Bias-Free Copper-Nickel Alloy/Graphene/Si Heterojunction as Catalytic Photocathode for Photovoltaic-Assisted Ammonia Production

With the rise of world’s population, the demand of energy is also getting higher, which further accelerates the consumption of fossil fuels and the destruction of natural ecosystems. Solar energy is an alternative to fossil fuels because it has the highest energy content. However, the development of solar energy is limited by its intermittency and volatility, therefore storing solar energy becomes an important issue. Since ammonia is a suitable compound for storing solar energy, and it is a carbon-free chemical with high energy density, the production of ammonia becomes an important goal in our research.<br/>The mainstream production of ammonia in the industry relies on the Haber-Bosch process, which requires harsh operating conditions at high temperatures and pressure. An alternative technique based on the green synthesis route of ammonia by catalytic reactions has recently attracted tremendous interest.<br/>With efficient catalyst materials, ammonia is produced from nitrogen or nitrate by sequential reactions at ambient conditions. However, nitrogen is an inert molecule and has strong triple bond, causing a lower faradaic efficiency under nitrogen (N₂) reduction reaction (NRR). Furthermore, the potential barrier to triggering ammonia reduction is still high; thus, external energy, such as electric potential, is always required to drive the reaction. The potential barrier to triggering ammonia reduction is still high; thus, external energy, such as electric potential, is always required to drive the reaction. Here we report an improved ammonia reduction system of Cu-Ni coated graphene/HJT photocathode promoted by the photovoltaic effect. A commercial Si solar cell was attached with a monolayer of graphene where Cu-Ni catalyst was then directly grown on it by electrochemical deposition.<br/>We attempted to electrochemically electrodeposition Cu-Ni nanostructures on graphene/HJT as an electrocatalyst for NtRR. Cu and Ni have the same crystal structure (FCC), similar valence, close values of atomic radii, and electronegativity, allowing the formation of homogeneous solutions over the entire composition range. Next, we test the catalytic performance of Cu-Ni /Gr/HJT in the electrochemical nitrate reduction reaction. Our results show that graphene is an excellent supporting material for Cu-Ni catalysts and an efficient medium for transferring photogenerated electrons from Si solar cell to Cu-Ni nanostructure.<br/>As a result, the additional solar energy boosted the reaction kinetic of Cu-Ni catalysts for ammonia reduction. Under AM 1.5G solar simulator illumination, the overpotential required for ammonia reduction is suppressed at least by 100mV, leading to improved Faraday efficiency from 80% to over 90% and yield rate from 525 μg h^-1cm^-2to 750 μg h^-1cm^-2.<br/>To achieve bias-free reaction, we intend to replace batteries with solar cell. Additionally, we replace Pt (anode material) with Ni-Fe LDH since it possesses better performance for oxygen evolution reaction (OER).