Editorial pubs.acs.org/acscatalysis
Best Practices in Pursuit of Topics in Heterogeneous Electrocatalysis addition. Accordingly, each potential breakthrough in the field needs to be easily reproducible among different laboratories. To facilitate this, the intrinsic electrocatalytic activity should be reported on the basis of a proper evaluation of the electrochemical reaction rate, which assumes normalization of the measured reaction rates. Such normalization requires a proper measurement of the electrochemically active surface area (ECSA) of the working electrode. This is critical for Ptbased materials, including Pt nanoparticles of exotic shapes as well as Pt alloys. Some common approaches used for measuring ECSA for Pt-based materials, such as adsorption/desorption of underpotentially deposited hydrogenated species and/or electrochemical oxidation of adsorbed CO, do come with specific challenges, as discussed in the literature.4,5 For submissions to ACS Catalysis, it is expected that authors will properly evaluate ECSA and normalize the kinetic rates with respect to the measured ECSA. Furthermore, similar thoroughness will be required when reporting the performance (reaction rates) of platinum-group metal-free (PGM-free) electrocatalysts (for example, carbonbased materials). While it is more challenging to properly measure ECSA of these systems, moving the field forward requires an increased level of rigor. If it is impossible to directly measure electrochemically active surface areas, normalization of the reaction rates based on the volume of the catalyst and comparison of these volume-based rates to the volume-based rates of Pt standards is expected.
he field of electrocatalysis is undergoing significant growth, as reflected by the substantial increase in the number of published articles on this topic from research groups around the world. In general, electrocatalysis is focused on electrochemical reactions that occur on the surface of an electrode. In addition to the electrode surface properties, the reaction rate also depends on the double layer that is formed in close proximity to the surface. The physical properties of the electrode material along with the nature and properties of the double layer are considered the main constituents of an electrified interface. Most published research has focused on electrode properties, while the liquid side of the interface, including the double layer, has not been as deeply addressed in the literature. Moreover, the ability to resolve key physical properties of electrode materials such as the composition, morphology, surface structure, and the presence of defects can be very challenging, especially in the case of nanomaterials. For the reasons above, it is important for the authors to adopt research approaches that recognize and address experimental obstacles that could lead to misinterpretation of the results. The purpose of this Editorial is to point out best practices for experimental measurements and data analysis in heterogeneous electrocatalysis. This adds to a series of earlier editorials and perspectives that define the journal scope and best practices to ensure that published articles present meaningful and impactful contributions to the scientific literature.1−3
T
■
■
BENCHMARKING CATALYSTS The importance of benchmarking in catalysis has been discussed in a previous Perspective in ACS Catalysis.1 As mentioned there, electrocatalytic performance (current density, overpotential, catalyst stability, etc.) can be affected by many experimental parameters. These include, but are not limited to, the configuration of the electrochemical cell, cleanliness of the electrolyte, operating conditions (temperature, partial pressure or concentration of reactants, rotating speed of electrode, etc.) as well as the skills of the operator. To achieve meaningful comparisons of results from different laboratories, it is essential that an appropriate benchmark catalyst is used and its performance is reported in the manuscript. The benchmarking catalysts should be those that are generally accepted by the community as standards, such as the commercially available 20 or 40 wt % Pt/C catalyst. All authors should demonstrate that the experimental setup and operating conditions are correct, and therefore their results are meaningful, by achieving the expected electrochemical performance of the appropriate benchmark catalyst, which should be in line with the literature data.
