The development of rechargeable lead-acid batteries and their use in automobiles has prompted research into new battery chemistries with superior properties. As a result, nickel-metal hydride (NiMH) and lithium-ion batteries were created with higher energy density and longer lifecycles. Energy storage technology contributes to cleaner and more sustainable societies by increasing access to both efficient power sources and renewable energy as well. Substituting internal combustion engines with electric-based alternatives aims to reduce greenhouse gas emissions and air pollution. These shifts towards cleaner technologies are reflected in the strong growth of the battery market, which is projected to quadruple between 2021 and 2030 [1].
Electrochemical Raman (EC-Raman) spectroscopy is a powerful analytical tool that increases understanding of the working of energy storage devices by monitoring physicochemical changes. This Application Note explains EC-Raman insights discovered during simulated charging and discharging of a nickel-metal hydride battery.
Raman spectroscopy is used for in-situ, real-time monitoring of chemical reactions, among other things. When combined with electrochemical techniques (e.g., EC-Raman), this allows researchers to monitor physicochemical changes occurring in electrolytes at different depths, at electrode surfaces, and more. This powerful combination of techniques (otherwise known as spectroelectrochemistry or SEC) provides further insight into the progress of electrochemical reactions.
All materials used in this study are listed in Table 1. The experiment was conducted using an i-Raman Prime 532H and a PGSTAT302N. The Metrohm EC-Raman Starter Solution will achieve comparable results. Experimental design was based on a study by Yeo and Bell [2].
Table 1. Instruments, electrodes, and chemicals used herein.
| Experimental Materials | |
|---|---|
| Potentiostat | PGSTAT302N (Metrohm Autolab) |
| Raman Instrument | i-Raman Prime 532H, BAC151C video microscope system with 50x objective, BAC150B probe holder, BWSpec Software (B&W Tek) |
| EC-Raman Cell | EC-Raman Flow Cell with gold WE, platinum wire CE, and Silver/Silver chloride RE (Ag/AgCl, 3 mol/L KCl) (RedoxMe) |
| Electrode | Cable Connector for Screen-Printed Electrode (CAC4MMH) and 220BT Electrode with gold printed WE and CE and silver printed RE (Metrohm DropSens, Figure 1) |
| Chemicals | 0.1 mol/L KCl, 0.01 mol/L Ni(NO3)2, and 0.1 mol/L NaOH (Sigma Aldrich) |
Electrochemical roughening of WE surface
Add 5 mL of 0.1 mol/L KCl to the EC-Raman cell (adjust volume according to cell specifications). Ensure that there are no bubbles on the WE surface. Drop 100 μL of 0.1 mol/L KCl onto the printed surface of 220BT, ensuring that all electrodes are submerged. WE roughening follows the procedure detailed in Table 2 [3]. During the process, the electrode should change color, as shown in Figure 2. After the roughening process is complete, rinse the substrate with deionized (DI) water. The WE is now ready for electrochemical deposition of Ni(OH)2.
Table 2. Operational settings for the simulated discharge/charge of a Ni(OH)2 electrode. Settings may change based on experimental results.
| PGSTAT302 Operation Settings | |
|---|---|
| Task | Autolab Control |
| Electrochemical Roughening of WE | Repeat 25x*: chronoamperometry (CA) at -0.3 V vs. Ag/AgCl (30 s), Linear sweep voltammetry (LSV) from -0.3 to 1.2 V at 10 mV/s, CA at 1.2 V (60s), LSV from 1.2 to -0.3 V at 10 mV/s. |
| Electrochemical Ni(OH)2 deposition on WE | Chronopotentiometry (-100 μA, 300 s) in a two-electrode cell (WE = Au disk, CE/RE = Pt)** |
| Simulated Cycle of Ni(OH)2 electrode | CV between -0.4 and 1.5 V vs. Ag/AgCl (start/stop at 0V, scan rate 10 mV/s)†. |
* Increased repetitions will create a rougher surface.
** A longer deposition time will thicken the Ni(OH)2 coating.
† Adjusted time according to the Raman operation setting.
Electrochemical deposition of Ni(OH)2 on WE
Connect all cables to the EC-Raman cell and add 5 mL of 0.01 mol/L Ni(NO3)2 while checking that there are no bubbles on the surface. Drop 100 μL of 0.01 mol/L Ni(NO3)2 onto the printed surface of 220BT, ensuring that all electrodes are submerged. Ni(OH)2 deposition on the WE surface is achieved through precipitation of the Ni(NO3)2 salt by chronoamperometry (CA) according to the settings described in Table 2. After deposition, disconnect the EC-Raman cell and 220BT SPE and rinse the substrate with DI water. The electrodes are ready for the electrochemical discharging and charging cycle.
