Understanding the nature of the reactive sites of CO2 reduction catalysts is crucial to developing efficient and selective materials to help mitigate the greenhouse effect. In this research, materials based on cobalt phthalocyanine supported by carbon black and pyrolyzed at various temperatures under argon are fabricated and tested for CO2 electroreduction. The results show that the high reactivity of the catalysts for the electroreduction of CO2 to CO is maintained for materials prepared at temperatures up to 700 °C, with CO Faradaic efficiencies of >85% and CO current densities consistently at >40 mA cm–2 at −0.86 V vs RHE. The materials annealed up to 900 °C are also remarkably active, with CO Faradaic efficiencies of >40% and CO current densities of >12 mA cm–2. The combination of X-ray diffraction, infrared and Raman spectroscopies, and X-ray absorption analysis show that the annealed materials exhibit chemical structures drastically different from those of the original CoPC and unsupported pyrolyzed catalyst while highlighting the role of the carbon black support in the formation of active species. These results give crucial insight into the reactive structure of CoPC and open the way for the development of pyrolyzed Co-N4 macrocycles as a new class of materials efficient for the electroreduction of CO2.
Fabricating electrochemical energy storage devices demands significant energy for drying cell components to ensure optimal performance. The development of new, water-tolerant materials would represent a tremendous advance in cost savings and sustainability. Although it is generally established that water deteriorates cell performance, there are few systematic studies on the maximum amount of tolerable water contamination, and most of the studies employed the electrolyte salt $\mathrm{LiPF_6}$, which inevitably decomposes upon exposure to humidity. In this work, the potential of using the non-hydrolyzing salt LiFSI is explored with respect to its performance in Li-ion capacitor cells based on activated carbon (AC) and pre-lithiated graphite (Gr). Water is deliberately added in various amounts (950, 2300, and 6000 ppm), and its effect on the electrochemical performance and aging of AC and Gr electrodes is systematically studied. While the addition of 950 ppm water has no evident impact on capacity retention (96% after 2000 cycles), the addition of 2300 ppm water or 6000 ppm water shows a distinct capacity fade, attributed to a significant loss of lithium inventory and increased resistivity due to irreversible reactions of the water with lithiated Gr. Post-mortem analysis reveals that water promotes the oxidative and reductive decomposition of LiFSI on AC and lithiated Gr, respectively. A significant thickening of the SEI on Gr is observed as the water concentration is increased.
As the desire to for carbon neutral processes increases with the prominence of global warming, the electrochemical reduction of carbon dioxide has the ability to not only reduce atmospheric CO 2 levels, but also produce value added chemicals such as CO, C 2 H 4 , and CHOOH using renewable energy. Polycrystalline copper is a popular electrocatalyst for the CO 2 reduction reaction (CO 2 RR) as it is the only pure metal catalyst to reduce CO 2 to products that require more than two electrons, at reasonable faradaic efficiencies [1]. This has led to the development of many novel copper-based catalysts, such as oxide derived copper, copper nanoneedles and copper nanospheres to enhance the activity and selectivity of the CO 2 RR [2-5]. Often when developing these unique catalysts, polycrystalline copper will be used as a reference material to show how the new material is more active and has better selectivity [2-5]. However, the literature shows substantial inconsistencies for the selectivity of the CO 2 RR on polycrystalline copper (under identical conditions), which brings into question the reliability of the results on the more complicated materials. For example, on polycrystalline copper, Mistry et al. reported a faradaic efficiency for methane and ethylene of 43% and 31%, respectively, at -1 V vs RHE in CO 2 saturated 0.1 M KHCO 3 solution [6], whereas Hori et al. reported a faradaic efficiency of 0% for both methane and ethylene under