1. Complex Anode Kinetics Chronocoulometry Evidence
Solar Thermal Decoupled Electrolysis: Reaction Mechanism of Cobalt Oxidation William Prusinski, Joshua Grade, Daniel Kotfer, Carol Larson, Dr. Robert Palumbo1, Dr. Jonathan Schoer2, Dr. Nathaniel Leonard1 1Department of Mechanical Engineering, Valparaiso University, Valparaiso, IN Department of Chemistry, Valparaiso University, Valparaiso, IN 46383
Abstract
Complex Anode Kinetics
From this observation, we conclude that the cathodic peak relies upon a preceding electrochemical step: the adsorbed species forms nucleation sites at which the cathodic and anodic reactions can then proceed.
In the proposed mechanism, the first step is for the dissolved Co(OH)2 to diffuse from the bulk solution to the surface of the anode. The electron transfer shown in Eq. 2 can then proceed at a potential near V vs. Ag/AgCl at 298 K as predicted by thermodynamic calculations. Figure 2 shows two cyclic voltammograms (CVs) that illustrate the complicated behavior of the anode.
The oxidation of Co(OH)2 at the anode of the H2 producing electrolytic cell was investigated via cyclic voltammetry (CV) and chronocoulometry to develop an explicit description of the reaction mechanism. It was found that the behavior at the anode is very complex; by varying the switching potentials and number of cycles in the CV, the shapes of the voltammograms change. Chronocoulometry studies provide evidence of surface adsorption. From the CV studies, it was also discovered that Co(OH)2 is oxidized to CoOOH at a potential close to the thermodynamically predicted value of V vs Ag/AgCl (3M NaCl) at 298 K.
Chronocoulometry Evidence
Figure 4 shows the result of double potential step chronocoulometry. The resulting plot contains evidence of an adsorption process.
Introduction
Because the industrial viability of the solar thermal decoupled electrolysis process relies in part on the rate at which H2 can be produced, the kinetic behavior of the process must be understood. Before quantitative data about the rate constants of electron transfer and mass transfer can be calculated, the mechanism of the reaction must be determined.
Knowing that the presence of Co(OH)2 dissolved in KOH solution decreases the potential required for the reduction of water to H21, our interest turned to the anode reaction. A mechanism found in the literature2 formed the basis of our kinetic studies, and is shown below.
Co(OH)2 (solution)  Co(OH)2 (surface) (1)
Co(OH)2 (surface) ↔ [Co(OH)2]+ + e- (2)
[Co(OH)2]+  CoOOH + H+ (3)
Though CoO is used in the current efficiency studies and is the product of the solar reduction step, CoO forms Co(OH)2 spontaneously in KOH. However, cyclic voltammetry suggested that the reaction mechanism was more complicated than reactions 1-3. We then turned toward conducting fundamental studies to better understand the system.
Figure 2. CV1 and CV2, both single-cycle, from to 0.12 V, 1 mV/s scan rate.
In CV1, no distinct anodic peak is present, but a distinct cathodic peak (Pc) is observed. This provides evidence that the cathodic peak does not require the anodic peak. In the next scan, CV2, Pc is observed with a lower peak current than in CV1. Two anodic peaks (Pa1 & Pa2) are observed, likely corresponding to the oxidation of Co(OH)2 to CoOOH. This shows that the Pa1/Pa2 reaction must be preceded by either the Pc reaction or a process occurring at a potential greater than that of the Pa1/Pa2 reaction. The cross-over of the anodic and cathodic scans of CV1 and the double peak of CV2 are indicative of an adsorption process3. To test this hypothesis further, another set of CVs were performed, with varying switching potentials.
Figure 4. Chronocoulometry response: Pre-step = V, 1 s. Step 1 = V, 4 s. Step 2 = V, 4 s
The regression lines of the linear region of the curves have similar slopes. The y-intercept for step 1 is mC, the y-intercept for step 2 is mC. Taking the difference in magnitude between the two intercepts removes the charge due to the capacitance, leaving only the charge due to the adsorbed species, 3.4 mC.
Conclusions
Co(OH)2 reacts to produce CoOOH at potentials above V vs. Ag/AgCl.
The anode reaction is likely more complex than as proposed in the literature.
There is strong evidence that this complexity is associated with an electrochemical adsorption process.
The cathodic reaction requires the cell be polarized to at least V.
Experimental
Adsorption Mechanism
The voltammetry studies were conducted using the following experimental setup:
Future Work
Electrolyte – 40 wt% KOH with dissolved Co(OH)2, sparged for 30 minutes with Ar prior and during experiments
Anode – 2 Ni leaves (A ~10 cm2), with ~0.070 g Co(OH)2 particles sandwiched between the leaves
Cathode – 3 Pt leaves (A ~11 cm2)
Reference – Ag/AgCl in 3M NaCl
Temperature – Room temperature ~298 K
Instrument – Gamry Potentiostat/Galvanostat
Integrate findings into a descriptive mechanism
Obtain values for diffusion coefficients and rate constants
References
Palumbo, R., Diver, R.B., Larson, C., Coker, E.N., Miller, J.E., Guertin, J., Schoer, J., Meyer, M., Siegel, N.P., Solar thermal decoupled water electrolysis process I: Proof of concept. J. Chemical Engineering Science 84,
Elumalai, P., Vasan, H.N., Munichandraiah N., Electrochemical studies of cobalt hydroxide – an additive for nickel electrodes. J. of Power Sources 93,
Southampton Electrochemistry Group. Instrumental Methods in Electrochemistry; Woodhead Publishing Limited, 2001.
Bott, A., Heineman, W Chronocoulometry. Bioanalytical System, Inc
Figure 3. CV1: to V. CV2: to V. Both single-cycle, 1 mV/s scan rate.
Figure 1. Electrolytic cell for voltammetry experiments
In CV1, no anodic or cathodic peaks are observed at potentials that peaks had been observed previously. In CV2, after a potential greater than -0.18V is reached, an anodic current is observed and a cathodic peak is observed in a position similar to Figure 2.