Combining theory and experiment in electrocatalysis: Insights into materials design

TLDR: A unified theoretical framework highlights the need for catalyst design strategies that selectively stabilize distinct reaction intermediates relative to each other, and opens up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions.

Abstract: BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.

Figures

Figure 7. Emerging reactions of interest. (A) Volcano plot for hydrogen peroxide production on metals and alloys. Reproduced with permission from (149). (B) Volcano plot for carbon dioxide reduction on metals. Reproduced with permission from (154, 155). (C) Volcano plot for nitrogen reduction on metals, with that of HER overlaid for comparison. Reproduced with permission from (170).

Figure 3. Hydrogen evolution reaction. (A) HER volcano plot for metals and MoS2. Reproduced with permission from (20, 32). (B) TOFavg plots with linear sweep voltammograms of various HER catalysts. Data obtained from (10, 32, 37, 51, 52, 59). State-of-the-art Ni-based homogeneous catalysts are also included for comparison (67-69). (C) Chronological trend in overpotential of MoS2 based and phosphide HER catalysts. Data obtained from (10, 50-59) and references within. (D) Representative microscopy images of HER catalysts. Reproduced with permission from (10, 32-34, 36, 39, 42, 49, 50).

Figure 4. Oxygen reduction reaction. (A) ORR volcano plot for metals. Reproduced with permission from (71). (B) ORR theoretical limiting potential plot for fcc (111) and (100) facets of metals and alloys. Reproduced with permission from (73). (C) Chronological trend in overpotential of Pt-based ORR catalysts. Data obtained from (74, 82-92, 98, 104) and references within. (D) Representative microscopy images of ORR catalysts. Reproduced with permission from (79, 82, 83, 103, 104).

Figure 5. Oxygen evolution reaction. (A) OER volcano plot for metal oxides. Experimental/theoretical data obtained from (118)/(126) for CoOx-(a), NiCoOx, CoFeOx, (118)/(123) for NiOx, NiFeOx, (129)/(130) for LaMO3 perovskites (applied +0.21 eV reverse solvation correction to ΔGO – ΔGOH for consistency with other data), (113)/(117) for IrO2(110), (113)/(119) for IrO2(100), (107)/(109) for PtO2 (calculation assumes beta-(CaCl2) phase, which is more stable than the rutile phase according to (108)), (119) for IrOx/SrIrO3 (calculation assumes IrO3/SrIrO3 active site), and (131) for FeCoW (calculation assumes FeW-doped CoOOH as active site). Current densities (extrapolated at constant Tafel slope where necessary) were normalized to surface area by capacitance in (118) and by Brunauer-Emmett-Teller measurements and scanning electron microscopy in (129, 131). IrO2 and IrOx/SrIrO3 films were found to be flat by atomic force microscopy. PtO2 (10-9 mol cm-2 loading) and TiO2 (1.2 nm thick film) were assumed to be flat. The volcano itself corresponds to ΔGOOH = ΔGOH + 3.2 eV (109). (B) Chronological trend in overpotential of various OER catalysts in acid and alkali. Data obtained from (65, 105, 114-116, 118, 119, 122, 131) and references within. Points marked with * are normalized to oxide area.

Figure 1. Sustainable energy future. Schematic of a sustainable energy landscape for the future based on electrocatalysis.

Figure 6. Microkinetic modeling for oxygen reduction reaction. (A) Free energy diagrams for O2 reduction to H2O and H2O2 on Pt(111) at 0.9 V vs. RHE, showing the pathway for reduction to H2O2 and the dominating pathway to H2O proceeding through electrochemical reduction of OOH∗A. Reproduced with permission from (136). (B) Simulated polarization curve and kinetic current density on Pt(111) at 1600 rpm rotation speed. Experimental polarization curves on Pt(111) at room temperature in 0.1 M HClO4 at a rotation speed of 1600 rpm are shown for comparison (141, 142). The inset shows Tafel plot with a 59 mV/dec slope indicated. Reproduced with permission from (136). (C) Simulated kinetic volcano at 0.9 V vs. RHE compared to the potential limiting volcano and experiments on (111) facets, with experimental labels for Cu/Pt(111), Pt(111), Pt3Ni(111) and Pd(111). Reproduced with permission from (136).

