Voltage Matters When Reducing CO2 in an Electrochemical Flow Cell
Voltage Matters When Reducing CO2 in an Electrochemical Flow Cell
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DOI：
10.1021/acsenergylett.9b02356
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发表时间：
2020-01-01
影响因子：
22
通讯作者：
Berlinguette, Curtis P.
中科院分区：
文献类型：
--
作者：
Salvatore, Danielle;Berlinguette, Curtis P.
CO2 into useful fuels and chemicals. 1 The economic viability of a CO2 electrolyzer is contingent on several factors, including the selectivity of the CO2 reduction reaction (CO2RR) and the rates of product formation. 2, 3 Operational voltage is also an important performance metric because it defines the efficiency of the electrolyzer. Commercial units will likely need to operate below 3 V while maintaining current densities (J) in excess of 200 mA/cm2 (Figure 1). Despite the importance of this metric, the full-cell voltages of pilot-scale CO2RR electrolysis reactors, or “flow cells”, are rarely reported. Moreover, very little is known about where the voltage losses occur in CO2RR flow cells. This gap in the literature prompted us to design and build an analytical flow cell capable of resolving voltage drops across individual electrode and membrane components during CO2RR electrolysis [with the oxygen evolution reaction (OER) occurring at the anode]. Measurements recorded with this apparatus point to the largest voltage losses occurring at the membrane and not at the catalyst layers. These results, which apply to both “membrane” and “hybrid” flow cell configurations common to the field (Figure 2), illuminate that a better understanding of membranes and the membrane− catalyst interface is needed. Most CO2 electrolyzer architectures reported to date can be described as either “hybrid” or “membrane” flow cell reactors (Figure 2). 1, 5− 14 In a hybrid flow cell, the electrochemistry at the cathode is influenced by the catholyte that separates the cathode gas diffusion electrode (GDE) and the membrane. This catholyte helps mediate efficient CO2RR at low half-cell overpotentials, but this liquid layer will ostensibly be responsible for a significant voltage drop across the cell. This voltage is typically not measured, however, and is simply estimated by the ionic resistance of the electrolyte. The membrane reactor architecture more closely matches that of a conventional zero-gap water electrolyzer, 15 wherein a polymer electrolyte membrane separates the cathode from the anode. 1 The CO2 feedstock can be delivered to the cathode directly in the gas phase, 5, 8, 16 as a saturated solution in an aqueous electrolyte, 10 or as a bicarbonate/carbonate solution that converts CO2 at a catalyst layer in an acidic environment. 17 Regardless of the feedstock, CO2 is electrochemically converted to a CO2RR product at the interface between the membrane and the cathode gas diffusion electrode. Membrane reactors should, in principle, not suffer from the same voltage losses incurred by the catholyte layer in the hybrid reactor. Moreover, the catholyte governs the cathode chemistry in a hybrid reactor, while the membrane figures more prominently in the cathodic electrochemistry of the membrane reactor. The polymer electrolyte membrane for both reactor types serves to separate the anode and cathode, mediate the flow of ions from electrode to the other, and prevent product crossover. 18 Cation exchange membranes (CEMs), anion exchange membranes (AEMs), and bipolar membranes (BPMs) are all being actively tested in CO2 flow cell systems. 1, 5− 14, 19 The choice of membrane dictates the reaction environments at the electrodes: CEMs and AEMs transport cations (eg, H+) and anions (eg, OH−), respectively, between electrodes. BPMs, which consist of an AEM laminated to a CEM, dissociate water at the AEM| CEM interface under reverse bias conditions to enable H+ transport to the cathode and OH− to the anode. Many of the highest performance flow cells are those that use an AEM, but product crossover between compartments remains a significant issue. 5, 18, 20 BPMs …
