(327i) Mechanistic Electrochemistry at the Center of Energy Science: From Catalysts to Batteries
Srini Ramakrishnan, University of California Berkeley, Berkeley CA 94720
I strongly believe that fundamental electrochemical science and engineering are at the heart of the push towards a sustainable energy ecosystem, from revolutionizing transportation with electric vehicle batteries, to agriculture with electro-catalyzing ammonia synthesis. With my formal training in chemistry, particularly in the electrochemistry of transition metal-based systems for catalysis and energy storage, I am interested in working on fundamental scientific challenges aimed at hastening these grand-scale transitions.
Postdoctoral Work: Probing Degradation Mechanisms in Advanced Lithium-ion Battery Chemistries
As a postdoctoral scholar in the lab of Prof. Bryan McCloskey in the Chemical Engineering department at UC Berkeley, Iâm studying the fundamental chemical processes at the electrode-electrolyte interface causing performance degradation in high energy lithium ion batteries using Online Electrochemical Mass Spectrometry (OEMS) and other spectroscopic techniques. Iâm studying Li-rich Ni, Mn, Co (NMC) oxide cathode materials which have theoretical capacities greater than 250 mAh/g, and therefore, can provide a game-changing boost to the energy density of current Li-ion batteries. Their high capacities stem for the participation of the oxygen anions of the oxide in charge compensation during Li-extraction at voltages above 4.5 V vs. Li+/0, as a result of which, they suffer from rapid capacity and voltage fade during battery cycling, with concomitant surface O2 release, electrolyte decomposition, phase segregation etc. Depending on the specific process, interfacial degradation leads to the release of various gases such as CO2, O2 and C2H4 during battery charging and discharging, which I measure qualitatively and quantitatively by OEMS.
As an example, I found that removing Li2CO3 impurities on the particle surface significantly suppresses electrolyte decomposition as well as O2 evolution during the first charge with no loss of extractable capacity, resulting in superior longer-term cycling performance. Ex-situ acid titrations of charged electrodes and gas analysis with mass spectrometry revealed the presence of peroxo-like species stemming from O-redox at much lower potentials than previously imagined. Analogous studies on isotopically labelled electrodes revealed a surface-to-bulk propagation of these peroxo-like O-species with increasing state-of-charge. Spectroscopic studies of the treated particles revealed a lithium deficiency on the surface, which we ascribe to the reduction in interfacial reactivity.
An alternate strategy to prevent interfacial degradation that I am currently pursuing involves coating the electrode surface with insulating transition metal oxides with atomic layer deposition, which I have found to result in a similar suppression of gas evolution during battery cycling. Mechanistic studies to understand the origins of this suppression of interfacial reactivity are currently underway.
An often-overlooked aspect of battery degradation is the electrolyte. Using a combination of NMR spectroscopy, X-ray photoelectron spectroscopy and OEMS, I am studying the electrochemical non-innocence of fluoridic species such as LiF, generated from the decomposition of LiPF6 on the electrode surface.
PhD work: Mapping Free Energies to Design Molecular Electrocatalysts on a Computer
Density functional theory based computational methods have traditionally been used to rationalize the thermodynamics and kinetics of a particular catalyst or its local derivatives with the luxury of extensive experimental benchmarking. In my PhD work at Stanford University, advised by Prof. Christopher E. D. Chidsey and co-advised by Prof. Robert M. Waymouth, I explored turning this workflow around, that is, using state-of-the-art density functional theory to guide experimental electrocatalyst design. For the specific example of the two-electron reduction of CO2 to CO and formate, and of protons to H2, I demonstrated how the use of two thermodynamic descriptors, viz. the free energies of two key intermediates relative to the lowest energy product, streamlines the search for promising catalyst candidates from a large library of transition metals and organic ligands. The predictions were validated by subsequent experimental synthesis and electrochemical studies of an iron complex that was found to be active towards CO2 and proton reduction at the expected reduction potential. The experimental studies, however, indicated that the in-silico method was underestimating the free energy of CO2 binding to the electrochemically reduced iron complex, thereby calling for further refinements and validations of the density functionals to adequately model this key step that leads to CO production.
This work, however, was the culmination of several experimental and computational endeavors to understand the electrocatalytic reactivity of transition metal complexes towards CO2 reduction at low driving forces, some of which included the a) unraveling of a new pattern of CO2 activation by singly reduced Ru-complexes, b) engineering single step two-electron reductions in first row transition metal complexes to minimize overpotentials associated with electron transfer, c) highlighting a key deactivation step when using transition metal hydrides as alcohol-oxidation catalysts in fuel cells.
Prior Research (MSc And BSc)
I studied the disparity in the low temperature microwave spectrum of the benzene-H2O dimer and the benzene-H2S dimer using the van Vleck contact transformation to the Vibrational-Rotational-Translational (VRT) Hamiltonian (IISc Bangalore).
I used hyper-spectral imaging and electron microscopy to study the growth of higher order gold nanostructures in L. acidophilus bacteria isolated from whey (IIT Madras).
I served as the Head teaching assistant (TA) for a first-of-its-kind Electrochemical Measurements laboratory course with Prof. Chidsey at Stanford University, designed for upper-class chemistry majors and graduate students. As part of curriculum development for this course, I designed and assembled a custom âopen-boxâ potentiostat with tunable circuit elements connected to a data acquisition device controlled by MATLABâ¢, with the aim of helping the students develop a circuit-level understanding of electrochemical measurements including potentiometry, voltammetry and electrochemical impedance spectroscopy.
My lecturing experience includes a three-part lecture series titled âDiscovering the World of Batteriesâ for the Science Circle High School Program at Stanford University, and a guest lecture on âDensity Functional Theory for Organometallic Chemistsâ as part of the graduate level Advanced Inorganic Chemistry course at Stanford University.
I have served as a TA trainer at Stanford University, training first-year graduate students for taking on teaching roles in the chemistry department. As a TA myself, I have taught several lecture and laboratory-based undergraduate chemistry courses.
- Ramakrishnan, Chidsey âInitiation of Electrochemical Reduction of CO2by a Singly Reduced Ruthenium(II) Bipyridine Complexâ, Chem. 2017, 56(14), 8326-8333.
- Waldie, Ramakrishnan, Kim, Maclaren, Chidsey, Waymouth âMulti-Electron Transfer at Cobalt: Influence of the Phenylazopyridine Ligandâ Am. Chem. Soc. 2017, 139(12), 4540-4550.
- Ramakrishnan, Chakraborty, Brennessel, Jones, Chidsey âRapid Oxidative Hydrogen Evolution from a Family of SquareâPlanar Nickel Hydride Complexesâ Sci. 2016, 7, 117-127.
- Ramakrishnan, Waldie, Warnke, de Crisci, Batista, Waymouth, Chidsey âExperimental and Theoretical Study of CO2 Insertion into Ruthenium Hydride Complexesâ Chem. 2016, 55(4), 1623-1632.
- McLoughlin, Waldie, Ramakrishnan, Waymouth âProtonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagentâ, J. Am. Chem. Soc. 2018, 140(41), 13233-13241.
- Ramakrishnan, Moretti, Chidsey âMapping Free Energy Regimes in Electrocatalytic Reductions with Transition Metal Complexesâ,
- Ramakrishnan, Park, McCloskey âSuppressing Electrolyte Degradation and O2 Release in Li-rich NMC Cathodes with a Surface Acidic Treatmentâ, in prep.