Low-Voltage Hydrogen Production via Hydrogen Peroxide Oxidation Facilitated by Oxo Ligand Axially Coordinated to Cobalt in Phthalocyanine Moiety
Authors: Lee Jisu , Song Hyun-Kon 
Published: February 2024

The global energy sector is shifting from fossil-based systems of energy production and consumption to renewable-based ones like wind and solar energy. Hydrogen has emerged as one of the energy carriers for the renewable-based economy, storing electrical energy generated from the renewables and being used as a fuel to generate power. Water electrolysis, powered by renewable energies, is one of the most feasible green hydrogen production technologies. At least 1.23 V and typically > 2 V is required between hydrogen and oxygen evolution reactions (HER and OER) for splitting water. OER is notorious for its slow kinetics so that the OER determines hydrogen production rate. Therefore, it has been emphasized to develop high-performance and low-cost OER electrocatalysts for reducing overpotentials required for electrolyzing water. Even if up-to-date upgraded electrocatalysts have decreased the kinetic overpotentials, it is clearly evident that it is impossible to split water molecules at the cell voltage less than 1.23 V. To decrease the potential required for water electrolysis, water oxidation reaction to drive OER was suggested to be replaced by oxidation of organic molecules or nitrogen hydrides such as formic acid and hydrazine. However, even if the more negative E
o of the organic molecules allowed the open-circuit potential difference between cathodic and anodic processes to be reduced, it sounds self-contradictory to employ carbon-based organic molecules and therefore produce carbon dioxide for producing green hydrogen.
Suggesting hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, in this work, we present a cobalt phthalocyanine having electron-poor CoN4 (+δ) in its phthalocyanine moiety as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR), demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allowed CoN4 to have the highly HPOR-active monovalent oxidation state and facilitate HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co-OOH- encouraged an axially coordinated cobalt oxo complex (O=CoN4) to form, the O=CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction (OER) guaranteeing low-voltage hydrogen production was successfully demonstrated in the presence of the metal-oxo complex having electron-deficient CoN4. Hydrogen production by 391 mA cm-2 at 1 V and 870 mA cm-2 at 1.5 V was obtained.
Also, we evaluated the techno-economic benefit of hydrogen peroxide as a hydrogen carrier by comparing hydrogen peroxide with other hydrogen carriers. Here, even though water (H2O) can be considered as a carrier, about 61 kWh/kg of energy is required for converting transported H2O to H2 again. Thus, in this case, if renewable energy can be supplied from the demand site, H2 can be produced directly on-site from the demand site without having to transport the H2 produced from another renewable energy complex. If renewable power cannot be supplied from the demand site, all the required energy for water electrolysis should be supplied with existing fossil fuel-based energy, it is like losing the eco-friendly significance of H2 energy, and considering energy consumption, using other carriers will be more economical and environmental. In this situation, H2O2 can act as a hydrogen carrier as a hydrogen/oxygen simultaneous receptor. When generated hydrogen is stored and transported, high cost arises. To lower the cost, many countries are paying attention to LOHC (=liquid organic hydrogen carrier) and ammonia which are materials to storage hydrogen energy. For LOHC, methylcyclohexane 6.1 wt.% and 47 kg H2/m3, dibenzyltoluene 6.2 wt.% and 57 kg H2/m3, ethylcarbazole 5.8 wt. % and 57 kg H2/m3, with a hydrogen storage capacity of 47 - 57 kg H2/m3. For ammonia, it has a hydrogen storage capacity of 17.6 wt. %, and 108 kg H2/m3. In the case of H2O2, it has 5.9 wt.% and 85.3 kg H2/m3, which belongs to the high hydrogen storage capacity compared to other hydrogen storage materials. To evaluate the economic analysis, we conducted to identify that H2O2 can be utilized for green H2 supply by comparing diverse hydrogen storage and transportation pathways. Furthermore, the scenario analysis was performed for CGH2, LH2, NH3, toluene-based LOHC, and H2O2.

Figure 1