materials

Review
Nanocomposite Electrocatalysts for Hydrogen Evolution
Reactions (HERs) for Sustainable and Efficient Hydrogen
Energy—Future Prospects
Ahmed Hussain Jawhari and Nazim Hasan *

Department of Chemistry, Faculty of Science, Jazan University, Jazan 45142, Saudi Arabia;
ahjawhari@jazanu.edu.sa
* Correspondence: hhasan@jazanu.edu.sa

Abstract: Hydrogen is considered a good clean and renewable energy substitute for fossil fuels. The
major obstacle facing hydrogen energy is its efficacy in meeting its commercial-scale demand. One of
the most promising pathways for efficient hydrogen production is through water-splitting electrolysis.
This requires the development of active, stable, and low-cost catalysts or electrocatalysts to achieve
optimized electrocatalytic hydrogen production from water splitting. The objective of this review is
to survey the activity, stability, and efficiency of various electrocatalysts involved in water splitting.
The status quo of noble-metal- and non-noble-metal-based nano-electrocatalysts has been specifically
discussed. Various composites and nanocomposite electrocatalysts that have significantly impacted
electrocatalytic HERs have been discussed. New strategies and insights in exploring nanocomposite-
based electrocatalysts and utilizing other new age nanomaterial options that will profoundly enhance
the electrocatalytic activity and stability of HERs have been highlighted. Recommendations on future
directions and deliberations for extrapolating information have been projected.

Keywords: hydrogen production; metal electrocatalysts; hydrogen evolution reactions; nanocomposites

Citation: Jawhari, A.H.; Hasan, N.

1. Introduction
Nanocomposite Electrocatalysts for Sustainable human growth is centered around energy and the environment [1]. Oil,
Hydrogen Evolution Reactions coal, and natural gas still supply over 85% of the world’s energy. Chemical industries use
(HERs) for Sustainable and Efficient fossil fuels. These non-renewable fuels may deplete quickly if burned. Fossil fuel combus-
Hydrogen Energy—Future Prospects. tion releases greenhouse gases such as CO2 , NO2 , and SOx , which harm the environment.
Materials 2023, 16, 3760. https:// Renewable carbon-neutral energy should replace fossil energy [2]. Hydrogen energy is
doi.org/10.3390/ma16103760 a possible replacement for fossil fuels due to its advantages, such as abundance, high
Academic Editor: Hasi Rani Barai gravimetric energy density, and carbon-free emission [3,4]. In addition, 95% hydrogen gas
is procured from fossil fuels, which causes serious environmental issues. Water electrolysis
Received: 27 March 2023 using renewable energy sources such as sun or wind can produce ecofriendly hydrogen
Revised: 8 May 2023
for the hydrogen economy. Only solar energy can replace fossil fuels. Sunlight is diffusive,
Accepted: 10 May 2023
which is why energy capture and storage are considered important [3].
Published: 16 May 2023
Photovoltaic (PV) cells convert solar energy into electricity; however, expensive energy
storage devices such as batteries are requited for storage and distribution purposes. As
an alternative is this, solar energy is converted to chemical energy, such as H2 . H2 is easy
Copyright: © 2023 by the authors.
to store and transport, has a high energy density (approx. 140 MJ/Kg at 700 atm), and
Licensee MDPI, Basel, Switzerland. emits no carbon (the only combustion product of H2 is water) [4]. H2 is both a chemical
This article is an open access article feedstock and a fuel for hydrogen fuel cells [5]. PV-based electricity-driven water splitting is
distributed under the terms and promising for H2 production, especially because PV electricity is cheaper (0.15 USD kWh@1
conditions of the Creative Commons (data from LONGi Silicon Materials Corp., Xi’an, China)). Renewable H2 production may
Attribution (CC BY) license (https:// be best achieved by coupling solar PV electricity with a water electrolyzer (PV-E) [6].
creativecommons.org/licenses/by/ Current water electrolysis technology includes: (1) proton exchange membrane (PEM)
4.0/). electrolysis; (2) alkaline electrolysis; and (3) high-temperature solid oxide water electrolysis.

Materials 2023, 16, 3760. https://doi.org/10.3390/ma16103760 https://www.mdpi.com/journal/materials

Materials 2023, 16, x FOR PEER REVIEW 2 of 16

production may be best achieved by coupling solar PV electricity with a water electrolyzer
Materials 2023, 16, 3760
(PV-E) [6]. 2 of 16
Current water electrolysis technology includes: (1) proton exchange membrane
(PEM) electrolysis; (2) alkaline electrolysis; and (3) high-temperature solid oxide water
electrolysis. PEM-based electrolysis cells split water in acidic conditions. Lower gas per-
PEM-based electrolysis cells split water in acidic conditions. Lower gas permeability and
meability and strong proton conductivity are advantages of this state. It produces hydro-
strong proton conductivity are advantages of this state. It produces hydrogen quickly and
gen quickly and efficiently [7]. OER electrocatalysts in acidic media are noble metal and
efficiently [7]. OER electrocatalysts in acidic media are noble metal and noble metal oxide
noble metal oxide catalysts. This need makes the cell expensive [8]. Alkaline electrolysis
catalysts. This need makes the cell expensive [8]. Alkaline electrolysis cells divide water.
cells divide water. Alkaline water splitting allows non-noble metals or metal oxides to be
Alkaline water splitting allows non-noble metals or metal oxides to be used as electrocata-
used as electrocatalysts. Alkaline media usually have 2–3 orders of magnitude less HER
lysts. Alkaline media usually have 2–3 orders of magnitude less HER activity than acidic
activity than acidic media [9]. Thus, designing low-cost, high-catalytic-activity, and dura-
media [9]. Thus, designing low-cost, high-catalytic-activity, and durable electrocatalysts
ble electrocatalysts for water splitting (electrocatalytic) in diverse mediums is difficult but
for water splitting (electrocatalytic) in diverse mediums is difficult but has always been
has always been attempted [10–13]. Figure 1 represents the schematic of the photocatalytic
attempted [10–13]. Figure 1 represents the schematic of the photocatalytic water splitting
water splitting method for H2 production.
method for H2 production.

