Opportunity
Zinc-nitrate batteries have emerged as promising energy storage systems that combine electricity supply, ammonia (NH₃) electrosynthesis, and sewage disposal (nitrate removal) into a single device, aligning with sustainable development goals. However, existing zinc-nitrate batteries face significant technical hurdles that limit their practical application. A major problem is their limited energy density and poor rechargeability. Current designs often function only as non-rechargeable galvanic cells, requiring the periodic replacement of sacrificial zinc anodes, which is inefficient and impractical for grid-level energy storage or electric vehicles. Furthermore, the cathode catalysts, typically copper-based, are optimized solely for the nitrate reduction reaction (NO₃RR) and exhibit poor activity and stability for the oxidation reactions needed during battery recharging. This forces reliance on the oxygen evolution reaction (OER) for charging, which has a high overpotential, reduces energy efficiency, and poses safety risks due to oxygen gas generation. Consequently, there is a critical need to develop a rechargeable zinc-nitrate battery system with high energy density, long cycle life, and efficient bifunctional catalysis for both discharge (NO₃RR) and charge processes.
Technology
This patent addresses the existing challenges by introducing a rechargeable zinc-nitrate/ethanol battery centered on a novel bifunctional cathode catalyst and an optimized electrolyte system. The core innovation is the use of a tetraphenylporphyrin (tpp)-modified, heterophase rhodium-copper alloy metallene (RhCu M-tpp) as the cathode catalyst. This material features an ultrathin, two-dimensional nanostructure with a unique combination of crystalline and amorphous domains. The technology employs a "molecule-metal relay catalysis" strategy. During discharge (NO₃RR), nitrate ions are first adsorbed and reduced to nitrite on the surface tpp molecules at a low overpotential (less than -0.1V vs. RHE). The nitrite intermediate then diffuses to copper sites on the RhCu alloy, where subsequent hydrogenation to ammonia is efficiently assisted by surrounding rhodium atoms. This relay mechanism significantly enhances reaction kinetics and ammonia selectivity. For the charging process, ethanol is introduced into the catholyte. The RhCu M-tpp catalyst also actively catalyzes the ethanol oxidation reaction (EOR), which replaces the high-overpotential OER. This results in a substantially lower charge plateau. The assembled battery uses a zinc plate anode, a bipolar membrane separator, and specific electrolytes: an anolyte of 1 M KOH with zinc acetate and a catholyte of neutral Na₂SO₄/NO₃⁻ solution mixed with ethanol. This integrated approach of advanced catalyst design and electrolyte optimization enables efficient bifunctionality for both discharge and charge cycles.
Advantages
- Achieves an ultra-high energy density of at least 110,000 Wh kg⁻¹ (catalyst mass) and a peak power density of at least 1.5 mW cm⁻².
- Enables efficient rechargeability with a significantly reduced charge plateau (approximately 130 mV lower at 0.1 mA cm⁻²) compared to systems using OER, leading to higher energy efficiency.
- Demonstrates exceptional long-term cycling stability, operating steadily for approximately 400 cycles at a high rate.
- The RhCu M-tpp catalyst facilitates high-selectivity ammonia production from nitrate at low overpotentials, achieving a Faradaic efficiency above 70% at potentials greater than -0.4V vs. RHE.
- The molecule-metal relay catalysis mechanism decouples reaction steps, optimizing kinetics and suppressing competing hydrogen evolution.
- The synthesis method for the catalyst is scalable and uses commercially available chemicals, promising feasibility for large-scale production.
- Multifunctional operation simultaneously provides electrical energy, synthesizes valuable ammonia (and ammonium acetate upon cycling), and removes nitrate pollutants from water.
Applications
- Distributed Stationary Energy Storage: As a high-energy-density, rechargeable battery system for storing renewable energy (e.g., solar, wind) on the grid.
- Electric Transportation: Potential power source for electric vehicles due to its high energy density and power output.
- Decentralized Ammonia Production: Enables on-site, electrochemical ammonia synthesis from nitrate-containing wastewater or agricultural runoff, serving as a complementary method to the Haber-Bosch process.
- Wastewater Treatment: Integrated device for the simultaneous removal of nitrate pollutants from water while generating electricity and valuable chemicals.
- Power for Consumer Electronics: Demonstration shows capability to power devices like digital clocks, indicating potential for portable power applications.
