Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane David A. Vermaas and Wilson A. Smith* Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Van der Maasweg 9, 2629 HZ Delft, The Netherlands S Supporting Information *
ABSTRACT: The electrochemical conversion of CO2 and water to value-added products still suffers from low efficiency, high costs, and high sensitivity to electrolyte, pH, and contaminants. Here, we present a strategy for this reaction using a silver catalyst for CO2 reduction in a neutral catholyte, separated by a bipolar membrane from a nickel iron hydroxide oxygen evolution catalyst in a basic anolyte. This combination of electrolytes provides a favorable environment for both catalysts and shows the effective use of bicarbonate and KOH to obtain low cell voltages. This architecture brings down the total cell voltage by more than 1 V compared to that with conventional use of a Pt counter electrode and monopolar membranes, and at the same time, it reduces contamination and improves stability at the cathode.
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remains challenging in aqueous environments due to conflicting electrolyte requirements. Here, we present a stable and selective CO2/H2O reduction device that incorporates an earth-abundant (NiFeOx) OEC in an alkaline environment and a Ag CO2 reduction catalyst in a neutral environment. To match these individually wellinvestigated catalytic materials and eliminate transport of species in the anodic electrolyte to the cathodic electrolyte, we introduce a bipolar membrane (BPM). This membrane is composed of an anion and cation exchange layer (Figure 1), which facilitates ion transport via water dissociation into H+ and OH−. The protons are transported to the cathode, while the hydroxides are transported to the anode, thereby balancing the consumed protons and hydroxides at the electrode reactions. A BPM, traditionally used for the production of acid and base,20−22 has recently been demonstrated to enable complementary cathode and anode catalysis for electrochemical,23,24 photoassisted,25 and photodriven water splitting.26 The use of a BPM for the electrochemical reduction of CO2 was recently shown;27 however, insights into the mechanism for how the BPM works in such an environment were not well understood. Using a BPM with a catholyte at near-neutral pH (typical for the CO2 reduction reaction) is suspected to favor ion crossover instead of water dissociation,21,28 while its operation during electrolysis suggests a dependency on ion type and concentration.25,26,29 The ion transport mechanism in the
lectricity production from wind and solar resources has been increasing across the world.1 To maximize the use of these renewable energy supplies, which fluctuate daily and seasonally, robust technologies are required to store their energy for on-demand use when they ebb. Channeling excess power during peak production into electrochemical conversion of water and CO2 to high energy density fuels is an appealing solution that takes advantage of the enormous global infrastructure available for the storage and transportation of hydrocarbons.2−5 However, the electrochemical reduction of CO2 faces many challenges associated with the low solubility of CO2 in alkaline environments and competitive formation of hydrogen in acidic environments.2 CO2 reduction catalysts are also sensitive to contaminants such as Co, Ni, or Fe, which can deposit on the cathode and thereby increase hydrogen evolution while decreasing the CO2 reduction rate.2,6 These factors constrain the overall system architecture and the choice of accompanying oxygen evolution catalyst (OEC). As a result, to date, precious metal counter electrodes such as Pt7−12 and IrO25 have been typically used to carry out coupled electrochemical water oxidation due to their stability in a neutral environment. Even when using these precious metals as counter electrodes, the combination of reduction and oxidation electrodes easily requires a total bias of at least 3 V to produce a selective product at a significant yield (>10 mA/cm2).13 At the same time, many highly efficient earth-abundant OECs are known to operate stably in an alkaline environment.14−18 The use of earth-abundant OECs coupled with CO2 reduction was recently shown for operation in nonaqueous liquids19 but © XXXX American Chemical Society
Received: October 26, 2016 Accepted: November 6, 2016 Published: November 7, 2016 1143
DOI: 10.1021/acsenergylett.6b00557 ACS Energy Lett. 2016, 1, 1143−1148
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http://pubs.acs.org/journal/aelccp
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ACS Energy Letters
Figure 1. (A) FE for CO and H2 evolution as a function of the applied potential at Ag. Each potential was probed for at least 2 h, from which the data of the last hour (i.e., 5 data points) were used to obtain an average FE with standard error. (B) Configuration of the electrochemical cell with water oxidation on one side and CO2 reduction on the other. (C) Ion transport mechanism in the BPM, including possible H2CO3/ HCO3− buffering (indicated by 1) and H2CO3 permeation (indicated by 2). (D) Cyclic voltammetry scan for the Ag cathode, recorded at a scan rate of 50 mV/s, where the arrows indicate the scan directions. (E) Current density as a function of the membrane potential difference. The dotted lines indicate the theoretical (reversible) membrane voltage, given by 0.059 · ΔpH. The membrane potential was recorded in the flow cell in conjunction with cyclic voltammetry of the Ag cathode (third returning scan, 50 mV/s), using a tripotentiostat. Similarly, the potential at the anode in (F) was recorded at the same time, for Pt foil in different electrolytes and for NiFe hydroxide in 1 M NaOH. All data were obtained from the cell described in Figure S1. Unless stated otherwise, a BPM and NiFe hydroxide anode were used.