Figure
Figure1.1.Hydrogen
Hydrogenenergy
energyobtained
obtainedby
byphotocatalytic
photocatalyticwater
watersplitting.
splitting.

Waterelectrolysis
Water electrolysishas hastwo
twohalf-cell
half-cellreactions:
reactions:aahydrogen
hydrogenevolution
evolutionreaction
reaction(HER)
(HER)
andan
and anoxygen
oxygenevolution
evolution reaction
reaction (OER).
(OER). The The cathode
cathode reduces
reduces waterwater
to Hto2 and
H2 and the an-
the anode
ode oxidizes
oxidizes it to Oit2.toHigh
O2 . overpotentials,
High overpotentials,
whichwhich
measure measure
kinetickinetic
energyenergy barriers,
barriers, slow OERslow
OER
and HERandreaction
HER reaction rates, making
rates, making water splitting
water splitting impractical.
impractical. OERs and OERs
HERs and HERs on
depend de-
pend on catalysis. To produce H and O , efficient catalysts
catalysis. To produce H2 and O2, efficient catalysts must reduce OER and HER overpoten-
2 2 must reduce OER and
HER overpotentials.
tials.
Nanometal-based composites
Nanometal-based composites have
haveimpacted
impacted various
variousaspects of electrocatalysis;
aspects hence,
of electrocatalysis;
there are huge opportunities and plenty of unexplored aspects.
hence, there are huge opportunities and plenty of unexplored aspects. While platinum While platinum is the
is
most effective catalyst for HERs, its high cost and its limited
the most effective catalyst for HERs, its high cost and its limited abundance hinder theabundance hinder the
widespreaddeployment
widespread deploymentofofPt-based
Pt-basedelectrolysis
electrolysisdevices.
devices.ItItisisininthis
thisdirection
directionthat
thatcom-
com-
posite/nanocomposite electrocatalysts are being considered alternative options in order toto
posite/nanocomposite electrocatalysts are being considered alternative options in order
reducethe
reduce thecost
costas aswell
wellasasenhance
enhancethe
thecatalytic
catalyticproperties.
properties.
The objective of the following review is
The objective of the following review is to briefly to brieflypresent
presentan anoverview
overviewofofthethecurrent
current
status of the metal-based electrocatalysts for HERs, and the advantages and disadvantages
status of the metal-based electrocatalysts for HERs, and the advantages and disadvantages
of these are presented. The options for HER electrocatalysts from noble and non-noble
metals have been listed. The composite/nanocomposite options, both metal-based and non-
metal-based, have been gathered, consolidated, and presented. The nano-metal options
Materials 2023, 16, 3760 3 of 16

for HER electrocatalyst have also been compiled, and the future scope and challenges
are presented.

2. Basic Reactions Involved in HERs

Two half-reactions make up electrochemical water splitting (EWS): the cathodic hy-
drogen evolution reaction (HER) and the anionic oxygen evolution reaction (the HER is
a two-electron transfer reaction that involves the adsorption of H2 O (alkaline solution)
or H2 (acidic solution) species on the cathode (the Volmer step) and the desorption of H2
from the cathode via chemical (the Tafel step) or electrochemical (the Heyrovsky step)
processes. Experimental and computational Tafel slope values reveal the HER mechanism.
The four-electron transfer OER mechanism is more complex and highly dependent on elec-
trocatalysts. The two most widely known OER mechanisms are lattice oxygen involvement
and adsorbate development. The lattice oxygen involvement process hypothesizes that the
oxygen partly comes from the lattice oxygen in metal oxides, while the adsorbate evolution
mechanism suggests that the water in electrolytes generates oxygen molecules [14]. H*, O*,
HO*, and HOO* are important reaction intermediates in HERs and OERs.
The two-electron transfer half-reaction HER produces hydrogen at the cathode in
water electrolysis. The environment strongly affects this HER’s mechanism. Three HER
reaction stages are feasible in acidic media.

H+ + e− = Had , H+ + e− = Had , (1)

H+ + e− + Had = H2 , H+ + e− + Had = H2 , (2)

2Had = H2 . 2Had = H2 . (3)

The first Volmer step (1) produces adsorbed hydrogen. Then, the HER is preceded
by the Heyrovsky step (2) or the Tafel step (3), or both, to produce H2 . In alkaline media,
two possible reaction steps, i.e., the Volmer step (4) and the Heyrovsky step (5), are possible,
as shown in the following equations, respectively:

H2 O + e− = OH− + Had , H2 O + e− = OH− + Had , (4)

H2 O + e− + Had = OH− + H2 . H2 O + e− + Had = OH− + H2 . (5)

Theoretical simulations linked HER activity to hydrogen adsorption (Had ). Hydrogen
evolution materials are usually described by their adsorption of free hydrogen energy
(∆GH). The HER benefits from moderate hydrogen binding energy. Pt is the best HER
catalyst in both media with excellent hydrogen adsorption energy and a high exchange
current density. Alkaline media had lower HER activity than acidic media [9]. The slow
water dissociation phase slows the process by 2–3 orders of magnitude. Industrial plants
prefer alkaline electrolysis. Figure 2 presents an overview of the basic HER.
Materials 2023, 16,
Materials 2023, 16, 3760
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of 16
16

