Carbon catalysts with metal and nitrogen
dopants hold significant
promises for an electrochemical CO2 reduction reaction
(CO2RR). However, the fabrication of these carbon catalysts
normally requires an energy-intensive synthesis process. Traditionally,
2D graphene and 1D carbon nanotubes (CNTs) are the most widely used
carbon supports, but graphene tends to aggregate and CNTs suffer from
low density of active sites on the surface. In this work, we developed
a 3D hybrid carbon nanosheet/nanotube catalyst with nickel (Ni) and
nitrogen (N) co-doped active sites for the CO2RR by a one-step
chemical vapor deposition (CVD) method. Both single atomic sites and
nanoparticles of Ni were observed on the hybrids, but the Ni nanoparticles
were encapsulated by graphitic carbon layers during the CVD process,
and as a result, the competing hydrogen evolution reaction was suppressed
and high CO selectivity was achieved. The as-prepared catalyst with
20 min CVD delivered a stable CO Faradaic efficiency of 91% with a
partial current density of 28.9 mA/cm2 at −0.74
V in an H-cell setup. The same catalyst achieved a commercially viable
current density of 600 mA/cm2 in a flow cell with CO selectivity
above 85%, at an applied voltage of −2.0 V vs reversible hydrogen
electrode without iR compensation. To the best of our knowledge, these
results are among the best performances in the literature in terms
of both current density and CO selectivity for the CO2RR
by carbon-based catalysts. Furthermore, catalysts developed in this
work are synthesized at a moderate temperature without any acid/oxidant
pretreatment or post-washing. The energy-efficient and environmentally
benign synthesis and the significantly high performance of catalysts
are essential to future large-scale CO2RR applications.
The electrochemical CO2 reduction reaction (CO2RR) is a promising approach of using renewable power sources such as wind and solar to convert CO2 into value-added products. However, conventional methods of synthesizing high-performance CO2RR catalysts usually produce wastes and are not environmentally friendly. Herein, we developed a sustainable catalyst synthesis method by using cheap, abundant cornstarch as the feedstock, and doping it with nickel (Ni) from a simulated metalcontaining wastewater, before finally doping it with nitrogen (N) to create a highly efficient metal-nitrogen-carbon (M-N-C) catalyst that is dominated by single atomic Ni sites without the need for an acid wash post-treatment. The cornstarch-based catalyst demonstrated a high faradaic efficiency (FE) of 92% for CO production with a CO current density of 11.6 mA/cm 2 at −0.8 V versus reversible hydrogen electrode (RHE). At the same Ni content under the same testing conditions, a catalyst prepared via conventional wet impregnation only attained a CO current density of 9.3 mA/cm 2 , and a catalyst prepared using more expensive graphene oxide achieved a CO current density of 11.5 mA/cm 2 but with a lower FE (CO) at 81%. Findings from this work provide insights into using low-cost sustainable biomaterials and non-waste producing methods to produce effective electrochemical CO2RR catalysts.
Metal-and nitrogen-doped carbon (M−N−C) is a promising material to catalyze electrochemical CO 2 reduction reaction (CO 2 RR). However, most M−N−C catalysts in the literature require complicated synthesis procedures and produce small quantities per batch, limiting the commercialization potential. In this work, we developed a simple and scalable synthesis method to convert metal-impurity-containing commercial carbon nanotubes (CNTs) and nitrogen-containing organic precursors into M−N−C via one-step moderate-temperature (650 °C) pyrolysis without any other treatment nor the need to add metal precursors. Batches of catalysts in varied mass up to 10 g (150 mL in volume) per batch were synthesized, and repeatable catalytic performances were demonstrated. To the best of our knowledge, the 10 g batch is one of the largest batches of CO 2 RR catalysts synthesized in the literature while requiring minimal synthesis steps. The catalyst possessed single-atomic iron−nitrogen (Fe−N) sites, enabling a high performance of >95% CO product selectivity at a high current density of 400 mA/cm 2 and high stability for 45 h at 100 mA/cm 2 in a flow cell testing. The catalyst outperformed a benchmark noble-metal nanoparticle catalyst and achieved longer stability than many other reported M−N−C catalysts in the literature. The scalable and cost-effective synthesis developed in this work paves a pathway toward practical CO 2 RR applications. The direct utilization of metal impurities from raw CNTs for efficient catalyst synthesis with minimal treatment is a green and sustainable engineering approach.
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