pyrolysis. J Electroceram 14:103–111
Radjenovic J, Escher BI, Rabaey K (2011) Electrochemical degradation of the β-blocker metoprolol by Ti/Ru0.7Ir0.3O2 and Ti/SnO2-Sb electrodes. Water Res 45:3205–3214
Riihelä D, Ritala M, Matero R, Leskelä M (1996) Electronics, optics and opto-electronics, introducing atomic layer epitaxy for the deposition of optical thin films. Thin Solid Films 289:250–255
Rodgers JD, Jedral W, Bunce NJ (1999) Electrochemical oxidation of chlorinated phenols. Environ Sci Technol 33:1453–1457
Rosales E, Pazos M, Longo MA, Sanromán MA (2009) Influence of operational parameters on electro-Fenton degradation of organic pollutants from soil. J Enviro Sci Health Part A: Toxic/Hazard Subst Environ Eng 44:1104–1110
Sakalis A, Vanĕrková D, Holčapek M, Jandera P, Voulgaropoulos A (2007) Electrochemical treatment of a simple azodye and analysis of the degradation products using high performance liquid chromatography-diode array detection-tandem mass spectrometry. Chemosphere 67:1940–1948
Sala M, Gutiérrez-Bouzán MC (2012) Electrochemical techniques in textile processes and wastewater treatment. Int J Photoenergy 2012:1–12
Scialdone O (2009) Electrochemical oxidation of organic pollutants in water at metal oxide electrodes: a simple theoretical model including direct and indirect oxidation processes at the anodic surface. Electrochim Acta 54:6140–6147
Scialdone O, Randazzo S, Galia A, Filardo G (2009a) Electrochemical oxidation of organics at metal oxide electrodes: the incineration of oxalic acid at IrO2–Ta2O5 (DSA-O2) anode. Electrochim Acta 54:1210–1217
Scialdone O, Randazzo S, Galia A, Silvestri G (2009b) Electrochemical oxidation of organics in water: role of operative parameters in the absence and in the presence of NaCl. Water Res 43:2260–2272
Simond O, Schaller V, Comninellis C (1997a) Theoretical model for the anodic oxidation of organics on metal oxide electrodes. Electrochim Acta 42(13–14):2009–2012
Simond O, Schaller V, Comninellis C (1997b) Anodic oxidation of organics on Ti/IrO2 anodes using Nafion® as electrolyte. Electrochim Acta 42(13–14):2013–2018
Soloman PA, Basha CA, Velan M, Ramamurthi V, Koteeswaran K, Balasubramanian N (2009) Electrochemical degradation of remazol Black B dye effluent. Clean 37(11):8–900
Song S, Zhan L, He Z, Lina L, Tu J, Zhang Z, Chen J, Xu L (2010a) Mechanism of the anodic oxidation of 4-chloro-3-methyl phenol in aqueous solution using Ti/SnO2–Sb/PbO2 electrodes. J Hazard Mater 175:614–621
Song S, Fan J, He Z, Zhan L, Liu Z, Chen J, Xu X (2010b) Electrochemical degradation of azo dye C.I. Reactive Red 195 by anodic oxidation on Ti/SnO2–Sb/PbO2 electrodes. Electrochim Acta 55:3606–3613
Soni BD, Ruparelia JP (2012) Studies on effects of electrodes for decontamination of dyes from wastewater. J Environ Res Dev 6:973–979
Trasatti S (2000) Electrocatalysis: understanding the success of DSR®. Electrochim Acta 45:2377–2385
Vicent F, Morallón E, Quijada C, Vâzquez JL, Aldaz A, Cases F (1998) Characterization and stability of doped SnO2 anodes. J Appl Electrochem 28:607–612
Wang YH, Chan KY, Li XY (2006) Electrochemical degradation of 4-chlorophenol at nickel-antimony doped tin oxide electrode. Chemosphere 65:1087–1093
Wang YQ, Gu B, Xu WL (2009) Electro-catalytic degradation of phenol on several metal-oxide anodes. J Hazard Mater 162:1159–1164
Wang KS, Wei MC, Peng TH, Li HC, Chao SJ, Hsu TF, Lee HS, Chang SH (2010a) Treatment and toxicity evaluation of methylene blue using electrochemical oxidation, fly ash adsorption and combined electrochemical oxidation-fly ash adsorption. J Environ Manage 91:1778–1784
Wang Y, Shen Z, Li Y, Niu J (2010b) Electrochemical properties of the erbium–chitosan–fluorine-modified PbO2 electrode for the degradation of 2,4-dichlorophenol in aqueous solution. Chemosphere 79:987–996
Watts RJ, Wyeth MS, Finn DD, Teel AL (2008) Optimization of Ti/SnO2–Sb2O5 anode preparation for electrochemical oxidation of organic contaminants in water and wastewater. J Appl Electrochem 38:31–37
Xu H, Li AP, Qi Q, Jiang W, Sun YM (2012) Electrochemical degradation of phenol on the La and Ru doped Ti/SnO2-Sb electrodes. Korean J Chem Eng 29:1178–1186
Yang X, Zou R, Huo F, Cai D, Xiao D (2009) Preparation and characterization of Ti/SnO2–Sb2O3–Nb2O5/PbO2 thin film as electrode material for the degradation of phenol. J Hazard Mater 164:367–373
Yeo IH, Wen S, Mho SI (2010) Effect of interfacial oxides on the electrochemical activity of lead dioxide film electrodes on a Ti substrate. Ananl Sci 26:39–44
Yusta FJ, Hitchman ML, Shamlian SH (1997) CVD preparation and characterization of tin dioxide films for electrochemical applications. J Mater Chem 7(8):1421–1427
Zaviska F, Drogui P, Blais JF, Mercier G (2009) In situ active chlorine generation for the treatment of dye-containing effluents. J Appl Electrochem 39:2397–2408
Zhao WR, Xu XH, Shi HX, Wang DH (2003) Degradation mechanism of cationic red X-GRL by ozonation. Chin Chem Lett 14:1309–1312
Zhoua M, He J (2008) Degradation of cationic red X-GRL by electrochemical oxidation on modified PbO2 electrode. J Hazard Mater 153:357–363
Zhuo Q, Deng S, Yang B, Huang J, Yu G (2011) Efficient electrochemical oxidation of perfluorooctanoate using a Ti/SnO2-Sb-Bi anode. Environ Sci Technol 45:2973–2979
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We thank the Department of Chemistry, Kuvempu University, Shankaraghatta for their support.
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Subba Rao, A.N., Venkatarangaiah, V.T. Metal oxide-coated anodes in wastewater treatment. Environ Sci Pollut Res 21, 3197–3217 (2014). https://doi.org/10.1007/s11356-013-2313-6
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DOI: https://doi.org/10.1007/s11356-013-2313-6
Keywords
- Dimensionally stable anodes
- Dopant
- Incineration
- Electrochemical oxidation
- Electrocatalytic activity
- Dip coating
- Sol–gel