Additionally, the i−v curves for oxygen evolution show a significantly steeper slope when using 1 M NaOH compared to 0.1 M KHCO3 due to the higher electrolyte conductivity and a more stable (local) pH in the more alkaline environment. As a consequence, the potential for oxygen evolution at 10 mA/cm2 using this NiFe hydroxide in 1 M NaOH is even slightly lower than that when using the best available OEC (IrO2) in a traditional neutral environment (e.g., 0.5 M NaHCO3 at both sides of the membrane).5 To make use of this favorable alkaline condition for oxygen evolution, a BPM is required to facilitate different electrolyte conditions for the cathode and for the anode. The use of a BPM to allow such an alkaline anolyte in combination with a 0.1 M KHCO3 catholyte results in a significantly higher electrical voltage to drive water dissociation (>0.6 V; see Figure 1E) than that with Nafion. However, when taking into account the chemical potential difference of 7 units pH difference over the BPM (0.059 · ΔpH, with pH ≈ 6.8 vs 13.7), the effective (electrochemical) membrane voltage is only 0.25 V at 10 mA/ cm2. In other words, the membrane voltage can be split into a reversible chemical potential difference (0.41 V) and a minor irreversible voltage loss (0.25 V at 10 mA/cm2). For smaller pH gradients over the BPM, such as the 0.1 M KHCO3−BPM−0.1 M KHCO3 case (ΔpH ≈ 1 due to CO2 gas in the catholyte only), the electrical potential is smaller, but the irreversible electrochemical potential difference increases, as indicated by the larger difference between the dotted solid blue lines in Figure 1E. Similarly, the electrochemical potential difference over the BPM is smallest when an extreme pH gradient is used (i.e., 1 M acid vs 1 M base, Figures S2 and S3),21,29 which is the
BPM is of particular importance for CO2 reduction due to the high sensitivity on pH and contaminations that affect the reaction efficiency and selectivity. Electrochemical CO2 reduction experiments were performed in a flow cell with 2 mm electrode−membrane spacing, illustrated in Figure S1. Silver foil, a well-studied catalyst for the selective reduction of CO2 to CO, operated as the cathode in 0.1 M KHCO3. For the anolyte, we explored both 0.1 M KHCO3 and 1 M NaOH (the latter instead of KOH to better distinguish the effect of cation crossover). The voltage−current characteristics of the individual catalysts in this configuration are shown in Figure 1, as well as the product selectivity at different potentials. The highest Faradaic efficiency (FE) for CO evolution was obtained at an applied potential of −1.1 V vs RHE at the silver electrode (Figure 1A), in agreement with earlier observations.9 The FE for CO at more negative potentials remained high, though Hatsukade et al.9 observed a significant decrease in FE at more negative potentials, which they ascribed to mass transport limitations at strongly negative potential (i.e., high current densities, Figure 1D). The use of forced electrolyte flow in our experiments appears to have overcome these mass transport limitations and is recommended for future research using CO2 reduction catalysts with larger current densities. For the oxygen evolution reaction (OER), NiFe hydroxide outperforms Pt by far, even at the same pH (Figure 1F), while Pt is the most frequently used counter electrode in electrochemical CO2 reduction.7−12 This performance of NiFe hydroxide is in agreement with the widely studied use of NiFe hydroxide as an OEC for alkaline water oxidation.14,18,30 1144
DOI: 10.1021/acsenergylett.6b00557 ACS Energy Lett. 2016, 1, 1143−1148
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ACS Energy Letters
Figure 2. CO2 reduction performance over 10 h. (A) Total cell voltage and (B) FE for CO as a function of time, for the cases with Ag− Nafion−Pt (black line), Ag−BPM−Pt (2× blue lines) and Ag−BPM−NiFe (red line), with an applied voltage of −1.1 V vs RHE at the Ag foil. The notation for the electrolyte indicates the anolyte; the catholyte is 0.1 M KHCO3 for all cases. The corresponding pH for the cases with (C) Ag−Nafion−Pt and (D,E) Ag−BPM−NiFe is plotted as a function of time.
