Electrochemical Reduction of Carbon Dioxide Using Multiwalled Carbon Nanotubes Supporting Nickel.

[NNadir]

The paper I'll discuss in this post is this one: Acidic Electrochemical Reduction of CO2 Using Nickel Nitride on Multiwalled Carbon Nanotube as Selective Catalyst (Kang et al ACS Sustainable Chem. Eng., 2019, 7 (6), pp 6106–6112).

It is a fundamental law of physics that the storage of energy wastes it; it is related to the fact that every energy transformation from one form into another generates heat. Anyone who owns a lap top computer can feel this if she or he places her or his hand on the battery pack while recharging it; it will be warm to the touch.

I point this out repeatedly, and am almost uniformly ignored, and in the majority of times I am not ignored, I am criticized, sometimes on the grounds that I'm rude and often insulting. Regrettably, the laws of physics do not change based on the personality of the person stating them.

Denial is not a river in Egypt; on the contrary, it is a powerful, if baleful, practice intrinsic to human nature, a dangerous practice, but a common one all the same.

Of course, people who nominally self identify as "environmentalists" without much scientific intuition about this issue of the complexity of the environment, love to prattle on about batteries, and other electrochemical transformations of electrical energy into stored chemical energy; a popular one is hydrogen produced by electrolysis. This is because many of these people are trained, in a Pavlovian fashion, by largely scientifically illiterate journalists, to believe that so called "renewable energy" will save the day; at least once we find a way to overcome the issue of its intrinsic intermittent nature by storing energy.

The pop enthusiasm for so called "renewable energy" as a meaningful tool for addressing climate change has led to the "investment" of trillions of dollars on this failed and increasingly absurd effort. I say "failed" because these "investments" of trillion dollar sums, beginning about the year 2001 and continuing to the present day have done nothing at all to address climate change. The percentage and absolute quantities of energy produced by dangerous fossil fuels has risen in the 21st century. This fact is expressed in the most dire fashion, in the reported rate of rise of the concentration the dangerous fossil fuel waste carbon dioxide in the planetary atmosphere is the highest ever observed. Since 2001, this rate with some statistical "noise," has been 2.2 ppm/year; in the last 5 years, the rate has averaged 2.6 ppm/year, and three of the five largest increases since measurement began at the Mauna Loa carbon dioxide observatory in 1958, were observed in the last 5 years. One such value (2015) exceeded 3.0 ppm/year for the first time since recording began. (The 1959 figure was 0.94 ppm over 1958.)

The reality is that energy storage of so called "renewable energy" will make it even less useful than it already is, and if your criteria for your self identification as an "environmentalist" consists of wanting to have the rate of change of carbon dioxide concentrations to be less than or equal to zero, the rise in the application so called "renewable energy" at a cost amounting to trillions of dollars has been useless. Arguably it has made things worse not better, if only for fostering delusional mythology, if not for the huge amounts of physical mass applied to it; it costs energy - almost all of it coming from dangerous fossil fuel related processing - to make silicon and steel and aluminum and neodymium iron boride magnets for wind turbines and electric cars.

A caveat to the true statement that storage of energy wastes energy is that one can obtain an energetic advantage if the stored energy is energy that is recovered that would have been wasted in any case. Consider the example of a gas turbine that is shut when the wind is blowing on a bright sunny day because so called "renewable energy" is briefly producing some large percentage of electrical demand; a figure that is likely to be dutifully reported in a blog or website by some breathless airhead. (We’ve seen these kinds of posts here lots of times.) If the gas powering the turbine is shut off the turbine will not immediately stop; it will continue to rotate, since it has stored energy in the form of rotational momentum, and this energy will be lost slowly to the friction and the resistance in the air or exhaust gases surrounding it; this energy being converted slowly into heat. However if the turbine/generator system is braked by using the residual electricity from the turbine/generator system that can no longer be applied to the grid (as it is going out of phase) to charge a battery, then some of this energy will remain available for future use. This is recovered energy. If the wind stops blowing at night, of course, additional energy - more energy than the turbine contained when it contained initially was being turned off - will be required to get it back up to the speed at which it must turn to produce appropriately phased electricity.


