Synthesis and Performance Characterizations of Transition Metal Single Atom Catalyst for Electrochemical CO2 Reduction

Summary

Here, we present a protocol for the synthesis and electrochemical testing of transition metal single atoms coordinated in graphene vacancies as active centers for selective carbon dioxide reduction to carbon monoxide in aqueous solutions.

Abstract

This protocol presents both the synthesis method of the Ni single atom catalyst, and the electrochemical testing of its catalytic activity and selectivity in aqueous CO₂ reduction. Different from traditional metal nanocrystals, the synthesis of metal single atoms involves a matrix material that can confine those single atoms and prevent them from aggregation. We report an electrospinning and thermal annealing method to prepare Ni single atoms dispersed and coordinated in a graphene shell, as active centers for CO₂ reduction to CO. During the synthesis, N dopants play a critical role in generating graphene vacancies to trap Ni atoms. Aberration-corrected scanning transmission electron microscopy and three-dimensional atom probe tomography were employed to identify the single Ni atomic sites in graphene vacancies. Detailed setup of electrochemical CO₂ reduction apparatus coupled with an on-line gas chromatography is also demonstrated. Compared to metallic Ni, Ni single atom catalyst exhibit dramatically improved CO₂ reduction and suppressed H₂ evolution side reaction.

Introduction

Converting CO₂ into chemicals or fuels using clean electricity is becoming increasingly important as a potential route to prevent further CO₂ emissions^1,2,3,4,5,6. However, this practical application is currently challenged by the low activity and selectivity of CO₂ reduction reaction (CO₂RR) due to the high kinetic barriers and the competition with hydrogen evolution reaction (HER) in aqueous media. Most of the traditional trans....

Protocol

1. Preparation of Ni Single Atom Catalyst (NiN-GS)

1. Preparation of electrospinning precursor solution
   1. Take a 20 mL scintillation vial, dissolve 0.5 g of polyacrylonitrile (M_w=150,000), 0.5 g of polypyrrolidone (M_w=1,300,000), 0.5 g of Ni(NO₃)₂·6H₂O, and 0.1 g of dicyandiamide (DCDA) in 10 mL of dimethylformamide (DMF).
   2. Heat the DMF mixture to 80 °C and keep the mixture at 80 °C with constant stirring until all polymers and salt are dissolved and a clear solution is observed.
2. Electrospinning to produce polymer fibers
   1. Set convent....

Results

Scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping images are shown in Figure 1 for the morphology characterization of NiN-GS. Three-dimensional atom probe tomography (3D-APT) results are shown in Figure 2 for the direct identification of single Ni sites distribution as well as their neighboring chemical environment. On-line electrochemical G.......

Discussion

In the above electrospinning process, two important steps should be noted in material synthesis procedures: 1) heating the DMF mixture (step 1.1.2), and 2) the pump rate adjusting (step 1.2.2) to match the spinning rate. The SEM image in Figure 1A shows the obtained carbon nanofibers interconnected with each other (~200 nm in diameter). They were broken into small pieces by ball milling for characterizations as shown in Figure 1B. Ni nanoparticles were uniformly.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
syringe pump	KD Scientific	KDS-100
tube furnance	Lindberg/Blue M	TF55035A-1
ball miller	SPEX SamplePrep	5100
electrochemical work station	BioLogic	VMP3
pH meter	Orion	320 PerpHecT	2 points calibration before use
gas chromatograph	Shimadzu	GC-2014	a combined seperation system consisting of molecular sieve 5A, Hayesep Q, Hayesep T, and Hayesep N
mass flow controller	Alicat Scientific	MC-50SCCM-D/5M
ultrapure water system	Millipore	Synergy
vacuum desiccator	PolyLab	55205
polyacrylonitrile	Sigma-Aldrich	181315	Mw=150,000
polypyrrolidone	Sigma-Aldrich	437190	Mw=1,300,000
Ni(NO3)26H2O	Sigma-Aldrich	244074
dicyandiamide	Sigma-Aldrich	D76609
dimethylformamide	Sigma-Aldrich	227056
carbon fiber paper	AvCarb	MGL370
Nafion 117 membrane	Fuel Cell Store	117	used as proton exchange membrane in H-cell
KHCO3	Sigma-Aldrich	431583	further purified by electrolysis
platinum foil	Beantown Chemical	126580
saturated calomel electrode	CH Instruments	CHI150
glassy carbon electrode	HTW GmbH	SIGRADUR	1 cm × 2 cm
wax	Apiezon	W-W100
Nafion 117 solution	Sigma-Aldrich	70160	used as ionomer in catalyst ink preparation
forming gas	Airgas	UHP	5% H2 balanced with Ar
carbon dioxide	Airgas	LaserPlus
sandard gas	Airgas	customized	500 ppm CO, 500 ppm CH4, 1000 ppm H2 balanced with Ar
sandard gas	Air Liquide	customized	100 ppm H2, 100 ppm CO and other alkanes balanced with Ar