(6dd) Mechanistic Electrochemistry Driving Energy Science: From Catalysts to Batteries

Vision

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 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.

Research Experience

Postdoctoral Work: Suppression of Interfacial Degradation in High-Energy Lithium-Ion Batteries
As a postdoctoral scholar in Prof. Bryan McCloskey’s research group in the Chemical Engineering department at UC Berkeley, I’m studying fundamental degradation processes at the cathode-electrolyte interface in advanced Li-ion batteries. Cathode materials with high specific charge capacities at high voltages are needed to significantly push the energy density of Li-ion batteries, which will serve to lower the associated cost per unit of energy. While there are several cathode materials that meet this requirement, such as the layered Li-rich NMC (Ni, Mn, Co) oxide with specific capacities greater than 250 mAh/g, degradation processes at high voltages (> 4.5 V vs. Li^+/0) severely affect cycle life and overall battery performance. Specifically, the decomposition of the organic electrolyte at the cathode surface causes CO₂ release with the concomitant deposition of insulating films on the cathode surface. Cathode surface impurities such as Li₂CO₃ also decompose to produce CO₂. In addition, the participation of the lattice oxide atoms in charge compensation (“O-redox”) during charging (Li⁺ removal), results in surface O₂ release above 4.5 V. The evolution of gases such as CO₂ and O₂ is therefore indicative of interfacial degradation at high voltages. Electrochemical interfaces in batteries are, however, buried and therefore difficult to mechanistically probe.

1. Quantification of High-Voltage Interfacial Degradation: I have utilized online electrochemical mass spectrometry (OEMS) to quantify processes leading to interfacial degradation in high-energy Li-ion cathode materials, as a function of state-of-charge of the metal oxide. Ex-situ acid titrations of charged electrodes with isotopically labelled cathode particles and gas analysis with OEMS quantify the peroxo-like species generated during O-redox. The studies revealed a surface-to-bulk propagation of these peroxo-like O-species with increasing state-of-charge.
2. Interface Engineering to Suppress Degradation: With the tools in hand to quantify interfacial degradation manifesting in gas release, I have pursued methods to “reactively” passivate the electrode surfaces against such degradation, i.e. by utilizing a surface chemical reaction to produce a uniform passivation layer. These can be classified as:
   1. Additive: Deposition of electro-inactive coating layers on the cathode surface by atomic layer deposition (ALD). I found that uniform layers of early transition metal oxides such as MoO_x and WO_x are effective in suppressing high-voltage gas release leading to superior long-term cycling performance.
   2. Subtractive: I found that a controlled acidic treatment of the metal oxide surface to produce a few nm thick Li-deficient reaction layer on the surface leads to a near-complete suppression of gas release at high voltages, with no loss of extractable capacity, along with significantly improved long term cycling and rate performance.

The electroanalytical studies of the cathode materials in their pristine and surface-treated forms were supplemented with electron microscopy, X-ray diffraction, inductively coupled plasma mass spectrometry as well as surface and bulk-sensitive spectroscopic techniques to provide a generalizable model for interface passivation. I also extended these studies to other cathode materials such as nanoscale-LiCoO₂, Li₃RuO₄ and the Ni-rich NCM oxides.

PhD work: Mapping Free Energies to Screen Molecular Electrocatalysts on a Computer
Density functional theory based computational methods have traditionally been used in molecular catalysis to post-facto study the thermodynamics and kinetics of catalysts with 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 for a priori molecular electrocatalyst design. Specifically, for the competing two-electron reductions 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 catalytic intermediates, streamlined the screen for promising catalyst candidates from a large library of transition metals and organic ligands. The predictions from the in silico screen 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.

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 a) the 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 (IIT Madras).

Research Interests:

I am broadly interested in studying fundamental electrochemical processes in energy storage materials. My research laboratory would strive to answer two scientific questions: (a) how to engineer cooperativity between transition metals and p-block elements to achieve multi-electron redox at low driving forces; and (b) how to rationally passivate electrode-electrolyte interfaces in high-voltage batteries.