Figure 3. Ion transport through Nafion (A) and BPM (B). As the concentration of K+ far exceeds that of H+, K+ is transported through Nafion. The BPM (B) consists of an anion exchange layer (AEL) and a cation exchange layer (CEL) that block ions from the electrolyte and facilitate water dissociation at the AEL−CEL interface.
native environment of this BPM, but would not be compatible with the favorable catholyte for CO2 reduction. The low electrochemical potential difference over the BPM when using KHCO3 as the catholyte is crucial for efficient application in CO2 reduction. Previous research has shown that for several electrolytes (acid−base,23,26 phosphate buffer,23 sodium chloride19) the electric potential over the BPM is at least 0.8 V at any significant current, which would imply an irreversible electrochemical potential of at least 0.4 V when
using a 0.1 M KHCO3 catholyte and a 1 M NaOH anolyte (i.e., ΔpH ≈ 7). Our results demonstrate that much lower potentials than 0.8 V are possible over the BPM, observed for all cases and steady over at least 24 h (Figure S4). The slope of the BPM i−v curve with a 1 M NaOH anolyte is steeper than that when using 0.1 M KHCO3 at either side, despite the larger specified membrane resistance (3 Ω·cm2 for BPM and 1.8 Ω·cm2 for Nafion in potassium-containing electrolyte31). The steeper slopes for the BPM curves are partly 1145
DOI: 10.1021/acsenergylett.6b00557 ACS Energy Lett. 2016, 1, 1143−1148
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ACS Energy Letters
In contrast to traditional monopolar membranes (e.g., Nafion), a BPM does not allow (cat)ions to pass the membrane. Instead, water is dissociated in the interface layer of the BPM and supplies H+ to the cathode compartment and OH− to the anode compartment, which matches production at the electrodes (Figure 3B). This is reflected in a stable pH, even when using 1 M NaOH as the anolyte against 0.1 M KHCO3 as the catholyte (Figure 2D,E). As a consequence, all cases using a BPM showed only a slight decrease in FE for CO production in the first 2 h (due to deposition of minor contaminants from the catholyte), while a globally constant CO selectivity was observed for the remaining 8 h (Figure 2B) as the pH and electrolyte concentration were maintained. The advantage of ion blockage in the BPM is accentuated when considering the transport of contaminants from the anolyte to the catholyte. With Nafion separating the NiFe hydroxide anode in neutral pH from the Ag cathode, massive Ni deposition and significant Fe deposition were observed at the cathode after 10 h of operation (Figure S5). Such severe contamination catalyzes the evolution of H2 almost exclusively and thereby diminishes CO product selectivity.2,6 In contrast, with the BPM, Ni and Fe species do not contaminate the cathode surface (Figure S5), even though the anolyte contains traces of Ni and Fe ions (Table S3). In addition, the total cell voltage significantly decreases when a pH gradient is provided over the BPM (dark blue line in Figure 2A), reflecting the lower electrochemical potential difference over the membrane (Figure S2) and the lower overpotential for Pt (Figure 1D) in 1 M NaOH compared to that in 0.1 M KHCO3. Similarly, the total cell voltage for a Ag−BPM−NiFe configuration is remarkably 1 V lower than the Ag−Nafion− Pt reference case at the start of operation, and this gap even increases up to 1.5 V during the first 10 h of operation (Figure 2A) as the cell voltage for Ag−BPM−NiFe remains constant over at least 24 h (Figure S4). The cell voltage for Ag−BPM− NiFe is very close to a practical minimum considering Ag foil as the CO2 