Full text

General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright
owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
 You may not further distribute the material or use it for any profit-making activity or commercial gain
 You may freely distribute the URL identifying the publication in the public portal
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from orbit.dtu.dk on: Aug 10, 2022
Combining theory and experiment in electrocatalysis: Insights into materials design
Seh, Zhi Wei; Kibsgaard, Jakob; Dickens, Colin F.; Chorkendorff, Ib; Nørskov, Jens K.; Jaramillo,
Thomas F.
Published in:
Science
Link to article, DOI:
10.1126/science.aad4998
Publication date:
2017
Document Version
Peer reviewed version
Link back to DTU Orbit
Citation (APA):
Seh, Z. W., Kibsgaard, J., Dickens, C. F., Chorkendorff, I., Nørskov, J. K., & Jaramillo, T. F. (2017). Combining
theory and experiment in electrocatalysis: Insights into materials design. Science, 355(6321), [eaad4998].
https://doi.org/10.1126/science.aad4998

1
Combining Theory and Experiment in Electrocatalysis:
Insights into Materials Design
Zhi Wei Seh
1,2,3
, Jakob Kibsgaard
1,2,4
, Colin F. Dickens
1,2
, Ib Chorkendorff
4
, Jens K. Nørskov
1,2
,
Thomas F. Jaramillo
1,2*
Abstract
Electrocatalysis plays a central role in clean energy conversion, enabling a number of sustainable
processes for future technologies. This review discusses design strategies for state-of-the-art
heterogeneous electrocatalysts and associated materials for several different electrochemical
transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize
catalyst performance. By examining the common principles that govern catalysis for different
electrochemical reactions, we describe a systematic framework that helps to understand trends in
catalyzing these reactions, serving as a guide to new catalyst development, while highlighting
key gaps that need to be addressed. We conclude by extending this framework to emerging clean
energy reactions including hydrogen peroxide production, carbon dioxide reduction and nitrogen
reduction, where the development of improved catalysts could allow for the sustainable
production of a broad range of fuels and chemicals.
______________________________________________________________________________
1
SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering, Stanford
University, Stanford, CA 94305, USA
2
SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, Menlo
Park, CA 94025, USA
3
Institute of Materials Research and Engineering, Agency for Science, Technology and Research
(A*STAR), 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore
4
Department of Physics, Technical University of Denmark, Kongens Lyngby, DK-2800, Denmark
*To whom correspondence should be addressed: jaramillo@stanford.edu

2
Creating a global-scale sustainable energy system for the future while preserving our
environment is one of the most crucial challenges facing humanity today (1-3). According to the
International Energy Agency, our global energy demand reached 18 TW in 2013, the vast
majority (~ 80%) of which was derived from fossil resources (coal, oil and gas) (4). With a
growing world population and expanding industrialization, the global energy demand is
projected to further increase from 18 TW in 2013 to 24 or 26 TW in 2040 under the “new
policies” or “current policies” scenario respectively, with a corresponding rise in carbon dioxide
emissions from 32 Gt yr
-1
in 2013 to 37 or 44 Gt yr
-1
in 2040 (4). As a result, major concerns
have been raised over the energy supply, particularly in regards to climate change associated
with the use of fossil fuels. Thus, a serious impetus exists to diversify our energy sources,
reducing our reliance on fossil fuels by turning to renewable energy such as solar, wind and
hydroelectric power.
Greater penetration of renewables into the electricity sector is important, as it accounts for 12%
of global energy demand (2.1 out of 17.6 TW in 2010) (5). Other key energy sectors where
developing sustainable pathways is needed include transportation and the chemical industry. In
2010, transportation accounted for 19% (3.3 TW) of global energy (5). Although approximately
43% (1.4 TW) of transportation energy demand involved light-duty vehicles where
electrification is already playing a role to help decarbonize the system, the remaining 57% (1.9
TW) was used for commercial transportation where electrification is much more challenging,
e.g. heavy-duty vehicles, marine, aviation and rail (5). Projections indicate that energy demand
for light-duty transportation will likely remain relatively flat in the coming decades; however
energy use for commercial transportation will grow by approximately two-thirds between 2010
and 2040, from 1.9 to 3.2 TW (5). This is a strong motivation for the development of sustainable
pathways to chemical fuels that are a more natural fit for this sector. Similarly, the current energy
demand for the production of industrial chemicals in 2010 was 8% (1.5 TW) of global energy,
almost all of which was derived from fossil fuels (5). Energy use in the chemical industry is also
expected to rise by about two-thirds between 2010 and 2040, to the 2.5 TW needed to produce
the products demanded world-wide such as plastics and fertilizers (5). A sustainable, fossil-free
path to produce industrial chemicals of global importance, such as hydrogen (50 Mt yr
-1
),
hydrogen peroxide (2.2 Mt yr
-1
), ethylene (115 Mt yr
-1
), propylene (73 Mt yr
-1
), methanol (40 Mt
yr
-1
), ammonia (175 Mt yr
-1
), among many others, could play a substantial role in reducing
carbon dioxide emissions while providing the chemicals needed to make the products used
globally on a daily basis (6-8).
Fig. 1 shows possible sustainable pathways for the production of important fuels and chemicals,
including hydrogen, hydrocarbons, oxygenates and ammonia, by either replacing or working in
concert with conventional energy production. The Earth’s atmosphere provides a universal
feedstock of water, carbon dioxide and nitrogen which can potentially be converted into the
aforementioned products via electrochemical processes coupled to renewable energy, if
electrocatalysts with the required properties can be developed. For instance, the water splitting

