Molecular tuning of CO2-to-ethylene conversion
The paper I'll discuss is this one: Molecular tuning of CO2-to-ethylene conversion. (Sargent et al, Nature volume 577, pages 509–513(2020))
In the case where carbon dioxide is reduced to make ethylene, the starting material for polyethylene and other polymers, the carbon so obtained is sequestered.
As I noted in a previous post, plastics have their own risks, huge risks in a purely environmental sense, but long term use of plastics as opposed to single use, will sequester carbon dioxide. Nothing is ever black and white.
The paper's abstract begins with the usual paean to the regrettably popular "renewable energy" Gods, although all the worship focused on them has not made them to do anything at all to address the rapidly rising use of dangerous fossil fuels, coupled intimately with the rapidly rising rate of the destruction of the planetary atmosphere. This said, one of the limitations of and reason for the failure of the so called "renewable energy" industry in addressing climate change is that it produces electricity at times that it is not needed, making the electricity generated have no value whatsoever while destroying the value of electricity produced by reliable sources, thus driving overall electricity prices up, not down. (There is a reason that Germany and Denmark have the highest consumer electricity prices in the OECD.)
Everybody loses when electricity prices go negative despite rhetoric to the contrary.
To address fluctuations in demand, however, most power grids maintain generation that is not utilized and not sold: This is termed "spinning reserve" and is designed to address demand surges. In a sensible world, as opposed to the world in which we actually live, "spinning reserve" might actually be utilized for production purposes with a capability to be rapidly switched to a grid when needed. This would tend to reduce electricity prices, since it would allow for the utilization of continuous reliable baseload power systems such as those produced by nuclear energy, which is measurably the most reliable system of electricity production in the world and also the cleanest, to produce value at all times.
Thus this paper is interesting.
From the text:
Recently we found that an N-aryl-substituted tetrahydro-4,4?-bipyridine organic thin film, formed by reductive electro-dimerization of an N-aryl pyridinium additive (Fig. 1a; see Supplementary Information for details), facilitated selective CO2RR to multi-carbon products on Cu foils11. However, the selectivity and partial current density for ethylene are low (about 40% and 0.5 mA cm?2) for practical applications. We sought to clarify factors contributing to the selectivity enhancement to enable further design of new functional molecules with better performance.
These pyridinium type systems build on the work of Emily Cole and Andrew Bocarsly at Princeton University from some years ago. (Dr. Cole's company built on this technology failed however.) Their work is referenced in this paper. I'm glad to see this. I met Dr. Cole once, and I liked her very much.
Here are the structures of the pyridinium systems utilized in the paper:
The caption:
a, Reaction describing the electro-dimerization process that converts an N-arylpyridinium salt to a mixture of N-aryl-substituted tetrahydro-bipyridines. b, Molecular structures of additives 1–11. OTf?– is trifluoromethanesulfonate. Cl? and OTf? are the counter-ions of the derivatives. c, Trend for ethylene FE and calculated Bader charge for the nitrogen atom of the N-aryl-substituted tetrahydro-bipyridines prepared from 1–11. Owing to the symmetric molecular structure of the tetrahydro-bipyridines, a hydrogen atom was used to replace half of the dimer unit (see Supplementary Fig. 6 for details). A spread of Bader charges for the nitrogen, covering the limiting values of the para,para and ortho,ortho structures, was plotted. The circles correspond to the average contribution from both the para,para and ortho,ortho isomers where their ratio could be determined by 1H NMR spectroscopy (see Supplementary Note 1 for details). The error bars for ethylene FE uncertainty represent one standard deviation based on three independent samples. The corresponding error bars for ethylene FE uncertainty were arbitrarily placed in the middle of the limiting values for those tetrahydro-bipyridines for which the para,para versus ortho,ortho ratio could not be reliably determined by 1H NMR spectroscopy.
The authors continue:
Noting that local environment plays a role in electrocatalysis through tuning interactions among reactants/intermediates12,13,14,15,16, we postulated that the N-arylpyridinium-derived film may affect the selectivity of CO2RR by interacting with the reaction intermediate(s). To test this hypothesis, we first prepared a library of N-arylpyridinium salts (1–11, Fig. 1b, Supplementary Figs. 1 and 2) expected to display different electronic properties. We then electrodeposited these N-arylpyridinium precursors onto a porous polytetrafluoroethylene gas diffusion layer17 with a sputtered Cu layer serving as both current collector and catalyst. The as-electrodeposited thin film is water-insoluble and consists of a mixture of both constitutional isomers and stereo isomers of N-aryl-substituted tetrahydro-bipyridine species (Fig. 1a, Supplementary Note 1, Supplementary Figs. 3–5). As expected, Bader charge analysis points to different electron donating abilities of these tetrahydro-bipyridines (Supplementary Fig. 6). Coating of the tetrahydro-bipyridine film onto the Cu electrode does not substantially change its morphology, crystallinity, electronics or wettability, nor does it retard the transport of reactants, ions and products, which is needed in electrocatalytic processes (Supplementary Note 2, Supplementary Figs. 7–10).
