Metallic Porous Electrodes Enable Efficient Bicarbonate Electrolysis

<p>We demonstrate here that a porous free-standing silver foam cathode in an electrolytic flow electrolyzer mediates efficient electrolysis of 3.0 M bicarbonate solutions into CO. These results have direct implications for carbon capture schemes where OH- solutions react with CO2 to form bicarbonate-rich solutions that need to be treated to recycle the sorbent and recover the CO2. Our study shows a viable path for replacing the high-temperature thermal process currently used to recover CO2 from these carbon</p><p>capture solutions by using electricity to drive the conversion of bicarbonate into CO2 and subsequently into CO. The use of free-standing porous silver electrodes was found to yield electrolysis performance parameters (e.g., a Faradaic efficiency for CO production, FECO, of 95% at 100 mA cm2; <3% performance loss after 80 h operation) that are superior to results obtained in bicarbonate electrolyzers that utilize conventional carbon-based gas diffusion electrodes (GDEs) designed for gaseous CO2 fed electrolyzers. This liquid-fed bicarbonate electrolyzer achieves high CO formation rates with the added benefit of not requiring an energy-intensive CO2 regeneration step that would be necessary for the electrolysis of gaseous CO2. These findings represent a potentially important step in closing the carbon cycle.</p>

Metallic Porous Electrodes Enable Efficient Bicarbonate Electrolysis

<p>We demonstrate here that a porous free-standing silver foam cathode in an electrolytic flow electrolyzer mediates efficient electrolysis of 3.0 M bicarbonate solutions into CO. These results have direct implications for carbon capture schemes where OH- solutions react with CO2 to form bicarbonate-rich solutions that need to be treated to recycle the sorbent and recover the CO2. Our study shows a viable path for replacing the high-temperature thermal process currently used to recover CO2 from these carbon</p><p>capture solutions by using electricity to drive the conversion of bicarbonate into CO2 and subsequently into CO. The use of free-standing porous silver electrodes was found to yield electrolysis performance parameters (e.g., a Faradaic efficiency for CO production, FECO, of 95% at 100 mA cm2; <3% performance loss after 80 h operation) that are superior to results obtained in bicarbonate electrolyzers that utilize conventional carbon-based gas diffusion electrodes (GDEs) designed for gaseous CO2 fed electrolyzers. This liquid-fed bicarbonate electrolyzer achieves high CO formation rates with the added benefit of not requiring an energy-intensive CO2 regeneration step that would be necessary for the electrolysis of gaseous CO2. These findings represent a potentially important step in closing the carbon cycle.</p>

Na3[Ru2(µ-CO3)4] as a Homogeneous Catalyst for Water Oxidation; HCO3− as a Co-Catalyst

In neutral medium (pH 7.0) [RuIIIRuII(µ-CO3)4(OH)]4− undergoes one electron oxidation to form [RuIIIRuIII(µ-CO3)4(OH)2]4− at an E1/2 of 0.85 V vs. NHE followed by electro-catalytic water oxidation at a potential ≥1.5 V. When the same electrochemical measurements are performed in bicarbonate medium (pH 8.3), the complex first undergoes one electron oxidation at an Epa of 0.86 V to form [RuIIIRuIII(µ-CO3)4(OH)2]4−. This complex further undergoes two step one electron oxidations to form RuIVRuIII and RuIVRuIV species at potentials (Epa) 1.18 and 1.35 V, respectively. The RuIVRuIII and RuIVRuIV species in bicarbonate solutions are [RuIVRuIII(µ-CO3)4(OH)(CO3)]4− and [RuIVRuIV(µ-CO3)4(O)(CO3)]4− based on density functional theory (DFT) calculations. The formation of HCO4− in the course of the oxidation has been demonstrated by DFT. The catalyst acts as homogeneous water oxidation catalyst, and after long term chronoamperometry, the absorption spectra does not change significantly. Each step has been found to follow a proton coupled electron transfer process (PCET) as obtained from the pH dependent studies. The catalytic current is found to follow linear relation with the concentration of the catalyst and bicarbonate. Thus, bicarbonate is involved in the catalytic process that is also evident from the generation of higher oxidation peaks in cyclic voltammetry. The detailed mechanism has been derived by DFT. A catalyst with no organic ligands has the advantage of long-time stability.