We often hear statements like "Scientists say..." 'x' or 'y' or 'z' in which the generic "scientists" are presented as oracular. This description of scientists as oracular is nonsense; scientists are all human beings; all have biases; and all can buy into popular mythology, at least unintentionally if not for manipulative reasons. (An example of a manipulative reason would be appealing to popular mythology in order to have their projects funded.) Many scientists include statements about how their work can be applied to improving the application of so called "renewable energy" even if the data shows it has not worked, and is not working, and thus powerfully suggests that it will not work, since repeating an experiment multiple times in hopes of getting a different result is either a reflection of scientific incompetence.

After several decades of considering the question of what we currently describe as "renewable energy" - a period over which I changed my mind about many things as I read more and more and more again - I am far more confident is stating "will not work" as something I regard as a fact than many other scientists.

The paper cited at the outset is an example. In the first sentence it makes the now traditional obeisance to "renewable energy," to wit, in it's introductory paragraph:

Increasing CO2 emission is urging researchers to develop renewable energy alternatives. CO2 can serve as a renewable carbon source for fuels or commodity chemicals.1?5 Electrochemical reduction of carbon dioxide is a promising strategy for sustainable production of chemicals under mild conditions. 2 However, because of the extreme stability of the linear CO2 molecule, electrochemical reduction of CO2 requires high activation energy to form a CO2 •? intermediate, which causes large overpotential and competitive formation of H2.6,7 Lowering the barrier of CO2 activation is necessary for developing new catalysts.

The first sentence is absurd; the second is most definitely true; the third is highly questionable since there are many other superior carbon dioxide reduction strategies that are likely to be superior. The fourth is true and is a statement of poor thermodynamic efficiency, although it is questionable that overpotentials producing hydrogen is a bad thing, since a mixture of hydrogen and carbon monoxide is key to replacing the noxious and dangerous fossil fuel petroleum, since syn gas can be used to synthetically produce equivalents of every product made from oil, including the horrible substance gasoline, and many alternatives to gasoline which are vastly superior, dimethyl ether for example. Finally the fifty statement is a statement of truth.

The next paragraph is a laundry list of other references to the (largely) electrochemical reduction of carbon dioxide focusing heavily on approaches involving nickel; it is hardly comprehensive since one can find many, many, many references to this approach, including thousands referring to other elements in the periodic table as catalysts:

Numerous CO2 reduction catalysts based on metal and metal oxides,8?13 chalcogenides,14 nitrogen-doped/-functionalized carbons,15?17 and molecular complexes18?20 have been developed. Among them, earth abundant metals have been increasingly explored as CO2 electroreduction catalysts, such as Ni,21?25 Co13,26 and Fe.27,28 Nickel-based materials have been an important class of catalysts for CO2 reduction. Ni-based complexes have been reported; for example, nickel-1,4,8,11- tetraazacyclotetradecane (Ni-cyclam) and its derivatives have shown considerable selectivity in electroreduction of CO2 to CO at Hg electrode.29?31 Recently, Li et al. reported Ni single atoms distributed in nitrogen-doped porous carbon (Ni SAs/ N?C) for selective reduction of CO2 to CO, and the highest selectivity for CO production was achieved at an overpotential of 0.89 V with Faradaic efficiency (FE) of 71.9%.32 Ni?N4 structure catalyst exhibited excellent activity for CO2RR with high FE over 90% for CO in the potential range from ?0.5 to ?0.9 V.24 Bao et al. reported high nickel loading materials by pyrolysis of Zn/Ni bimetallic ZIF-8 which yields high CO current density of 71.5 mA cm?2 at ?1.03 V (vs RHE), and high CO FE of over 90%.23 Kamiya and co-workers prepared nickel?nitrogen-modified graphene for CO2RR with CO FEs over 90% in weakly acidic and neutral solutions.33 Strasser et al. investigated Ni?N?C catalysts with CO FEs of over 80%.34 Still, developing Ni-based catalyst is highly challenging because Ni-based nanocomposites are highly efficient for H2 evolution reaction (HER).35,36