Figure 2. The basic scheme of reaction involved in HERs in alkaline and acidic environments.
Figure 2. The basic scheme of reaction involved in HERs in alkaline and acidic environments.
3. Electrocatalysts in HERs
3. Electrocatalysts in HERs
HER electrocatalysts fall within two categories, either noble- or non-noble-metal-
HER electrocatalysts fall within two categories, either noble- or non-noble-metal-based
based catalysts. Several techniques are being researched to improve the HER and lower
catalysts. Several techniques are being researched to improve the HER and lower the price
the price of noble metal electrocatalysts, especially Pt-based catalysts. For instance, alloy-
of noble metal electrocatalysts, especially Pt-based catalysts. For instance, alloying Pt with
ing Pt with low-cost transition metals could boost Pt consumption and change electrical
low-cost transition metals could boost Pt consumption and change electrical surroundings,
surroundings, thus improving activity. Coupling Pt with other water dissociation promot-
thus improving activity. Coupling Pt with other water dissociation promoters improves
ers improves alkaline HER activities, which is useful in industries. Due to their low cost
alkaline HER activities, which is useful in industries. Due to their low cost and availability,
and availability, non-noble metal HER electrocatalysts have received a lot of attention [15].
non-noble metal HER electrocatalysts have received a lot of attention [15].
3.1. Noble
3.1. Noble Metal Electrocatalysts
Metal Electrocatalysts
Platinum group
Platinum group metals
metals (PGMs),
(PGMs), including
including Pt,Pt, Pd,
Pd, Ru,
Ru, Ir, and Rh,
Ir, and Rh, hold
hold aa reputation
reputation as as
excellent HER catalyzers. Pt tops the volcanic curve. These noble-metal-based
excellent HER catalyzers. Pt tops the volcanic curve. These noble-metal-based catalysts are catalysts
are expensive
expensive and and
hardhard to store,
to store, limiting
limiting theirtheir commercial
commercial use. use. Alloying
Alloying Pt with
Pt with transition
transition met-
metals improves Pt usage changes the electronic microenvironment
als improves Pt usage changes the electronic microenvironment to boost HER electroactivity. to boost HER electro-
activity.variations/combinations
Several Several variations/combinationsof PGMsof PGMs
have been have been
tested fortested for electrocatalyst
electrocatalyst ap-
applications.
plications.
Sun Sun et al. demonstrated
et al. demonstrated the in situ development
the in situ development of an ultrafine-PtNi-nanopar-
of an ultrafine-PtNi-nanoparticle-decorated
ticle-decorated
Ni nanosheet array Ni nanosheet
on a carbon array on a(PtNi-Ni
cloth carbon cloth
NA/CC) (PtNi-Ni
withNA/CC)
ultralow with ultralow
loading load-
Pt (7.7%)
ing Pt
and (7.7%) and
improved HER improved HER
activity (38 mVactivity
in 0.1 M(38 mV at
KOH in 10
0.1mA cm−2 )atcompared
M KOH 10 mA cmto−2)Pt/C
compared
(20%),
to Pt/C
with (20%), with
long-term long-term
durability. The durability.
downshift The
of downshift
Pt’s D-band of center
Pt’s D-band center
decreases thedecreases
adsorp-
the adsorption energy of oxygenated species (OH*) on the
tion energy of oxygenated species (OH*) on the surface Pt atom, which promotes HER surface Pt atom, which pro-
motes HER performance
performance [16]. [16].
For Pt electrocatalysts, HER activity in alkaline media is usually lower than in acidic
media [17]. Inefficient
Inefficientwater
waterdissociation
dissociationononthe thePtPtsurface
surface reduces
reducesHERHER activity. To in-
activity. To
crease alkaline
increase alkalineHER
HERactivity,
activity,PtPtisisusually
usuallycoupled
coupledwith withwater
waterdissociation
dissociationpromoters
promoters [15].
[15].
Controlling the metal composition on the surface of Pt-based HER electrocatalysts [18–20]
Controlling
improves
improves electrocatalytic
electrocatalyticactivity.
activity.Subbharaman
Subbharamanetet al.al.
[21] produced
[21] produced nano
nanoNi(OH) 2 clusters
Ni(OH) 2 clus-

on
tersPtonelectrode surfaces
Pt electrode and showed
surfaces and showed a factor of 8 increase
a factor in HER
of 8 increase activity
in HER compared
activity to Pt.
compared
Ni(OH) ad intermediates
2 cluster
to Pt. Ni(OH) edgesedges
2 cluster dissociate waterwater
dissociate to generate M–HM–H
to generate ad intermediateson Pt,
onand the
Pt, and
adsorbed
the adsorbed hydrogen
hydrogen intermediates
intermediates produce
produceH2H . 2Zhao
. Zhaoetetal.al.created
created surface-engineered
surface-engineered
PtNi–O nanoparticles with an enhanced NiO/PtNi interface via Ni(OH)2 and Pt(111)
Materials 2023, 16, 3760 5 of 16

PtNi–O nanoparticles with an enhanced NiO/PtNi interface via Ni(OH)2 and Pt(111)
synergy. In alkaline environments, this interface structure becomes Ni(OH)2 , forming a
Pt(111)-like interface on the surface. The catalyst has a low HER overpotential of 39.8 mV at
10 mA cm−2 with 5.1 µgPt cm−2 loading of Pt [22]. Doping Pt-based compounds with other
metals can also boost HER catalytic performance, minimizing Pt consumption. N-modified
PtNi nanowire catalysts increase water dissociation kinetics via N-induced orbital tuning,
revealing an ultralow overpotential of 13 mV at 10 mA cm−2 in alkaline environments [14].