identical conditions [7]. Here, we show that the discrepancy is likely not due to a single factor alone, but it is made up of several contributing factors. These include surface pre-treatments of the polycrystalline copper [7, 8] (such as sanding, mechanical polishing, electropolishing and etching), natural variation in the crystal orientation on the copper surface [9-11], and the way that iR compensation is applied and subsequently corrected for post-experiment. It was found that the faradaic efficiency for methane and ethylene could increase by up to 140% and 124%, respectively, over a 0.035 V range, which if iR compensation is not correctly adjusted for post experiment, could lead to significant errors in the reported results. We also show that the solution resistance can change by 51% over a 6 hour period of electrolysis, requiring the solution resistance to be regularly measured throughout the experiment to be able to correctly adjust the potential post experiment. Single crystal CO 2 RR experiments have also shown differences between the mechanism for reducing CO on a Cu(111) surface, compared to a Cu(100) surface, which leads to different activities and selectivities [12]. Interestingly, we show electron backscatter diffraction (EBSD) measurements for different pieces of the same polycrystalline copper that have different crystal orientation distributions. References [1] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B. Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Norskov, T. F. Jaramillo, I. Chorkendorff, Chem. Rev., 119 (2019) 7610−7672. [2] M. Ma, K. Djanashvili, W. A. Smith, Angew. Chem. Int., 55 (2016) 6680−6684. [3] L. Mandal, K. R. Yang, M. R. Motapothula, D. Ren, P. Lobaccaro, A. Patra, M. Sherburne, V. S. Batista, B. S. Yeo, J. W. Ager, J. Martin, T. Venkatesan, ACS Appl. Mater. Interfaces, 10 (2018) 8574−8584. [4] D. Raciti, K. J. Livi, C. Wang, Nano Lett., 15 (2015) 6829−6835. [5] A. Loiudice, P. Lobaccaro, E. A. Kamali, T. Thao, B. H. Huang, J. W. Ager, R. Buonsanti, Angew. Chem. Int., 55 (2016) 5789−5792. [6] H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser, B. R. Cuenya, Nat. Comms. 7 (2016) 12123. [7] Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc., Faraday Trans. 1, 85 (1989) 2309−2326. [8] K. Jiang, Y. Huang, G. Zeng, F. M. Toma, W. A. Goddard, A. T. Bell, ACS Energy Lett., 5 (2020) 1206−1214. [9] I. Takahashi, O. Koga, N. Hoshi, Y. Hori, J. Electroanal. Chem., 533 (2002) 135−143. [10] Y. Hori, I. Takahashi, O. Koga, N. Hoshi, J. Phys. Chem. B, 106 (2002) 15−17. [11] Y. Huang, A. D. Handoko, P. Hirunsit, B. S. Yeo, ACS Catal. 7 (2017) 1749−1756. [12] K. J. P. Schouten, Z. Qin, E. Perez Gallent, M. T. M. Koper, J. Am. Chem. Soc., 134 (2012) 9864−9867.
Understanding the structure-activity relationship of materials that are active for the CO2 electrochemical reduction reaction (CO2ERR) is crucial for developing stable, high-performance catalysts. In this research, it is first shown that ball-milling is a highly efficient way to disperse cobalt phthalocyanine (CoPC) onto carbon black without influencing the CO2 electroreduction performance of the resulting materials. Then, the link between the loadings of the CoPC precursor on the carbon support and the CO2ERR activity of the pyrolyzed materials is demonstrated. With CO current efficiencies higher than 45 % and CO current densities as high as -20 mA cm−2 at -0.77 V vs. RHE, the materials with CoPC loadings of 7.9 wt% and higher were still surprisingly active after pyrolysis at 800 and 900°C. On the other hand, the CO2ERR activity of the materials containing less than 6.1 wt% CoPC was drastically reduced after pyrolysis at these temperatures, with CO current efficiencies lower than 10%. X-ray powder diffraction revealed that only the materials containing crystalline CoPC before pyrolysis showed good CO2ERR activity after pyrolysis at 800 and 900°C. Furthermore, X-ray absorption spectroscopy showed that the loading of the CoPC precursor influenced the structure of the active sites in the pyrolyzed CoPC/C materials. Overall, this study highlights the importance of the dispersion of CoPC when forming a material that is catalytically active for CO2ERR after pyrolysis.
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