Editorial: Advanced water splitting technologies development: Best practices and protocols

Editorial: Advanced water splitting

technologies development: Best

practices and protocols

Brendan Bul

, Marcelo Carmo

, Roel Van de Krol

Julie Mougin

, Kathy Ayers

*, Karl J. Gross

, Olga A. Marina

George M. Roberts

, Ellen B. Stechel

* and Chengxiang Xiang

Department of Mechanical and Process Engineering, ETH Zurich, Zurich, Switzerland,

Nel Hydrogen,

Wallingford, CT, United States,

Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und

Energie, Berlin, Germany,

University Grenoble Alpes, CEA, Grenoble, France,

Technology Consulting,

Alamo, CA, United States,

Paci

fi

c Northwest National Laboratory, Richland, WA, United States,

ASU

LightWorks

and the School of Molecular Sciences, Arizona State University, Tempe, AZ, United States,

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA,

United States

KEYWORDS

hydrogen, water splitting, benchmarking, low temperature electrolysis, high temperature

electrolysis, photoelectrochemical, solar thermochemical water splitting, protocol

Editorial on the Research Topic

Advanced water splitting technologies development: Best practices and

protocols

As the level of deployment and utilization of renewable energy sources, including wind

and solar, continues to rise, large-scale, long-term energy storage technologies that could

accommodate weekly and seasonal energy

fl

uctuations will play a signi

fi

cant role in the

overall deployment of renewable energies in the future. Harnessing and storing renewable

energy resources

via

electrochemical, photoelectrochemical, or thermochemical processes by

converting renewable energy into sustainable (energy storage) fuels have the potential to

meet the long-term, terawatt scale energy storage challenge. Renewable hydrogen production

is the cornerstone for sustainable fuel production and deep decarbonization of multiple

sectors in our society. Cost-competitive clean hydrogen provides value to applications, such

as 1) in the transportation sector for fuel cell vehicles, 2) in the electric grid sector for system

stability and load balancing, and 3) in the industrial sector with metal re

fi

neries, cement

production, and biomass upgrading (carbon-free fertilizer production). In addition, coupling

clean renewable hydrogen with the carbon and nitrogen cycles enables known and well-

established thermal-chemical processes to generate renewable hydrocarbon fuels and

ammonia. The Advanced Water Splitting Technologies (AWST): low temperature

electrolysis (LTE), high temperature electrolysis (HTE), photoelectrochemical (PEC) and

solar thermo-chemical hydrogen (STCH) provide four unique and parallel approaches to

produce low cost, low greenhouse gas (GHG) emission hydrogen at scale (

Figure 1

). Cost

competitive clean hydrogen production using these four technologies is a current high

priority focus for governments and industry. In June of 2022, the U.S. Department of Energy

(DOE) launched the

fi

rst in a series of Earthshot Initiatives. The Hydrogen Shot,

“

111

”

aims

to reduce the cost of clean hydrogen by more than 80% to one dollar per one kilogram in

1 decade ($1/kg H

). The European Green Deal and the International Energy Agency (IEA)

OPEN ACCESS

EDITED AND REVIEWED BY

Fenglong Wang,

Shandong University, China

*CORRESPONDENCE

Kathy Ayers,

kayers@nelhydrogen.com

Olga A. Marina,

Olga.Marina@pnnl.gov

Ellen B. Stechel,

Ellen.Stechel@asu.edu

Chengxiang Xiang,

cxx@caltech.edu

SPECIALTY SECTION

This article was submitted to

Hydrogen Storage and Production,

a section of the journal

Frontiers in Energy Research

RECEIVED

22 January 2023

ACCEPTED

14 February 2023

PUBLISHED

06 March 2023

CITATION

Bul

fi

n B, Carmo M, Van de Krol R,

Mougin J, Ayers K, Gross KJ, Marina OA,

Roberts GM, Stechel EB and Xiang C

(2023), Editorial: Advanced water splitting

technologies development: Best

practices and protocols.

Front.EnergyRes.

