Nitrogen-doped carbon nanotube-encapsulated nickel nanoparticles assembled on graphene for efficient CO2 electroreduction

Swapping CO2 electro-reduction active sites on a nickel-based hybrid formed on a "guilty" covalent triazine framework

A homogeneous and almost monodisperse Ni/CTFph composite of ultrasmall Ni NPs (∼2.2 nm) has been prepared by Metal Vapor Synthesis (MVS) and deposited on a highly porous and high specific surface area covalent triazine network. Metal doping was deliberately carried out on a metal-free system exhibiting superior CO2RR selectivity towards the challenging CO2-to-HCOOH electroreduction. Electrochemical studies aimed at shedding light on the CO2RR performance of the ultimate composite have allowed speculation on the synergistic or exclusive action of the two potentially active phases (N-doped C-network vs. Ni NPs). In contrast to the generally exclusive CO2-to-CO reduction mechanism described for the state-of-the-art Ni NP-based CO2RR electrocatalysts, Ni/CTFph has unveiled the unprecedented ability of Ni NPs to promote the alternative and more challenging 2e- CO2-to-HCOOH reduction pathway, even at moderately reducing potentials (-0.3 V vs. RHE).

MOF-Derived N-Doped Carbon Nanotube-Confined Ni Nanoparticles for the Simultaneous Electrochemical Detection of Cu²⁺ and Hg²⁺ with High Sensitivity and Stability

Heavy metal pollution has posed a serious threat to the ecological environment and human health. Thus, the development of accurate and effective methods for their detection is crucial. In this study, a novel electrochemical sensor was fabricated to detect Cu2+ and Hg2+, based on N-doped carbon nanotube-wrapped Ni nanoparticle (Ni@N-CNT) sensing material, which was derived from the pyrolysis of Ni2+ doped ZIF-8. For electrode material design, the packaging structure not only protected the encapsulated Ni nanoparticles from electrochemical corrosion in the acid electrolyte but also provided excellent electro-catalytic activity and electrical conductivity by controlling their size. Thanks to the overall performance of the Ni@N-CNT composite, the proposed sensor exhibited excellent analytical performance for Cu2+ and Hg2+ detection, with ultra-low detection limits of 33.3 ng⋅L-1 and 33.3 ng⋅L-1, respectively. The sensor also demonstrated good repeatability, reproducibility and selectivity. In addition, the method was successfully applied to the electrochemical analysis of Cu2+ and Hg2+ in actual Chinese cabbage samples with satisfactory recovery, confirming its practical applicability.

Confined Ni nanoparticles in N-doped carbon nanotubes for excellent pH-universal industrial-level electrocatalytic CO2 reduction and Zn-CO2 battery

Electrocatalytic reduction of CO2 (ECR) offers a promising approach to curbed carbon emissions and complete carbon cycles. However, the inevitable creation of carbonates and limited CO2 utilization efficiency in neutral or alkaline electrolytes result in low energy efficiency, carbon losses and its widespread commercial utilization. The advancement of CO2 reduction under acidic conditions offers a promising approach for their commercial utilization, but the inhibition of hydrogen evolution reaction and the corrosion of catalysts are still challenging. Herein, Ni nanoparticles (NPs) wrapped in N-doped carbon nanotubes (NixNC-a) are successfully prepared by a facile mixed-heating and freeze-drying method. Ni100NC-a achieves a high Faraday efficiency (FE) of near 100 % for CO under pH-universal conditions, coupled with a promising current density of CO (>100 mA cm-2). Especially in acidic conditions, Ni100NC-a exhibits an exceptional ECR performance with the high FECO of 97.4 % at -1.44 V and the turnover frequency (TOF) of 11 k h-1 at -1.74 V with a current density of 288.24 mA cm-2. This excellent performance is attributed to the synergistic effect of Ni NPs and N-doped carbon shells, which protects Ni NPs from etching, promotes CO2 adsorption and regulates local pH. Moreover, Ni100NC-a could drive the reversible Zn-CO2 battery with a high power-density of 4.68 mW cm-2 and a superior stability (98 h). This study presents a promising candidate for efficient pH-universal CO2 electroreduction and Zn-CO2 battery.

An orientated mass transfer in Ni-Cu tandem nanofibers for highly selective reduction of CO2 to ethanol

Electrochemically reducing CO2 to ethanol is attractive but suffers from poor selectivity. Tandem catalysis that integrates the activation of CO2 to an intermediate using one active site and the subsequent formation of hydrocarbons on the other site offers a promising approach, where the control of the intermediate transfer between different catalytic sites is challenging. We propose an internally self-feeding mechanism that relies on the orientation of the mass transfer in a hierarchical structure and demonstrate it using a one-dimensional (1D) tandem core-shell catalyst. Specifically, the carbon-coated Ni-core (Ni/C) catalyzes the transformation of CO2-to-CO, after which the CO intermediates are guided to diffuse to the carbon-coated Cu-shell (Cu/C) and experience the selective reduction to ethanol, realizing the orientated key intermediate transfer. Results show that the Faradaic efficiency for ethanol was 18.2% at -1 V vs. RHE (VRHE) for up to 100 h. The following mechanism study supports the hypothesis that the CO2 reduction on Ni/C generates CO, which is further reduced to ethanol on Cu/C sites. Density functional theory calculations suggest a combined effect of the availability of CO intermediate in Ni/C core and the dimerization of key *CO intermediates, as well as the subsequent proton-electron transfer process on the Cu/C shell.

