|Voltage curves of the first discharge−charge cycle, K−O2 battery (0.5 M KPF6 in DME) at a current density of 0.16 mA/cm2. The dash lines indicate the calculated thermodynamic potentials for the batteries. Credit: ACS, Ren and Wu. Click to enlarge.|
Researchers at Ohio State University (OSU) have demonstrated the concept of a potassium-air (K−O2) battery with low overpotentials. In a paper published in the Journal of the American Chemical Society, they reported a charge/discharge potential gap smaller than 50 mV at a current density of 0.16 mA/cm2—the lowest ever reported in metal-oxygen batteries, according to the team.
While lithium-air (Li–O2) batteries are widely seen as promising future energy storage systems, especially for electric vehicles, development and subsequent commercialization of the technology still faces a number of hurdles, including the large overpotentials of the discharge (formation of Li2O2) and charge (oxidation of Li2O2) reactions, which undermine the energy efficiency.
...lithium−oxygen battery research is facing a lot of challenges. The discharge process involves the reduction of oxygen to superoxide (O2−), the formation of LiO2 and its disproportionation into Li2O2 and O2; while the charge process is the direct oxidation of Li2O2 into O2. As a result of the asymmetric reaction mechanism, battery charge has a much higher overpotential (∼1−1.5 V) than that of discharge (∼0.3 V), which renders the system with a low round-trip energy efficiency around 60%.
Recently, researchers have also found out the instability of electrolyte and carbon electrode under the high charging potential (>3.5 V), which contributes to the low rechargeability. Different electrocatalysts have been explored to lower the overpotentials. But the necessity of catalysts has been argued, because the catalyst on carbon may not be able to work once its surface is blocked. Moreover, side reactions can be facilitated by the catalysts. On the other hand, the insulating nature of bulk Li2O2 (band gap larger than 4 eV, calculated values) can hinder the charge transfer reactions and result in a limited battery capacity.
...Therefore, new chemistry is needed to solve the problems in Li−O2 batteries....In contrast with LiO2 and NaO2, KO2 is thermodynamically stable and commercially available.—Ren and Wu
|Potassium, an alkali metal similar to lithium (and sodium) can be used in a rechargeable battery.|
|In a 2004 paper published in the Journal of Power Sources, Ali Eftekhari noted that “the potential of the potassium anode and lithium anode are approximately the same with only a 0.12V difference. Indeed, among alkali metals, the potential of potassium anode is the closest one to that of lithium anode.”|
Xiaodi Ren and Yiying Wu at OSU started their investigation by carrying out electrochemical measurements to verify the influence of cations on the redox chemistry of oxygen. They found that the oxygen reduction and oxidation potential gap in the electrolyte with K+ is much smaller than that in the electrolyte with Li+, implying that a K−O2 battery may operate at much lower overpotentials than a Li-O2 battery, and thus with higher roundtrip efficiences.
They then fabricated a K−O2 battery containing a potassium metal foil, a glassy fiber separator and a porous carbon electrode, with 0.5 M KPF6 in an ether solvent (1,2- dimethoxyethane (DME) or diglyme) as the electrolyte. For comparison, they built a Li−O2 battery in a similar manner.
Cyclic voltammetry in the two-electrode battery setup showed the small difference between the onset potentials of oxygen reduction and oxidation for the K-O2 battery. They determined that the oxidation process can be complete within the potential range where the carbon electrode and the electrolyte are relatively stable.
The smaller discharge overpotential of the K−O2 battery may result from the better conductivity of KO2 (>10 S/cm2, room temperature) than Li2O2, they suggested.
More importantly, in the subsequent charging process, the voltage is as low as 2.50− 2.52 V for the K−O2 battery. The charge overpotential ηchrg of ∼20−40 mV is significantly smaller than the Li−O2 battery. Moreover, within this small charging potential range, almost 90% of the discharged product can be oxidized. In contrast, for the Li−O2 battery, only half of the product was able to be removed even when the voltage reaches 4.0 V, where the ether electrolyte and the carbon electrode become unstable. The charge/discharge potential gap of about 50 mV is the lowest one that has ever been reported in metal−oxygen batteries. Compared with the typical Li−O2 battery, which has a potential gap larger than 1 V, our K−O2 battery can can provide an exceptional round-trip energy efficiency of >95%.
...As a final note, very recently during the preparation of this paper, another group of researchers has published their results about a Na−O2 battery. [Earlier post.] Although it shares a similar mechanism with our K−O2 battery, a notable difference is that KO2 is both kinetically and thermodynamically stable, while NaO2 is only kinetically stable. This can bring some advantages to our K−O2 battery. For example, as shown earlier, a KO2-loaded carbon electrode can be prepared as the artificially discharged cell, which allows us to separately study the charging step and the discharging step. This is helpful for an in-depth understanding of the battery processes. Despite this difference, the promising results from both K−O2 and NaO2 batteries indicate the potential of the superoxide batteries.—Ren and Wu
Xiaodi Ren and Yiying Wu (2013) A Low-Overpotential Potassium–Oxygen Battery Based on Potassium Superoxide. Journal of the American Chemical Society doi: 10.1021/ja312059q
Pascal Hartmann, Conrad L. Bender, Miloš Vračar, Anna Katharina Dürr, Arnd Garsuch, Jürgen Janek & Philipp Adelhelm (2012) A rechargeable room-temperature sodium superoxide (NaO2) battery. Nature Materials doi: 10.1038/nmat3486
Ali Eftekhari (2004) Potassium secondary cell based on Prussian blue cathode. Journal of Power Sources, Volume 126, Issues 1–2, Pages 221-228, doi: 10.1016/j.jpowsour.2003.08.007