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Energy and the Environment
Nickel Based Membrane Electrodes Enable High Rate Electrochemical Ammonia Recovery Dianxun Hou, Arpita Iddya, Xi Chen, Mengyuan Wang, Wenli Zhang, Yifu Ding, David Jassby, and Zhiyong Jason Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01349 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018
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Environmental Science & Technology
Nickel Based Membrane Electrodes Enable High Rate Electrochemical Ammonia Recovery by Dianxun Hou1, Arpita Iddya2, Xi Chen1, Mengyuan Wang3, Wenli Zhang4, Yifu Ding3, David Jassby2, Zhiyong Jason Ren1 * 1. Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, CO 80303, USA 2. Department of Civil and Environmental Engineering, University of California, Los Angeles, California 90095, United States 3. Department of Mechanical Engineering, University of Colorado Boulder, Boulder, CO 80309, USA 4. Materials Science and Engineering, Physical Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal 239556900, Kingdom of Saudi Arabia
* Corresponding author address: Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, CO, USA; Tel: +1 (303) 492 4137; Fax: +1 (303) 492 7317; E-mail:
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Abstract
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Wastewater contains significant amounts of nitrogen that can be recovered and
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valorized as fertilizers and chemicals. This study presents a new membrane electrode
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coupled with microbial electrolysis that demonstrates very efficient ammonia recovery
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from synthetic centrate. The process utilizes the electrical potential across electrodes to
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drive NH4+ ions towards the hydrophilic nickel top layer on a gas-stripping membrane
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cathode, which takes advantage of surface pH increase to realize spontaneous NH3
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separation and recovery using the membrane electrode structure. Compared with a
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control configuration with conventionally separated electrode and hydrophobic
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membrane, the integrated membrane electrode showed 40% higher in NH3-N recovery
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rate (36.2 ± 1.2 gNH3-N/m2/d) and 11% higher in current density. The energy
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consumption was 1.61 ± 0.03 kWh/kgNH3-N, which was 20% lower than the control and
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70-90% more efficient than competing electrochemical nitrogen recovery processes (5-
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12 kWh/kgNH3-N). Besides, the negative potential on membrane electrode repelled
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negatively charged organics and microbes thus reduced fouling. In addition to
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describing the system’s performance, we explored the underlying mechanisms
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governing the reactions, which confirmed the viability of this process for efficient
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wastewater ammonia recovery. Furthermore, the nickel-based membrane electrode
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showed excellent water entry pressure (∼41 kPa) without leakage, which was much
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higher than PTFE/PDMS based cathodes (~1.8 kPa). The membrane electrode also
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showed superb flexibility (180o bend) and can be easily fabricated at low cost (1100 mgNH3-N/L), total Kjeldahl
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nitrogen (TKN, >1300 mgN/L), orthophosphate (>200 mgPO4-P/L), and organics (up to
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2000 mg/L)
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primary treatment units. However, the small volume of centrate (< 1% total volume) can
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contribute up to 30% of nutrient loading, as well as elevated energy demand, pipe
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scaling (from struvite), which leads to an increase in the environmental footprint of
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mainstream treatment facilities12, 13. Therefore, direct ammonia recovery from centrate
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represents a good opportunity for wastewater nutrient recovery.
11
. Generally, wastewater treatment plants recycle centrate back to the
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Air stripping, ion exchange, and membrane processes have been used for ammonia
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recovery from wastewater
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demand and/or require large quantities of chemicals for regeneration
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many of these technologies have low selectivity. Besides ammonium, phosphate and
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other non-nutrient ions (e.g., Na+, Cl-) are also recovered and mixed with ammonium,
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which downgrades product quality, reduces efficiency
14-17
. However, these technologies suffer from high energy
13, 19-23
2, 18
. Moreover,
and leads to scaling (e.g.,
21, 22, 24
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struvite)
. In this regard, electrochemical processes carry a good advantage for
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product selection by utilizing low-cost electrons to realize phase separation
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an electrochemical system, the electrolysis of water produces hydroxide ions on the
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10, 20, 25-28
. In
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cathode
. By separating the anode and cathode using an ion exchange membrane
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(IEM), the high pH in the cathode chamber can drive the conversion of ammonium ion
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(NH4+) to ammonia (NH3), which can be recovered using air stripping or gas-permeable
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hydrophobic membranes 13, 19, 31. Based on this hypothesis, Kuntke et al. firstly achieved
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ammonia recovery (3.29 gNH3-N/m2/d) from a membrane electrode assembly (MEA) air
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cathode in a microbial fuel cell (MFC) via gas diffusion
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fabricated at low cost, such gas diffusion structure was generally thick (>1 mm) with low
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flexibility, and it could not withhold high water pressure ( 99%) with the COD in the
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recovery solution maintained below 10 mg/L.
