Formation of ammonium ions by electrochemical oxidation of urea with a boron-doped diamond electrode

At a Glance

Metadata	Details
Publication Date	2020-01-01
Journal	New Journal of Chemistry
Authors	Norihiro Suzuki, Akihiro Okazaki, Kai Takagi, Izumi Serizawa, Genji Okada
Institutions	Tokyo University of Science
Citations	9
Analysis	Full AI Review Included

6CCVD Technical Analysis: Electrochemical $\text{NH}_{4}$⁺ Generation using Boron-Doped Diamond (BDD)

Executive Summary

This research validates the use of Boron-Doped Diamond (BDD) electrodes as a superior, energy-efficient platform for the synthesis of ammonium ions ($\text{NH}{4}$⁺) via the electrochemical oxidation of urea, a critical step toward sustainable ammonia ($\text{NH}{3}$) production from waste streams.

Core Achievement: Demonstrated almost complete decomposition of urea (2 wt%) in simplified artificial urine within 18 hours using BDD at ambient temperature and pressure.
Material Superiority: BDD proved uniquely effective, achieving complete mineralization and successfully suppressing the formation of toxic byproducts (e.g., hydrazine, $\text{N}{2}\text{H}{4}$), a challenge faced by conventional $\text{Ti}/\text{IrO}_{2}$ electrodes.
Mechanism: The BDD’s exceptionally wide electrochemical potential window enables high-voltage water oxidation, generating highly reactive oxygen species (ROS), primarily hydroxyl radicals ($\text{OH}^{\bullet}$), crucial for urea decomposition.
Energy Efficiency: The process operates successfully at room temperature and atmospheric pressure, offering a significantly lower energy footprint compared to the energy-intensive Haber-Bosch process (400-600 °C, 20-40 MPa).
Hybrid Enhancement: Integrating a mesoporous $\text{TiO}{2}/\text{BDD}$ photocatalyst and 207 nm UV irradiation enhanced selectivity, promoting $\text{NH}{4}$⁺ accumulation in the counter cell over the formation of undesirable nitrate ($\text{NO}_{3}$^-).
Application Potential: This technique provides a scalable, environmentally benign method for recycling nitrogen from waste (urine) into valuable, reactive $\text{N}$ sources for fertilizer or hydrogen carrier applications.

Technical Specifications

Hard data extracted from the experimental methodology and results:

Parameter	Value	Unit	Context
Electrode Material (Working)	Boron-Doped Diamond (BDD)	-	Electrochemical catalyst source for ROS ($\text{OH}^{\bullet}$) generation.
Applied Current	75	mA	Constant current utilized during electrochemical treatment.
Initial Urea Concentration ($C_{0}$)	2	wt%	Concentration in the reaction cell electrolyte.
Electrolyte Base	1	wt%	NaCl aqueous solution (simulating a main urine sub component).
Urea Decomposition Time (99% removal)	~18	hours	Time required for nearly complete urea removal using BDD only.
Operational Temperature	Room	°C	Ambient operating condition, enabling low energy usage.
Photocatalysis Wavelength ($\lambda$)	207	nm	Wavelength of Kr-Br excimer lamp for hybrid treatment.
UV Irradiance	2.0	$\text{mW}_\text{cm}^{-2}$	Irradiance applied to the $\text{TiO}_{2}/\text{BDD}$ photocatalyst.
Membrane Separator	Nafion® NRE-212	-	Used to separate oxidation (BDD) and reduction (Pt) cells.
Achieved Product	$\text{NH}_{4}$⁺ (Ammonium Ion)	-	Target high-value product, accumulated in the counter cell.

