Nanowire structured non-noble metal electrocatalyst for hydrogen production

Abstract

As the worlds energy dema nd is significantly increasing, various eco-friendly energies are emerging to replace fossil fuels. Among various kinds of energy, solar energy is attracting enormous attention from its high energy production potential. Hydrogen is receiving attention for storing solar energy due to its high energy density and clean combustion with only producing water. Hydrogen production using photovoltaic-electrolysis (PV-EC) systems is being actively pursued. PV-EC offers a significant advantage with its high solar-to-hydrogen conversion efficiency compared to other methods, and its durability has also been proven to a certain extent. Consequently, PV-EC is gaining attention as a next-generation hydrogen production method. However, the high costs associated with the system (solar panels, noble-metal electrocatalysts) currently present the biggest obstacle to the commercialization of electrochemical hydrogen production. To address this issue, two approaches are being explored to improve hydrogen production efficiency: developing low-cost electrocatalysts and optimizing electrolytes. The development of low-cost electrocatalysts focuses on optimizing catalytic activity using non-precious metals and maximizing the electrochemical surface area through nanostructure formation. Improving electrolytes involves enhancing system efficiency by substituting the oxygen evolution reaction with the hydrazine oxidation reaction and transitioning from H-cell electrolyzers to anion exchange membrane water electrolysis (AEMWE). In Chapter 2, various metals (Fe, Mo, Co, V) doped Ni2P catalysts were synthesized using a facile method and applied to a solar hydrogen production system (PV-EC). The synthesized electrocatalysts featured a nanowire structure, with each element doped into the Ni2P crystalline structure. In the oxygen evolution reaction (OER) performance test, NiFeP showed the highest performance, requiring only an overvoltage of 279 mV to achieve a current density of 100 mA cm-2. In the hydrogen evolution reaction (HER) activity, NiMoP showed the highest performance, requiring an overvoltage of 67mV to obtain 10 mA cm-2. When these two catalysts were connected to complete water splitting, a cell voltage of 1.57V was required at 10 mA cm-2. Finally, the PV-EC system was completed by combining it with perovskite solar cells, exhibiting a high solar to hydrogen (STH) efficiency over 14%. These results demonstrated the possibility of achieving high efficiency above the commercialization standard (STH 10%) with using low cost solar cells and electrocatalysts. In Chapter 3, we investigated a hydrazine oxidation reaction (HzOR) as a replacement for the oxygen evolution reaction (OER) in an efficient solar hydrogen production system. The OER consumes a significant amount of electricity during water decomposition and produces oxygen, which is not the target substance. By replacing the OER with HzOR, the voltage required for hydrogen production can be significantly reduced, while also providing the benefit of hydrazine purification in wastewater. CoFeP nanowire electrocatalyst was synthesized using the method developed in Chapter 2, and CoFeP/CF required a voltage of -8 mV to achieve a current density of 10 mA cm-2. A new PV-EC system was fabricated using the CoFeP/CF electrocatalyst and HzOR. Unlike conventional PV-EC systems that require two solar cells in series, this PV-EC system could be operated with only one solar cell when using hydrazine oxidation. This study demonstrated a PV-EC system that doubles the efficiency and maintains long-term stability compared to existing systems. In Chapter 4, we explored the development of a hierarchical nanowire electrocatalyst and a large-scale hydrogen production system using an anion exchange membrane (AEM) to enhance hydrogen production efficiency. The surface area was further increased by adding a WO microwire support to the previously developed NiFeP nanowire. This NiFeP-WOx electrocatalyst required a voltage of 1.519 V to achieve a current density of 100 mA cm-2 during the oxygen evolution reaction. For the urea oxidation reaction (UOR) and hydrazine oxidation reaction (HzOR), the required voltages were 1.388 V and 0.005 V to achieve 100 mA cm-2, respectively. Finally, these electrocatalysts were combined with NiMo-WOx as a HER catalyst and PiperION as an AEM to complete the AEMWE system. The hydrazine oxidation reaction achieved an efficiency of 0.66 V at 1 A cm-2, which is significantly lower compared to the OER (1.96 V) and UOR (1.93 V). This result demonstrated the feasibility of a large-scale and low-cost hydrogen production system using hydrazine. In summary, we introduced a new synthesis method that provides a high electrochemical surface area with a nanowire structure. This facile synthesis method is also possible to easily synthesize various catalysts. These electrocatalysts demonstrated high efficiency in electrocatalytic reactions, including HER, OER, UOR, and HzOR. To enhance electrolyte performance, we developed an electrocatalyst with high activity under hydrazine oxidation and urea oxidation conditions, and implemented an AEMWE hydrogen production system using a polymer electrolyte membrane and catalyst. This research is expected to significantly contribute to the development of catalysts for the commercialization of electrochemical solar hydrogen production.