3.2. Non-Noble Metal Electrocatalysts

Transition metal carbides (TMCs) are widely used to develop non-noble metal electro-
catalysts. Mo2 C and WC exhibit significant HER catalytic activity. Besides strong electrical
conductivity, their hydrogen adsorption and d-band electronic density state (similar to Pt)
are optimum, which is thought to be a fundamental reason for their high HER activity [23].
Carbon atoms in lattice interstitials provide them with D-band electronic density states
akin to the Pt benchmark [24]. Vrubel and Hu (2012) [25] found good HER catalytic activ-
ity in acidic and alkaline conditions in commercial molybdenum carbide microparticles
(com-Mo2 C) with a cathodic current of 10 mA cm−2 and an overpotential of 90–230 mV.
In an attempt to expose more active areas to optimize Mo2 C catalysts, materials were
nanoengineered. Xiao et al. [26] used the hydrogen carburization of anilinium molybdate
to make porous Mo2 C nanorods; commercial Mo2 C is less active than nanorods. After
2000 cycles, the activity is steady, indicating a good cycling life. In a 1M KOH alkaline
environment, the Mo2 C nanorod outperforms commercial Mo2 C. High-conductivity and
well-defined porous shapes make Mo2 C nanorods competitive for HERs in acidic and
alkaline conditions. Ni nanoparticles could also boost catalytic activity. The HER could
be improved as a hybrid nano-electrocatalyst by depositing molybdenum carbides on
carbon-based materials. Liu et al. [27] grew molybdenum carbide (Mo2 C) nanoparticles in
situ on graphene nanoribbons (GNRs) via hydrothermal synthesis and high-temperature
calcination. In acidic, basic, and neutral conditions, the Mo2 C-GNR combination was highly
electrocatalytic and durable. GNRs as templates for in situ carbide growth are intriguing
because the interconnected GNR network structure forms the platform for multiple con-
ductive pathways for fast electron transportation with an extensive active surface area with
increased active sites, enhancing catalytic activity in all acidic, basic, and neutral media.
Transition metal phosphides (TMPs) make up one of the fastest-growing fields for
producing electrocatalysts with excellent catalytic activity and stability in acid and base
solutions (pH-universal). Due to its high conductivity and unusual electrical structure,
P atoms are of importance with respect to TMP. Ni2 P catalysts for the HER were first
discovered in 2005. Liu and Rodriguez used a density functional theory (DFT) to study
electrocatalysts [28]; their results confirmed that [NiFe] hydrogenase exhibited the highest
activity toward the HER, followed by [Ni(PNP)2 ]2+ > Ni2 P > [Ni(PS3 *)(CO)]1− > Pt > Ni
in a decreasing sequence. Strong bonding increases hydrogen desorption energy from
metal surfaces. Popczun et al. demonstrated that Ni2 P nanoparticles placed onto a Ti foil
substrate with good HER activity had an exchange density of 3.3 × 10−5 mA cm−2 and a
Tafel slope of 46 m V dec−1 [29]. In alkaline electrolytes, the Ni2 P/Ti electrode’s stability
was poor. Hu et al. [30] developed a bimetallic-structured phosphide electrocatalyst, a
NiCo2 Px pH-universal HER catalyst, which was durable and stable in various electrolytes.
In acidic, alkaline, and neutral media, NiCo2 Px performed well. In the alkaline electrolyte,
NiCo2 Px had the lowest overpotential of 58 mV at 10 mA cm−2 compared to CoPx (94 mV),
NiPx (180 mV), and commercial Pt (70 mV). After 5000 cycles under diverse conditions,
NiCo2 Px ’s catalyst structure remained intact. The dangling P atom (P− ) and H atom interact,
similar to the under-coordinated metal center (M+, M = Ni, Co) and the O atom. Water
molecules dissociate into H atoms and OH− , weakening the H–OH bond. Adsorbed H
atoms unite to create molecular H2 when the H atom is moved to a neighboring unoccupied
metal site [14].
Materials 2023, 16, x FOR PEER REVIEW 6 of 16
Materials 2023, 16, 3760 6 of 16

Transition metal
Transition metal chalcogenides
chalcogenides(sulphides
(sulphides andandselenides)
selenides)are are
alsoalso
alternative options.
alternative op-
MoS 2 is a promising HER electrocatalyst and has free energy similar to Pt. Chorkendorff
tions. MoS2 is a promising HER electrocatalyst and has free energy similar to Pt. Chork-
et al. [31]
endorff etproduced triangular
al. [31] produced MoS2 single
triangular MoScrystals of varied sizes on Au(111) substrates
2 single crystals of varied sizes on Au(111)
to locate the structure’s active site. The MoS 2 catalyst’s edge locations linearly affect elec-
substrates to locate the structure’s active site. The MoS2 catalyst’s edge locations lin-
trocatalytic
early HER activity. Xie
affect electrocatalytic HERet activity.
al. [32] identified flaws
Xie et al. [32] in MoS2 flaws
identified ultrathin nanosheets
in MoS to
2 ultrathin
boost their electrocatalytic
nanosheets HER performance.
to boost their electrocatalytic HERThe defect-rich structure’s
performance. activestructure’s
The defect-rich edge sites
were created by the partial breaking of the catalytically inert plane.
active edge sites were created by the partial breaking of the catalytically inert plane.In another study, Jin
In an-
et al. [33] produced CoS with tunable film, microwire (MW), and nanowire
other study, Jin et al. [33] produced CoS2 with tunable film, microwire (MW), and nanowire
2 (NW) mor-
phologies.
(NW) They extensively
morphologies. analyzed their
They extensively structures,
analyzed theiractivities, andactivities,
structures, stabilities,and andstabili-
found
that the unique morphologies enhance activity and stability. CoS NWs
ties, and found that the unique morphologies enhance activity and stability. CoS2 NWs
2 have the best HER
catalytic
have the performance and stability
best HER catalytic becauseand
performance of their highly
stability effective
because ofelectrode
their highly surface area.
effective
HER activity
electrode can area.
surface be increased via heteroatom
HER activity doping. via
can be increased Oxygen atom inclusion
heteroatom doping. and Oxygencon-
trolledinclusion
atom disorder engineering
and can govern
controlled disorder the electrical
engineering structure
can govern of MoSstructure
the electrical 2 ultrathin of
nanosheets, increasing conductivity and HER activity [34]. The disordered
MoS2 ultrathin nanosheets, increasing conductivity and HER activity [34]. The disordered structure pro-
vides huge
structure amounts
provides of unsaturated
huge sulfur atomssulfur
amounts of unsaturated as active
atomssites for HERs
as active sitesand
for aHERs
quasiperi-
and a
odic nanodomain
quasiperiodic layout forlayout
nanodomain rapidforinterdomain electron transport.
rapid interdomain Figure 3 Figure
electron transport. gives an over-
3 gives
view
an of the various
overview metals and
of the various non-metallic
metals catalyst supports
and non-metallic used thusused
catalyst supports far for
thus HER
far ap-
for
plications.
HER applications.

