Electrolysis of Lithium-Free Molten Carbonates

[NNadir]

The paper I'll discuss in this post is this one: Electrolysis of Lithium-Free Molten Carbonates (Xiang Chen, Zhuqing Zhao, Jiakang Qu, Beilei Zhang, Xueyong Ding, Yunfeng Geng, Hongwei Xie, Dihua Wang, and Huayi Yin ACS Sustainable Chemistry & Engineering 2021 9 (11), 4167-4174)

I've argued before in this space that electricity is a thermodynamically degraded form of energy: Synthesizing Clean Transportation Fuels from CO2 Will at Least Quintuple the Demand for Electricity.

I have also argued, sometimes while addressing the stupid but oft discussed solar thermal energy fantasy, that the thermochemical splitting of carbon dioxide into carbon monoxide - from which pretty much any industrially produced carbon compound can be made (just add water) - and oxygen is the most efficient approach to carbon dioxide reduction: Cerium Requirements to Split One Billion Tons of Carbon Dioxide, the Nuclear v Solar Thermal cases.

This lab scale and thus hardly optimized result raises the cerium requirements by a huge amount. It means that for the nuclear case, the requirement for exactly one billion tons of carbon dioxide to be split would be 7,566,562 metric tons of cerium (as the metal) and that for the solar thermal case, it would be 45,399,372 metric tons also as the metal.

Although for certain reasons I am very fond of the cerium based thermochemical splitting of carbon dioxide, many other such catalytic systems are known for this highly endothermic reaction.

The international symbol for dangerous fossil fuel free energy has become a wind turbine or solar cell graphic object, which is frankly as silly as waving a graphic of a Roman executioner's cross as a symbol for the way to address immorality; both are faith based. It has been experimentally verified, at a cost of trillions of dollars, that wind turbines and solar cells are not even remotely capable of addressing climate change. In fact, the worship of them - and let's be clear that it's nothing more than faith based worship - has led to the acceleration of climate change. The average concentration of the dangerous fossil fuel waste carbon dioxide in the atmosphere measured at the Mauna Loa Carbon Dioxide Observatory during the week beginning on March 19, 2000 was 370.98 ppm. This morning, the latest figure was 417.67 ppm. The current 12 month running average of weekly measured increases over carbon dioxide increases over the last ten years is 24.24 ppm, 2.42 ppm/year. The same 12 moth running average for the week beginning March 19, 2000 was 15.14 ppm, 1.51 ppm/year.

These are facts. Facts matter.

Heckuva job humanity at addressing climate change with all those wind turbines, solar cells and electric cars, heckuva job.

The graphic attached to the introduction of this paper is nonetheless the equivalent of the Roman execution's cross as applied to so called "green" energy. Here it is:



What I like about this paper is that it is cognizant of the fact that matter, specifically the individual elements in the periodic table, is not "renewable." The claim that we can save the world with batteries is no different than the claim that supplies of dangerous petroleum, dangerous natural gas, and dangerous coal are infinite, as well as the claim that places to put the waste, chiefly, but hardly limited to, carbon dioxide is unlimited.

They start out talking about lithium and then consider elements that are far more available, one of which, strontium, catches my eye. Overall this discussion of batteries clearly has some relevance to the "batteries will save us" fantasy.

From the introduction to the paper:

Molten carbonate electrolysis offers an efficient route to convert CO2 or carbonate to C/CO at a rapid rate and high product selectivity(1?11) without using rationally designed catalysts to overcome the kinetic barrier.(12?17) Currently, most molten carbonate eletrolyzers employ lithium-containing salts due to the thermodynamic favorability of Li2CO3, relatively low eutectic temperature, and adequate solubility of Li2O.(18?24) However, lithium resource is limited in the Earth’s crust (e.g., lithium’s reserve is only less than one-thousandth of that of calcium in the Earth’s crust, Figure 1a) and will become scanter with increasing demand for lithium-based energy-storage devices.(25?27) Thus, exploiting lithium-free molten carbonate electrolyzers is of great importance to expand the novel systems and achieve the practical viability of molten carbonate electrolysis.

They have a nice graphic on the point:



The caption:

Figure 1. (a) Profiles of the abundance of alkali/earth alkali metal in the crust; data are derived from ref (43). (b) Schematic illustration of the electrolysis of molten carbonates and CO2 electrochemical conversion in molten carbonates. (c) Potential profiles as a function of temperature in molten MCO3–Na2CO3–K2CO3 (M = Mg, Ca, Sr, and Ba); all thermodynamic data are obtained from HSC Chemistry 6.0.

