Hy­dro­gen Econ­omy is not dead

– Some Re­cent De­vel­op­ments In Hy­dro­gen Gen­er­a­tion, Stor­age, Trans­port And Us­age As En­ergy Car­rier

Chemical Industry Digest - - What’s In? - Dr N C Datta

- Dr N C Datta, Con­sul­tant, Mod­i­con Pvt Ltd.

Hy­dro­gen eco­comy has been touted for some time as a su­pe­rior al­ter­na­tive to the hy­dro­car­bon econ­omy we are in to­day. This ar­ti­cle cov­ers var­i­ous pro­duc­tion pro­cesses, its trans­porta­tion and stor­age as­pects, par­tic­u­larly in terms of the lat­est ad­vances in these ar­eas.

“I be­lieve that wa­ter will one day be em­ployed as fuel, that hy­dro­gen and oxy­gen which con­sti­tute it, used singly or to­gether, will fur­nish an in­ex­haustible source of heat and light, of an in­ten­sity of which coal is not ca­pa­ble.” - Jules Verne (The Mys­te­ri­ous Is­land, pub­lished in 1874, Chap­ter 33) Ab­stract

Hy­dro­gen eco­comy has been touted for some time as a su­pe­rior al­ter­na­tive to the hy­dro­car­bon econ­omy we are in to­day. Hy­dro­gen is of­ten seen as more at­trac­tive and cleaner than the con­ven­tional fuels be­cause whether it is used in a fuel cell with air to pro­duce elec­tric­ity or burned to pro­duce heat, the only by-prod­uct is wa­ter rather than car­bon diox­ide or other green­house gases and par­tic­u­lates. Much as hy­dro­gen is a clean fuel and abun­dantly avail­able in wa­ter, its pro­duc­tion, stor­age and trans­porta­tion poses many chal­lenges.

This ar­ti­cle cov­ers var­i­ous pro­duc­tion pro­cesses, its trans­porta­tion and stor­age as­pects, par­tic­u­larly in terms of the lat­est ad­vances in these ar­eas.

Hy­dro­gen econ­omy is the vi­sion of us­ing hy­dro­gen as the source of en­ergy for sev­eral pur­poses. Cur­rently, more than 70% of the crude oil is used in trans­porta­tion. Pro­por­tion­ate amounts of CO , un­burnt hy­dro

2 car­bons, and NOx are re­leased into the at­mos­phere, lead­ing to global warm­ing. Hence, for any mean­ing­ful abate­ment of global warm­ing, it is nec­es­sary that a suit­able sub­sti­tute of oil is found, hy­dro­gen can be the best al­ter­na­tive.

The con­cept was pro­posed al­most 100 years ago in a pa­per pre­sented by the fa­mous sci­en­tist J.B.S. Hal­dane (1892-1964) be­fore the Cam­bridge So­ci­ety – The Heretics1. It was res­ur­rected in 1970 when the first signs of an im­pend­ing oil cri­sis loomed at the hori­zon. In a lec­ture at the Tech­nol­ogy Cen­tre of Gen­eral Mo­tors, the cel­e­brated elec­tro­chemist, John O’M. Bock­ris (1923-2013), elab­o­rated on the con­cept, and coined the term “Hy­dro­gen econ­omy”. Later in 1975,

1 he pub­lished a book, on the sub­ject, en­ti­tled, “En­ergy: So­lar Hy­dro­gen Al­ter­na­tive”.

There are no two opin­ions that hy­dro­gen is the clean­est fuel on earth. It burns in O /air, form­ing only

2 wa­ter, which, though a green­house gas in vapour form, is turned eas­ily to liq­uid wa­ter. En­ergy out­put wise, 1 kg of hy­dro­gen is al­most equiv­a­lent to about 3.3 M3 of nat­u­ral gas / 3.8-3.9 L of gaso­line / 3.3-3.4 L of high speed diesel2. To il­lus­trate more, a fuel cellpow­ered ve­hi­cle may travel upto 60 miles in USA con­di­tions with 1 kg of hy­dro­gen in the fuel tank, or the same quan­tity of hy­dro­gen may pro­vide elec­tric­ity to an av­er­age USA house­hold for 12 hours.

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How­ever, even to­day, the econ­omy is driven al­most com­pletely by fos­sil fuels all around the world with lit­tle vis­i­bil­ity of hy­dro­gen as an al­ter­na­tive. To be spe­cific, al­most 85% of all en­ergy re­quire­ments are still met from fos­sil fuels. Re­gard­ing the other en­ergy re­sourc

4 es, nu­clear en­ergy is used only 2%, and re­new­ables, 13%, with the fol­low­ing break up: biomass (wood, etc.) 10.2%, wind 0.2%, hy­dropower 2.3%, marine 0.0002%, geother­mal 0.1%, and so­lar just 0.1% 4.

In 2016, about 65 mil­lion met­ric tons of hy­dro­gen was pro­duced world­wide, of which 10 mil­lion met­ric tons were pro­duced in USA alone. Of this quan

5 tity, 48% was used in petroleum re­fin­ing for a process known as hy­dro­c­rack­ing, 43% was used in am­mo­nia man­u­fac­ture, about 4% in methanol pro­duc­tion, and the bal­ance 5% was used in metal fab­ri­ca­tion, elec­tron­ics man­u­fac­ture and food pro­cess­ing. No hy­dro­gen

5 was used as fuel. One rea­son for this is the eco­nom­ics. The prices of var­i­ous en­ergy car­ri­ers and re­sources, as of 2009, are shown in Fig.1. Hy­dro­gen is one of the costli­est in com­par­i­son with other en­ergy car­ri­ers. The other rea­sons are tech­ni­cal, as will be dis­cussed.