10, 25, 28, 58
. The ammonia recovery rate and energy
2, 10, 25, 36, 58
. The membrane
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The higher performance observed in Run 3 was believed to be associated with the
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mutual benefits caused by the active ammonia harvesting at the membrane electrode
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surface, which alleviated the accumulation of NH3/OH- and reduced cathode over-
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potential
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According to the Nernst equation (Section S10 in SI), increasing pH by one unit can
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lead to an increase of over-potential by 59 mV. Therefore, the low catholyte pH in Run 3
27, 59
; this, in turn, led to a higher current density and ammonia recovery.
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might have reduced the apparent over-potential by 17.7 and 75.5 mV compared to Run
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2 and Run 1, respectively. In order to further illustrate the effect of active ammonia
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recovery on the over-potential, the membrane electrode was operated with and without
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active ammonia recovery (Figure 3B). It was shown that the over-potential difference
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was 0.11 V, suggesting that at a same current density, the actual pH on the electrode
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surface with active ammonia recovery was 1.86 lower than that operated without
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ammonia recovery. It is interesting to note that the pH variance at the cathode surface
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was larger than that of the bulk (1.28, Figure S10A). This indicated that the pH
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difference between the cathode surface and bulk solution during ammonia recovery was
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comparatively lower than that without ammonia recovery. The reduced pH gradient was
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due to the active ammonia harvesting, which could effectively remove ammonia from
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the electrode surface thus altered the NH4+/NH3 equilibrium and mitigated the
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accumulation of NH3/OH- on the electrode surface. The reduced pH on the electrode
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surface hence mitigated cathodic potential losses and might theoretically enhance the
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overall electrical efficiency by 8.8% 30, 60.
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3.3. Ammonia Mass Balance and Mechanism for Improved Recovery
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Figure 3 shows the overall mass balance of the NH3-N present in different operating
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conditions. While for all runs good mass balances were achieved, the NH3-N transport
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in different runs varied significantly. Driven by the current, NH4+ was continuously
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transported from the anolyte to the catholyte (Figure 3A) and the loss rate in the anolyte
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was positively correlated with the current in the reactor (Figure 3C). This is in
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agreement with our previous findings in microbial resource recovery, in which ion
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transport was determined by the current as well as its relative abundance in the solution
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21
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chamber can be balanced with the NH3-N increase in the cathode chamber. However,
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up to 85 mg NH3-N overall loss from the system was observed, presumably due to the
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formation of struvite precipitate due to increased cathode pH
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the corresponding loss of PO4-P (Figure S9).
. For Run 1 without ammonia harvesting, most of the NH3-N loss in the anode
13, 24
; this is supported by
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In contrast, when ammonia harvesting was applied in Run 2 and Run 3, NH3-N
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concentrations dropped in both the anolyte and catholyte. This simultaneous drop was
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due to the ammonia harvesting from the cathode to the recovery solution (Figure 3A).
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Overall, in Run 3, where the cathode chamber was equipped with the membrane
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electrode (on which NH4+-N was electro-adsorbed and pH was much higher than the
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bulk pH 61, 62), the direct deprotonation and capture greatly enhanced ammonia recovery
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by 40% as compared to Run 2. Benefiting from the active ammonia recovery using the
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membrane electrode, the local pH on the membrane electrode surface was lowered,
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which reduced the cathodic potential loss (Figure 3B) and enhanced current production
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(Figure 2). Moreover, the catholyte bulk pH was also maintained below 8.06 during Run
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3 (Figure S10A), which mitigated ammonia/phosphate precipitation (Figure S11).
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Importantly, the ammonia recovery in Run 3 was directly correlated with the current, as
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shown in Figure 3D. This is likely because NH4+ ions served as proton shuttles during
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operation 17, 22, 62, in which NH4+ ions carried protons to the membrane electrode surface,
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and lost protons when they were converted to NH3. In this study, the NH4+ ions
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accounted for 36.9±0.7% of all proton shuttles, which agreed with the findings from a
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previous study 62.
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During MEC operation, the effective concentration (Ceffective,i) of a cation on the electrode
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surface can be determined using the Nernst-Plank equation
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are pulled via diffusion and migration from the bulk solution and accumulate at the
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electrode-solution interface, where NH4+ concentration is higher than that in the bulk
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(Figure 4)
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accumulates on the electrode surface, leading to localized pH increase and the
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deprotonation of NH4+ 29, 30. Researchers previously employed hydrophobic membranes
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for ammonia recovery from the catholyte
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NH3/OH- from the cathode surface to the bulk largely governed the cathodic potential
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losses thus current production, which may reduce the overall recovery performance
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Moreover, buffer ions (e.g., HCO3- and HPO42-) can further reduce the efficiency by
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countering catholyte pH change
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we took advantage of the local pH increase on the membrane cathode surface to
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enable in situ NH3 generation and separation, which mitigated over-potential and
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eliminated the transport limitation of NH3/OH- from the cathode surface to the bulk for
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further recovery
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OH- and promoted NH4+ transport from the bulk toward the electrode to enable
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continuously high efficient NH3 harvesting (Figure 4). The active ammonia recovery also
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reduced the electrode-solution interface pH, which reduced the over-potential
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associated with electrolysis and enhanced current production 30.