What is different in this particular case are two things. One is that this material is being studied for use in the gas phase, although in the present case it's explored in solution. Many other electrochemical processes take place in aqueous (or other solvent) solutions. The second is that it can tolerate acid, which is a good thing, since carbon dioxide is an acidic species, a "Lewis Acid" which accepts electrons quite readily: In water it forms "carbonic acid" which is usually partially ionized to a proton and the bicarbonate ion, HCO3-. The electrodes here are nickel nitride embedded in multiwalled carbon nanotubes, otherwise known as MCNT. The reason for the use of nanotubes is to increase the exposure surface of the nickel nitride:

Reducing CO2 under acidic conditions is important for gas phase CO2 reduction. However, reducing CO2 under acidic conditions is very challenging and much less reported. In gas phase CO2 reduction, catalyst is commonly pressed onto Nafion membrane which is highly acidic in nature, requiring high selectivity for the catalyst. In Newman’s study, Ag nanoparticles loaded on Nafion only generated H2, unless they were separated with Nafion using a buffer layer.37 Making the catalyst surface with more basic sites could increase CO2 adsorption on surface and buffer the local pH. Nickel nitride (Ni3N) has low electrical resistance and decent corrosion resistance and has been used as electrocatalyst for water splitting38,39 and electrode material for supercapacitors.40 In this work, we used Ni3N/MCNT (multiwalled carbon nanotube) nanocomposites of small particle sizes...

...Using MCNT as substrate is very important for even distribution of Ni3N particles. Without MCNT, bare Ni3N was found to be large, aggregated particles (Figure S2b), suggesting that using MCNT as substrate can prevent aggregation during ammonolysis. Yet, at higher Ni loading, Ni3N/MCNT-2 became more aggregated on MCNT with larger sizes.

There is a discussion, somewhat vague, of the preparation of this electrode. The carbon nanotubes are prepared using self-assembly type molecules and this cartoon evokes the process for impregnating the nanotubes with nickel nitride.

Scheme 1. Preparation of Ni3N/MCNT

Here's some images connected with the prepared electrodes:




The caption:

Figure 1. TEM (a, b), HR-TEM (c, d), and HADDF-STEM images of Ni3N/MCNT-1 and its corresponding element mapping for C, N, and Ni (e).

The X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) patterns:



The caption:

Figure 2. (a) XRD patterns of Ni(OH)2/MCNT, Ni3N/MCNT-1, and bare Ni3N; XPS spectrum for N 1s (b), Ni 2p (c) of Ni3N/MCNT-1, and N 1s (d) for N-MCNT as control.

XPS identifies elements by the method of Henry Moseley, who had his head blown off at Gallopoli in the so called "Great War," - as if wars can be "great" - and didn't live past his twenties, and thus ran out of time to collect what surely would have been a Nobel Prize.

The pore sizes as determined by nitrogen absorption isoterms:



The caption:

Figure 3. (a) Nitrogen adsorption isotherms and (b) pore size distributions of Ni(OH)2/MCNT-1 and Ni3N/MCNT-1 nanocomposites.

A graphic showing some aspects of the electrochemical performance of these electrodes with respect to the reduction of carbon dioxide to carbon monoxide.

The caption:

Figure 4. (a) CVs of Ni3N/MCNT-1 (black), Ni3N/MCNT-2 (red), and Ni(OH)2/MCNT (green) in CO2-saturated 0.5 M NaHCO3 solution at scan rate of 100 mV s–1; Faradaic efficiencies for Ni3N/MCNT-1 (b), Ni3N/MCNT-2 (c), and Ni(OH)2/MCNT (d); (e) partial current densities for CO production; (f) Tafel plots using CO partial current densities for Ni3N/MCNT-1 and Ni3N/MCNT-2.

