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Abstract:
This transcript captures a live "Ask Us Anything about Electrochemistry" session hosted by Pine Research. After introductory remarks covering stream logistics, YouTube memberships, rules, and Pine Research offerings like Aftermath Live simulation software, the hosts address viewer questions. Key topics include the rationale for slow scan rates in Linear Sweep Voltammetry (LSV) for gas evolution reactions and determining polarization resistance (RP), minimizing non-faradaic current, and potential instrument limitations at very slow speeds. They discuss the distinction between diffusion speed (a fluid property) and diffusion phenomena related to electrode geometry (planar vs. porous) and size. Methods for determining RP are compared, favoring slow LSV/LPR over Electrochemical Impedance Spectroscopy (EIS) due to consistency issues, with explanations on interpreting impedance data (semicircle width) if used. Other topics include adjusting Open Circuit Potential (OCP) via reference electrode choice, managing gas bubbles during hydrogen evolution, interpreting Distribution of Relaxation Times (DRT) analysis from EIS data, its limitations (especially with Warburg impedance), and how its results (area under peaks) relate to impedance compared to traditional circuit fitting. The fundamental nature of diffusion (random motion vs. concentration gradient driven) is debated. Finally, the use of Cyclic Voltammetry (CV), scan rate dependence, and simulation software (Aftermath Live) for elucidating reaction mechanisms, potential issues with material loss during cycling, and the advantages of frequency-domain analysis (EIS) over time-domain (DC techniques) for deconvoluting processes occurring at different speeds are explained. The session concludes with plugs for Pine Research workshops.
Ask Us Anything: Electrochemistry Q&A Highlights
0:00:10 Stream Introduction & Logistics: The hosts outline how the "Ask Us Anything" live stream works, including how to ask questions, priority for YouTube members (Tier 1 & 2), and rules about avoiding spam.
0:01:18 Purpose of the Live Stream: Created to answer electrochemistry questions in more detail than comments or phone calls allow, utilizing visual tools. They note that very specific or overly broad questions can be difficult to answer effectively in this format and point viewers to other resources like specific videos or courses.
0:02:50 Pine Research & Aftermath Live: Introduction to Pine Research Instrumentation's equipment and their new electrochemical simulation software, Aftermath Live, highlighting its utility for research (mechanism support) and learning. A discount code (PINE YOUTUBE10) is offered.
0:04:26 Stream Schedule & Etiquette: Streams are typically Fridays 1-3 PM US Eastern, recordings are available with timestamps, apologies for potential name mispronunciations, and a strict no-spam policy.
0:08:03 Slow Scan Rates for LSV (HER/OER): Question about why slow scan rates (1-5 mV/s) are used for Hydrogen/Oxygen Evolution Reactions (HER/OER).
Takeaway: Slow scans minimize non-faradaic current (like double-layer capacitance charging, which is proportional to scan rate), ensuring the measured current is primarily attributed to the faradaic process of interest (kinetics of HER/OER). This is crucial for accurate analysis like Tafel plots.
0:10:15 Electrode Geometry & Diffusion Speed: Question comparing planar vs. porous electrodes regarding the "fastest diffusion."
Key Distinction: The speed of diffusion is determined by the diffusion coefficient (D, cmΒ²/s), a property of the molecule and the fluid (viscosity, temperature), not the electrode geometry.
Electrode Geometry Impact: Electrode geometry (planar, spherical, microelectrode) affects the type of diffusion (planar vs. radial) and the resulting electrochemical response (e.g., peak shape vs. S-shape in CV). Porosity affects how molecules move through the electrode structure but isn't typically called "geometry."
0:15:30 Determining RP via LSV & Scan Rate: Question on using LSV (+/- 10 mV around Ecorr) at 1 mV/s to find Polarization Resistance (RP) for aluminum corrosion, and whether this scan rate is too fast.
Recommendation: 1 mV/s is often an upper limit for LPR. Slower rates (e.g., 0.1-0.5 mV/s) are generally preferred to further minimize non-faradaic effects.
Instrument Consideration: Very slow scans in standard CV/LSV modes might not be ideal as the potentiostat makes discrete steps. A dedicated Linear Polarization Resistance (LPR) experiment mode often uses chronoamperometry steps with averaging for better accuracy at slow effective scan rates.
0:24:29 RP Dependence on Scan Rate: Follow-up question: Should RP change with scan rate?
Ideal Answer: No, if scanned slowly enough to eliminate capacitive effects, RP (a property of the corrosion kinetics) should ideally be independent of scan rate.
Troubleshooting: If RP changes predictably with scan rate, it might indicate residual non-faradaic contributions or issues with the measurement technique (e.g., using slow CV instead of a proper LPR method). Random variation might just be measurement noise. Doing multiple scan rates can test this.
0:27:11 Decreasing OCP for a Bioanode: Question on how to decrease the Open Circuit Potential (OCP) of a Prussian blue/glucose oxidase bioanode from +0.1 V to a negative value.
Easiest Method: Change the reference electrode. The OCP is measured relative to the reference. Using a reference electrode with a more positive potential (e.g., Mercury Sulfate vs. Calomel) will make the measured OCP of the working electrode appear more negative. Pine Research's website has a calculator for this.