Why hy­dro­gen?

(1) No other en­ergy car­rier is as in­fi­nite as hy­dro­gen, be­cause it can be ob­tained from wa­ter, and car­bo­hy­drates (biomass), both of which are re­new­able re­sources.

(2) Hy­dro­gen is non-toxic.

(3) Recharg­ing of hy­dro­gen-pow­ered ve­hi­cles may be rel­a­tively easy – it may need just re­place­ment of the ex­hausted hy­dro­gen stor­age unit by a re­fill.

(4) H sys­tem can be in­te­grated well into the power

2 grid and be very use­ful in grid sta­bil­i­sa­tion dur­ing de­mand fluc­tu­a­tions, as ex­cess power gen­er­a­tion could be utilised in elec­trol­y­sis of wa­ter to make more hy­dro­gen and oxy­gen, and any short­fall in power sup­ply may be augmented from hy­dro­gen-pow­ered fuel cell stacks. This grid sta­bil­i­sa­tion through flex­i­ble in­put and out­put has be­come a ne­ces­sity to­day in ad­vanced coun­tries be­cause of sharply di­min­ish­ing prices of al­ter­nate en­ergy re­sources and change of user pref­er­ences.

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(5) Like petroleum crude / oil, hy­dro­gen may be trans­ported over long dis­tances through pipe­lines and vast quan­ti­ties of hy­dro­gen may be stored in large un­der­ground cav­erns.

(6) Some of the ad­van­tages of hy­dro­gen are equally pos­si­ble with other en­ergy car­ri­ers such as methanol

and ethanol, which, too, may be ob­tained from re­new­able re­sources like biomass. How­ever, hy­dro­gen is unique and su­pe­rior to other en­ergy car­ri­ers be­cause of one fun­da­men­tal rea­son.

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Ta­ble 1 shows the val­ues of max­i­mum avail­able use­ful en­ergy (DG) that could be ob­tained when some of these co­va­lent chem­i­cal bonds such as H – H, C – H, C – C, and N – H, as present in dif­fer­ent en­ergy car­ri­ers, are bro­ken by re­ac­tion with O . Ta­ble 1 shows also

2

(i) the num­ber of elec­trons (n) in­volved in each of these re­ac­tions, if the re­ac­tions are car­ried out elec­tro­chem­i­cally, and (ii) the cor­re­spond­ing cell volt­age (E), which is a mea­sure of the avail­able use­ful bond en­ergy per elec­tron. This avail­able use­ful bond en­ergy per elec­tron is an im­por­tant pa­ram­e­ter, be­cause fuel cells work only through flow of elec­trons.

On this ba­sis, H (or H – H bond) con­tains the maxi

2 mum avail­able use­ful bond en­ergy per elec­tron (1.23 V) in com­par­i­son with other co­va­lent chem­i­cal bonds. Of course, the re­ac­tions 4, 7, and 8 of Ta­ble 1, viz, ox­i­da­tion of NH by O to form N and H O, and re­duc

3 2 2 2 tion of CO to form CH OH and C H OH, may have

2 3 2 5 sim­i­lar use­ful bond en­ergy per elec­tron with E = 1.17, 1.213, and 1.145 V, re­spec­tively, but these re­ac­tions are not com­mer­cially vi­able be­cause CO is present in

2 the at­mos­phere at a con­cen­tra­tion of about 400 ppm only, and the ox­i­da­tion of NH3 in­volves the han­dling of a highly haz­ardous sub­stance. Also, the syn­the­sis of NH3 re­quires a huge amount of en­ergy and pure H .

2 (7) The split­ting of wa­ter mol­e­cule into H and O

2 2 is the eas­i­est; thanks to ab­sence of any side re­ac­tions, faster ki­net­ics and rel­a­tively lower ac­ti­va­tion bar­rier.

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Crit­i­cal Is­sues with hy­dro­gen

(1) Be­ing the light­est gas, it oc­cu­pies a very large vol­ume in gaseous state. There­fore, for trans­porta­tion in ve­hi­cles as fuel tanks, it must be com­pressed to very high pres­sure and / or liq­ue­fied. For liq­ue­fac­tion, hy­dro­gen must be cooled to be­low its crit­i­cal tem­per­a­ture, 33 K. There­fore, ad­e­quate cryo­genic cool­ing is nec­es­sary for stor­age and trans­porta­tion of liq­uid hy­dro­gen.

(2) Hy­dro­gen is an ex­tremely in­flammable gas, may form ex­plo­sive mix­ture with air, and ex­plode if heated. In air, it has very wide flamma­bil­ity lim­its: 4 – 75% (v/v), and det­o­na­tion lim­its: 13 – 70% (v/v).

(3) Hy­dro­gen may dis­place oxy­gen rapidly and with­out no­tice, caus­ing suf­fo­ca­tion.

(4) It burns with pale blue flame, which is al­most in­vis­i­ble in day light. While burn­ing, it does not pro­duce any in­fra-red ra­di­a­tion, but pro­duces a lot of UV ra­di­a­tion – so any per­son stand­ing nearby would not ex­pe­ri­ence any heat, but would ex­pe­ri­ence sun-burn like ef­fect on the skin due to the ex­po­sure to UV ra­di­a­tion.