Figure 3. An overview of the metals and non-metals that are used to make composites/nanocompo-
Figure An overview
sites for3.HER of the metals and non-metals that are used to make composites/nanocomposites
applications.
for HER applications.
4. Composite HER Electrocatalysts
4. Composite HER Electrocatalysts
Due to their activity, quantity, and accessibility, nonprecious metals have helped de-
velop newtoenergy
Due their activity,
materialsquantity, and accessibility,
[35]. Single-metal-atom nonprecious
materials metals have
have excellent atom helped
utiliza-
develop
tion efficiency, high activity, and well-defined active sites, making them promising atom
new energy materials [35]. Single-metal-atom materials have excellent cata-
utilization efficiency,
lysts [36,37]. Suitablehigh activity,
supports and
(e.g., well-defined
metal active sites, making
sulfides, hydroxides, g-C3N4, them promising
etc.) are needed
catalysts [36,37].
to disseminate andSuitable
stabilizesupports (e.g.,sites
metal active metal
withsulfides, hydroxides,
high surface g-C3[36,38].
free energy N4 , etc.)
Manyare
needed to disseminate and stabilize metal active sites with high surface free
single-atom catalysts (SACs) have shown significant catalytic activity and structural sta- energy [36,38].
Many single-atom
bility [39,40]. Qi etcatalysts
al. [41] (SACs) have
presented a shown
covalentlysignificant
bondedcatalytic
atomic activity and structural
cobalt array interface
stability [39,40].
catalyst via Qi et
a phase al. [41]
change inpresented a covalently
MoS2 to metallic bonded
D-1T from atomic cobalt2H.
semiconductive array interface
Yi et al. [42]
catalyst via a phase change in MoS2 to metallic D-1T from semiconductive 2H. Yi et al. [42]
Materials 2023, 16, 3760 7 of 16

observed that cobalt SACs with CoeN4 moiety had good HER performance, with a 21 mV
onset overpotential (h0) and a 50 mV decade1 Tafel slope. These examples demonstrate the
potential of SACs towards large-scale water electrolyzers.
Du et al. [43] co-electrodeposited nanoparticles of Ni, HG, and rGO layers on Ni foam
to create Ni-HG-GO-Ni foam catalysts. The catalysts performed well in alkaline solution
due to their datura-like structure and excellent charge transfer between Ni and HG-rGO.
Besides placing active components on conductive supports, heteroatom doping boosts
TM catalytic activity. Jin et al. [44] optimized the Ni metal HER activity using a multidi-
mensional heteroatom-doping technique and found that N,P-co-doped Ni NPs performed
best (h10 = 24 mV). Theoretical studies revealed that the doping-induced redistribution of
charges on the Ni surface results in upgraded characteristics. Alloying is a smart technique
that is used to boost TM catalytic activity and lifetime by combining electroactive metals or
advancing the ratio of real-to-geometric surface areas [45]. Hundreds of trace metal alloys,
especially Fe-, Ni-, and Co-based alloys, perform well for EWS. Hsieh et al.’s cobalt SACs
with CoeN4 [46] had strong HER and OER activities in basic media and a current density
of 10 mA cm2 for water splitting at 1.47 V, outperforming the Pt/C and IrO2 pair.
Gao et al. [47] designed a modular electrocatalyst with OER/HER catalytic activity by
rationally targeting various metal oxide components. The bifunctional CoeCueW oxide cata-
lyst that could bring about water splitting in alkaline water electrolyzers had low overpoten-
tial, good faradaic efficiencies, and long stability. Metal-layered double hydroxides (LDHs),
such as CoSe@NiFe LDH nanoarrays [48] and Ni@NiFe LDH [49], follow these design
ideas. Metal chalcogenides (e.g., sulphides, selenides, and telluride), are more promising as
HER catalysts than the corresponding metal oxides/hydroxides [50]. Electrodeposition–
annealing–nitridation produced NiCo-nitride/NiCo2 O4 -supported graphite fibers; this
three-component system supported electroactive sites, such as NiCo2 O4 , CoN, and Ni3 N, at
the interfaces between components. The 3D oxide nitride/graphite fibers were established
as pH-universal bifunctional electrocatalysts for water splitting [51]. EWS-performing
hybrids include Co4N@nitrogen-doped carbon [52] and CoP/Ti3 C2 MXene [53].

4.1. Nanocomposites for HER Applications

4.1.1. Pt—Composites
Zhao et al. annealed PtNi/C structures to create octahedral nanomaterial with
Ni(OH)2 -Pt(111)-like interfaces. PtNi-O/C material has a mass activity of 7.23 mA/µg
at 70 mV, which is almost eight times greater than that of commercial Pt/C catalysts [22].
Zhang et al. showed how regulated synthesis could affect a catalyst’s electrocatalytic
activity and stability towards the HER under alkaline circumstances. They synthesized
well-crystalline lotus–thalamus-shaped Pt-Ni anisotropic superstructures using a solvother-
mal technique and recorded an overpotential of 27.7 mV at 10 mA cm−2 and a turnover
frequency of 18.63 H2 s−1 at 50 mV [54]. Koo et al. used alkyltrimethyl-ammonium bro-
mide, K2 PtCl4 , and NaBH4 to make different-sized Pt nanocubes. The crop-casting of
8, 20, and 25 nm Pt nanocubes was carried out on a FTO/glass substrate. Under optimum
conditions, intermediate-size nanocubes, surrounded by {100} facets with some corners
chopped to expose {110} facets, produced 1.77 A mg−1 at 50 mV and 0.54 A mg−1 at 100 mV.
Nanocubes and cuboctahedra were observed [55], and catalytic performance was found to
depend on shape effect, and, as reported in catalysis, size is crucial [56,57].
Zhang et al. used electrochemical deposition to immobilize Pt SAs over two-dimensional
inorganic material (MXene-Mo2 TiC2 Tx) nanosheets using Mo vacancies. The catalyst’s high
HER catalytic activity, with modest overpotentials of 30 and 77 mV to achieve 10 and
100 mA cm−2 , was attributable to strong covalent connections between the Pt SAs and
the support. This prevented atoms from aggregating. The material had roughly 40 times
greater mass activity than a Pt/C catalyst [58]. Yu et al. facilitated the chemical binding of
2,6:20 ,2”-terpyridine onto a 3D carbon support in a single step, which was submerged in an
aqueous K2 [PtCl4 ] solution with 0.5 ppm Pt2+ for 2 h at room temperature to create Pt SAs.
They achieved a very low metal loading (0.26 ± 0.02 µg·cm−2 of Pt) with a mass activity
Materials 2023, 16, 3760 8 of 16

