1. Solar-Thermal Decoupled Electrolysis: Assessment of MnxOy Systems
Justin Buiter, Rachel Silcox, Daniel Kotfer, Guadalupe Villagran, Carol Larson, Dr. Jonathan Schoer, Dr. Robert Palumbo
Abstract
Experimental
Bulk Electrolysis of MnxOy Systems
We used voltammetry, bulk electrolysis (BE), and x-ray diffraction (XRD) analysis to assess the viability of various MnxOy redox systems for the production of H2 in electrolytic cells. We determined that the Mn3O4/Mn2O3 system is not appropriate for H2 production, primarily because of solubility issues. We determined that the MnO/MnO2 system is also not viable as a source of H2 as a fuel, because the current density is too low and the potential at which the oxidation occurs is too close to the thermodynamic value for conventional electrolysis. However, the MnO/MnO2 system is potentially viable for producing MnO2 or H2 as chemical commodities.
The voltammetry and bulk electrolysis studies were conducted using an experimental apparatus similar to that shown in figure 2.
Figures 5 and 6 show representative results for our bulk electrolysis of the MnxOy system. The figures show the MnO/MnO2 system in 2M H2SO4 at 348 K. The initial peak in figure 5 is due to capacitive charging in the system. The current slowly decays as the MnO is oxidized. The current density is about 1 mA/cm2, far less than our target. However, Figure 6 shows the results of the X-Ray Diffraction (XRD) analysis in which we see MnO2 was successfully produced. Similar BE results observed for the Mn3O4/Mn2O3 system.
The experimental conditions were a combination of the following:
Electrolyte: KOH, H2SO4, HCl, or HNO3 of various concentrations
Anode: Pt wire (A >1 cm2) or Pt foil (A ≈ 4 cm2)
Cathode: Pt wire (A >1 cm2) or 1-6 Pt foil leaves (A ≈ 8-50 cm2)
Temperature: K
Instrument: Gamry Potentiostat/Galvanostat
Figure 5. Bulk electrolysis of MnO to MnO2 in 2M H2SO4 at 348 K. After the initial charging current, the current of the cell slowly decreases as the MnO is oxidized. The current density is about 1 mA/cm2 far less than our target.
Introduction
The solar-thermal decoupled electrolysis project at Valparaiso University is investigating a two-step cyclic process in which concentrated sunlight is used to produce H2, Figure 1. In the water electrolysis step (Figure 1), H2 is produced at the cathode while MOx-α (the reduced form of a metal oxide) is oxidized to MOx (the oxidized form of the metal oxide) at the anode. Ideally, a potential substantially lower than that required for direct electrolysis of water drives the process. In the solar-thermal reactor step, the MOx is reduced with concentrated solar heat to MOx-α releasing O2 and completing the cycle. Overall, the process represents an innovative approach to converting solar energy to chemical energy (H2) or to produce H2 as a chemical commodity.
Figure 2. Electrolytic cell for voltammetry experiments.
Figure 6. XRD results of the material produced at the anode in 2M H2SO4 saturated with MnO. The four main peaks align with the peaks of MnO2.
Solubility of MnxOy Systems Assessed by LSV
Figures 3 and 4 show typical results for our solubility studies based on linear sweep voltammetry (LSV). To meet our definition of “soluble”, a solution containing the metal oxide of interest must generate a current density 1mA/cm2 greater than that for a blank solution. In Figure 3 Mn3O4 is considered insoluble in 13.5 M KOH (blue), because the current density of the blank and Mn3O4 are similar. In Figure 4, the difference in current density is significant and we consider Mn2O3 (gray) and Mn3O4 (orange) to be soluble in 2M H2SO4 at 358 K.
Conclusions
Manganese oxides have varying solubility.
In bases, such as KOH they are generally insoluble
In acids, such as H2SO4 and HCl they are moderately soluble
Mn3O4/Mn2O3 system not viable for production of H2 as a fuel
Complete solubility requirement mismatch
The maximum voltage criteria (1.23 V) is not met
Low current densities
Insignificant H2 production
MnO/MnO2 system is not appropriate for production of H2 as a fuel
The solubility requirements are mismatched in some electrolytes
Overall process is complex
Figure 3. Linear sweep voltammograms (LSV) for a 13.5 M solution of KOH (blue) and for a 13.5 M KOH solution saturated with Mn3O4 (red). The similar appearance of the two LSVs indicate that Mn3O4 is insignificantly soluble in 13.5 M KOH for our purposes.
Figure 1. Metal oxide-assisted solar-thermal, decoupled, electrolysis cycle for the production of hydrogen from water.
For a metal oxide system to be commercially useful for the H2 producing electrolysis step (enclosed by the box in Figure 1), it must meet the following set of parameters:
MOx-δ must be soluble in the electrolyte.
MOx must be insoluble in the electrolyte.
For the production of H2 as a fuel, the potential required to drive the metal oxide-assisted electrolysis must be significantly lower than thermodynamic potential for splitting water, 1.23 V.
For the production of H2 as a commodity, the potential to drive the electrolysis must be significantly less than the voltages currently used in commercial environments, ~2 V.
The current density for the electrolysis needs to be at least 50 mA/cm2.
Additionally, the following properties of the metal oxide system are desired :
Low health and safety risk
Low equipment complexity
Low cost
Figure 4. LSV for 2.0 M H2SO4 (blue), Mn2O3 in 2.0 M H2SO4 (gray), and Mn3O4 in 2.0 M H2SO4 (orange) at 358 K. The LSV show the relative amounts of sample in solution compared to the blank. Both Mn2O3 (~1 mA/cm2) and Mn3O4 (1.6 mA/cm2) exhibit a current density at least 1 mA/cm2 greater than the blank and are considered soluble in 2.0 M H2SO4 .
Future Work
Focus additional work on cobalt oxide systems
Utilize rotating disk electrode techniques to investigate mechanisms
Investigate better electrolyte and electrodes with porous surface area
Investigate potential of MnxOy systems for commercial MnO2 production
Investigate potential of MnxOy systems for commodity production of H2
Acknowledgements
National Science Foundation, Award Number
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