They continue:

Thermodynamically, alkaline-earth metal carbonates (AEMCs) can be electrochemically reduced to C/CO at a potential more positive than that of metal deposition. Thus, AEMCs are alternatives to replace Li2CO3, thereby preventing the use of strategic lithium salts. The use of CaCO3 or BaCO3 in Li2CO3-containing(28?30) or chloride melts(31?33) has been studied, which proves that AEMCs are suitable candidates to be reduced in molten salt systems. However, it is difficult to find a low-cost inert oxygen-evolution anode in the molten halides dissolved with AEMCs.(34?38) To date, a lithium-free molten carbonate system has not been systematically studied. Therefore, it is necessary to study the underlying mechanisms of the lithium-free molten carbonate electrolysis employing more abundant and inexpensive electrolytes.

The use of molten calcium salts is the essence of the FFC Cambridge process for the electrochemical reduction of metals, which I personally believe should be world changing, even if it is true that electricity is a thermodynamically degraded form of energy.

In saying this, they do raise some issues to be addressed:

In essence, the active species in molten carbonate electrolyzers is CO32– derived from the dissociation of carbonates or/and the captured CO2 by O2– rather than CO2 because the solubility of CO2 in molten carbonates is low.(39,40) The reduction of CO32– at the cathode releases O2– to capture CO2 and replenish CO32– or combine with alkaline-earth cation to generate the corresponding alkaline-earth metal oxides (MO).(41) Meanwhile, CO32– and O2– could directly diffuse to the anode and then be oxidized to O2 if the anode is inert (Figure 1b). However, there are two different reaction processes of molten carbonate electrolysis, which depends on the solubility of alkaline-earth metal oxides (MO) in the molten carbonates. If MO is insoluble in the molten AEMCs, MCO3 will be converted to MO and C at the cathode and O2 at the anode. In this case, the molten carbonate electrolyzer consumes MCO3. If the MO is soluble in the molten AEMCs, the dissolved MO will absorb CO2 to replenish carbonate. In this scenario, the molten carbonate electrolyzer consumes CO2, while the composition of the electrolyte remains unchanged. However, fundamental data of the solubility of MO in molten AEMCs and electrochemical properties of molten AEMCs urgently need exploring in lithium-free molten carbonates. In addition to atmospheric CO2, AEMCs (e.g., CaCO3) are one of the major CO2 hosts with a huge reserve in the forms of limestone and magma,(42) promising another way to valorize CO2 in terms of carbonates.

These factors are what they examine in the paper, and then propose an electrode to reduce carbon dioxide to carbon, in effect reversing the combustion of the dangerous fossil fuel coal.

This process, which requires the input of a thermodynamically degraded form of energy, electricity, as well as heat suggests an important point that is often over looked, which is this: In order to capture carbon dioxide in air one needs to put more energy into the system than all of the original energy ever released by the original combustion of the dangerous fossil fuel produced. This reality should sober up all the drunken handwaving and wishful thinking that goes on when energy and the environment is discussed, but I confidently predict, on the basis of being a tired old man, that it won't.

Their important concern is thermodynamics:

During the electrolysis of the ternary molten MCO3–Na2CO3–K2CO3 (M = Mg, Ca, Sr, and Ba) mixture, several reactions may take place as follows:

(1)

(2)

If the MO in eq 2 is soluble, then the carbonization reaction between MO and CO2 spontaneously occurs and forms fresh carbonate to maintain the concentration of CO32– of the electrolyte:

(3)

The overall reaction of eqs 2 and 3 is

(4)

Thermodynamically, alkaline-earth metal carbonates (MCO3, M = Mg, Ca, Sr, and Ba) can be electrochemically converted to carbon at the potential prior to the deposition of the alkaline-earth metal (Figure 1c). Therefore, the deposition of carbon is thermodynamically easier than that of metal. Moreover, the thermodynamic favorability of generating carbon is different depending on the different alkaline-earth metal cations. For example, the thermodynamic deposition potential sequence follows CaCO3 < SrCO3 < BaCO3. In other words, CaCO3 can be reduced to carbon at the potential more positive than that of SrCO3 and BaCO3. Note that the potentials of carbon generation in Na2CO3 and K2CO3 are more negative than that of the deposition of the corresponding alkali metals. Thus, Na2CO3 and K2CO3 are usually employed as the supporting electrolyte.(39,44) Therefore, alkaline-earth carbonates are the solute to be reduced for the carbon generation in MCO3–Na2CO3–K2CO3 (M = Mg, Ca, Sr, and Ba).