(5) So, any hy­dro­gen leak­age must be de­tected. The de­tec­tion may be done by an elec­tronic sen­sor or by an odor­ant. For ef­fi­cient de­tec­tion, an odor­ant should have sim­i­lar molec­u­lar weight and dif­fu­sion char­ac­ter­is­tics as the bulk gas so that it spreads at the same rate. So far no odor­ant has been found which has sim­i­lar speed (1.78 km/s) and dif­fu­siv­ity (0.61x10- m2/s) as hy

4 dro­gen.

(6) Hy­dro­gen ex­hibits a pos­i­tive Joule-Thompson ef­fect at tem­per­a­tures above 193 K, which is its in­ver­sion tem­per­a­ture. It means that the tem­per­a­ture of the hy­dro­gen gas in­creases upon de­pres­sur­iza­tion, and this may lead to its ig­ni­tion. Hy­dro­gen has a very low ig­ni­tion en­ergy 0.0019 Joule.

(7) At el­e­vated tem­per­a­tures and pres­sures, hy­dro­gen, be­ing a tiny mol­e­cule, dif­fuses in­side the metal

ma­trix of the stor­age con­tainer. As hy­dro­gen spreads in­side the metal, grad­u­ally the metal loses its duc­til­ity and be­comes brit­tle. This is hy­dro­gen em­brit­tle­ment. This fail­ure of metal is a se­ri­ous con­cern in any sit­u­a­tion in­volv­ing stor­age or trans­fer of hy­dro­gen gas un­der pres­sure.

How pal­la­dium is use­ful

In view of the prob­lems of stor­ing and trans­porta­tion of com­pressed and liq­ue­fied hy­dro­gen, re­searches have been done to de­velop solid ab­sorbents, which would ab­sorb a large vol­ume of hy­dro­gen and des­orb it re­versibly on user de­mand. Sev­eral me­tals and al­loys have been de­vel­oped for this pur­pose. These me­tals and al­loys ab­sorb H to form hy­drides and these hy

2 drides de­com­pose at higher tem­per­a­tures, lib­er­at­ing the ab­sorbed hy­dro­gen and re­gen­er­at­ing the orig­i­nal me­tals and al­loys. Ta­ble 2 shows the tem­per­a­ture and pres­sure re­quired for the for­ma­tion of some of these metal hy­drides, their com­po­si­tion, and quan­tity of H

2 these may carry.

It is es­sen­tial that the molec­u­lar H should dis­soci

2 ate into atoms be­fore it is in­cor­po­rated into the metal / al­loy lat­tice to form the hy­drides. It is com­monly es­tab­lished that pal­la­dium has an ex­tra­or­di­nary abil­ity to dis­so­ci­ate molec­u­lar H rapidly, and this prop­erty of

2 pal­la­dium is at the root of its use as a very ef­fi­cient cat­a­lyst for hy­dro­gena­tion re­ac­tions in or­ganic syn­the­sis.

As shown in Ta­ble 2, most me­tals and al­loys, other than pal­la­dium, re­quire some pres­sure and / or tem­per­a­tures to over­come an ac­ti­va­tion bar­rier. But pal­la­dium ab­sorbs hy­dro­gen un­der am­bi­ent con­di­tions upto 900 times of its own vol­ume, form­ing pal­la­dium hy­dride of com­po­si­tion: PdHx, where x varies from 0.015 to 0.607. Still, pal­la­dium is not ac­cept­able as a hy­dro­gen stor­age ma­te­rial be­cause it is too ex­pen­sive, and the to­tal quan­tity of hy­dro­gen that can be stored in Pd is not very high – it is just 0.56% by weight. But Pd has the po­ten­tial to play a ma­jor role in all ar­eas of hy­dro­gen econ­omy such as hy­dro­gen pu­rifi­ca­tion, stor­age, de­tec­tion, and fuel cells.

(a) Hy­dro­gen Stor­age

The US Depart­ment of En­ergy has con­cluded that for a good hy­dro­gen stor­age de­vice: (i) it must be able to ab­sorb at least 5.5 wt% hy­dro­gen for the time be­ing and should be able to ab­sorb upto 9 wt% later af­ter fur­ther de­vel­op­ment, (ii) it should be light-weight, in­ex­pen­sive and read­ily avail­able, (iii) the sorp­tion - des­orp­tion ki­net­ics should be fast and re­versible, and (iv) it should have long-term sta­bil­ity af­ter re­peated re­cy­cling. From Ta­ble 2, it is ap­par­ent that MgH meets 7

2 all these cri­te­ria, but it is highly sus­cep­ti­ble to be at­tacked by both acids and al­ka­lis. Also, the rate of H

2 sorp­tion by Mg is very slug­gish, and the hy­dro­gen gets des­orbed only at tem­per­a­tures higher than 300oC.

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All these prob­lems may be solved, if Mg is al­loyed first with Ti, form­ing a thin film of an al­loy of com­po­si­tion MgyTi1- y, where y = 0.80 op­ti­mally, and then if Pd is de­posited elec­tro­chem­i­cally on this al­loy upto a thick­ness of 3-4 nm. This cap­ping of MgyTi1- y flm by Pd makes it not only acid – al­kali re­sis­tant, but also its hy­dro­gen sorp­tion – des­orp­tion ki­net­ics be­come rea­son­ably fast. It has been ob­served that the hy­dro­gen stor

8 age ca­pac­ity of this film of Pd-capped MgyTi1- y al­loy ap­proaches 1750 mAh/g, when used in fuel cell, and this is equiv­a­lent to 6.4 wt% of hy­dro­gen stor­age. Pd

8 capped Mg-Sc al­loys of sim­i­lar com­po­si­tion also have shown iden­ti­cal prop­er­ties.