CHOICE OF COUNTER ELECTRODE Experimental evaluations of electrocatalytic activity are usually done in a three-electrode cell configuration. Accordingly, the reference, working, and counter electrodes (CE) are employed within the electrochemical cell to assess voltammetry and estimate the intrinsic catalytic activity of the material that is acting as a working electrode. In terms of stability and performance, Pt is considered one of the best electrocatalysts in aqueous electrolytes for a number of reactions. As a result, Pt has also been commonly used as a counter electrode in the form of wire, mesh, sponge, plate, or other configurations. However, despite being traditionally considered highly stable, recent developments of in situ and ex situ techniques have revealed significant dissolution of Pt under electrochemical cycling in both alkaline and acidic electrolytes.6−8 Such dissolution can potentially impact the quality of electrochemical measurements, especially in the case of PGM-free materials. While this issue has been addressed by research groups that are utilizing carbon-based counter electrodes,9 the global electrocatalysis community is only slowly diverting away from the standard use of a Pt CE. A main reason appears to be reliance on earlier published work that refers to the deployment of Pt CE even for electrochemical evaluations of PGM-free materials. Considering that the total number of articles focused on PGMfree materials in electrocatalysis is rapidly increasing, we feel it
■
ELECTROCHEMICAL SURFACE AREA Over the past decade, the electrochemical community has rapidly developed new catalytic materials, and the prolific publication rate has presented the community with the challenge of distinguishing findings that significantly move the field forward from those that provide only an incremental © XXXX American Chemical Society
6392
DOI: 10.1021/acscatal.7b02839 ACS Catal. 2017, 7, 6392−6393
ACS Catalysis
Editorial
is important to address the need to avoid use of a Pt CE in reports studying PGM-free catalytic materials. To this end, manuscripts submitted to ACS Catalysis will be scrutinized for use of best practices, and we encourage authors to avoid the use of a Pt CE, in particular when PGM-free materials are tested. If a Pt CE is used in conjunction with PGM-free catalysts the authors should provide justification along with evidence that the presence of the Pt CE did not cause altered electrochemical performance. We hope that these general guidelines will help the community standardize the evaluation of heterogeneous electrocatalysts. We also emphasize that we will still evaluate all submissions on a case-by-case basis, as we seek to publish the contributions of high technical rigor and impact in electrocatalysis.
Jingguang G. Chen, Associate Editor Columbia University
Christopher W. Jones, Editor-in-Chief Georgia Institute of Technology
Suljo Linic, Associate Editor University of Michigan
Vojislav R. Stamenkovic, Associate Editor
■
Argonne National Laboratory
AUTHOR INFORMATION
ORCID
Jingguang G. Chen: 0000-0002-9592-2635 Christopher W. Jones: 0000-0003-3255-5791 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
■
REFERENCES
(1) Bligaard, T.; Bullock, R. M.; Campbell, C. T.; Chen, J. G.; Gates, B. C.; Gorte, R. J.; Jones, C. W.; Jones, W. D.; Kitchin, J. R.; Scott, S. L. ACS Catal. 2016, 6, 2590−2602. (2) Chang, S.; Fornasiero, P.; Gunnoe, T. B.; Jones, C. W.; Linic, S.; Ooi, T.; Williams, R.; Zhao, H. ACS Catal. 2016, 6, 4782−4785. (3) Jones, C. W.; Smith, D. J.; Gunnoe, T. B.; Zhao, H.; Sautet, P.; Scott, S. L.; Xu, B. ACS Catal. 2014, 4, 2827−2828. (4) Moniri, S.; Van Cleve, T.; Linic, S. J. Catal. 2017, 345, 1−10. (5) van der Vliet, D. F.; Wang, C.; Li, D.; Paulikas, A. P.; Greeley, J.; Rankin, R. B.; Strmcnik, D.; Tripkovic, D.; Markovic, N. M.; Stamenkovic, V. R. Angew. Chem., Int. Ed. 2012, 51, 3139−3142. (6) Lopes, P. P.; Strmcnik, D.; Tripkovic, D.; Connell, J. G.; Stamenkovic, V. R.; Markovic, N. M. ACS Catal. 2016, 6, 2536−2544. (7) Chen, R.; Yang, C.; Cai, W.; Wang, H.-Y.; Miao, J.; Zhang, L.; Chen, S.; Liu, B. ACS Energy Lett. 2017, 2, 1070−1075. (8) Rodriguez, P.; Tichelaar, F. D.; Koper, M. T. M.; Yanson, A. I. J. Am. Chem. Soc. 2011, 133, 17626−17629. (9) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443−437.
6393
DOI: 10.1021/acscatal.7b02839 ACS Catal. 2017, 7, 6392−6393
