Effect of alkali-metal cations on the electrochemical reduction of carbon dioxide to formic acid using boron-doped diamond electrodes

At a Glance

Metadata	Details
Publication Date	2017-01-01
Journal	RSC Advances
Authors	Norihito Ikemiya, Keisuke Natsui, Kazuya Nakata, Yasuaki Einaga
Institutions	Tokyo University of Science, Keio University
Citations	40
Analysis	Full AI Review Included

6CCVD Technical Documentation: Advanced BDD Electrodes for High-Efficiency CO₂ Reduction

Executive Summary

This research validates the use of highly stable, Boron-Doped Diamond (BDD) electrodes manufactured via Microwave Plasma Assisted Chemical Vapor Deposition (MPCVD) for the efficient electrochemical conversion of carbon dioxide (CO₂) to formic acid. 6CCVD’s BDD materials are ideally suited to replicate and scale this robust process.

Peak Efficiency Confirmed: A Faradaic Efficiency (FE) of 71% was achieved (and optimized to 86%) for formic acid production, significantly surpassing efficiencies obtained with traditional metal electrodes (Sn, Pb) while offering superior stability.
Optimal Electrolyte Identification: The study definitively demonstrated the critical role of alkali-metal cations, finding that Rubidium ($\text{Rb}^{+}$) solutions (0.075 M, pH 6.2) maximize FE by suppressing competing hydrogen evolution reactions (HER).
Material Durability: The MPCVD BDD electrodes exhibited extreme stability, showing no degradation in FE or morphology after 48 hours of continuous high-potential operation at 2 mA cm⁻².
High Productivity: Utilizing higher current densities, the process achieved high productivity, generating over 1 g L⁻¹ of formic acid per hour, addressing a key industrial scaling challenge.
6CCVD Advantage: This process requires highly conductive, stable BDD films (resistivity 2 mΩ cm). 6CCVD specializes in delivering custom, heavily doped BDD wafers and plates tailored for high-stability electrochemical applications.

Technical Specifications

Parameter	Value	Unit	Context
Max Faradaic Efficiency (FE)	71 (Optimized to 86)	%	Conversion of CO₂ to HCOOH using 0.075 M Rb⁺
BDD Electrical Resistivity	2	mΩ cm	Highly conductive, high-doped BDD film
B/C Ratio (Doping)	1.0	w/w	High boron doping concentration in the source gas
BDD Film Structure	sp³ Diamond	-	Confirmed by Raman peak at 1324 cm⁻¹
Electrolysis Stability	48	hours	Continuous operation without FE degradation
Standard Current Density	2	mA cm⁻²	Standard test condition
Maximum Current Density Tested	20	mA cm⁻²	Used for increased productivity studies
Formic Acid Productivity	> 1	g L⁻¹ hr⁻¹	Achieved at elevated current densities
Optimum Catholyte pH	6.2	-	Neutralized solution maximizing $\text{HCO}_{3}^{{-}}$ species
Electrolysis Potential Range	-2.2 to -3	V	Highly negative potential versus Ag/AgCl reference

Key Methodologies

The synthesis and electrochemical testing relied on precise control over BDD film growth and subsequent cell chemistry, confirming the requirement for high-specification material inputs.

1. BDD Material Synthesis via MPCVD

Deposition Technique: Microwave Plasma Assisted Chemical Vapor Deposition (MPCVD).
Substrate: Si(100) wafers.
Carbon Source: Acetone, utilized as both the carbon source and solvent for the boron precursor.
Boron Source & Doping: $\text{B(OCH}{3}){3}$ (Trimethyl borate) resulting in a high B/C atomic ratio of 1.0 w/w.
Power Input: Chamber maintained at 5 kW.
Resulting Film: High quality, highly conductive BDD thin films (resistivity 2 mΩ cm).