reduction catalyst, which is illustrated by the voltage contributions in Figure 4. This figure shows that only 0.4 V is used for the overpotential of the OEC and the (ohmic) losses in the BPM and electrolyte together, which is already close to the overpotentials of the best-available OEC only.17 Hence, when applying NiFe hydroxide and a BPM and thereby reducing the overpotential at the oxygen evolution side by more than 1 V, the overall efficiency now mainly depends on the overpotential of the CO2 reduction catalyst. The ability to separate anode and cathode compartments with a BPM has great promise for the electrochemical conversion and production of numerous chemicals. For example, Chen et al.11 demonstrated highly selective CO formation at approximately 10 mA/cm2 with an overpotential of 300 mV on oxide-derived Au. Combining such a catalyst with a BPM and NiFe hydroxide as an anode would bring down the total cell voltage to 2.1 V only (Figure 4). This arrangement could easily be applied to other systems where the stability, activity, and selectivity of anodic and cathodic reactions have conflicting electrolyte dependences. For example, this demonstration using a neutral pH electrolyte for CO2 reduction and an alkaline electrolyte for water oxidation could be a model for the electrochemical synthesis of ammonia, wherein the nitrogen reduction reaction is not favored in an alkaline environment. Furthermore, the increased stability of the pH and electrolyte concentration (Figure 2) when using a BPM is of major
due to the higher electrolyte conductivity of the NaOH solution (approximately 60 mV lower ohmic loss at 10 mA/ cm2) and partly because the conductivity of the membrane itself increases in more conductive solutions.32,33 The steep i−v curves indicate that the ohmic loss of the BPM itself is insignificant for CO2 reduction at these current densities. To demonstrate the synergistic effects of a NiFe-based OEC, a BPM, different electrolytes, and a Ag CO2 reduction catalyst, we explored four different configurations for CO2 reduction. Figure 2 shows the total cell voltage, FE for CO, and the pH in both compartments during 10 h of operation. Upon replacing a Nafion membrane with a BPM while leaving the electrodes and electrolytes on both sides the same (black and light blue lines in Figure 2A), the total cell voltage was initially higher due to the higher electrochemical potential difference as observed in Figure 1C. However, the cell voltage for the traditional Nafion configuration increased over time, to a similar cell voltage of about 3.8 V after 7 h. This increasing cell voltage for the reference case with a Nafion membrane in the first 5 h coincided with a decrease in FE (Figure 2B), a large decrease in pH of the anolyte, and a slight increase in pH in the catholyte (Figure 2C). Nafion is permeable not only to H+ but also to other cations, such as K+. Because the initial concentration of K+ exceeds the H+ concentration by 7 orders of magnitude, K+ permeates through the Nafion membrane, as illustrated in Figure 3A. At the same time, H+ is produced at the anode and consumed at the cathode, which acidifies the anolyte while the catholyte becomes more alkaline. As the bicarbonate initially buffers the pH in the anolyte (HCO3− + H+ ⇄ H2CO3), the pH decreases gradually in the first 5 h. When the deionization of the anolyte continues beyond the pKa of this reaction, this buffering effect disappears and the pH decreases rapidly after 6 h. Finally, the decrease slows down due to the logarithmic scale of the pH and the corresponding H+ transport. The low conductivity of the deionized anolyte (