The insights derived from this endeavor will aid materials’ design for electrochemical energy conversion and storage processes including high-energy batteries and electrocatalytic fuel syntheses. The proposed research will involve a multi-pronged experimental effort involving synthetic, electrochemical, spectroscopic and analytical tools, aided by synergistic collaborations.

Teaching Experience

I served as the head teaching assistant 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 ‘open-box’ potentiostat with tunable circuit elements connected to a data acquisition device controlled by MATLAB™, with the aim of helping the students develop a circuit-level understanding of electrochemical measurements.

Apart from being a teaching assistant (TA) for several lecture and laboratory-based undergraduate chemistry courses at Stanford University, I also served as a TA trainer for first-year graduate students taking on teaching roles in the chemistry department.

My lecturing experience includes a three-part lecture series titled ‘Discovering the World of Batteries’ for the Science Circle High School Program at Stanford University, and a guest lecture on ‘Density Functional Theory for Organometallic Chemists’ as part of the graduate level Advanced Inorganic Chemistry course at Stanford University.

Apart from teaching in the formal settings outlined above, I particularly enjoy mentoring younger graduate students, undergraduates and high school students towards their scientific advancement.

Teaching Interests:

I am interested in teaching lecture-based as well laboratory-based courses related to energy science and engineering, such as Electrochemistry, Inorganic Chemistry, Solid-State Chemistry, Thermodynamics, Kinetics, Quantum Chemistry, Electrochemical Energy Storage and Conversion. I am also interested in teaching courses that broadly cover topics related to renewable energy conversion and storage technologies.

Publications

1. Ramakrishnan, Moretti, Chidsey “Mapping Free Energy Regimes in Electrocatalytic Reductions with Transition Metal Complexes”, Chem. Sci. 2019 ASAP (doi: 10.1039/C9SC01766F)
2. McLoughlin, Waldie, Ramakrishnan, Waymouth “Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent”, J. Am. Chem. Soc. 2018, 140(41), 13233-13241.
3. Ramakrishnan, Chidsey “Initiation of Electrochemical Reduction of CO₂ by a Singly Reduced Ruthenium(II) Bipyridine Complex”, Inorg. Chem. 2017, 56(14), 8326-8333.
4. Waldie, Ramakrishnan, Kim, Maclaren, Chidsey, Waymouth “Multi-Electron Transfer at Cobalt: Influence of the Phenylazopyridine Ligand” J. Am. Chem. Soc. 2017, 139(12), 4540-4550.
5. Ramakrishnan, Chakraborty, Brennessel, Jones, Chidsey “Rapid Oxidative Hydrogen Evolution from a Family of Square–Planar Nickel Hydride Complexes” Chem. Sci. 2016, 7, 117-127.
6. Ramakrishnan, Waldie, Warnke, de Crisci, Batista, Waymouth, Chidsey “Experimental and Theoretical Study of CO₂ Insertion into Ruthenium Hydride Complexes” Inorg. Chem. 2016, 55(4), 1623-1632.

Publications (in preparation)

7. Ramakrishnan, Park, Wu, Yang, McCloskey “Suppression of Interfacial Outgassing and Improved Cycling in High Energy Li-rich NMC Cathodes enabled by an Acid-treated, Li-deficient Surface”.
8. Ramakrishnan, Asundi, Park, Bent, McCloskey “Effective Interface Passivation of Li, Mn-rich high energy cathodes with Atomic Layer Deposited Molybdenum Oxide”.
9. Ramakrishnan, Hu, Cabana, McCloskey “Improved Interfacial Stability with Gradient Al₂O₃ Coatings on LiCoO₂ Nanoplates”.
10. Li, Ramakrishnan, McCloskey, Cabana “Definition of Redox Centers in Reactions of Lithium-Intercalation in Li₃RuO₄Polymorphs”.