of 77.1 A·mgPt−1 at 50 mV [59]. Elmas et al. synthesized a platinum-group metal-selective

electropolymerizable monomer (4-(terthiophenyl)-terpyridin) with a pendant terpyridine unit
by utilizing a metal-selective ligand approach. The metallopolymer had highly active Pt SAs
that catalyzed the HER [60].
Cheng et al. used ab atomic layer deposition to produce Pt SAs and clusters on
nitrogen-doped graphene nanosheets for the HER and found that almost all the Pt atoms
were used, which was economically appealing and 37 times more active than commer-
cial Pt/C catalyst [61]. Sun et al. used atomic layer deposition to immobilize Pt SA on
graphene nanosheets (undoped). Their catalytic activity was 10 times higher than that
of the commercial Pt/C catalyst [62]. According to Shi et al., site-specific electrodeposi-
tion on two-dimensional transition metal dichalcogenides supports yielded Pt SAs (MoS2 ,
WS2 , MoSe2 , and WSe2 ). By tuning the metal’s d-orbital state, the anchoring chalcogens
(S and Se) and transition metals (Mo and W) can synergistically regulate Pt SAs’ elec-
tronic structure. Pt SAs outperformed MoSe2 with 34.4 A mg−1 under an overpotential of
100 mV [63]. Despite significant breakthroughs, wet chemical synthesis is preferred because
of its simplicity [39,64].

4.1.2. Palladium-Based Composites

Pd’s atomic size is similar to Pt’s, making it a good HER catalyst. Pd has various
efficient synthetic methods for size- and shape-controlled nanocrystal creation [65–68].
Pd nanoctahedra bordered by (111) facets have a high H loading of 0.90, making them at-
tractive for HER applications [69]. Li et al. created a core@shell PdCu@Pd nanocube catalyst
for efficient HERs. The system needed an overpotential of 10 mV to reach 68 mA cm−2 [70].
Wang et al. used a polyol technique with polyvinylpyrrolidone as a stabilizer to make
Pd icosahedral NPs with 0.22 nm lattice spacing in the (111) plane. After 130,000 cyclic
voltammetric cycles, the overpotential was 32 mV at 10 mA cm−2 , confirming its durability
and activity [71].

4.1.3. Nickel-Based Composites

Due to its chemical characteristics and group numbers being similar to Pt, Ni is a
potential non-noble metal choice for HERs. It is cheaper and more abundant than Pt
and Pd. In alkaline circumstances, the rate-determining step is hydrogen adsorption (the
Volmer step), and the low-valence-state oxide of Ni improves the HER [72]. The reaction is
interesting for Ni–Pt and Ni–Mo alloys, as well as Ni-nitride-, Ni-oxide-, Ni-phosphide, and
Ni-sulfide-based catalysts [73]. Li et al. published an intriguing study based on the Cu–Ni
alloy. Selective wet chemical etching produced edge-notched, edge-cut, and mesoporous
Cu–Ni nanocages. The highly catalytic (111) facets were etched from the last two materials.
The Cu–Ni nanocages had increased HER activity under alkaline circumstances (a current
density of 10 mA cm−2 under an overpotential of 140 mV). The density functional theory
showed that Ni–Cu alloys are more promising than pure Ni [74]. Nanocubes of Ni(OH)2
and Ni–Fe modified with Pt atoms were proposed for the process to reduce noble metal
loading [75,76]. After alloying with Pt, Kavian et al. generated 9 nm Pt–Ni octahedral
nanocrystals with 15-fold higher specific activity and roughly 5 times more mass activity in
alkaline media than the Pt/C commercial catalyst [77]. Considering the overpotential, Tafel
slope, and turnover frequency, Seo et al. found that spherical nickel phosphide nanocrystals
with (001) facets had higher HER activity than rod-shaped ones with (210) facets [78].
Xiang et al. produced nickel phosphide nanowires using a one-step hydrothermal method
with an overpotential of 320 mV and a Tafel slope of 73 mV dec−1 for HERs [79,80]. Table 1
summarizes a comprehensive list of the various electrocatalysts that have been used for
HER applications.
Materials 2023, 16, 3760 9 of 16

Table 1. The HER performance of predominant electrocatalysts.

