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OXYGEN REDUCTION REACTION Recommended practice to report selectivity in electrochemical synthesis of H2O2

Two diferent types of H2O2 selectivity are reported for the electrochemical synthesis of H2O2: molar fraction selectivity and Faradaic selectivity. Here we revisit their defnitions and discuss the best way to report H2O2 selectivity, which can help to avoid misunderstandings or unfair performance comparisons in this growing feld. Chuan Xia, Jung Yoon (Timothy) Kim and Haotian Wang

ydrogen peroxide (H2O2), as 1.6 one of the most important basic – 2e -ORR (H O ) chemicals in the world, is widely 2 2 H 4e–-ORR (Fuel Cell) used in chemical synthesis, bleaching, wastewater treatment, disinfection, and so on1. Until today, more than 95% of 1.4 global H2O2 production came from the well-established anthraquinone process, which was developed in 1939 by Riedl and Pfleiderer2 and later scaled-up in chemical 1.2 plants in 1953. However, this synthesis process has a large carbon footprint, as well as organic waste. Additionally, centralized Normalized number of publications

H2O2 production requires transportation and storage of unstable and hazardous 1.0 bulk H2O2 solutions. Electrochemical H2O2 2015 2016 2017 2018 2019 synthesis from oxygen reduction or Year oxidation, with energy input from renewable electricity, offers a green and sustainable Fig. 1 | Trends of 2e–-ORR and 4e–-ORR publications in recent years. The normalized number of 3–6 route for on-site H2O2 generation . 2e–-ORR and 4e–-ORR publications per year over the past five years, with the publication numbers However, the challenge here is to develop in 2015 normalized as 1. While the publication numbers for both reactions have continued to grow highly selective catalysts that can direct – over the years, the past two years show a clear surge in H2O2 interest compared to traditional 4e -ORR. the reaction pathway towards the desired The counts were obtained from Web of Science. product. In the oxygen reduction reaction

(ORR), for example, O2 molecules can be 8,9 electrochemically reduced to either H2O2 via Many more classes of materials, including or spectroscopic quantification methods . – – a 2e pathway, or H2O via a 4e pathway: metal alloys, carbon, single-atom catalysts, While these two types of selectivity have and so on, were discovered in recent years completely different definitions and report O2 2Hþ 2e� H2O2; E 0:68 V 1,4,5,8 þ þ ! ¼ to show extraordinary H2O2 selectivity . different data sets (Fig. 2), which will be 1 As an exponential increase in the number discussed in detail in the following section, ð Þ of reports on H2O2 catalysts is expected, the worrying part is that they are sometimes a standard and normative way to report used interchangeably, especially in recent O2 4Hþ 4e� H2O; E 1:23 V þ þ ! ¼ H2O2 selectivity will help researchers publications, suggesting that confusion 2 to have fair comparisons of different exists in the growing research community. ð Þ catalysts, and tremendously benefit the Therefore, we consider it the right time to

As most ORR studies over the past healthy development of the field. However, revisit the definition of H2O2 selectivity – decades have focused on developing 4e we noticed that different types of H2O2 and discuss the most reasonable way to catalysts for fuel cell applications, the H2O2 selectivity are reported depending on the report it, which could serve as a standard pathway has long been treated as the side quantification techniques used, which for fair comparisons, and help to avoid any reaction to be avoided until a recent surge could cause confusion for researchers, confusions or mistakes in future reports on of interest, especially over the past two particularly for those coming from different the electrochemical synthesis of H2O2. – years, in searching for selective 2e -ORR research backgrounds. One type of H2O2 catalysts (Fig. 1). selectivity is reported as molar fraction, H2O2 selectivity quantifcation The first reported catalyst that was which is widely used in rotating ring-disc The RRDE is one of the most common 4 purposely developed for selective H2O2 electrode (RRDE) measurements , and the setups for quantifying catalysts’ H2O2 generation via ORR was reported by other one is reported as Faradaic efficiency, selectivity4,10,11. Electrochemically generated 7 Traube in 1887 using a Hg–Au catalyst . which typically comes from either titration H2O2 on the disc electrode is flushed to a

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1.0 selectivity of 90%, its actual Faradaic

90% selectivity is only about 81.8%. This big difference needs urgent emphasis in the 0.8 field to avoid any unfair comparisons