11:1149688.

doi: 10.3389/fenrg.2023.1149688

fi

n, Carmo, Van de Krol,

Mougin, Ayers, Gross, Marina, Roberts,

Stechel and Xiang. This is an open-access

article distributed under the terms of the

Creative Commons Attribution License

(CC BY)

. The use, distribution or

reproduction in other forums is

permitted, provided the original author(s)

and the copyright owner(s) are credited

and that the original publication in this

journal is cited, in accordance with

accepted academic practice. No use,

distribution or reproduction is permitted

which does not comply with these terms.

Frontiers in

Energy Research

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TYPE

Editorial

PUBLISHED

06 March 2023

DOI

10.3389/fenrg.2023.1149688

have implemented a strong focus on green hydrogen production for

a clean and secure energy future.

In LTE, an electrochemical process produces hydrogen gas from

water. H

is produced at the cathode and O

at the anode

electrochemically under a voltage bias, typically operating

between 50 and 80

C. In commercial systems, either a membrane

or a porous fabric-like separator separates the cathode from the

anode. Among various LTE con

fi

gurations, three types of LTE

systems include alkaline, proton exchange membrane (PEM)

based, and hydroxide exchange membrane based water

electrolysis. These con

fi

gurations are the most utilized systems.

Alkaline and PEM electrolyzers have been deployed commercially

at multi-megawatt scale (thousands to tens of thousands of metric

tons per year H

production capacity for the largest projects) while

hydroxide membrane electrolyzers are mostly pre-commercial.

The operating principle of HTE is similar to LTE. The HTE cells

include a cathode for water reduction and hydrogen production, an

anode for oxygen generation, and a solid ceramic electrolyte for

selective transport of oxygen or proton ions at elevated temperatures

of 400

–

850

C. HTE systems operate at low voltages of

≤

1.3 V and

current densities of 1

–

1.5 A cm

-2

and can achieve 90%

–

95% stack

electrical ef

fi

ciency. Current SOEC technology is pre-commercial;

the largest demonstrated scale for HTE is 0.72 MW projected to

increase to a 2.6 MW demo plant in 2023, which corresponds to a

generation rate of ~ 525 metric tons per year.

While LTE and HTE use renewable electrons from solar, wind,

and/or hydroelectric power (or carbon free electrons from nuclear)

to generate clean hydrogen, PEC and STCH produce clean H

directly from sunlight. PEC water-splitting integrates light

absorption, photo-generated carrier transport, electrocatalysis,

ionic transport, and product separation in an integrated

photoelectrochemical device for hydrogen generation, (water plus

sunshine-in and renewable-hydrogen-out solar panel). In a more in-

depth perspective: PEC devices often operate at much lower current

densities compared with LTE or HTE to match the solar

fl

ux, while

high current density PEC devices with an operating current density

close to 1 A cm

-2

have been demonstrated in conjunction with solar

concentrators. A portfolio of PEC devices have achieved a solar to

hydrogen conversion ef

fi

ciency of

>

10% with an overall device

stability ranging from tens of hours to hundreds of hours. For

the moment, PEC devices have only been demonstrated at a

laboratory scale

<<

1 kg/day H

In contrast, solar thermochemical hydrogen (STCH) cycles use

the heat from sunlight to produce hydrogen and oxygen from water.

A popular STCH pathway uses two-step redox active metal oxide

(MO

) thermochemical cycles to produce H

and O

sequentially in

two different chemical reactions. STCH has been demonstrated

−

1 kg/day. In the two-step cycle, a redox-active MO

fi

rst

heated, generally using concentrated solar radiation, to temperatures

typically exceeding 1500 K and often close to 1800 K at a low partial

pressure of oxygen (pO

), at which point the material becomes

reduced to a more O-poor metal oxide. In the second step, the

reduced metal oxide cools to a temperature where re-oxidation is

favorable when exposed to superheated H

O vapor (aka steam),

which leads to water splitting and regeneration of the original MO

Off-stoichiometric metal oxides form and

fi

ll oxygen vacancies

during the thermal reduction and water splitting (or re-

oxidation) steps, respectively, without undergoing major

structural bulk phase transitions, thus promoting faster kinetics,

cyclability, and durability.