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(b) Hy­dro­gen de­tec­tion7

Pd may be used to make some very ef­fi­cient sen­sors to de­tect hy­dro­gen. In one type of sen­sors, its elec­tri­cal re­sis­tiv­ity in­creases sharply as hy­dro­gen gets ab­sorbed in Pd. In an­other type of sen­sors, Pd is coated with an op­ti­cally ac­tive ma­te­rial, which sends an op­ti­cal sig­nal pro­por­tional to the con­cen­tra­tion of hy­dro­gen ab­sorbed. In both types of sen­sors, Pd must be in nano-form.

(c) Hy­dro­gen pu­rifi­ca­tion

Among all tran­si­tion me­tals and metal ox­ides, plat­inum has been found to be the most ef­fec­tive cat­a­lyst in all types of fuel cells, but it is ex­tremely sus­cep­ti­ble to poi­son­ing by CO, H S, and other poi­sons.

2

When H is ob­tained by re­form­ing hy­dro

2 car­bons such as steam meth­ane re­form­ing re­ac­tion (SMR) or from car­bo­hy­drates by ox­i­da­tion, some quan­ti­ties of CO and CO

2 are in­vari­ably formed in course of the reac-

tions. Even af­ter strin­gent pu­rifi­ca­tion, some resid­ual CO re­main in the prod­uct hy­dro­gen, and the Pt cat­a­lyst in the fuel cell is ir­re­versibly poi­soned, if H feed

2 con­tains more than 10 ppm of CO. This is one ma­jor road block for use of fuel cells in au­to­mo­biles, be­cause most of the hy­dro­gen is made by SMR as on now. It has been ob­served that the pal­la­dium or pal­la­dium al­loy based mem­branes may be use­ful to make 99.9999% pure hy­dro­gen. But there is some prob­lem here, too. H ad­sorp­tion in Pd is ac­com­pa­nied by phase

2 change and lat­tice ex­pan­sion. At lower con­cen­tra­tion of H-ab­sorp­tion, it forms an α-phase, and as the ab­sorp­tion in­creases, the lat­tice grad­u­ally ex­pands and forms a β-phase. Fi­nally, be­yond a cer­tain crit­i­cal limit of the lat­tice ex­pan­sion, the mem­brane cracks and breaks into pieces. This also is called as hy­dro­gen em­brit­tle­ment.

It has been found that if the ab­sorp­tion of H oc­curs

2 at 570 K and above, there is no lat­tice ex­pan­sion and no hy­dro­gen em­brit­tle­ment. But ab­sorp­tion of H at 570

2

K and above would re­duce the quan­tity of ab­sorbed hy­dro­gen fur­ther – also, this would in­volve an ex­pen­di­ture of en­ergy. How­ever, this tem­per­a­ture of 570 K may be re­duced to lower tem­per­a­tures, say, to 393 K, by al­loy­ing Pd with Ag (23 wt% Ag) or with Cd (15

9 at% Cd) 10. Such al­loy­ing not only pre­vents hy­dro­gen em­brit­tle­ment, but also re­duces the cost of hy­dro­gen stor­age by us­ing less ex­pen­sive me­tals.

(d) Pd as cat­a­lyst in fuel cells

Plat­inum is the es­tab­lished elec­trode ma­te­rial in all types of fuel cells. A pro­ton ex­change mem­brane or poly­mer elec­trolyte mem­brane fuel cell (PEMFC) is shown in Fig. 2.

But very high cost of plat­inum, its lim­ited sup­ply, and sus­cep­ti­bil­ity to poi­son­ing are its ma­jor lim­i­ta­tions. Also, the ca­thodic oxy­gen re­duc­tion re­ac­tion

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(see Fig.2) is not very fast on Pt, al­though Pt is the fastest cat­a­lyst for this re­ac­tion among most me­tals. Pd al­loys and com­bi­na­tions of Pd with other plat­inum group me­tals such as Ru, Ir, Pt, etc. have been widely in­ves­ti­gated in fuel cells us­ing methanol, ethanol, or formic acid as fuel. Pd-Pt bimetal­lic cat­a­lysts have

12 been found to be bet­ter than Pt in many re­ac­tions, and Pd is shown to be a far su­pe­rior cat­a­lyst than Pt in formic acid ox­i­da­tion. For the oxy­gen re­duc­tion re­ac­tion, Pd-al­loys have also demon­strated im­proved per­for­mance when com­pared to Pt. The change from Pt to

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Pd-based cat­a­lysts in fuel cells is be­ing con­sid­ered se­ri­ously, but the price of pal­la­dium has in­creased dras­ti­cally in re­cent times due to in­creased us­age and other geopo­lit­i­cal rea­sons. It is not clear if such a change will bring down ul­ti­mately the fuel cell cost.

Hy­dro­gen gen­er­a­tion

Hy­dro­gen is the most abun­dant el­e­ment in the uni­verse – 75% of all mat­ter in the uni­verse is made of hy­dro­gen, but the earth’s at­mos­phere con­tains just 1 ppm of H . There­fore, it has to be ob­tained al­ways from its

2 com­bined forms such as wa­ter, hy­dro­car­bons, and car­bo­hy­drates, which are avail­able in plenty. The var­i­ous com­mer­cial pro­cesses, which are presently used to make H , are: re­form­ing of nat­u­ral gas or steam meth

2 ane re­form­ing (SMR), gasi­fi­ca­tion of coal or biomass in air /O , py­rol­y­sis of coal or biomass in ab­sence of O ,

2 2 and elec­trol­y­sis of wa­ter.

