Application of Boron-doped Diamond Electrodes - Focusing on the Electrochemical Reduction of Carbon Dioxide
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-05-27 |
| Journal | Electrochemistry |
| Authors | Yasuaki Einaga |
| Institutions | Keio University |
| Citations | 8 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâBoron-doped Diamond (BDD) electrodes are validated as a highly durable, next-generation material for advanced electrochemical applications, particularly focusing on efficient CO2 reduction (CO2R).
- High Efficiency CO2R: Achieved near-quantitative Faradaic Efficiency (FE) for Formic Acid (HCOOH) production, reaching 94.7% in an optimized two-chamber flow cell system.
- Durability and Stability: BDD electrodes maintain high durability and stability over long-term operation, addressing a major limitation of traditional metal catalysts (Sn, Pb, Cu) which suffer from toxicity and corrosion.
- Selectivity Control: Product selectivity (HCOOH vs. Carbon Monoxide, CO) is precisely controlled by tuning the Boron concentration in the BDD film and selecting the electrolyte (KCl favors HCOOH; KClO4 favors CO, achieving up to 68% FE).
- Mechanism Elucidation: In situ ATR-IR spectroscopy confirmed that electrolyte choice controls the adsorption strength of the intermediate CO2 anion radical (CO2â˘â) on the BDD surface, dictating the final product.
- Scale-Up Validation: Successful transition to a medium-sized reactor (30 cm2 electrode area) using a novel intermittent flow system, which maintained high HCOOH FE (96.1%), proving industrial feasibility.
- Versatile Applications: Beyond CO2R, BDD is proven for practical electrochemical sensors (e.g., free chlorine), biomedical microsensors for in vivo monitoring, and efficient generation of active radicals for organic synthesis.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Max HCOOH Faradaic Efficiency (FE) | 94.7 | % | Optimized small flow cell (9.6 cm2) using 0.1% BDD and KCl. |
| HCOOH FE (Scale-up) | 96.1 | % | Medium flow cell (30 cm2) using intermittent flow system. |
| Max CO Faradaic Efficiency (FE) | Up to 68 | % | Optimized using 1% BDD and KClO4 electrolyte. |
| HCOOH Production Rate (High Current) | 473 | Âľmol m-2 s-1 | BDD performance, exceeding Sn/Pb electrodes (440 Âľmol m-2 s-1). |
| Optimized Boron Concentration (HCOOH) | 0.1 | % | Concentration yielding maximum HCOOH selectivity. |
| Optimized Boron Concentration (CO) | 1 | % | Higher concentration favoring CO production. |
| Applied Current Density (Standard CO2R) | -2.0 | mA cm-2 | Standard condition for initial HCOOH optimization. |
| Optimized Catholyte Flow Rate | 200 | mL min-1 | Flow rate used to achieve 94.7% HCOOH FE. |
| CO2R Potential (CO Optimization) | -2.1 | V | vs. Ag/AgCl, used in KClO4 solution. |
| Intermittent Flow Cycle Time | 1 | second | Frequency at which solution pressure cycles to zero during scale-up. |
| BDD Microelectrode Tip Diameter | ~40 | Âľm | Used for in vivo biomedical sensing applications. |
| ATR-IR Peak (CO2â˘â Adsorption) | 1634 | cm-1 | Observed in KClO4 solution, indicating adsorbed CO2â˘â intermediate. |
Key Methodologies
Section titled âKey MethodologiesâThe electrochemical reduction of CO2 was primarily conducted using BDD electrodes in a controlled flow environment, with detailed analysis of material and electrolyte effects.
- BDD Synthesis: Boron-doped Diamond films were deposited onto substrates using Microwave Plasma-Assisted Chemical Vapor Deposition (MPCVD). Boron concentration was varied (0.01% to 2%) to study its effect on electronic properties and CO2R selectivity.
- Electrolysis Setup: A two-chamber flow cell was employed, separated by a Nafion NRE-212 cation exchange membrane to isolate the cathode (BDD) and anode (Pt plate) compartments.
- Electrolyte Control: Aqueous solutions of alkali metal halides (MX, where M = Li, Na, K, Rb, Cs) and potassium salts (KNO3, K2SO4, KCl, KBr, KI, KClO4) were used as catholytes, saturated with CO2 gas.
- Reaction Control: Experiments were run under galvanostatic control (constant current density, typically -2.0 mA cm-2) or potentiostatic control (constant potential, e.g., -2.1 V vs. Ag/AgCl).
- Scale-Up System: For medium-sized cells (30 cm2), a continuous liquid-fed intermittent flow system was implemented using a synchronized dual-phase double-action cylindrical pump. This system ensured the solution pressure dropped to zero every 1 second, optimizing mass transfer.
- In Situ Characterization: Attenuated Total Reflection Infrared Spectroscopy (ATR-IR) was performed in situ during electrolysis to monitor the adsorption state of the CO2â˘â intermediate on the BDD surface, confirming the role of the electrolyte (e.g., KClO4 vs. KCl) in controlling the reaction pathway.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique properties of BDD electrodesâwide potential window, low background current, high chemical inertness, and exceptional durabilityâmake them suitable for several high-value engineering sectors.
| Application Area | BDD Value Proposition | Specific Examples |
|---|---|---|
| Carbon Capture & Utilization (CCU) | High Faradaic efficiency and long-term stability for industrial CO2 conversion. | Sustainable production of commodity chemicals like Formic Acid (HCOOH) and Carbon Monoxide (CO) from waste CO2. |
| Environmental Sensing | Wide potential window allows detection of species that overlap with oxygen evolution on traditional electrodes. | Commercialized free chlorine sensors; highly sensitive detection of hypochlorite ions (ClO-). |
| Biomedical Diagnostics | Miniaturization capability and high sensitivity for real-time, localized measurements in complex biological environments. | In vivo real-time monitoring of drug kinetics (e.g., bumetanide) and neurotransmitters (e.g., dopamine) using BDD microsensors. |
| Advanced Oxidation Processes (AOP) | Efficient electrochemical generation of powerful oxidizing agents (e.g., OH radicals). | Waste water treatment and degradation of persistent organic pollutants (electrochemical incineration). |
| Electrochemical Synthesis | Controlled generation of specific active radical species in aqueous and organic solvents. | Synthesis of complex organic molecules (e.g., licarin A) via methoxy radical generation. |
View Original Abstract
Boron-doped diamond (BDD) electrodes are next generation electrode materials and their electrochemical applications have been actively developed in recent years. They are expected to be useful electrode materials for improving the environment and for bio-medical applications. Here, examples of practical applications as electrochemical sensors, the development of in vivo real time measurements, and electrochemical organic synthesis using BDD electrodes are briefly introduced. In the second part, our recent work on the production of useful chemicals by means of the electrochemical reduction of CO2 using BDD electrodes is described. The work has attracted particular attention for its potential contribution to carbon neutrality and carbon recycling.