CV here stands for an electrochemical measurement known as "cyclic voltammetry," which is a plot of current vs voltage, and describes the voltage required to induce a chemical reaction, in this case, the reduction of carbon dioxide to the monoxide.

Linear Sweep Voltametry data and Faradaic efficiency:



The caption:

Figure 5. (a) LSV scans at 50 mV s?1 for Ni3N/MCNT-1 in CO2-saturated NaCl solution at different pHs (adjusted with HCl or NaHCO3, ionic strength = 0.5 M). (b) FEs for CO (blue) (sic) and H2 (red) (sic) by Ni3N/MCNT-1 at various pHs.

Some commentary from the text on this figure:

The electrolyte pH value has significant effect on the performance of CO2RR. Figure 5a shows LSVs at different pHs from 2.5 to 7.2. Although the pH values were different, which should change the reduction potential, the LSVs mostly overlapped with each other, suggesting that the ratedetermining step is less sensitive to pH changes. Figure 5b shows the maximum FEs for CO at different pHs. The applied potentials were between ?0.9 and ?1.0 V. Although more acidic conditions generated more H2, still at pH 3.7, the CO FE remained at 85.7%. Yet, at pH 2.5, CO FE dropped to 50.1%. Furthermore, CO FE was only 8% at pH 1, suggesting that overly acidic pH still favors hydrogen evolution.

I'm not necessarily familiar with this technique for measuring the electrochemically active surface area:




The caption:

Figure 6. (a) Charging current densities vs CV scan rates; (b) CO2 adsorption isotherms for Ni(OH)2/MCNT and Ni3N/MCNT-1.

This text gives a feel for what is going on in this graphic:

Electrochemical surface areas (ECSAs) were determined by measuring the double-layer capacitance in CV experiments. In Figure 6a, Ni3N/MCNT-1 showed the highest ECSA of 10.87 mF cm?2, which was ca. 1.5-fold of Ni3N/MCNT-2. Increased ECSA indicates more active catalytic sites, which could contribute to increased catalytic activity for Ni3N/MCNT-1. Also, measurement of CO2 adsorption revealed that Ni3N/ MCNT-1 absorbs more CO2 (ca. 120 mg·g?1) than Ni(OH)2/ MCNT (ca. 90 mg·g?1), indicating that the nitriding strategy increased available base sites (Figure 6b).

An interesting read, I think. There's regrettably no discussion of the fate of oxygen in this paper, which, along with production bottlenecks for multiwalled carbon nanotubes, may limit any practical import for this work.

It is unlikely that this technology even were these limitations overcome would be anywhere near as thermally efficient as thermochemical approaches to the reduction of carbon dioxide to either carbon or carbon monoxide, including, but not limited to, the oxidation of biomass and other waste carbon componds with carbon dioxide at high temperatures available in nuclear energy settings.

However, there are some esoteric settings in which it might prove useful. One application concerns "spinning reserve." Most power grids require spinning reserve to adjust for unexpected fluctuations in demand. These are turbines that turn continuously, almost always dangerous natural gas driven turbines, as back up to meet sudden surges in demand without creating power dips that can damage sensitive equipment. Of course, if a grid's power supply is unstable because of surges and dips in wind speed, or the movement of clouds over solar cells, the requirement is somewhat more exigent, which is why either more gas powered spinning reserve or environmentally dubious batteries are required in the self defeating enthusiasm for so called "renewable energy."

In a sensible, nuclear powered world, one in which there was less hatred for science, electricity might be a side product of thermochemical processes designed to replace the awful petroleum industry, and one can imagine that situations might arise where gas pressure needed to be discharged, ideally against a turbine, in periods of low electricity demand, whereupon the waste electricity might be captured - albeit at a thermodynamic penalty as with any battery - by reducing stored carbon dioxide.

Have a pleasant Saturday evening.