Ta­ble 3 shows the ef­fi­cien­cies of en­ergy con­ver­sion in var­i­ous tech­nolo­gies of H pro­duc­tion, as cal

2 cu­lated in the Hy­dro­gen Tools Por­tal of the Pa­cific North­west Na­tional Lab­o­ra­tory with sup­port from the US Depart­ment of En­ergy. As shown in Ta­ble 3, in all

14 such pro­cesses, al­most 30-60% of en­ergy is wasted.

14 There­fore, any process to use hy­dro­gen as en­ergy car­rier would be eco­nom­i­cally vi­able only if the en­ergy to isolate hy­dro­gen from its com­pounds is avail­able cheaply. And what could be cheaper source than the en­ergy from the Sun, which is avail­able freely and abun­dantly around the Globe?

There are three pro­cesses by which CO, CO -free

2

H may be made us­ing so­lar ra­di­a­tions. These are: (1)

2 wa­ter split­ting by di­rect con­cen­trated so­lar ra­di­a­tion,

as­sisted by pho­to­cat­a­lysts, (2) so­lar ther­mo­chem­i­cal hy­dro­gen (STCH) cy­cles, and (3) elec­trol­y­sis of wa­ter, as­sisted by elec­tro­cat­a­lysts, us­ing elec­tric­ity gen­er­ated by pho­to­volat­ics. These pro­cesses are dis­cussed briefly in the fol­low­ing.

(a) Wa­ter split­ting by pho­to­cat­a­lysts

Di­rect split­ting of wa­ter by so­lar ra­di­a­tion, as­sisted by pho­to­cat­a­lysts, has been a dream for decades. A large num­ber of metal ox­ides, sul­phides, ni­trides, nano-com­pos­ites, doped ma­te­ri­als and organo-metal­lic com­plexes have been tried with vary­ing de­grees of suc­cess. So far TiO , and cat­a­lysts based pri­mar­ily

2 on TiO have been found to be most suc­cess­ful. But

2 no process has been found to be vi­able for com­mer­cial­i­sa­tion as yet be­cause of (i) wide band gap (~ 3.2 eV), (ii) large over­po­ten­tial for hy­dro­gen evo­lu­tion, and (iii) rapid re­com­bi­na­tion of elec­tron-hole pairs in TiO based cat­a­lysts. Re­cently a nano-hy­brid of Au on

15

2

TiO has been found to make as high as 647,000 mol

2 of H per hour per gram of the cat­a­lyst, but it is still in

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2 lab­o­ra­tory level only. In fact, as on now, the other two meth­ods, viz. ther­mo­chem­i­cal cy­cle and pho­to­voltaics based elec­trol­y­sis ap­pear to be more promis­ing than the pho­to­cat­alytic split­ting of wa­ter. (b) Wa­ter split­ting by ther­mo­chem­i­cal Cy­cle

In a ther­mo­chem­i­cal cy­cle, one highly en­dother­mic de­com­po­si­tion re­ac­tion is car­ried out at a very high tem­per­a­ture us­ing so­lar ra­di­a­tion, which is in­tensely con­cen­trated by a ring of par­a­bolic mir­rors. O

2 is evolved dur­ing this de­com­po­si­tion re­ac­tion. This is called the re­duc­tion step. In the next step, one of the de­com­po­si­tion prod­uct is re­acted with wa­ter at a rel­a­tively lower tem­per­a­ture or elec­trol­ysed in aque­ous medium gen­er­at­ing H and

2 the orig­i­nal re­ac­tant. Since H is elim­i­nated from wa­ter

2 in this step, this is called an ox­i­da­tion step. The process is shown schemat­i­cally in Fig. 3. When elec­trol­y­sis is done in the ox­i­da­tion step, it is called a hy­brid cy­cle.

In­nu­mer­able ther­mo­chem­i­cal pro­cesses are pos­si­ble on the ba­sis of ther­mo­dy­namic data, but only a few are con­sid­ered to be com­mer­cially vi­able. Some of the promis­ing ther­mo­chem­i­cal cy­cles are shown in Ta­ble 4. Two such pro­cesses are dis­cussed be­low for il­lus­tra­tion and these are: (a) zinc ox­ide cy­cle, which is a di­rect ther­mo­chem­i­cal cy­cle, mean­ing all steps are chem­i­cal,

and (b) hy­brid sul­phur cy­cle.

Zinc Ox­ide Cy­cle: As shown in Ta­ble 4, in the re­duc­tion step zinc ox­ide is dis­so­ci­ated into Zn pow­der and O at a very high tem­per­a­ture of 1800-2000oC by

2 in­tensely con­cen­trated so­lar ra­di­a­tion. In the next step, zinc ox­ide is re­gen­er­ated and H is formed by hy­dro

2 ly­sis of zinc pow­der with H O at 450oC. This cy­cle has

2 at­tracted con­sid­er­able at­ten­tion be­cause zinc ox­ide is a non-haz­ardous, eas­ily avail­able, and a rel­a­tively benign ma­te­rial. But the ma­jor tech­ni­cal prob­lems are the re­com­bi­na­tion of Zn pow­der with O in­side the re­ac­tor

2 to form ZnO back again, and the rapid de­te­ri­o­ra­tion of the re­ac­tor ma­te­ri­als at such high tem­per­a­tures. The prob­lem of back re­ac­tion to ZnO could be solved by rapid quench­ing of Zn pow­der in ar­gon, but this led to the loss of some sen­si­ble heat. A 10-kW demon­stra­tion plant was es­tab­lished18, but the ac­tual ef­fi­ciency of the process in the pi­lot plant was found to be much less than the the­o­ret­i­cal ef­fi­ciency and the prob­lem of re­ac­tor dam­age could not be solved, too. There­fore,

19 there is some skep­ti­cism on the com­mer­cial vi­a­bil­ity of this process, and ac­cord­ing to some re­cent stud­ies, the non-sto­i­chio­met­ric per­ovskites, which lose O at lower

2 tem­per­a­tures, may prob­a­bly be a more promis­ing op­tion.