63
. Accordingly, NH4+ ions
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. Due to water reduction during electrolysis on the MEC cathode, OH-
30, 37
26, 35
, but the diffusion and/or migration of
30
.
and decreasing the ammonia fraction. In this study,
30
. This approach enhanced the local generation and consumption of
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In addition to ammonia, hydrogen gas was also recovered from the system. In Run 1
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without ammonia recovery, the system was operated similarly to a traditional two-
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chamber MEC with a nickel foam cathode, so H2 was stably produced at 0.15 ± 0.01
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m3-H2/m3/d with a cathode efficiency of 100 ± 7%. The cathodic efficiency was
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approximately 40% higher than our previously reported result in a single-chamber MEC
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cathode efficiency to 0.13 ± 0.02 m3-H2/m3/d and 75 ± 9%, respectively, though the
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current was much higher as shown in Figure 2 and Table S3. However, with active
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ammonia recovery using the membrane electrode, Run 3 exhibited improved H2
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production rate (0.19 ± 0.01 m3-H2/m3/d) and cathode efficiency (85 ± 2%) than Run 2.
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The variation of H2 production is believed to be associated with ammonia recovery thus
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pH and free ammonia in the catholyte, which can greatly affect the activities of hydrogen
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consumption microbes (e.g., sulfate reducing bacteria and methanogens)
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discussions can be found in Section S13 in SI.
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3.4. Low Fouling Behavior
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Figure 5 shows the membrane surface in Run 2 and membrane electrode in Run 3
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following the ammonia harvesting experiments. The bare PP membrane used in Run 2
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displayed a yellowish color (Figure S7C), and its SEM images with EDX analysis
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showed the presence of organic fouling with significant crystal deposition (Figure 5A).
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The deposited organics reduced the porosity of the PP membrane (Figure 5A-2)
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which could reduce the ammonia recovery efficiency. A more severe issue on PP
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membrane in Run 2 can be membrane wetting, which can result from organic matter
. It is interesting to note that ammonia recovery in Run 2 reduced H2 production and
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. Detailed
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,
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depositing inside the membrane’s pores.68 This can result in the acid leaking and
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reduced gas separation. In contrast, the membrane electrode in Run 3 did not suffer
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from organic fouling (Figure 5B-1 and Figure S7A-2). This benefit may be a result of the
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negative potential on the membrane electrode during operation, which repelled the
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negatively charged organics and/or microbes
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nickel thin layer greatly reduced the roughness (~35 nm vs ~120 nm of the PP
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membrane, Figure S3) and hydrophobicity (CA: 21.4±0.5°, Table S1) of the composite
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membrane electrode, both of which help mitigate organic fouling and biofouling
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However, the membrane electrode did show inorganic scaling (Figure S7A-2 and Figure
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5B-1), which, based on XPS analysis, was found to be similar to the composition of
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NH4MgPO4 (struvite)
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believed to be attributed to the heterogeneous/surface crystallization on the electrode
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surface, which was exacerbated by the pH change due to electrolysis
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electrolysis, the negative potential can promote the formation of an electric double layer
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on the membrane surface
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adsorbs cations (e.g., NH4+ and Mg2+)
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fouling and promoting ammonium transfer, the side effect is the increased pH due to
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OH- accumulation, which could promote crystallization
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scaling did not significantly hinder the performance of the membrane electrode, with
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current production dropped from 61.2 to 56.5 mA (7.7%), and ammonia recovery
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decreased from 37.4 to 34.9 gNH3-N/m2/d (6.8%) , respectively, in 7 batches (Figure 2
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and Figure 3A). Moreover, the deposited crystals only appeared on the membrane
24, 70
39, 40
. In addition, the electrodeposited
17, 69
.
and/or Mg3(PO4)2 or Ca3(PO4)2 71, 72 (Figure S8A). This was
70-72
. During
73, 74
. Though the double layer repels anions and but also 74
. Even though it has benefits of anti-organic
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. Fortunately, inorganic
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surface and was easily cleaned with 0.1% H2SO4 (Figure 5B-2, Figure S7A-3 and
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Figure S8A).
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3.5. Implications and Outlook
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The study demonstrated in both reaction mechanisms and lab scale experiments that
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the nickel based membrane electrodes can achieve high rates of electrochemically-
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driven ammonia recovery. The layered structure enabled both efficient catalysis on the
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hydrophilic side and gas separation on the hydrophobic side, and the negative potential
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and local OH- accumulation reduced fouling and facilitated ammonia generation. In
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addition to the data presented, we also found the membrane electrode endured higher
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water pressure (41.4 ± 0.4 kPa) than traditional air cathodes (∼12 kPa for PVDF based
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cathode, and 0.18 ± 0.02 m for PTFE and PDMS based cathodes)
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enter pressure was able to prevent gas channel flooding and grant the membrane
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electrode better scalability or modulation (Section S5 in SI). Furthermore, the
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membrane electrode was very thin (
