This is a very important topic Dr. Zhou. I agree with you that little attention have been given to Fe3+/Fe2+ redox cycle in electrocatalysis (EF and related processes). Based on my experience, the two carbon materials that are highly efficient for the regeneration of Fe2+ from Fe3+ are carbon-felt and graphite felt. carbon cloth, carbon sponge, carbon brush also showed good performance. However other cathodes like GDEs (irrespective of the based material) bulk graphite etc. doesn't regenerate Fe2+. one thing that is common to those carbon materials that regenerate Fe2+ is the large surface area. This incluse the carbon cloth and others. Our group (Oturan group Marne-la-Vallee, France) have hypothesized that the regeneration of Fe2+ on this materials is due to their large surface area which contains enough active sites for both Fe2+ recycling and H2O2 generation as well as their small interfacial charge resistance (Rt in EIS). However, rencently we prepared a carbon material (results yet to be published) which has almost similar or even better electrochemical characteristic with carbon and graphite felt (lower Rt, better H2O2 generation, better electroactive surface area, simiar surface function group but lower surface area).When applied in EF, it showed almost no Fe2+ regeneration capacity even though it performed better than carbon-felt interms of H2O2 generation and mineralization of organic, but require higher amount of Fe2+ and when perform experiment with Fe3+, the efficiency reduces by more than 30%. In fact, in clear solution we can see the deposition of the iron hydoxide/oxide as electrolysis progress because the surface of the cathode change to dirty yellow. Analysis of the use cathode suggest the same. Thus, we agreed that surface area play more important role in regeneration of Fe2+ more than the interfacial charge resistance. Finally, I think more analytical techniques are require to reveal more insight to the mechanisms of Fe2+ regenerationand what charateristic of the cathode contribute more.

Dr. Soliu. I agree with you that little attention have been given to Fe3+/Fe2+ redox cycle in electrocatalysis (EF and related processes)

That is a really awesome question, which has troubled me for quite a long time. To be honest, we have paid too much attention to the cathodic H2O2 accumulation and unquestionably, it is crucial in the electro-Fenton process. In fact, electro-Fenton comprises two Fenton’s reagents: one is H2O2 and another is VALID Fe2+. The role of the valid Fe2+ is important as well since Fe3+/Fe2+ is the rate-limiting step in the classical Fenton reaction. We could notice as we pave our way to improve H2O2 accumulation to fabricate various cathodes, among which gas diffusion electrodes (GDEs) are regarded as one of the most efficient ones for H2O2 formation, however, hydroxyl radical generation in the GDEs-based electro-Fenton is not that promising. Because iron precipitation happened near-cathode vicinity as H+ was consumed too much by an oxygen reduction reaction (Journal of Power Sources, 2020, 466: 228342). As a result, we need to balance H2O2 formation and Fe3+ reduction to maximize the radical generation if one single cathode used for both reactions.

Regarding Fe3+ reduction, in my view, we need to consider it from two aspects.

One is about its chemical nature before it migrates to the cathode surface, I mean, iron tend to complex with various ions (like -OH) or chelating agents with function groups (-O, -N) such as humic substances polycarboxylates, etc, commonly presented in the aqueous environment. Those ligands donate a pair of electrons to the Fe atom and then redox-properties of Fe3+/Fe2+ were modified.

Another one is what you have mentioned is the nature of the cathode itself once iron approaches to the cathode. Research about it is quite limited and as far as I knew, cathode surface modified with electron-donating moieties, like phenolic -OH, quinoid -C=O, carbon-centered persistent free radicals favors Fe3+ reduction.

Thanks for your discussion on the topic Dr. Deng. Fengxia Deng I have read your recent work, it is an excellent work which proposes that the H2O2 formation and Fe3+ reduction should be considered synergestically to maxmize the EF performance. Also thanks for your views on Fe3+ reduction, which is my knowledge blind spots.

Best,

Wei Zhou

Hello Dr. Ganiyu, I appreciate your discussion on the topic. Yes, I also agree that the specific surface area of carbon cathode play an important role in Fe2+ regeneration. In my recent work on carbon-assisted water electrolysis (CAWE, the system also use Fe3+/Fe2+ cycle, and the reduction of Fe3+ is the rate-determing step of cathodic H2 production), I found oxidized carbon can not effectively reduce Fe3+ to Fe2+, as many papers suggested. So, I am confused that in electro-Fenton process, O-doped carbon cathode are beneficial to Fe3+ electroreduction, but in CAWE, oxidized carbon are bad for Fe3+ reduction. Possibly, one process is electroreduction, another is reduction. I am still trying to figure out this question.

Best,

Wei Zhou

Hi Wei Zhou. I agree with you. Even though we produce lots of H2O2 on the cathode, but we cannot activate the H2O2 to hydroxyl radicals without electro-donor liks Fe(II). But, the electroduction of Fe(III) to Fe(II) and the reduction of O2 to H2O2 are competiting reaction on the surface of cathode. The oxidized carbon surface might be reduced.

Hi Prof. Baek, thanks for your suggestions. Yes, 2eORR process for H2O2 formation and Fe3+ electroreduction are competitive reactions. From my perspectives, it is gradually becoming clear that some certain sites (O-containing sites such as carboxyl, ester, and other defective sites waiting to be unveiled) on carbon matrix are responsible for 2eORR process, and these sites are also possibly active for Fe3+ reduction (little research on this topic).