19, 20 The Hy­brid Sul­phur cy­cle or the HyS process: It con­sists of two steps: (a) a high tem­per­a­ture (at ~ 850oC) de­com­po­si­tion of H SO

2 4 to SO and O , fol­lowed

2 2 by (b) a low tem­per­a­ture (at ~ 100oC) elec­trol­y­sis step of ox­i­diz­ing SO to

2

H SO at the an­ode and

2 4 gen­er­at­ing pure H at the

2 cath­ode. The re­ac­tions are shown in Ta­ble 4. On the ba­sis of the stan­dard po­ten­tial of the over­all re­ac­tion (0.158 V), only 12.8% of the elec­tri­cal en­ergy is re­quired for the elec­trol­y­sis step of this cy­cle in com­par­i­son with the elec­tri­cal en­ergy re­quired for the elec­trol­y­sis of wa­ter (1.23 V).

The elec­tro­chem­i­cal ox­i­da­tion of sul­phur diox­ide was dis­cov­ered by West­ing­house in 1970s and has since been in­ten­sively in­ves­ti­gated on many elec­trode sys­tems us­ing plat­inum, gold, graphite, pal­la­dium, pal­la­dium ox­ide, plat­inum ox­ide, and plat­inum-gold al­loys in var­i­ous con­fig­u­ra­tions. It has been found that a high con­cen­tra­tion of sul­phuric acid is re­quired in the elec­trol­y­sis cell to max­i­mize the over­all en­ergy ef­fi­ciency of the cy­cle. But the Nafion

21 mem­brane in the elec­trol­yser cell, which re­quires to be hy­drated for pro­ton trans­fer across the cell, is also re­spon­si­ble for wa­ter mi­gra­tion into the an­ode com­part­ment. This, con­se­quently, leads to di­lu­tion of sul­phuric acid, and de­crease in cell ef­fi­ciency. How­ever, two de­vel­op­ments in re­cent years have given a push for a se­ri­ous con­sid­er­a­tion of this cy­cle: (1) the de­vel­op­ment of a bay­o­net-type re­ac­tor us­ing sil­i­con car­bide as ma­te­rial of construction to carry out ef­fi­ciently the ther­mal de­com­po­si­tion of the sul­phuric acid un­der so­lar ra­di­a­tion, (Fig.4), and (2) use of sul­phuric acid

22 doped poly­ben­z­im­i­da­zole-based mem­branes in place of Nafion in the elec­trol­y­sis part.

23

(c) Wa­ter split­ting by pho­to­voltaic elec­tric­ity

The split­ting of wa­ter into H and O by ap­ply­ing

2 2 elec­tric­ity is not new, but the gen­er­a­tion of elec­tric­ity at com­mer­cial level solely by us­ing sun­shine, and ap­ply­ing it to split wa­ter is a tech­nol­ogy un­der de­vel­op­ment for years. The suc­cess of hy­dro­gen econ­omy de­pends largely on how ef­fi­ciently the so­lar ra­di­a­tion is

con­verted to elec­tric­ity, and then how ef­fi­ciently this elec­tric­ity is used to gen­er­ate H in the elec­trol­yser cell,

2 or, in brief, on the so­lar-to-hy­dro­gen (STH) ef­fi­ciency. Thus, the de­vel­op­ment has two as­pects: one, the de­vel­op­ment of cheaper so­lar cell; two, de­vel­op­ment of a bet­ter elec­tro­cat­a­lyst that will re­duce the over­po­ten­tial of the O evo­lu­tion re­ac­tion.

2

The cost of H pro­duced by elec­trol­y­sis is still sig

2 nif­i­cantly higher than that pro­duced by steam meth­ane re­form­ing re­ac­tion (SMR). Ac­cord­ing to the US Depart­ment of En­ergy, for com­mer­cial vi­a­bil­ity, H

2 thresh­old cost should be USD 2.00–4.00 per gal­lon of gaso­line equiv­a­lent, whereas the most up-to-date re­ported H pro­duc­tion cost via elec­trol­y­sis is USD 3.26–

2

6.62 per gal­lon of gaso­line equiv­a­lent.

24

Among many de­vel­op­ments, men­tion may be made of a re­cently de­vel­oped pho­to­voltaic-elec­trol­y­sis sys­tem of a very high STH ef­fi­ciency. It con­sists of two poly­mer elec­trolyte mem­brane elec­trol­y­sers in se­ries with one triple-junc­tion so­lar cell which pro­duces a large-enough volt­age to drive both elec­trol­y­sers with no ad­di­tional en­ergy in­put. The triple junc­tion is made of InGaP (1.9 eV) / GaAs (1.4 eV) /GaInNAsSb (1.0 eV). The elec­trode assem­bly con­sists of car­bon pa­per/ plat­inum black/Nafion/Nafion mem­brane /Nafion/ irid­ium black/ti­ta­nium mesh. The sys­tem achieved a 48-h av­er­age STH ef­fi­ciency of 30%., and ac­cord­ing to the au­thors, this is the high­est ever ef­fi­ciency achieved so far.