Key Methodologies

The experiment utilized a highly controlled two-cell electrochemical reactor setup to achieve selective urea oxidation:

Cell Architecture: An H-type glass cell was employed, physically separating the oxidation half-cell (BDD working electrode) from the reduction half-cell (Pt counter electrode).
Electrode Separation: The two half-cells were connected solely by a Nafion® NRE-212 membrane, allowing ion exchange ($\text{NH}_{4}$⁺ migration) while preventing mixing of electrolytes and undesired reduction reactions.
Electrolyte: The reaction cell was charged with 50 mL of simplified artificial urine (2 wt% urea in 1 wt% NaCl solution). The counter cell was charged with 50 mL of 1 wt% NaCl aqueous solution.
Electrochemical Treatment: A constant current of 75 mA was applied to the BDD working electrode, driving water electrolysis and the generation of hydroxyl radicals ($\text{OH}^{\bullet}$).
Hybrid Photocatalysis (Optional): In integrated experiments, a mesoporous $\text{TiO}{2}/\text{BDD}$ catalyst was placed in the reaction cell and irradiated horizontally with 207 nm UV light (2.0 $\text{mW}_\text{cm}^{-2}$) to promote intermediate oxidation ($\text{NO}{2}$^- $\rightarrow$ $\text{NH}_{4}$⁺).
Monitoring: Solution aliquots were periodically sampled, and concentrations of urea, $\text{NH}{4}$⁺, $\text{NO}{2}$^-, and $\text{NO}_{3}$^- were quantified using HPLC and ion chromatography.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality Boron-Doped Diamond (BDD) in advanced electrochemical and photoelectrochemical systems. 6CCVD is an expert supplier of MPCVD diamond substrates necessary to replicate and scale this highly efficient $\text{NH}_{3}$ fabrication technology.

Applicable Materials

The successful replication and scaling of this work hinge on highly conductive, uniform BDD films.

Material Specification: Heavy Boron-Doped Polycrystalline Diamond (BDD) Wafers. 6CCVD delivers BDD with controlled doping concentrations, ensuring the low resistance and high stability required for effective constant-current operation and high ROS generation at the anode.
Substrate Support: For hybrid systems, 6CCVD provides custom Polycrystalline Diamond (PCD) substrates suitable for subsequent mesoporous $\text{TiO}_{2}$ deposition, ensuring the mechanical stability and thermal robustness needed for catalyst integration.

Customization Potential

The experimental setup requires electrodes tailored for H-type cell geometry and integrated catalyst layers. 6CCVD excels in meeting these specific engineering demands:

Required Specification	6CCVD Capability	Value Proposition
Electrode Dimensions	Custom dimensions up to 125 mm diameter (PCD).	Enables rapid scale-up from lab-scale cells to commercial flow reactors.
Film Thickness	BDD films available from 0.1 µm to 500 µm.	Allows engineers to optimize conductivity, lifetime, and cost specific to electrochemical load demands (75 mA).
Surface Finish	Custom polishing down to Ra < 5 nm (PCD).	Essential for repeatable thin-film deposition (e.g., $\text{TiO}_{2}$ photocatalyst layers) and maintaining uniform current density.
Metalization/Contacts	Internal capability for Au, Pt, Pd, Ti, W, and Cu metalization.	We provide reliable, low-resistance ohmic contacts (e.g., Ti/Pt/Au contact pads) required for robust current application (75 mA) to the BDD working electrode.
Prototyping & Geometry	Advanced laser cutting services.	Allows for precise cutting of BDD wafers into specific electrode shapes (e.g., rectangular plates) suitable for H-cell or custom reactor configurations.

Engineering Support

6CCVD’s in-house PhD material scientists specialize in CVD growth recipes that optimize diamond properties for electrochemical and photoelectrochemical applications. We are ready to assist engineers and researchers scaling this work, specifically in:

Doping Optimization: Fine-tuning boron concentration to maximize $\text{OH}^{\bullet}$ production efficiency while maintaining material integrity and wide potential window functionality.
Interface Engineering: Consulting on appropriate substrate preparation and metalization schemes for BDD electrodes used in complex, corrosive environments like urea oxidation.
Scale-Up Strategy: Providing support on material selection (large area PCD) and geometry for transitioning from laboratory H-cells to industrial flow cell architectures for energy-efficient $\text{NH}_{3}$ fabrication projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Ammonium ions were formed electrochemically from urea with a boron-doped diamond electrode and increased by using photocatalyst together.

Tech Support

Original Source

DOI: https://doi.org/10.1039/d0nj03347b