It is important to understand that LTE, HTE, PEC, and STCH

water splitting technologies have different technical readiness levels

and face different technical challenges. However, a common and

absolutely vital need in each and all of these (and many other) basic

research to commercial development efforts, is a concerted effort to

come together and produce a path for the most trust-worthy,

reliable, and reproducible results. This coordination will lead

ultimately to the most rapid development of these life-changing

bene

fi

cial technologies.

In this light, Benchmarking including the developing and

documenting the best practice procedures and protocols are vital

for creating an advanced R&D foundation for the broader research

community in all four pathways and, potentially, in the R&D

community in general. The development of standard protocols

that integrate and harmonize independently funded work across

the world is vital for accelerating the materials development as well

as for the commercialization of each technology. This statement is

particularly true given the enormous new interest in clean or green

hydrogen. There exists at this moment a critical need for consistency

in testing protocols, reference materials, and standard testing cells

within and across all four technological pathways. To present a clear

perspective, standard protocol development and the use of reference

materials and cells for testing at the materials level, component level,

and device level have proved to be critical in the development of

similar technologies including fuel cells, advanced batteries,

etc.

Standard protocols, when successfully established and utilized

within and across communities and technologies, can

signi

fi

cantly reduce inconsistencies in results from different

research groups due to different and varying testing procedures.

The incorporation and adherence to a set of standardized protocols

FIGURE 1

Advanced water splitting technologies for renewable hydrogen

production.

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enables true cross comparison among newly discovered or

developed materials and components. Standard protocols can

also quantitatively gauge the progress made in each technology

and are a vital part of developing an overall roadmap designed to

achieve high level $/kg H

goals that are desperately needed.

Through this collaborative Benchmarking, Protocol, Best

Practices, and Road mapping project, an improved ef

fi

ciency,

reliability, and most effective pathway towards solving serious

future energy issues has taken form and is being implemented.

Critical for this effort was bringing the four technologies together to

effectively facilitate cross-cutting opportunities. Low TRL

technologies with a current emphasis on materials level

development can learn scale up strategies, balance of system

designs, and methods of estimating cost from higher TRL

technologies, while high TRL technologies with the current

emphasis on the system level development can bene

fi

t from

fundamental materials knowledge gained in low TRL technologies.

To provide a speci

fi

c example of the bene

fi

ts and success in these

efforts; the standard protocols that benchmark membrane properties,

such as water content, gas permeability, and ion exchange capacity in

LTE, can be adapted by the PEC community to include additional

considerations on stability under illumination and conductivity for

cations and anions other than proton or hydroxide for near-neutral

pH operations. Similarly, HTE is learning from LTE about cell and

system scale-up, testing on comparable cell and device levels, and

corresponding testing protocols.

At the same time, through this widening of communications and

support, expertise in PEC has opened new possibilities for LTE. For

example, earth abundant electro-catalytic materials development in

the PEC community is largely translatable to LTE and can in

principle provide options for non-platinum group metal (PGM)

catalysts for incorporation in LTE. There is a lot of similarities in

materials requirements in HTE and STCH. Examples of common

R&D interests in system level considerations for PEC and STCH

include sunlight spectrum utilization, overall solar to hydrogen

conversion ef

fi

ciency, operational conditions, and constraints of

each system under sunlight, can cross-fertilize advances in both

technologies. Cross-communication, collaboration, and unilateral

support of R&D advancements towards a clean energy future is vital

for all our futures. This project and this publication platform provide

an opportunity to collaborate and collectively improve the

community

’

s R&D efforts to ensure that we are all working most

effectively towards a sustainable clean energy future.

Research Topic in Frontiers in Energy: This on-line, peer

reviewed journal, provides open access to rapidly developing

standardized testing protocols for the AWST R&D community.

In this issue, authors offer a collection of standard protocols in the

fi

elds of LTE, HTE, PEC, and STCH advanced water-splitting

technologies for clean hydrogen production. To track and report

on progress and to set global priorities for advanced water splitting

hydrogen research, this issue is a step intended to address the need to

gain consistency within and across individual technology pathways

so that researchers can reliably evaluate and compare the potential

for each pathway. In addition, time is not our friend, and the lack of

consistency and agreed upon benchmarks, protocols, standards, or

roadmaps creates a large activation barrier for entry, for

communicating to decision makers, and for general outreach.