2. Electrochemical Cell Setup and Control

Cell Configuration: Two-compartment H-type cell (100 mL volume).
Electrode Placement: BDD working electrode, Platinum (Pt) counter electrode, and Ag/AgCl reference electrode.
Gas Management: Initial 30 min purging with $\text{N}{2}$ (200 sccm) to remove oxygen, followed by 5 min saturation with $\text{CO}{2}$ (500 sccm).
Electrolyte Neutralization: The $\text{CO}{2}$ saturated alkaline solutions ($\text{KOH}$, $\text{RbOH}$, etc.) were neutralized to the optimal $\text{pH}$ 6.2 using $\text{HCl}$ to ensure the formation of $\text{HCO}{3}^{{-}}$ (the key precursor species).
Electrolysis Mode: Experiments performed at a controlled constant current (typically -10 mA, or 2 $\text{mA cm}^{{-2}}$ current density) for 1 hour at ambient temperature and pressure.

3. Analysis and Characterization

Film Quality: Confirmed via Raman Spectroscopy (1324 $\text{cm}^{{-1}}$ peak).
Morphology: Surface integrity confirmed via Scanning Electron Microscopy (SEM) after 30 hours of operation, showing clean diamond facets with no evidence of etching.
Product Analysis: Formic acid analyzed using High Performance Liquid Chromatography (HPLC) with an electroconductivity detector.

6CCVD Solutions & Capabilities

This breakthrough research demonstrates that BDD, when highly doped and structurally stable, is a superior alternative to unstable metal catalysts for CO₂ conversion. 6CCVD provides the specialized MPCVD diamond materials necessary to advance this field toward industrial scale.

Applicable Materials

The study requires Heavy Boron-Doped Diamond (BDD) films with high carrier concentration and excellent mechanical stability. 6CCVD offers materials tailored to meet these demanding specifications:

Heavy Boron Doped Polycrystalline Diamond (PCD BDD): Ideal for scaling up this electrochemical process. 6CCVD can match the required resistivity (2 mΩ cm) via precise control of the B/C doping ratio during MPCVD growth.
- Recommendation: Use BDD PCD wafers for large-area electrode fabrication, leveraging 6CCVD’s ability to produce plates up to 125mm.
Boron Doped Single Crystal Diamond (SCD BDD): For fundamental research focused on understanding the reaction mechanisms and adsorption phenomena at the atomic level, high-purity SCD BDD offers an unparalleled, defect-controlled platform.

Customization Potential

The experimental setup utilized small, customized BDD coupons. 6CCVD’s specialized engineering services directly support the challenges of both laboratory research and industrial scaling.

Requirement in Paper	6CCVD Capability & Solution	Value Proposition
Small Coupon Size	Custom Laser Cutting & Sizing	Precise, reliable electrode dimensions for H-cell or flow-cell experiments, ensuring reproducibility.
High Doping Level	Thickness & Doping Control	Guaranteed SCD/PCD thickness from 0.1µm to 500µm with precise, repeatable resistivity targets (down to < 1 mΩ cm).
Electrode Contacting	Custom Metalization Services	Application of robust electrode contacts (Au, Pt, Ti, W) specifically required for highly corrosive or aggressive electrochemical environments.
Surface Quality	Precision Polishing	Ultra-low roughness polishing available (Ra < 5nm for inch-size PCD), critical for minimizing parasitic side reactions and maximizing catalyst surface area integrity.

Engineering Support

6CCVD’s in-house team of MPCVD PhD material scientists provides consultative support essential for successful implementation of diamond technology. We assist customers with:

Material Selection: Determining the optimal balance between cost (PCD BDD) and structural purity (SCD BDD) for specific electrochemical reactors.
Recipe Optimization: Advising on material resistivity and thickness requirements for high-current density applications, ensuring long-term stability under continuous, negative potentials.
Integration Support: Providing expertise on mechanical integration, metal contacting, and thermal management for advanced electrochemical cells, such as those used in CO₂ reduction ($\text{CO}_{2}$RR) projects.

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

View Original Abstract

Rb<sup>+</sup> cations provide a greater effect on the electrochemical conversion of CO<sub>2</sub> to formic acid using BDD electrodes.

Tech Support

Original Source

DOI: https://doi.org/10.1039/c7ra03370b