Curvature Effect of Pyridinic N-Modified Carbon Atom Sites for Electrocatalyzing CO2 Conversion to CO

Carbon material is considered a promising electrocatalyst for the CO2 reduction reaction (CO2RR); especially, N-doped carbon material shows high CO Faradic efficiency (FECO) when using pyridinic N species as the active site. However, in the past decade, more efforts were focused on the preparation of various carbon nanostructures containing abundant pyridinic N species and few researchers studied the electronic structure modulation of the pyridinic N site. The curvature of the carbon substrate is an easily controllable parameter for modulating the local electronic environment of catalytic sites. In this research, carbon nanotubes (CNTs) with different diameters are applied to modulate the electronic environment of pyridinic N by the curvature effect. The pyridinic N sites doped on CNTs with the average curvature of 0.04 show almost 100% FECO at the current density of 3 mA cm-2 at -0.6 V vs RHE and 91% FECO retention after 12 h test, which is superior to most of the carbon-based electrocatalysts. As demonstrated by density functional theory simulation, the pyridinic N site forms a strong local electric field around the nearby C active site and protrudes out of the curved CNT surface like a tip, which remarkably enriches the protons around the adsorbed CO2 molecule.

The integration of bio-catalysis and electrocatalysis to produce fuels and chemicals from carbon dioxide

The dependence on fossil fuels has caused excessive emissions of greenhouse gases (GHGs), leading to climate changes and global warming. Even though the expansion of electricity generation will enable a wider use of electric vehicles, biotechnology represents an attractive route for producing high-density liquid transportation fuels that can reduce GHG emissions from jets, long-haul trucks and ships. Furthermore, to achieve immediate alleviation of the current environmental situation, besides reducing carbon footprint it is urgent to develop technologies that transform atmospheric CO₂ into fossil fuel replacements. The integration of bio-catalysis and electrocatalysis (bio-electrocatalysis) provides such a promising avenue to convert CO₂ into fuels and chemicals with high-chain lengths. Following an overview of different mechanisms that can be used for CO₂ fixation, we will discuss crucial factors for electrocatalysis with a special highlight on the improvement of electron-transfer kinetics, multi-dimensional electrocatalysts and their hybrids, electrolyser configurations, and the integration of electrocatalysis and bio-catalysis. Finally, we prospect key advantages and challenges of bio-electrocatalysis, and end with a discussion of future research directions.

Electrochemical Reduction of CO2 to CO over Transition Metal/N-Doped Carbon Catalysts: The Active Sites and Reaction Mechanism

Electrochemical CO2 reduction to value-added chemicals/fuels provides a promising way to mitigate CO2 emission and alleviate energy shortage. CO2 -to-CO conversion involves only two-electron/proton transfer and thus is kinetically fast. Among the various developed CO2 -to-CO reduction electrocatalysts, transition metal/N-doped carbon (M-N-C) catalysts are attractive due to their low cost and high activity. In this work, recent progress on the development of M-N-C catalysts for electrochemical CO2 -to-CO conversion is reviewed in detail. The regulation of the active sites in M-N-C catalysts and their related adjustable electrocatalytic CO2 reduction performance is discussed. A visual performance comparison of M-N-C catalysts for CO2 reduction reaction (CO2 RR) reported over the recent years is given, which suggests that Ni and Fe-N-C catalysts are the most promising candidates for large-scale reduction of CO2 to produce CO. Finally, outlooks and challenges are proposed for future research of CO2 -to-CO conversion.

From CO2 to Value-Added Products: A Review about Carbon-Based Materials for Electro-Chemical CO2 Conversion

The global warming and the dangerous climate change arising from the massive emission of CO2 from the burning of fossil fuels have motivated the search for alternative clean and sustainable energy sources. However, the industrial development and population necessities make the decoupling of economic growth from fossil fuels unimaginable and, consequently, the capture and conversion of CO2 to fuels seems to be, nowadays, one of the most promising and attractive solutions in a world with high energy demand. In this respect, the electrochemical CO2 conversion using renewable electricity provides a promising solution. However, faradaic efficiency of common electro-catalysts is low, and therefore, the design of highly selective, energy-efficient, and cost-effective electrocatalysts is critical. Carbon-based materials present some advantages such as relatively low cost and renewability, excellent electrical conductivity, and tunable textural and chemical surface, which show them as competitive materials for the electro-reduction of CO2. In this review, an overview of the recent progress of carbon-based electro-catalysts in the conversion of CO2 to valuable products is presented, focusing on the role of the different carbon properties, which provides a useful understanding for the materials design progress in this field. Development opportunities and challenges in the field are also summarized.