24

(d) Hy­dro­gen by en­zy­matic method25,

Among many new de­vel­op­ments, men­tion may be made of a purely bi­o­log­i­cal process be­cause of its spec­tac­u­lar pro­duc­tion of hy­dro­gen from biomass, though it does not use any so­lar ra­di­a­tion. This process is known as cell-free syn­thetic en­zy­matic path­way bio­trans­for­ma­tion or shortly, as SyPaB. It uses a com­bi­na­tion of 13 en­zymes to con­vert car­bo­hy­drate (C H O ) to CO and H2 by com­plex path­ways. But the

6 10 5 2 most strik­ing fea­tures are that the re­ac­tions take place at 30oC and at­mo­spheric pres­sure, and the hy­dro­gen yield is very high – about 12 mol­e­cules of H per glu

2 cose equiv­a­lent in place of the usual 4. Also, it may be able to pro­duce hy­dro­gen from mu­nic­i­pal sewage and in­dus­trial waste wa­ter con­tain­ing very high de­gree of or­gan­ics. It is es­ti­mated that af­ter full scale up, this tech­nol­ogy may be able to bring down the cost of H to

2 about USD 2 per kg, but as on now, the method is in the lab­o­ra­tory level.

25, 26

Con­clu­sion

Hy­dro­gen econ­omy com­prises three as­pects: hy­dro­gen gen­er­a­tion, stor­age & trans­port, and ex­trac­tion of en­ergy from hy­dro­gen by fuel cells. This ar­ti­cle has dis­cussed very briefly each as­pect and some of the re­cent de­vel­op­ments. For more in­for­ma­tion, in­ter­ested read­ers may visit the web­sites of the US Depart­ment of En­ergy, Of­fice of En­ergy Ef­fi­ciency and Re­new­able En­ergy. These web­sites27 pro­vide in de­tail the lat­est de­vel­op­ments in hy­dro­gen econ­omy and fuel cells. It is in­evitable that hy­dro­gen would be the main driver of the world econ­omy in fu­ture. In Jan­uary 2017, at the end of the Davos Sum­mit, a global ini­tia­tive has been taken by sev­eral lead­ing en­ergy, trans­port and in­dus­try com­pa­nies of the world, and Hy­dro­gen Coun­cil has been formed with a mis­sion “to po­si­tion hy­dro­gen among the key so­lu­tions of the en­ergy tran­si­tion” 28. Mankind took a huge num­ber of mil­len­nia to tran­sit from wood and an­i­mals to coal, and a few hun­dred years from coal to oil. It may take now just a few decades to tran­sit from oil to hy­dro­gen.

Ac­knowl­edge­ment

The au­thor ex­presses his deep grat­i­tude to Mr Amit Modi, Di­rec­tor, Mod­i­con Pvt Ltd, Mum­bai, for pro­vid­ing re­search op­por­tu­ni­ties so that this ar­ti­cle could be writ­ten. Ref­er­ences

01. J. B. S. Hal­dane, “Daedalus OR Sci­ence and the fu­ture”, pa­per read on 4th Fe­bru­ary, 1923, be­fore the Cam­bridge So­ci­ety, The Heretics. As cited in https://en.wikipedia.org/ wiki/Hy­dro­gen_e­con­omy, Ac­cessed on May 01, 2018

02. En­ergy Equiv­a­lency of Fuels (LHV) / Hy­dro­gen Tools: https://h2­tools.org/hy arc/hy­dro­gen-data/en­ergy-equiv­a­lency-fuels-lhv (Ac­cessed on May 01, 2018)

03. B. Pivo­var, N. Rustagi and S. Satya­pal, “Hy­dro­gen at scale (H2@Scale) Key to a clean, eco­nomic and sus­tain­able en­ergy sys­tem”, In­ter­face, pub­lished by the Elec­tro­chem­i­cal So­ci­ety, 2018, 27 (1), 47.

04. Spe­cial Re­port on Re­new­able En­ergy Sources and Cli­mate Change Mit­i­ga­tion (SRREN), 2008 (www.ipcc-wg3.de/srren-re­port, ac­cessed on May 9, 2018).

05. US Depart­ment of En­ergy, Fuel Cell Tech­nolo­gies Of­fice, Hy­dro­gen and Fuel Cells Progress Over­view, Dr Su­nita Satya­pal, May 23, 2017, https://www. en­ergy.gov/sites/prod/ files/2017/05/f34/fc­to_­may_2017_h2_s­cale_wk­sh­p_satya­pal. pdf (Ac­cessed on May 05, 2018)

06. Y.-H. P. Zhang,”A sweet out-of-the-box so­lu­tion to hy­dro­gen econ­omy – is the sugar-pow­ered car sci­ence fic­tion?”, En­ergy En­v­i­ron. Sci., 2009, 2, p. 272.

07. B. D. Adams and Aicheng Chen, “The Role of Pal­la­dium in Hy­dro­gen Econ­omy”, Ma­te­ri­als To­day, 2011, 14 (6), 282.

08. P. Ver­meulen et al, “Hy­dro­gen stor­age in metastable MgyTi1-y films”, Elec­trochem Com­mu­ni­ca­tions, 2006, 8 (1), 27.

09. Y. Sun et al., “Ag Nanowires coated with Ag/Pd Al­loy Sheaths and Their Use as Sub­strates in Re­versible Ab­sorp­tion and Des­orp­tion of Hy­dro­gen”, J. Am. Chem. Soc., 2004, 126 (19), 5940.

