Joule
ArticleHydrogen production with seawater-resilient bipolar membrane electrolyzers
Context & scale
Direct seawater electrolysis offers a pathway toward low-cost H2 and O2 using abundant water feedstocks. However, high concentrations of ions present in seawater complicate electrolyzer operation. Cl− ions, for example, can oxidize to corrosive and toxic byproducts. This presents a challenge to the safe and long-term operation of seawater electrolysis devices. In this work, we highlight fundamental principles of operation for a robust electrolysis system that leverages ion-selective membranes to control the transport and oxidation of Cl− under both simulated and real seawater electrolysis conditions. We observe significantly improved bipolar membrane device lifetimes compared with those of proton-exchange membrane devices under real seawater conditions. This work motivates efforts to further develop impurity-tolerant electrolyzers and to advance seawater electrolysis systems that could enable green H2 production and decarbonize critical sectors of the global economy.
Graphical abstract
Introduction
The sustainable generation of chemical fuels such as H2 offers a means to address the long-duration energy-storage challenge and to enable a next-generation, carbon-neutral chemical industry.1 To this end, low-temperature water electrolysis driven by renewable electricity is a promising route to inexpensive, sustainably produced H2 via the hydrogen evolution reaction (HER). While traditional membrane electrolyzers rely on ultrapure water feeds to generate H2 and O2, the direct electrolysis of impure water sources, e.g., seawater, could have inherent advantages, enabling broader access to water feedstocks while reducing capital costs by mitigating the need for on-site water purification.2,3,4,5,6 However, electrolyzing seawater to generate H2 and O2 introduces distinct challenges in comparison to electrolyzing ultrapure water. One critical challenge arises from the high concentrations of ionic species (e.g., Cl−, Na+, SO42−, Mg2+, Ca2+, etc.)—particularly Cl−—in seawater. The Cl− oxidation reaction (COR) generates corrosive “free chlorine” species (i.e., Cl2, HOCl, OCl−) at the electrolyzer anode. Although the electro-oxidation of Cl− to Cl2 (i.e., the chlorine evolution reaction [ClER]) is a critical industrial process (e.g., chlor-alkali process),7 COR poses significant challenges to the safety, efficiency, and durability of seawater electrolyzers during operation.8,9,10,11,12 Mitigating the COR with impure water feed also opens the possibility of making use of a pure O2 stream from the anode for uses including undersea operations and life-support.
The COR can, in principle, be suppressed by (1) decreasing anode catalyst selectivity for the COR, (2) modifying the anodic microenvironment to disfavor Cl− oxidation, and/or (3) decreasing Cl− access to the anode.12,13,14,15 Developing catalysts that strongly favor the 4-electron oxygen evolution reaction (OER) over the 2-electron COR at relevant applied potentials is an active area of research.16,17 IrOx is an efficient precious-metal OER catalyst in Cl−-free electrolytes, but in electrolytes with Cl− at concentrations as low as ∼30 mM it yields a high COR Faradaic efficiency (FE) of ∼86%.18 Non-precious metal oxide catalysts, such as MnOx, have shown promising OER selectivity (93%) in these same low-Cl−-concentration electrolytes, but material stability challenges in acidic pH are not yet resolved.19,20 Strategies (2) and (3) are complementary and can be leveraged using device architectures to create local electrolyte conditions that disfavor COR, for example, by creating an alkaline anode environment where the oxidation of Cl− is disfavored thermodynamically with respect to OER,17,21 or by selectively inhibiting the transport of Cl− to the anode, for example, using cation-selective membranes.
Here, we demonstrate how strategies (2) and (3) enabled near-complete suppression of Cl− oxidation, even with a seawater feed. Our approach uses a bipolar membrane (BPM), composed of a cation exchange layer (CEL) combined with an anion exchange layer (AEL), integrated into a BPM water electrolyzer (BPMWE) device.22,23,24 We found that an appropriately designed BPMWE, in concert with an asymmetric electrolyte feed in which seawater is only present at the cathode, mutually captured both the advantages of the CEL that limits Cl− crossover to the anode (due to cation transport selectivity) and the AEL that provides a local alkaline anode pH (where OER catalysts have high selectivity and mitigate the COR), resulting in an inherently ion-tolerant seawater electrolyzer (Figure 1A). We evaluated the ion-transport properties, performance, selectivity, and durability of the BPMWEs with saline water feeds and compared them with monopolar proton exchange membrane water electrolyzers (PEMWEs). We demonstrated BPMWE devices operating with real seawater—collected from the Pacific Ocean (Half Moon Bay, CA, USA)—during sustained electrolysis to generate H2 and O2 at current densities of 250 mA cm−2.
Section snippets
Results and discussion
BPMWE and PEMWE architectures were fabricated according to the designs in Figures 1 and S1. Extended descriptions of the experimental fabrication and electrochemical methods are provided in the experimental procedures section and in the supplemental information. Anion exchange membrane water electrolyzers (AEMWEs) were similarly fabricated, but significant Cl− crossover precluded robust comparison to BPMWE and PEMWE (see Figure S2). The AEMWE is not examined in detail in this work owing to its
Lead contact
Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Thomas F. Jaramillo, at [email protected].
Materials availability
This study did not generate new unique reagents.
Materials
Electrodes and membranes were prepared according to recent reports.27 NaCl (99.0% trace metals basis) was used as purchased from Sigma Aldrich in all saline water (0.5 M NaClaq) experiments. Cationic impurities of NaCl include Ca2+ (<0.002%), Mg2+ (<0.001%), K+ (<0.005%) and Fe 2+/3+ (<2
Acknowledgments
Primary funding was provided by the US Office of Naval Research under grant N00014-20-1-2517. Partial support for long-term seawater durability measurements was provided by the Stanford Doerr School of Sustainability Accelerator. Partial support for ICP-MS measurements and COR three-electrode measurements was provided by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science Program through the
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