We evaluated CO2RR properties of these tetrahydro-bipyridine-functionalized electrodes in a liquid-electrolyte flow cell system (Supplementary Fig. 11), using CO2-saturated 1 M aqueous KHCO3 as the supporting electrolyte. In this system, the abundant catalyst/electrolyte/CO2 triple-phase interfaces overcome the CO2 mass-transport limit17,18 and thus enable commercially relevant current densities19,20. We note that, although the large achievable current densities in the flow cell drive up local pH (Supplementary Fig. 12), the tetrahydro-bipyridine layer does not create a further pH gradient near the active Cu surface (Supplementary Note 2).
The authors synthesized a chemical library of pyridinium salts to test, a good idea. Chemical libraries have come a long way since the early days, days in which I personally had occasion to work on them.
A few more graphics:
The caption:
a, The relationship between the ethylene FE and the ratio of atop CO and bridge CO on Cu–x electrodes. The relative population of these two kinds of Cu-bound CO was calculated through the integrated areas of each band in the Raman spectra, which are proportional to the corresponding *CO coverage (see Supplementary Note 3 for more details). The error bars for ethylene FE uncertainty represent one standard deviation based on three independent samples. b, The relationship between the ratio of atop CO to bridge CO on Cu–x and the Bader charge for the nitrogen atom of the N-aryl-substituted tetrahydro-bipyridine formed from additive x. The Bader charges and associated uncertainty were calculated using the same protocol as in Fig. 1. The error bars for the ratio of COatop to CObridge in a and b represent one standard deviation based on two independent measurements. c, Energy barriers of the dimerization of two CO at both bridge sites and two CO at bridge and atop sites, respectively. IS, initial state; TS, transient state; FS, final state. d, Plots of electron density difference for the CO adsorption with one water layer and the tetrahydro-bipyridine formed from 1. The yellow and blue contours represent electron density accumulations and depressions, respectively. Dashed lines indicate hydrogen bond network. Red, O; grey, C; blue, N; white, H; pink, Cu.
The caption:
a, Reaction describing the electro-oligomerization of the N,N?
1,4-phenylene)bispyridinium salt 12 to form an N-aryl-dihydropyridine-based oligomer. b, FE of ethylene on Cu and Cu–12 using CO2-saturated 1 M KHCO3 as the supporting electrolyte. c, FEs of CO and ethylene on Cu and Cu–12 at the applied potential range of ?0.47 V to ?0.84 V. The error bars for FE uncertainty represent one standard deviation based on three independent samples.
The caption:
The operating current and ethylene FE were monitored for the device. Cu–12 and iridium oxide supported on titanium mesh were used as the cathode and anode, respectively. Humidified CO2 was flowed through the gas channels in the cathode, and 0.1 M aqueous KHCO3 solution was flowed through channels in the anode. The anode and cathode were separated by an anion exchange membrane to form the membrane-electrode assembly. The total geometric area of the flow field in the cathode is 5 cm2, of which 45% is the gas channel while the remaining 55% is the land area (Supplementary Figs. 27 and 28). Full-cell voltage was gradually increased from 3 V to 3.65 V and kept constant starting at time 0.
The authors conclusion:
Overall, this work presents a strategy to tune the stabilization of intermediates on heterogeneous electrocatalysts through the introduction of organic molecules. Using this strategy, implemented with N-aryl-substituted tetrahydro-bipyridine films and a related oligomeric film on a Cu catalyst, we achieved CO2-to-ethylene conversion with an ethylene FE of 72% and a full-cell energy efficiency of 20% in neutral media. In light of this performance, in combination with the long-term operating stability, this is a promising strategy for the use of renewable electricity to convert CO2 into value-added chemicals, thus storing the renewable energy (solar, wind) in the form of chemical energy.
Cool if it works, although as
energy storage, it is not particularly efficient, even less efficient than all those tons of batteries on which people want to foolishly bet the future of the planet.
I hope you're having a nice Saturday afternoon.
= new reply since forum marked as read