Direct Electrochemical Reduction of Bicarbonate to Formate Using Tin Catalyst

Nowadays, the self-accelerating increase in global temperatures strengthens the idea that the cutting of CO2 emissions will not be enough to avoid climate change, thus CO2 from the atmosphere must be removed. This gas can be easily trapped by converting it to bicarbonate using hydroxide solutions. However, bicarbonate must be converted into a more valuable product to make this technology profitable. Several studies show great efficiency when reducing bicarbonate solutions saturated with pure CO2 gas to formate. However, those approaches don’t have a real application and our objective was to obtain similar results without pure CO2 saturation. The method consists of electroreduction of the bicarbonate solution using bulk tin (Sn) as catalysts. Tin is a relatively cheap material that, according to previous studies performed in saturated bicarbonate solutions, shows a great selectivity towards formate. The 1H NMR analysis of bicarbonate solutions after electroreduction show that, without pure CO2 gas, the faradic efficiency is around 18% but almost 50% for saturated ones. The formate obtained could be used to power formate/formic acid fuel cells obtaining a battery-like system, with greater energy density than common lithium batteries, but electroreduction efficiency needs to be improved to make them competitive.

Electrolytic Conversion of Bicarbonate Solutions to CO at >500 mA cm-2 and 2.2 V

Electrolyzers that reduce carbon dioxide (CO2) into chemicals and fuels often use high-purity gaseous CO2 feedstocks that need to be isolated from upstream carbon capture units. If CO2 were to be captured directly from air, the eluent is likely to be an aqueous solution rich in bicarbonate ions (HCO3-). This scenario provides the impetus to electrolytically reduce these bicarbonate-rich carbon capture solutions into the same products as a CO2 electrolyzer. We report here an electrolyzer configuration that couples the conversion of bicarbonate to CO at the cathode with hydrogen oxidation at an anode. This unique system is capable of reaching a commercially-relevant current density of 500 mA cm-2 at merely 2.2 V, which is >0.5 V more efficient than any other reported electrolyzer that reduces HCO3- or CO2 at these current densities.

Electrolytic Conversion of Bicarbonate Solutions to CO at >500 mA cm-2 and 2.2 V

Electrolyzers that reduce carbon dioxide (CO2) into chemicals and fuels often use high-purity gaseous CO2 feedstocks that need to be isolated from upstream carbon capture units. If CO2 were to be captured directly from air, the eluent is likely to be an aqueous solution rich in bicarbonate ions (HCO3-). This scenario provides the impetus to electrolytically reduce these bicarbonate-rich carbon capture solutions into the same products as a CO2 electrolyzer. We report here an electrolyzer configuration that couples the conversion of bicarbonate to CO at the cathode with hydrogen oxidation at an anode. This unique system is capable of reaching a commercially-relevant current density of 500 mA cm-2 at merely 2.2 V, which is >0.5 V more efficient than any other reported electrolyzer that reduces HCO3- or CO2 at these current densities.

Metallic Porous Electrodes Enable Efficient Bicarbonate Electrolysis

We demonstrate here that a porous free-standing silver foam cathode in an electrolytic flow cell mediates efficient electrolysis of 3.0 M bicarbonate solutions into CO. These results have direct implications for carbon capture schemes where OH- solutions react with CO2 to form bicarbonate-rich solutions that need to be treated to recycle the sorbent and recover the CO2. Our study shows a viable path for replacing the high-temperature thermal process currently used to recover CO2 from these carbon capture solutions by using electricity to drive the conversion of bicarbonate into CO2 and subsequently into CO. The use of free-standing porous silver electrodes was found to yield electrolysis performance parameters (e.g., a Faradaic efficiency for CO production, FECO, of 78% at 100 mA cm2; <3% performance loss after 80 h operation) that are superior to results obtained in bicarbonate electrolyzers that utilize conventional carbon-based gas diffusion electrodes (GDEs) designed for gaseous CO2 fed electrolyzers. These performance metrics are comparable to any electrolytic flow cell fed directly with a CO2 feedstock, with the added benefit of not requiring an energy-intensive pressurization step that would be necessary for the electrolysis of gaseous CO2. These findings represent a potentially important step in closing the carbon cycle.

Metallic Porous Electrodes Enable Efficient Bicarbonate Electrolysis

We demonstrate here that a porous free-standing silver foam cathode in an electrolytic flow cell mediates efficient electrolysis of 3.0 M bicarbonate solutions into CO. These results have direct implications for carbon capture schemes where OH- solutions react with CO2 to form bicarbonate-rich solutions that need to be treated to recycle the sorbent and recover the CO2. Our study shows a viable path for replacing the high-temperature thermal process currently used to recover CO2 from these carbon capture solutions by using electricity to drive the conversion of bicarbonate into CO2 and subsequently into CO. The use of free-standing porous silver electrodes was found to yield electrolysis performance parameters (e.g., a Faradaic efficiency for CO production, FECO, of 78% at 100 mA cm2; <3% performance loss after 80 h operation) that are superior to results obtained in bicarbonate electrolyzers that utilize conventional carbon-based gas diffusion electrodes (GDEs) designed for gaseous CO2 fed electrolyzers. These performance metrics are comparable to any electrolytic flow cell fed directly with a CO2 feedstock, with the added benefit of not requiring an energy-intensive pressurization step that would be necessary for the electrolysis of gaseous CO2. These findings represent a potentially important step in closing the carbon cycle.