Supporting Information

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1 Supporting Information Simultaneous hydrogen generation and waste acid neutralization in a Reverse Electrodialysis System Marta C. Hatzell 1, Xiuping Zhu 2, and Bruce E. Logan 2* 1 Department of Mechanical and Nuclear Engineering 2 Department of Civil and Environmental Engineering, 131 Sackett Building, The Pennsylvania State University, University Park, PA 1682, USA. *Corresponding Author: E mail: blogan@psu.edu; phone:

2 S2 Power Density (W m -2 ) Power Density (W m -2 ) A B 6 ml min -1 (WA) 6 ml min -1 (AmB) 4 ml min -1 (WA) 4 ml min -1 (AmB) 2 ml min -1 (WA) 2 ml min -1 (AmB) 1 ml min -1 (WA) 1 ml min -1 (AmB) Current Density (A m -2 ) Figure S1: Effect of flow rate on gas evolving (O 2 /H 2 ) electrodes with ammonium bicarbonate and waste acid catholyte.

3 S3 Table S1: Model Parameters Parameter Constant Value Units Proton Diffusivity D H m 2 s 1 Chloride Diffusivity D Cl m 2 s 1 Ammonium Diffusivity D NH m 2 s 1 Bicarbonate Diffusivity D HCO m 2 s 1 Stack Initial Concentration (AmB) C s,i 1.1 M Catholyte Initial concentration (HCl) C cat,i.1 M Proton mobility u H cm 2 s 1 volt 1 Chloride mobility u Cl cm 2 s 1 volt 1 Ammonium mobility u NH cm 2 s 1 volt 1 Bicarbonate mobility u HCO cm 2 s 1 volt 1 Stack Flow rate 1 u s, cm s 1 Stack Flow rate 2 u s,2 8.6 cm s 1 Stack Flow rate 3 u s,3 4.3 cm s 1 Stack Flow rate 4 u s,4 2.1 cm s 1 Catholyte Flow rate 1 u c,1 cm s 1 Catholyte Flow rate 2 u c,2 cm s 1 Catholyte Flow rate 3 u c,3 cm s 1 Catholyte Flow rate 4 u c,4 cm s 1 Charge Proton z H 1 Charge Chloride z cl 1 Charge Ammonium z NH4 1 Charge Bicarbonate z HCO3 1 Faradays Constant F C mol 1 Universal Gas Constant R J K 1 mol 1 Temperature T 33 K

4 S4 Description of Model Results In order to further understand the transport (diffusion) mechanisms for acid neutralization the system was modeled with a two chamber (Stack and Catholyte) control volume (Figure S2). From the simulation results, if the system were operated in a single pass approach, decreasing the catholyte flow rate (producing a longer hydraulic retention time) allowed for greater changes in the catholyte exit ph than changing the stack flow rate (Figure S3). The greatest change in ph was produced by the slowest flow rate, mainly because of the longer residence time, which allowed for greater diffusion of protons across the membrane (catholyte to stack). The increased rate of neutralization with increased stack flow rates was due to the amplified removal of the transported protons out of the cell. When protons accumulated within the stack solution the proton gradient was diminished, reducing the rate of proton transport due mainly diffusion (Figure S5). Thus, increasing the stack flow rates is not only preferable for optimal system performance, but also to maintain a large ph gradient across the end membrane.

direction and magnitude of the flux of ammonium ions.

5 S5 (a) Figure S2: Ion concentration in the catholyte and RED channel directly adjacent to the cathode. The black arrows indicate direction and magnitude of the proton flux, and the white arrows indicate direction and magnitude of the flux of ammonium ions. The interface region is zoomed in to show the magnitude of the flux at the membrane surface. (a) Ammonium concentration (b) proton concentration. (b)

6 S A Catholyte ph B 6 ml min -1 4 ml min -1 2 ml min -1 1 ml min -1 Catholyte ph ml min -1 1 ml min -1 5 ml min -1 1 ml min Distance along channel length (m) Figure S3: Catholyte ph changes along the membrane surface where x= is the inlet and x=.2 m is the exit (Single Pass) when the (a) stack or b) catholyte flow rate is varied.

7 S7 Acid Neutralization Efficicney (%) /41 Hour Hour 1/4 Hour Hour Catholyte Flowrate (ml min -1 ) Figure S4: Acid neutralization efficiency based on the electrical current observed through an external load. The efficiency greater than 1% indicated that neutralization took place not only through the electrochemical reaction of protons to hydrogen gas.

8 S8 9 8 Catholyte ph ml min -1 1 ml min -1 5 ml min -1 1 ml min Distance along membrane (m) Figure S5: Catholyte ph changes along the membrane surface where x= is the inlet x=.57 m, 2.8 m, 5.4 m, 8.6 m is the exit (recycle mode) when the catholyte flow rate is varied.

9 S9 Moles of [H + ] removed from catholyte NH 4 + Flux H + Diffusion Rxn Total Measured 1 ml 2 ml 4 ml 6 ml min -1 min -1 min -1 min -1 Figure S6: Protons Stack transport Flow Rate mechanism during testing where the stack flow rate was varied and the catholyte flow rate was fixed at 1 ml min 1.

S1 12 1 Oxygen Evolution Energy Required for Hydrogen and Oxygen evolution (kj/mol) 8 6 4 2-2 -4-6 ~35 kj/mol ~2 kj/mol, 2&12 Hydrogen Evolution, 7-8 2 4 6 8 1 12 14 ph Figure S7: Energy consumed

10 S Oxygen Evolution Energy Required for Hydrogen and Oxygen evolution (kj/mol) ~35 kj/mol ~2 kj/mol, 2&12 Hydrogen Evolution, ph Figure S7: Energy consumed by electrode overpotential for hydrogen and oxygen evolution as a function of solution ph. Whole cell activation overpotentials indicated at two operating points (ph 7 and cathode=ph 2; anode=ph 12)

11 S11 12 CO 2 H 2 1 Gas Composition (%) AmB WA 1 ml min -1 AmB WA 2 ml min -1 AmB WA 4 ml min -1 AmB WA 6 ml min -1 Figure S8: Gas composition collected from catholyte headspace.

Basic overall reaction for hydrogen powering

Fuel Cell Basics Basic overall reaction for hydrogen powering 2H 2 + O 2 2H 2 O Hydrogen produces electrons, protons, heat and water PEMFC Anode reaction: H 2 2H + + 2e Cathode reaction: (½)O 2 + 2H +