Maximal difference = 17.2% between molar fraction selectivity and Faradaic selectivity, or any mixed use of 0.6 58.6% these two measures of selectivity. 81.8%

molar fraction Molar fraction or Faradaic efciency 2 0.4 O

2 As we now have a clear image of the big H gap between these two types of selectivity, 41.4% a natural question would be: what is the 0.2 best way for researchers to report H2O2 Fraction = 2FE/(1+FE) selectivity in the field of electrochemical 0.0 synthesis of H2O2? This is obviously an 0.0 0.2 0.4 0.6 0.8 1.0 open question to the whole community, H2O2 Faradaic efficiency and could have different answers. But here we would like to initiate this discussion by

Fig. 2 | The relationship between the two types of selectivity. The difference between H2O2 molar providing some of our own thinking and fraction selectivity and Faradaic efficiency (FE) selectivity under the same performance dataset can arguments. Our recommendation is that, reach a maximal of 17.2%. to avoid any confusions in the future and to make literature comparison easy, it would be more meaningful to directly report Faradaic

Pt ring, where it is partially oxidized, current (charge) producing H2O2 versus efficiency as H2O2 selectivity, instead of the according to the following reactions12: the total current (charge) invested, which molar fraction, no matter what experimental are typically used in H-cell or flow-cell setup is used. This is based on the following Disc : O2 2Hþ 2e� H2O2 þ þ ! 3 measurements. After a certain number of three points: O 4Hþ 4e� H O ð Þ charges pass through the electrochemical Firstly, using molar fraction as H O 2 þ þ ! 2 2 2 cell, generated H2O2 can be quantified selectivity only holds in cases where only by titration or spectroscopic methods, H2O2 and H2O are produced, without any Ring : H2O2 O2 2Hþ 2e� 4 followed by calculation of the H O other by-products. This will become invalid ! þ þ ð Þ 2 2 Faradaic efficiency. if there are other side reactions. For example,

By measuring the Pt ring current and its in acidic ORR to H2O2, there are catalysts collection efficiency, the H2O2 generation Inconsistent H2O2 selectivity that may need large — even current on the disc can be obtained and Due to the different testing methodologies, more negative than the reversible hydrogen 15 used to calculate its selectivity. In 1959, researchers typically report H2O2 molar electrode — to drive the reaction . In Alexander Frumkin et al. published their fraction as the selectivity in the RRDE test, some other cases, large polarizations are initial findings on a theoretical description and report H2O2 Faradaic efficiency as the needed to deliver industrially relevant of solution flow at the RRDE13, and selectivity in H-cell or flow cell tests. While currents3,16. Under these negative potentials, then introduced this technique to the these two types of selectivity have their the hydrogen evolution reaction becomes a community. Since then, clear definitions, confusion sometimes possible side reaction that needs to be taken RRDE setup has been extensively used kicks in, particularly in reports where the into consideration. Another example would in ORR studies, especially in H2/O2 fuel RRDE test was used as the standard test be the side reactions in catalysts, such as the cell catalysis where H2O2 is an undesired for catalysts’ H2O2 selectivity followed by well-known oxidized carbon catalysts, where by-product14. As the purpose of using the applying the catalysts onto gas diffusion surface oxygen can slowly get reduced under RRDE in 4e–-ORR is to quantify how much layer electrodes in cells for bulk . large overpotentials3.

H2O2 was produced during the ORR, H2O2 More importantly, due to inconsistency, Secondly, for electrosynthesis of H2O2 selectivity was conventionally defined as the H2O2 selectivity comparison in the field where electricity input is considered one molar fraction of ORR products based on may not be standardized. We found that the of the most important operational costs, the following equation: molar fraction selectivity can be up to 17.2% it makes more practical sense to focus on higher than that of Faradaic selectivity the current efficiency, namely Faradaic n H O X ðÞ2 2 H2O2 n H2O2 n H2O using the same dataset (Fig. 2). As Faradaic efficiency. This is also widely used in other ¼ ð Þþ ðÞ IR =N IR efficiency can be calculated as I ´ 100% in electrosynthesis areas, including CO2 N =2F D I I 5 R= R = the RRDE system, the relationshipI between reduction, N2 reduction, methane oxidation, ¼ N 2F ID N 4F ð Þ þðÞ� molar fraction and Faradaic efficiency (FE) hydrogen evolution, and many other 2IR=N I I =N can be derived as: organic electrochemical reactions, where the ¼ Dþ R product selectivity is generally equivalent to where I , I , N and F are the ring current, 2FE its efficiency17,18. It would also help R D Molar fraction 6 disc current, collection efficiency of the ¼ 1 FE ð Þ researchers coming from these areas to avoid setup and , respectively. þ possible confusions or mistakes.

