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Selective Conversion of Water and CO2 Using Interfacial Electrochemical Engineering

Periodic Reporting for period 2 - WU TANG (Selective Conversion of Water and CO2 Using Interfacial Electrochemical Engineering)

Reporting period: 2019-05-01 to 2020-10-31

CO2 concentrations are steadily growing in the atmosphere, mainly due to humans burning fossil fuels. The growing CO2 levels have already shown to be causing widespread consequences in the global climate, and as these levels grow, more devastation will follow. While humanity will undoubtably need enormous amounts of energy moving forward, we need to derive the materials and energy from non-fossil resources to slow down and ultimately reverse climate change.While CO2 is detrimental to the atmosphere, it is possible to use this as a resource to both reduce global CO2 levels, and to act as a non-fossil based resource of carbon based materials. If CO2 can be captured and converted using renewable energy (solar, wind), it is possible to reduce global CO2 concentrations and mitigate global climate change. In this project, we aim to use renewably produced electricity to reduce CO into chemicals and fuels, in order to reduce CO2 emissions from the extraction and utilization of fossil fuels. In particular, we are focused on the mechanisms by which CO2 is reduced, both at the catalyst and system level. Here, we address the problem of scaling up this process, by considering the effects of increasing the rate of CO2 reduction (via increased current density), and by looking at a system level to look at scalable architectures such as gas diffusion electrodes (GDEs), which are already in use in industrial electrochemical systems such as water electrolysis and chlor-alkali processes.The results of our project have shed light on the effect of increased current density, by showing that the local environment (pH) increases dramatically on Cu electrodes when increasing current density. These results improve our knowledge of how the electrolyte concentrations can affect product selectivity, and give new boundary conditions to computational and theoretical considerations to electrochemical CO2 reduction. In addition, through modelling techniques, we show that this local pH will increase even further when moving to a scalable architecture of GDE’s, again showing the community that the environment for scaling up this process is much different than previously known.
The major achievements of this project are: (1) experimentally measuring the local pH near a Cu catalyst surface during electrocatalysis, and (2) the computational modeling of local pH as a function of current density for aqueous and GDE based catalyst architectures.To measure the local pH during electrolysis, we developed a novel operando spectroelectrochemical technique that can observe the vibrational modes (and thus concentrations and compositions) of species close to an electrode surface. Using this technique, we could see the different protonated forms of phosophate ions to. Determine the local pH within 5-10 nm of the electrode surface. Using this technique, we show that in an aqueous system, the buffer breaks down very quickly, and after a few tens of mA/cm2 of current, the local pH becomes highly alkaline (pH>10), while the majority of the current goes to hydrogen evolution and not CO2 redcution.In addition, we modeled the local pH near the surface of electrodes in both an aqueous and GDE configuration. Again, we found that as current increases, the local pH also increases, and above 200mA/cm2 the pH is always above 13, regardless of the starting electrolyte.The work on this project directly lead to contributions in the following peer-reviewed articles:1.R. Kas, K. Yang, D. Bohra, R. Kortlever, T. Burdyny, W.A. Smith*, Electrochemical CO2 reduction on nanostructured metal electrodes: fact or defect?, Chemical Science, 11, 1738~1749 (2020)2.K. Yang, R. Kas, W.A. Smith*, In situ infrared spectroscopy reveals persistent alkalinity near electrode surfaces during CO2 electroreduction, J. Am. Chem. Soc., 141, 15891~15900 (2019)3.R. Kas, O. Ayemoba, N.J. Firet, W.A. Smith, A. Cuesta Ciscar, In-situ infrared spectroscopy applied to the study of the electrocatalytic reduction of CO2: Theory, practice and challenges, ChemPhysChem, 20, 2904~2925 (2019) 4.W.A. Smith*, T. Burdyny, D.A. Vermaas, J.C.C. Geerlings, Pathways to industrial-scale fuel out of thin air from CO2 electrolysis, Joule, 3, 1822~1834 (2019) 5.K. Liu, W. A. Smith, T. Burdyny, An introductory guide to assembling and operating gas diffusion electrodes for electrochemical CO2 reduction, ACS Energy Letters, 4, 639~643 (2019)6.T. Burdyny and W.A. Smith*, CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially relevant conditions, Energy and Environmental Science, 12, 1442~1453 (2019) (HOT article, Front Cover)
Prior to this project, the electrochemical CO2 reduction community was focused on aqueous based systems that used near-neutral pH’s. The use of a neutral pH was because if the pH is too low it preferentially produces H2 while if the pH is too high the solubility of CO2 is too low. As a result of the work our the WUTANG project, we have shown that as current density increases, the local pH increases and becomes highly alkaline, and that the only architecture to support this operation is a gas based GDE. Both of these results (high local pH and using GDEs) have changed the focus of the CO2 reduction community as they are highly unsuspected, yet now this has become the standard for the community as a result of the work our group made on this project.Visual representation of the effect of increasing current density on the local pH and CO2 concentrations near electrode surfaces during electrochemical CO2 reduction.As seen in: T. Burdyny and W.A. Smith*, CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially relevant conditions, Energy and Environmental Science, 12, 1442~1453 (2019)
Visual representation of the effect of increasing current density on the local pH and CO2 concentrat