Catalysts HER Performance Stability Reference

10 I mA cm−2 ;38η mV; Tafel slope of
PtNi-Ni NA/CC 90 h [16]
42 mV dec−1
10 I mA cm−2 ; 39.8η mV; Tafel slope of
PtNi-O/C 10 h [22]
78.8 mV dec−1
10 I mA cm−2 ; 13η mV; Tafel slope of
PtNi(N) NW 10 h [15]
29 mV dec−1
32 I mA cm−2 ; 200η mV; Tafel slope of
Mo2 C-R 2000 cycle [25]
58 mV dec−1
10 I mA cm−2 ; 266η mV; Tafel slope of
Mo2 C-GNR 3000 cycles [26]
74 mV dec−1
20 I mA cm−2 ; 130η mV; Tafel slope of
Ni2 P/Ti 500 cycles [29]
46 mV dec−1
10 I mA cm−2 ; 63η mV; Tafel slope of
NiCo2 Px 5000 cycles [30]
63.6 mV dec−1
13 I mA cm−2 ; 200η mV; Tafel slope of
Defect rich MoS2 10,000 s [32]
50 mV dec−1
10 I mA cm−2 ; 145η mV; Tafel slope of
CoS2 NW 3h [33]
51.6 mV dec−1
10 I mA cm−2 ; 158η mV; Tafel slope of
CoS2 NW 41 h [33]
58 mV dec−1
120η mV; Tafel slope of
Oxygenated MoS2 3000 cycles [34]
55 mV dec−1
Pt SAs over nanosheets of a Overpotential of 77 mV is 8.3 Amg−1 ,
two-dimensional inorganic 39.5 times more than commercial HER catalyst (40 wt% 10,000 cycles [58]
material MXene-Mo2 TiC2 Tx Pt/C, 0.21 Amg−1 ); Tafel slope of 30 mVdec−1
PdCu@Pd nanocube core@shell 10 I mA cm−2 ; 65η mV; Tafel slope of 35 mV dec−1 ND [70]
Polyvinylpyrrolidone as a
stabiliser to make 32 mV at 10 mA cm−2 130,000 cycles [71]
Pd icosahedral NPs
Ni2 P NW 320 mV and a Tafel slope of 73 mV dec−1 ND [79]
Current density of 10 mA cm−2(under an
Cu-Ni nanocages ND [74]
overpotential of 140 mV).
Pt SA on graphene nanosheets 10 times more active than commercial Pt/C 150 cycles [62]
Pt SAs and clusters
on nitrogen-doped 37 times more active than Pt/C 50 and 100 cycles [61]
graphene nanosheets
Pt nanocubes on FTO/
1.77 A mg−1 at 50 mV and 0.54 A mg−1 at 100 mV ND [55]
glass substrate
−10 and −100 mA cm−2 ; overpotentials of −50 and
Ni-HG-rGO/NF catalysts ND [43]
−132 mV; a low Tafel slope of −48 mV dec−1
21 mV onset overpotential (h0 ) and a
Cobalt SACs with a Co-N4 10 h [42]
Tafel slope of 50 mV dec−1
Ultralow overpotentials of
NiFeMo alloy 50 h [46]
33 and 249 mV; 500 mA cm−2
Materials 2023, 16, 3760 10 of 16

Table 1. Cont.

Catalysts HER Performance Stability Reference

Low Tafel slope of 58 mV dec−1 ;lowest
NiCo-nitrides/NiCo2 O4 /GF overpotentials (η) of 71 and 180 mV to obtain current over 40 h [51]
densities of 10 and 50 mA cm−2
10 I mA cm−2 ; 90η mV; Tafel slope of
Ni@Pd/PEI–rGO stack structures ND [81]
54 mV dec−1
10 I mA cm−2 ; 184η mV; Tafel slope of
MWCNTs@Cu@MoS2 1000 cycles [82]
62 mV dec−1
10 I mA cm−2 ; 200η mV; Tafel slope of
Nanoporous Ag2 S/CuS 1000 cycles [83]
75 mV dec−1
10 I mA cm−2 ; 310η mV; Tafel slope of
Wl8 O49 @WS2 NRs 750 cycles [84]
86 mV dec−1
10 I mA cm−2 ; 387η mV; Tafel slope of
RuCo/Ti foil >12 h [85]
107 mV dec−1
10 I mA cm−2 ; 122η mV; Tafel slope of
Rh2 S3 –Thick HNP/C 10,000 cycles [86]
44 mV dec−1
10 I mA cm−2 ; 158η mV; Tafel slope of
Fe1 -xCoxS2 /CNT >40 h [87]
46 mV dec−1
10 I mA cm−2 ; 182η mV; Tafel slope of
MoS2 @OMC ND [88]
60 mV dec−1
10 I mA cm−2 ; 48η mV; Tafel slope of
Pd2 Te NWs/rGO 48 h [89]
63 mV dec−1
NiAu@Au NPs Tafel slope of 36 mV dec−1 20,000 cycles [90]
ND—not determined.

5. Future Perspectives and Conclusions

Developing advanced low-cost electrocatalysts for water splitting is of great scientific
and industrial significance. The existing metal composites/nanocomposite electrocatalysts
used for HER applications were reviewed and discussed in this review. The main concern is
not a dearth in options; it instead rests on criteria such as cost-effective and environmentally
friendly electrocatalyst options. The realization of the successful commercialization of low-
cost electrocatalysts deserves attention. First, developing simple and scalable synthetic
methods that can enable mass production is mandatory when it needs to scale up for
large-scale applications. Secondly, in-depth knowledge on the strengths and weaknesses
of electrocatalyst options is required. Thirdly, catalyst stability with respect to industrial
protocols (e.g., time, temperature, mass loading) is another crucial mandate. Finally,
developing bifunctional/multifunctional catalysts in view of simplification, cost reduction,
and practical application is also necessary.
Metal-free catalysts are potential advanced energy sources due to their abundance,
cost-effectiveness, pH resistance, and environmental friendliness [91–93]. Carbon materi-
als, such as multiwalled/single-walled CNTs, graphene, carbon quantum dots, graphene
oxide (rGO), and graphdiyne, are used to make advanced electrocatalysts due to their
excellent electrical and thermal conductivity, low cost, large active surface area, good
mechanical/chemical strength, and tunable electronic structures [92,94–98]. Heteroatom
(N, O, S, P, B, or F) doping and defect structure fabrication are the most effective ways
to tune the electrical structure of bare carbon [99,100]. P doping increased graphite
layer HER activity [101]. Due to its high conductivity, configurable direct bandgap, and
anisotropic characteristics, phosphorene is becoming an electrocatalyst alternative to carbon
materials [80]. Phosphorene-based catalysts exhibit good HER and OER performance. Dop-
ing and defect engineering can boost phosphorene activity, like carbon [80,99–101]. Through
the course of the review, we observed that most of the studies with respect to material
Materials 2023, 16, 3760 11 of 16