The reduction behaviors at a Mo electrode in a variety of carbonates containing different alkaline-earth metals are studied. As shown in Figure 2a, no reduction peaks were observed before the cathodic limit in the pure molten Na2CO3–K2CO3, demonstrating that Mo is an inert material that does not involve in any electrochemical reactions in the selective potential range.

The preliminary experiments investigated the cyclic voltammograms of molten carbonate systems using molybdenum electrodes:



The caption:

Figure 2. Cyclic voltammograms recorded from Mo electrode in the electrolytes of Na2CO3–K2CO3 with/without (a) 5 wt % MgCO3, (b) 5 wt % CaCO3, (c) 5 wt % SrCO3, and (d) 5 wt % BaCO3 at 750 °C under argon atmosphere. Scan rate is 100 mV/s.

Carbon was indeed deposited under these conditions, at a temperature of 750°C in sodium/potassium carbonate melts containing 10% (by weight, surprisingly) of three alkali metal carbonates, those of calcium (as in the FFC Cambridge Process), strontium and barium.

Micrographs of the carbons formed are shown:



The caption:

Figure 3. (a) XRD patterns of the electrolytic carbon obtained from molten Na2CO3–K2CO3–MCO3 (M = Ca, Sr, and Ba). SEM images of carbon obtained in the electrolytes of (b) Na2CO3–K2CO3–CaCO3, (c) Na2CO3–K2CO3–SrCO3, and (d) Na2CO3–K2CO3–BaCO3 at 3.0 V under 750 °C.

The question next turned to finding a stable electrode for the oxidation side of the reaction:



The caption:

Figure 4. (a) Digital photos of Ni10Cu11Fe electrode before and after electrolysis at various carbonate electrolytes. (b) Gas chromatograms of the outlet gas before and during electrolysis at 3.0 V under 750 °C in BaCO3–Na2CO3–K2CO3 using the Ni10Cu11Fe anode.

Next the solubility of the various oxides, calcium, strontium, and barium were determined by the simple expedient of putting pellets of these oxides in the melts and observing the amount dissolved:



The caption:

Figure 5. Digital photos of (a) CaO, (b) MgO, (c) BaO, and (d) SrO before and after being soaked in their corresponding molten 10 wt % MCO3–Na2CO3–K2CO3 (M = Mg, Ca, Sr, and Ba) at 750 °C.

Quantitative tables of the solubility of these oxides are not given in the paper, but the general conclusion was that barium oxide was the most soluble and the solubility of its oxide and carbonate were evaluated in more detail along with the rate of dissolution, as shown in the following graphic:



The caption:

Figure 6. Profiles of solubility of (a) BaCO3 in Na2CO3–K2CO3 and (b) BaO in Na2CO3–K2CO3 at 750 °C. Dissolution equilibrium cures of (c) BaCO3 in Na2CO3–K2CO3 and (d) BaO in Na2CO3–K2CO3 with different concentrations BaCO3.

It would be nice, from my perspective, if a more complex carbonate system allowing for the use of strontium in this system were considered in subsequent optimization, since strontium is available from used nuclear fuel with a heat generating isotope, Sr-90, which might defray the environmental and economic cost of maintaining a melt, although there are many heat network setting which might address this problem at an acceptable environmental and economic cost.

In any case, the barium oxide can absorb gaseous oxygen, even at these high temperatures - the decomposition temperate of barium carbonate is around 1300°C.

If the goal is, however, to produce the oxides themselves of strontium and/or calcium - the latter oxide is a key constituent of concrete, concrete production being a major contributor to climate change, the insolubility of the oxides is actually a desirable outcome.

The authors write:

Molten carbonates can be directly split by electrolysis, producing metal oxide (MO) and carbon at the cathode and oxygen at the anode. Two reduction mechanisms are shown in Figure 7. The solubility of MO determines the real reduced species in the molten carbonate: if MO is soluble (e.g., BaO), the carbonates electrolysis converts CO2 to C and O2; if MO is insoluble (e.g., CaO, SrO), the electrolysis converts MCO3 to MO, C, and O2.