Electrochemical discharging/charging WE
Reconnect the EC-Raman cell, then add 5 mL of 0.1 mol/L NaOH to the EC-Raman cell and ensure that there are no bubbles on the Ni(OH)2 WE surface. Drop 100 μL of 0.1 mol/L NaOH onto the printed surface of 220BT. Check that all electrodes are submerged. Submit the deposited nickel film to cyclic voltammetry (CV) according to details in Table 2. Nickel is deposited with a +2 oxidation state as Ni(OH)2 and can be reversibly oxidized to Ni+3 (NiOOH) according to the following equation:
Ni(OH)2 + OH- ↔ NiOOH + H2O + e-
Monitoring WE surface with i-Raman Prime 532
Place the EC-Raman cell or 220BT on a BAC150B probe holder or BAC151C video microscope stage and position the laser on the WE surface. A 50x objective is used for this experiment; adjust magnification to the required depth of focus (Figure 3). EC-Raman spectra were acquired with the timeline function of BWSpec: integration time = 5s, laser power = 100%, and average = 1.
Electrochemical characterization of the nickel oxide film was performed with cyclic voltammetry. A typical voltammogram is displayed in Figure 4a with a pair of reversible peaks, characteristic of the reversible oxidation of Ni(OH)2 to NiOOH, observed around 0.50 V vs. Ag/AgCl. During the forward (anodic) scan, the main species present is Ni(OH)2 until the potential reaches 0.45 V vs. Ag/AgCl. After this potential, the film is mainly composed of NiOOH until the potential reaches approximately 0.35 V vs. Ag/AgCl on the reverse (cathodic) scan.
Note: The EC-Raman cell uses a true Ag/AgCl (3 mol/L KCl) reference electrode with a stable reference potential, while the 220BT SPE has an Ag pseudo-reference. The potential will shift with oxygen concentration in this cell: the O2 that is generated at high potentials saturates the electrolyte and defines a new reference potential for the Ag pseudo-reference.
To confirm the main species at the surface of the electrode, Raman spectra were collected throughout one full discharge/charge cycle of the EC-Raman cell. The Raman profile for three different voltages is presented in Figure 4b. At 0.0V vs Ag/AgCl, no Raman band associated with Ni(OH)2 was detected – a possible result of low Raman cross-section or deposit layer depth [2]. As the potential reaches the reversible NiOOH/Ni(OH)2 point, NiOOH-related bands at 476 and 556 cm-1 appear, allowing for clear identification of NiOOH in the thin film. These bands disappear again when voltage is reduced to 0.0V.
The Raman profile for the 220BT SPE was comparable to that of the EC-Raman cell, but with stronger peak intensity and lower redox potential. This is possibly caused by the experiment setup, e.g., electrode specification, laser focus/depth of electrolyte, glass window, dimension of the WE, RE type, etc. There are pros and cons of each configuration:
- The EC-Raman cell is a closed system which provides a controlled environment and can accommodate continuous flow reactions. The intensity of Raman peaks is lower for a closed cell and the Raman signal may be compromised by bubbles that form during the reaction.
- The 220BT screen-printed electrode is a more affordable open system that is less susceptible to interference from bubbles, but the sample is vulnerable to contamination and spilling. Moreover, the CV peak position may change due to the influence of oxygen at the working electrode or the reference electrode.
- i-Raman Plus 532 yields comparable results to i-Raman Prime.
- The EC-Raman cell and 220BT SPE can be stored for a few days after roughening.
- Bubbles can interfere with Raman measurement of an EC-Raman cell.
- CV can change depending on experiment parameters when using the 220BT.
- Optimal results are achieved by increasing electrochemical etching and Ni(OH)2 deposition on WE surface.
The physicochemical properties of Ni(OH)2 deposits on an electrode during a simulated discharge/charge cycle were monitored using a Metrohm hyphenated EC-Raman system. The changes in the intensity of Raman bands were indicative of oxidation and reduction of nickel during CV, confirming the system’s ability to monitor changes in energy storage materials by hyphenated EC-Raman.
- O’Dea, S. Battery market size worldwide by technology 2018-2030. Statista.
https://www.statista.com/statistics/1339880/global-battery-market-size-by-technology/ (accessed 2023-07-25). - Yeo, B. S.; Bell, A. T. In Situ Raman Study of Nickel Oxide and Gold-Supported Nickel Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Phys. Chem. C 2012, 116 (15), 8394–8400. https://doi.org/10.1021/jp3007415.
- Tian, Z.-Q.; Ren, B.; Wu, D.-Y. Surface-Enhanced Raman Scattering: From Noble to Transition Metals and from Rough Surfaces to Ordered Nanostructures. J. Phys. Chem. B 2002, 106 (37), 9463–9483. https://doi.org/10.1021/jp0257449.
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