Obviously, the molar fraction of H2O2 is As shown in Fig. 2, the molar fraction Thirdly, reporting H2O2 Faradaic quite different from its corresponding selectivity is always higher than Faradaic efficiency in 2e–-ORR could easily be

Faradaic efficiency. The Faradaic efficiency efficiency, except at 0% and 100%. integrated with water oxidation to H2O2, of H2O2 is defined as the amount of Under a typical benchmark molar fraction where molar fraction selectivity has never

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been used due to many possible side selectivity needs to be explained more 9. Shi, X. et al. Nat. Commun. 8, 701 (2017). ❐ 10. Jung, E. et al. Nat. Mater. 19, 436–442 (2020). reactions (OH radical, O3, catalyst oxidation, explicitly in research papers. 11. Gao, J. et al. Chem 6, 1–17 (2020). 6,9 and so on) . 12. Yang, S. et al. ACS Catal. 8, 4064–4081 (2018). We hope that our suggestions will Chuan Xia 1,2, Jung Yoon (Timothy) Kim1 13. Frumkin, A., Nekrasov, L., Levich, B. & Ivanov, J. J. Electroanal. 1,3 ✉ Chem. 1, 84–90 (1959). help researchers to normalize catalytic and Haotian Wang 14. Chong, L. et al. Science 362, 1276–1281 (2018). 1 performance while limiting the possibility Department of Chemical and Biomolecular 15. Barros, W. R., Reis, R. M., Rocha, R. S. & Lanza, M. R. of false-positive results. We believe that this Engineering, Rice University, Houston, TX, USA. Electrochim. Acta 104, 12–18 (2013). 2 16. Chen, Z. et al. React. Chem. Eng. 2, 239–245 (2017). practice should ultimately lead to a better Smalley-Curl Institute, Rice University, Houston, 17. Liu, M. et al. Nature 537, 382–386 (2016). 3 picture of the intrinsic catalytic properties TX, USA. Azrieli Global Scholar, Canadian 18. Foster, S. L. et al. Nat. Catal. 1, 490–500 (2018).

of new H2O2 . In future Institute for Advanced Research (CIFAR), Toronto, studies, more practical electrochemical Ontario, Canada. Acknowledgements ✉ This work was not directly funded. H.W. is a CIFAR systems for H2O2 generation — such as e-mail: [email protected] flow cell reactors or membrane electrode Azrieli Global Scholar in the Bio-inspired Solar Energy Program. C.X. thanks the support from assembly — will become crucial for the Published online: 14 August 2020 J. Evans Attwell-Welch Postdoctoral Fellowship. H.W. – applicability of 2e -ORR catalysts on larger https://doi.org/10.1038/s41929-020-0486-1 acknowledges support from the Welch Foundation scales. However, as RRDE testing has been Research Grant. and will still be a prominent method of References evaluating 2e–-ORR catalytic performances, 1. Perry, S. C. et al. Nat. Rev. Chem. 3, 442–458 (2019). Author contributions it is important to unify how ORR selectivity 2. Hans-Joachim, R. & Georg, P. US patent 2158525A (1939). The project was conceptualized by C.X. and H.W., and 3. Xia, C., Xia, Y., Zhu, P., Fan, L. & Wang, H. Science 366, in RRDE and other electrochemical supervised by H.W.; C.X. and H.W. wrote the manuscript 226–231 (2019). with input from J.K. methods is presented in scientific reports. 4. Lu, Z. et al. Nat. Catal. 1, 156–162 (2018). While we believe the two different types of 5. Kim, H. W. et al. Nat. Catal. 1, 282–290 (2018). 6. Xia, C. et al. Nat. Catal. 3, 125–134 (2020). selectivity need to be unified, we are also 7. Traube, M. Ber. Kgl. Akad. Wiss. 2, 1041–1050 (1887). Competing interests open to other suggestions. At least, H2O2 8. Jiang, K. et al. Nat. Commun. 10, 3997 (2019). The authors declare no competing interests.

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