This figure obviates that point:




The caption:

Figure 7. Two different reaction processes of molten carbonate electrolysis depending on the solubility of alkaline-earth metal oxides (MO) in the molten carbonates. Electrolysis of CO2 capture (left) and electrolysis of MCO3 (right).

They then write about capturing carbon dioxide from flue gas; if this gas is formed to generate electricity, they are then talking about a perpetual motion machine, but that doesn't totally negate the value of the paper itself.

I began this commentary by noting that electricity is a thermally degraded form of energy, but did not state the caveat that electricity captured by use of waste heat that would otherwise be rejected to the atmosphere is merely an improvement in overall energy efficiency. In the case where electricity is generated as a side product, using waste heat from a very high temperature process, the second law of thermodynamics is still operative - there is no physical way to make it inoperative - and increased exergy is obtained.

The conceit surrounding the so called "renewable energy" fantasy is that the intermittent nature of electricity can be addressed by net metering. On the left, we like to mock the statements of the assholes running ERCOT and the racist governor of Texas that the recent events in that State that led Senator Cancun Cruz to run away, the collapse of its power system was an outgrown of the so called "Green New Deal." To be perfectly clear and unambiguous, even as a lifelong Democrat, I don't think that the "Green New Deal" is going to be green or much of a deal. I have zero respect for the tired and old, and frankly experimentally failed energy ideas of Ed Markey, whether I otherwise agree with his other politics, as I often do. So called "renewable energy" is failing us now, and worse, is failing the future. It didn't work to address climate change. It isn't working to address climate change. It won't work to address climate change. Period. A key concept in the ideology of Green New Dealism is so called "net metering" which is the idea that the use of electricity follows its availability, its availability determining its price. If we are honest with ourselves, not that it is easy for anyone to be honest with themselves when honest confronts faith, including quasi religious faith, even absurd faiths, the extreme utility bills seen by some citizens of Texas are "net metering" run wild.

A heat network run by a high temperature nuclear plant, one producing temperatures high enough thermochemically split carbon dioxide - the cerium cycle running at maximum temperatures of 1400°C - will achieve the highest efficiency - the yield of exergy - as a constituent of a heat network, possibly producing a number of Brayton, Rankine and even Sterling cycles in sequence. If this is the case, electricity may be inevitably a side product. If this electricity is used to run combustion in reverse, which is what this paper is all about, the electricity can be switched to the grid whenever electricity prices rise high enough to justify the switching. In effect, a chemical plant is run as spinning reserve. This is not a new idea. Kaiser aluminum - aluminum production is an chemoelectric process - did this historically with hydro power in the Pacific Northwest.

Power demand on the grid fluctuates regularly, with the highest demand in the late afternoon and early evening, precisely when the sun goes down ironically enough, since so many people actually believe that solar electricity will save the world. It won't. It hasn't. Exploiting these fluctuations can be utilized to produce molecular carbon from CO2 gas or provide electricity to the grid, depending entirely on the value to the operator of the plant.

From the paper's conclusion:

Lithium-free molten carbonates (Na2CO3–K2CO3–MCO3, M = Ca, Sr, and Ba) are a family of promising electrolytes to replace Li-containing molten carbonate for the electrochemical conversion of CO2 and/or carbonate. The thermodynamic calculation shows that molten carbonate containing alkaline-earth metal cations can be used for producing carbon, and the low-cost Ni10Cu11Fe oxygen-evolution inert anode is stable in the molten carbonate containing various cations. More importantly, two reaction mechanisms are demonstrated based on the solubilities of alkaline-earth oxides (CaO, SrO, and BaO) in molten carbonates. The first mechanism is the conversion of MCO3 to MO, C, and O2 if the MO is insoluble in molten carbonates (M = Ca and Sr), and the second mechanism is the conversion of CO2 to C and O2 if the MO is soluble (the solubility of BaO ranges from 3 to 16 wt %) in molten carbonates. In addition, the electrolytic carbons exhibit superior energy-storage performances. Overall, this paper shed light on screening sustainable molten carbonates for the electrochemical conversion of inexpensive carbonate or CO2 to value-added carbon with a high product selectivity.

It's a very nice paper. I like it a lot.

I trust you are enjoying your weekend.