options for HER electrocatalysts report one material after the other in various combinations,
with varied dopants and defects; there is no systematic study at present that compares the
efficacy of these metal electrocatalysts and puts forth optimized catalyst suggestions. This
is ideally important when it comes to the strategic planning of next-generation catalysts.
Moreover, the catalytic mechanisms of many electrocatalysts, such as metal-based catalysts
for HERs in alkaline conditions, have been hardly researched compared to HERs in acidic
conditions. This is another aspect that deserves more research attention.
Nanotechnology has transformed every sector, but catalysis has benefited the
most [80,102]. Metal NPs seek customizable characteristics with controllable parame-
ters for better catalysis. A lack of theoretical models of electronic structures, the controlled
synthesis of the shape and presentation of specific facets, and a fundamental grasp of
catalytic processes hinder catalyst development. Electrocatalyst nanomaterials produce
surface species with suitable bonding energies. Transition metals (TMs), such as Fe-, Ni-,
and Co-based and metal-free catalysts, electrocatalyze HERs and/or OERs, while metal-free
catalysts (e.g., N, P, S, and O) incorporate carbon nanomaterials, and other options come
from TMs, TM alloys, and TMX (X = O, S, Se, Te, N, P, B, and C). Several of these low-cost
chemicals have EWS capabilities comparable to NM-based catalysts. We have reported
the effective use of Pt-Ag/Ag3 PO4 -WO3 nanocomposites for photocatalytic H2 produc-
tion from bioethanol [103]. Such metal-based nanocomposites hold huge relevance and
similar such trinary composites will combine multifold properties of materials involved,
and hence will be able to contribute significantly to this application. Different heterostruc-
tures based on WO3 with noble metals such as Pt/TiO2 /WO3 [104], TiO2 @WO3 /Au [105],
Ag3 PO4 /Ag/WO3−x [106], and Ag/Ag3 PO4 /WO3 [107] were reported to exhibit excellent
photocatalytic potential for the photodegradation of organic compounds. Such nanocom-
posites should be extended to electrocatalyst applications. We recently published a new
study [108], titled Noble Metals Deposited LaMnO3 Nanocomposites for Photocatalytic
H2 Production. Similar lanthanum-based nanocomposites need to be tested for HER
electrocatalyst applications too.
Previous authors report that similar morphologies of the same materials yielded differ-
ent tendencies. So, the question based on what is behind such findings remains. This will
require a proper understanding of the interactions between the surface facets/morphologies
of supports and active species. In situ studies are critical in order to obtain an advanced
comprehension of catalytic processes and to improve the catalysts’ design and performance.
Strategies that aim to regulate the internal and external characteristics of nanomaterials
are recommended, such as heteroatom doping, hybridization, defect engineering, phase
control, and nanostructure construction.
Our PubMed search using various keywords (metal catalysts in a hydrogen evolution
reaction, composite catalysts in a hydrogen evolution reaction, and nanocomposite catalysts
in a hydrogen evolution reaction) provided varying amounts of search results (2643, 760,
and 127, respectively) (Figure 4). This trend reflects the nature of research interests in
the corresponding areas. This survey clearly indicates that the use of nanocomposites is
significantly lagging. When nanotechnology becomes very promising in various aspects,
its use in this application decreases, which is a strange mismatch. Composites largely
benefit from their ability to combine unique properties of various materials. Metallic
electrocatalysts on non-metallic supports (CNTs and graphene) have been reported to
enhance HER applications. Integrating nano-concepts to composites is certainly a fulfilling
area, which we found has not been appropriately attempted. This certainly represents a
gap in the current setting. With the ever-expanding list of nanomaterials at our disposal,
there will be a huge waste of resources if we do not avail them. This review prompts the
incorporation of more nano-aspects into electrocatalysts and the improvisation of existing
composite catalysts with nanoforms. There is plenty of room when there is nano and
superior attributes to harness and benefit sustainable energy production.
Materials 2023,
Materials 2023, 16,
16, 3760
x FOR PEER REVIEW 12
12of 16
of 16

Figure
Figure 4.4. Graph
Graph showing
showingthethecurrent
currentresearch
researchtrends
trends with respect
with to composites
respect to compositesandand
nanocomposites
nanocompo-
for HER applications. A represents PubMed search term results on catalysts in hydrogen
sites for HER applications. A represents PubMed search term results on catalysts in hydrogen evolution
evo-
reaction; B represents search term results on composite catalysts in hydrogen evolution reaction,reac-
lution reaction; B represents search term results on composite catalysts in hydrogen evolution and
tion,
C and C represents
represents search termsearch
resultsterm results on nanocomposite
on nanocomposite catalysts
catalysts in the in the
hydrogen hydrogen
evolution evolution
reaction. The
reaction.
values areThe values
based are basedsearch
on PubMed on PubMed search
(as of April (as of April 2023).
2023).

Author Contributions: Conceptualization, A.H.J. and N.H.; data curation, A.H.J. and N.H.; writ-
Author draft, A.H.J.Conceptualization,
Contributions:
ing—original A.H.J. and
and N.H.; writing—review N.H.; data
and editing, A.H.J.curation,
and N.H.A.H.J. and N.H.;
All authors have
writing—original draft, A.H.J. and N.H.; writing—review
read and agreed to the published version of the manuscript. and editing, A.H.J. and N.H. All authors
have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable
Informed Consent Statement: Not applicable.
Data Availability Statement: Not Applicable
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflicts of interest.
Conflicts of Interest: The authors declare no conflict of interest.
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