In the interest of lowering the ba

rriers and disseminating best

practices in characterizing and benc

hmarking advanced water splitting

materials, creating a foundation in a

ccelerated materials, device, and

systems research, development and deployment for the broader

research community, this research topic asked the community for

articles that describe comparisons, ma

terials screening, characterization

protocols, benchmarks

, techno-economics, system analyses, and

roadmaps for any and all advanced water splitting pathways, in

which the primary energy is renew

able (or at least carbon-free).

Current LTE protocols focus on

ex-situ

material screening

methods as a

fi

rst step in any new exploration to ensure that

minimum criteria are met before investing additional time and

resources in more complex tests. Some of these measurements are

very technique dependent and require careful methods to obtain

accurate results, such as catalyst activity

via

rotating disk electrodes.

Other property measurements include ion exchange capacity, and

oxidative stability of anion exchange membranes, and physical

properties of porous transport layers. A

fi

nal protocol addresses

standard teardown of full cells for analysis. These protocols form a

framework to build from for in cell component testing and durability.

PEC currently has

fi

ve topical protocol reports that include

benchmarking solar-to-hydrogen conversion ef

fi

ciency of

photoelectrodes, measurements of ion exchange membranes for

solar fuel applications, comprehensive evaluation of optical,

electrical, photoelectrochemical and spectroscopic properties of

protective coatings, incident photo-to-current ef

fi

ciency

measurements and long term photoelectrode stability

measurement protocols for PEC water splitting.

Presented HTE protocols focus on methods of solid oxide cell

materials properties

’

characterization, such as measuring sample

density, determining oxide electrical properties, separating the

contributions of different charge carriers to the total conductivity

as well as describing effective ways of solid oxide cell sealing,

operating, and leak testing. In addition, one of the protocols

compares several different steam generators to ensure stable and

reliable steam supply to HTE, which is vital to ensure a uniform

hydrogen production rate and to report degradations accurately.

STCH protocols in this issue include one paper on performance

indicators (

“

Performance Indicators for Benchmarking Solar

Thermochemical Fuel Processes and Reactors.

”

) The solar reactor

for operationalizing STCH is the key component and the

performance of that reactor can be the deciding factor in

assessing technical and economic feasibility. Important indicators

discussed in this paper are conversion, selectivity, ef

fi

ciency, and

stability. The issue also includes a paper on determining the off-

stoichiometry with temperature and partial pressure of oxygen (

“

Thermogravimetric Temperature-Programmed Thermal Redox

Protocol for Rapid Screening of Metal Oxides for Solar

Thermochemical Hydrogen Production,

”

) The authors show that

temperature-programmed thermal reduction can provide a simple

thermogravimetric analysis-based single-run experiment that

measures the redox behavior of a specimen under thermal

reduction and reoxidation conditions relevant to STCH. Lastly,

the issue includes a paper on synchrotron-base characterization

(

“

Synchrotron-based techniques for characterizing STCH water-

splitting materials.

”

) Synchrotron radiation is a powerful tool for

characterizing STCH materials. X-ray absorption spectroscopy can

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identify those cations that are redox active and the extent of

reduction in quenched conditions.

In summary, this issue contains nineteen articles and ninety

different authors. Additional protocols are still in development and

will be published later. We look forward to extensive international

use of the protocols presented in this Research Topic and new ones

as they become available. We also highly encourage participation by

the wider community in improving and giving feedback on the

current protocols and the continued creation of advanced protocols

for AWST materials, component, and systems research and

development.

Author contributions

KA, OM, ES, and CX contributed to the manuscript equally.

Con

ict of interest

Authors MC, KA, and GR are employed by Nel Hydrogen. KJG

was employed by H

Technology Consulting.

The remaining authors declares that the research was conducted

in the absence of any commercial or

fi

nancial relationships that

could be construed as a potential con

fl

ict of interest.

Publisher

’

s note

All claims expressed in this articl

e are solely those of the authors

and do not necessarily represent those of their af

fi

liated organizations,

or those of the publisher, the editor

s and the reviewers. Any product

that may be evaluated in this article, or claim that may be made by its

manufacturer, is not guaranteed

or endorsed by the publisher.

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