3
reaction, which consists of the hydrogen and oxygen evolution half-reactions, has attracted great
attention as a sustainable source of hydrogen (9, 10). Hydrogen is an attractive energy carrier
which can be used to produce clean electricity in fuel cells, where the hydrogen oxidation and
oxygen reduction reactions convert chemical energy into electrical energy (11, 12). Hydrogen
peroxide, an essential chemical in the pulp-/paper-bleaching and water treatment industries, can
potentially be derived from the oxygen reduction reaction as well (13). Carbon dioxide captured
from the atmosphere or directly from point sources could become a feedstock for fuels, fine
chemicals and precursors to polymers and plastics via preliminary electroreduction (14).
Likewise the electroreduction of nitrogen to ammonia would allow for the production of
fertilizers sustainably and locally at the point of application and at the required concentration,
eliminating distribution costs stemming from the inflexibly large-scale, centralized Haber-Bosch
process and preventing environmental hazards associated with runoff (15). Crucial to enabling
this vision is the development of improved electrocatalysts with the appropriate efficiency and
selectivity for the chemical transformations involved.
There are generally two strategies to improve the activity (or reaction rate) of an electrocatalyst
system: (i) increasing the number of active sites on a given electrode (e.g. through increased
loading or improved catalyst structuring to expose more active sites per gram), or (ii) increasing
the intrinsic activity of each active site (10). These strategies (Fig. 2) are not mutually exclusive
and can ideally be addressed simultaneously, leading to the greatest improvements in activity. At
the same time, there are physical limits to how much catalyst material can be loaded onto an
electrode without affecting other important processes such as charge and mass transport (10). For
this reason, Fig. 2 shows a plateau effect observed in practice at high catalyst loadings. On the
other hand, increasing intrinsic activity leads to direct increases in electrode activity in a manner
that mitigates transport issues arising from high catalyst loadings; with improved intrinsic
activity, the catalyst loading can be decreased, which also saves on catalyst costs. Moreover,
catalyst activity is measured across many orders of magnitude; the difference between a good
catalyst and a poor catalyst can be more than 10 orders of magnitude apart, whereas the
difference between a high loading and a low loading catalyst might only be 1 to 3 orders of
magnitude (10).
The field of electrocatalysis has seen much progress in recent years, as evidenced by the rapidly
increasing number of publications on this subject. This review aims to focus on several
quintessential case studies of electrocatalysis for different energy conversion reactions,
surveying state-of-the-art catalyst materials, using theory as a means to rationalize trends in
performance. By examining multiple reactions involving water, hydrogen, and oxygen, we
describe a framework that helps to understand broader trends in electrocatalysis for clean energy
conversion.
We begin by presenting theoretical results with a focus on understanding catalytic trends using a
descriptor-based approach: a framework that aims to establish a select few, key properties of a
catalyst surface that are necessary but possibly not sufficient for high activity. We describe how

4
this relatively fast, simple, and straightforward approach has been implemented successfully in
recent years to develop advanced catalysts. The next major step would be to extend the modeling
capabilities to capture greater complexities regarding the catalyst and the electrode-electrolyte
interface in a manner that does not require excessive time and resources. Developing modeling
approaches that use minimal resources to rapidly and accurately predict reaction mechanisms and
rate data across a broad range of catalyst materials and reaction conditions represents an
important aim for future work. The same holds true for the development of more advanced
experimental methods that are capable of providing atomic- and molecular-scale depictions of
the electrode-electrolyte interface under operating conditions. At this point, we can provide a
description of current theoretical approaches to further these types of insights (e.g. on reaction
rates and mechanisms) with more detailed calculations performed using microkinetic models.
The combination of the descriptor-based approach to cover a broad set of systems coupled to
detailed studies of single systems has proven fruitful. Future efforts to advance both theory and
experiment will allow for a more detailed picture of catalysis on surfaces.
Hydrogen evolution/oxidation reactions
Active catalysts are required to minimize the overpotential necessary to drive the hydrogen
evolution reaction (HER; 2H
+
+ 2e
−
→ H
2
) (9, 10). The HER is a classic example of a 2-electron
transfer reaction with 1 catalytic intermediate, H
*
, where * denotes a site on the electrode
surface, and may occur through either the Volmer−Heyrovsky or the Volmer−Tafel mechanism
(9, 10).
Volmer step: H
+
+ e
-
+ * → H* (1)
Heyrovsky step: H* + H
+
+ e
-
→ H
2
+ *
(2)
Tafel step: 2H* → H
2
+ 2* (3)
The rate of the overall reaction is largely determined by the hydrogen adsorption free energy,
ΔG
H
(16, 17). If hydrogen binds to the surface too weakly, the adsorption (Volmer) step will limit
the overall reaction rate, whereas if the binding is too strong, the desorption (Heyrovsky/Tafel)
step will limit the rate. Thus, a necessary but insufficient condition for an active HER catalyst is
ΔG
H
≈ 0 (16, 17). In plotting experimentally-measured exchange current densities for a wide
range of catalyst materials against ΔG
H
at the appropriate coverage calculated from density
functional theory (DFT), a volcano relationship emerges, a quantitative illustration of the so-
called Sabatier principle (Fig. 3A) (18-20). An active catalyst binds reaction intermediate(s)
neither too strongly nor too weakly. Understanding how to control binding energies of reactive
intermediates on a surface is the key to designing materials with improved performance. The
volcano shown in Fig. 3A is the first of several described in this work, each representing a