0:30:13 Removing Bubbles in HER: Question about the best method to remove hydrogen bubbles during HER experiments using LSV.
Strategies: Rotate the electrode (RDE), angle the electrode so bubbles can escape via buoyancy, gently flicking/shaking the electrode (can disturb data momentarily), engineering the cell/electrode setup. It's a common challenge in gas-evolving reactions.
0:33:00 DRT Analysis for EIS (Battery Example): Addressing saved questions about Distribution of Relaxation Times (DRT) analysis on battery impedance data.
0:35:15 Warburg Region Exclusion: DRT assumes RC elements and is inappropriate for purely capacitive, inductive, or diffusional (Warburg) regions. Data points where impedance significantly increases along the imaginary axis (indicating Warburg) should be excluded to avoid false peaks/conclusions. Cutoff is subjective but should be strict.
0:38:48 Extrapolation Need: Extrapolating impedance data (e.g., fabricating points to complete a semicircle) is generally not recommended. If a semicircle is mostly complete (>~50%), DRT might still yield reasonable results. Circuit fitting can implicitly extrapolate, but the best approach is to collect data over a wider frequency range if possible.
0:41:21 Extracting Impedance from DRT: Yes, the area under each DRT peak correlates with the relative impedance (resistance) of the corresponding process (e.g., SEI formation, charge transfer). It allows comparison of the magnitude of different processes.
0:43:38 Discrepancy between DRT Area and EIS Fit: If the relative resistance suggested by DRT peak area (e.g., for SEI) disagrees with the value from EIS circuit fitting, trust the circuit fitting result more. Circuit fitting directly models the impedance data (assuming a correct model), while DRT is a transformation with assumptions. DRT is better for identifying the number and relative magnitude of processes rather than precise quantification.
0:46:45 Basic Explanation of Diffusion: Question asking for a basic explanation of diffusion.
Definition: Diffusion is a mode of mass transport arising from the random motion of molecules.
Concentration Gradient: While often described as movement due to a concentration gradient, the hosts argue the fundamental mechanism is random motion, which results in net movement from high to low concentration areas (like dropping food coloring in water). In electrochemistry, reaction at the electrode creates a local concentration gradient, driving diffusion towards the electrode via this random motion.
0:54:04 Material Loss During Scan Rate CV Study: Question about performing CV at decreasing scan rates (200 to 1 mV/s) on deposited graphene and whether the deposited amount will reduce.
Possibility: Yes, material loss (delamination, dissolution, side reactions) can occur during repeated cycling, potentially correlating with time/number of cycles rather than scan rate itself.
Testing: Repeat the entire scan rate sequence multiple times or change the order of scan rates. If a trend (e.g., decreasing capacity) persists regardless of order/scan rate, it likely indicates material degradation over time/cycling. Improving electrode preparation/adhesion might be needed.
0:59:51 Determining RP from EIS Circuit Fitting: Addressing a saved question on getting RP from EIS circuit fitting for corrosion.
Caveat: While EIS is good for R and C, LPR (slow DC scan) is often preferred for RP determination in corrosion due to better reproducibility and consistency compared to EIS, which can be sensitive to system changes during the measurement.
How To (If Using EIS): If reliable EIS data with a clear charge-transfer semicircle is obtained (often requires applying a small DC bias away from OCP), the RP corresponds to the width of that semicircle on the Nyquist plot (difference in real impedance between the high and low-frequency intercepts of the arc). It is not the high-frequency intercept (R_ohm or R_s).
1:09:03 Using CV for Reaction Mechanisms (Organic Synthesis): Question about using CV to understand organic reaction mechanisms.
Applicability: CV/electrochemistry is primarily useful for probing redox reactions (oxidation/reduction). It can reveal mechanisms involving electron transfer (E) coupled with chemical steps (C), such as EC, ECE, CE mechanisms.
Simulation (Aftermath Live Demo): The hosts demonstrate using simulation software to model different mechanisms (E, EC) and show how changing parameters (rate constants, potentials) and experimental conditions (scan rate) affects the CV shape (peak separation, relative peak heights, irreversibility). Comparing experimental CVs (especially scan rate dependence) to simulations helps elucidate mechanisms.
1:18:37 Time Domain vs. Frequency Domain (Why EIS?): Addressing a saved question on the importance of changing from the time domain (like CV) to the frequency domain (EIS).
Analogy: Compares electrochemistry to a symphony orchestra where many instruments (processes like charge transfer, diffusion, capacitance) play simultaneously. DC techniques hear the whole orchestra at once.
EIS Advantage: Different electrochemical processes occur at different characteristic speeds/rates. EIS uses AC signals of varying frequencies (inverse of time/speed) to probe these processes selectively. High frequency probes fast processes (like R_s, some charge transfer), while low frequency probes slow processes (like diffusion, capacitance).
Deconvolution: EIS allows deconvoluting these overlapping processes by separating their responses based on frequency, much like isolating individual instruments in the orchestra, providing more detailed information than often possible with DC techniques alone. Converting time-domain sine waves to frequency-domain peaks simplifies analysis.
1:27:59 Workshop Plug & Wrap-up: Announcement of the upcoming Pine/Villanova CV Boot Camp workshop and encouragement to check the Pine Research website for tools and resources. The session ends due to a lack of further live questions.