Frequently asked questions

Q1. What is the key to designing materials with improved performance?

Understanding how to control binding energies of reactive intermediates on a surface is the key to designing materials with improved performance. 

Q2. What are the contributions mentioned in the paper "Combining theory and experiment in electrocatalysis: insights into materials design" ?

This review discusses design strategies for state-of-the-art heterogeneous electrocatalysts and associated materials for several different electrochemical transformations involving water, hydrogen, and oxygen, using theory as a means to rationalize catalyst performance. By examining the common principles that govern catalysis for different electrochemical reactions, the authors describe a systematic framework that helps to understand trends in catalyzing these reactions, serving as a guide to new catalyst development, while highlighting key gaps that need to be addressed. The authors conclude by extending this framework to emerging clean energy reactions including hydrogen peroxide production, carbon dioxide reduction and nitrogen reduction, where the development of improved catalysts could allow for the sustainable production of a broad range of fuels and chemicals. 

Q3. What metals were found to lie near the top of the volcano plot?

Metals such as Ru, Rh, Mo and Fe were calculated to lie near the top of the volcano plot, binding nitrogen neither too strongly nor weakly (169). 

Q4. What is the way to assess catalyst durability?

Accelerated cyclic voltammetry tests, long-term stability studies that quantify the amount of catalyst leached into the electrolyte, and the use of thin film catalyst morphologies are helpful approaches to assess catalyst durability (10, 66). 

Q5. What is the main metric for catalyst activity?

In addition to catalyst activity, long-term stability is an equally important metric and should be reported in conjunction with activity. 

Q6. What is the way to engineer catalyst activity?

Lithium intercalation leads to chemical exfoliation of MoS2 and a phase transition from the 2H semiconducting polymorph to the 1T metallic polymorph, another means to engineer catalyst activity (Fig. 2, 3D). 

Q7. What is the way to prepare amorphous molybdenum sul?

Amorphous molybdenum sulfides can be prepared using electrodeposition (42) or wet chemical synthesis (43) without any thermal treatment, which makes them attractive for certain applications where avoiding high temperature sulfidation is desired, e.g. in the fabrication of photoelectrochemical devices. 

Q8. What is the key insight that the MoS2 edges are active?

The combination of theoretical andexperimental studies provided the key insight that only the MoS2 edges are active, motivating the development of MoS2 catalysts with a significant fraction of exposed edge sites. 

Q9. Why does the ORR volcano plot have a non-ideal overpotential?

Due to this non-ideal scaling between OOH* and OH*, even a catalyst calculated to be at the top of the ORR volcano plot with optimal ΔEO will have a non-zero theoretical overpotential of 0.3 to 0.4 V (71-73). 

Q10. What is the important factor in the development of catalysts with high TOFavg?

Homogeneous catalysts with high TOFavg have also been developed, though they typically require large overpotentials to reach appreciable current densities (Fig. 3B) (67-69). 

Q11. Why is Pt the pure metal catalyst for both the HER and HOR in acid?

As discussed in the preceding section, Pt is the best pure metal catalyst for both the HER and HOR in acid, essentially due to microscopic reversibility: both reactions involve the same steps, except in reverse. 

Q12. What is the likely reaction to be a weaker OOH*?

On the other hand, in the case of weak OOH* binding, the 2- and 4- electron volcano plots overlap each other, which indicates a compromise in activity for hydrogen peroxide selectivity with weaker OOH* binding (13, 149). 

Q13. What is the theoretical framework for a low overpotential ORR/OER catalyst?

O2 + * + H+ + e- → OOH* (12)OOH* + H+ + e- → H2O2 + * (13)As such, it is possible to find a catalyst with zero theoretical overpotential that has an optimal ΔGOOH, binding OOH* neither too strongly nor weakly (13).