10. B. D. Adams et al.,” Hy­dro­gen Elec­trosorp­tion into Pd-Cd Nanos­truc­tures”, Lang­muir, 2010, 26(10), 7632.

11. C. Sealy,” Prob­lem with plat­inum”, Ma­te­ri­als To­day, 2008, 11(12), 65.

12. S.Car­rion-Sa­torre et al, “Per­for­mance of car­bon-sup­ported pal­la­dium and pal­la­dium-ruthe­nium cat­a­lysts for al­ka­line mem­brane di­rect ethanol fuel cells”, Int. J. Hy­dro­gen En­ergy, 2016, 41 (21), 8954.

13. F. Al­caide et al, “Per­for­mance of car­bon-sup­ported PtPd as cat­a­lyst for hy­dro­gen ox­i­da­tion in the an­odes of pro­ton ex­change mem­brane fuel cells”, Int. J. Hy­dro­gen En­ergy, 2010, 35 (20), 11634.

14. https://www.h2­tools.org/hyarc/hy­dro­gen-data/hy­dro­gen­pro­duc­tion-en­ergy-conve rsion-ef­fi­cien­cies (Ac­cessed on May 10, 2018)

15. T. Ja­fari et al, “Pho­to­cat­alytic Wa­ter Split­ting – The Un­tamed Dream – A Re­view of Re­cent Ad­vances”, Mol­e­cules, 2016, 21, 900.

16. J. Zhang et al, “En­gi­neer­ing the Ab­sorp­tion and Field En­hance­ment Prop­er­ties of Au-TiO Nano-hy­brids via Whis­per­ing Gallery Mode Res­o­nances for Photo cat­alytic Wa­ter Split­ting” ACS Nano, 2016, 10(4), 4496 (as cited in Ref 21)

17. UNLV Re­search Foun­da­tion: So­lar Hy­dro­gen Gen­er­a­tion Re­search Fi­nal Re­port: https://www.osti.gov/servlets/ purl/1025597 (Ac­cessed on May 2, 2018)

18. R. Müller et al, “H2O-Split­ting Ther­mo­chem­i­cal Cy­cle Based on ZnO/Zn-Re­dox: Quench­ing the Ef­flu­ents from ZnO Dis­so­ci­a­tion”, Chem. Eng. Sci., 2008, 63, 217.

19. M. B.Gorensek et al, “So­lar Ther­mo­chem­i­cal Hy­dro­gen (STCH) Pro­cesses”, In­ter­face, pub­lished by the Elec­tro­chem­i­cal So­ci­ety, 2018, 27 (1), 53.

20. C. N. R. Rao et al, ”So­lar Ther­mo­chem­i­cal Split­ting of Wa­ter to Gen­er­ate Hy­dro­gen”, Proc. Na­tional Acad. Sci., 2017, 114 (51), 13385.

21. P. W. T. Lu et al, “An In­ves­ti­ga­tion of Elec­trode Ma­te­rial for the An­odic Ox­i­da­tion of Sul­phur Diox­ide in Con­cen­trated Sul­phuric Acid”, J. Elec­trochem. Soc., 1980, 127 (12), 2610.

22. M. B. Gorensek et al, “En­ergy Ef­fi­ciency Lim­its for a Re­cu­per­a­tive Bay­o­net Sul­phuric Acid De­com­po­si­tion Re­ac­tor for Sul­phur Cy­cle Ther­mo­chem­i­cal Hy­dro­gen Pro­duc­tion”, Ind. Eng. Chem. Res., 2009, 48, 7232; R. Moore et al, US Patent 764,5437 B1 (2010).

23. J. V. Jayaku­mar et al.,“Poly­ben­z­im­i­da­zole Mem­branes for Hy­dro­gen and Sul­phuric Acid Pro­duc­tion in the Hy­brid Sul­phur Elec­trol­yser”, ECS Elec­trochem. Lett., 2012, 1, F44.

24. J. Jia et al, “So­lar Wa­ter Split­ting by Pho­to­voltaic-Elec­trol­y­sis with a So­lar-to_Hy­dro­gen ef­fi­ciency over 30%”, Na­ture Com­mu­ni­ca­tions, Oc­to­ber 2016, DOI: 10.1038/ncomms13237

25. Y.-H.P. Zhang, “Hy­dro­gen Pro­duc­tion from Car­bo­hy­drates – A Mini Re­view”, in “Sus­tain­able Pro­duc­tion of Fuels, Chem­i­cals and Fibers from For­est Biomass”, Eds. J. Zhu et al, ACS Sym­po­sium Se­ries, Amer­i­can Chem­i­cal So­ci­ety, Wash­ing­ton D.C., 2011, Chap­ter 8.

26. Y.-H.P. Zhang et al, “High Yield Hy­dro­gen Pro­duc­tion from Starch and Wa­ter by a Syn­thetic En­zy­matic Path­way”, PLOS One (DOI: 10.1371/jour­nal.pone.0000 456), 2007, May 23, 2, e456.

27. https://www1.eere.en­ergy.gov/li­brary/de­fault.aspx

28. http://www.fch.eu­ropa.eu/news/launch-hy­dro­gen-coun­cil; http://hy­dro­gen­coun­cil.com/.

Data source: Ref. 6

Source: Mod­i­fied from Ref.3 (F in Col­umn 5: Fara­day con­stant = 96,485 coulombs)

Fig 2. Schematic di­a­gram of a pro­ton ex­change mem­brane fuel cell

Source: Mod­i­fied in SI units from Ref. 14

Fig 3. Schematic pre­sen­ta­tion of a so­lar ther­mo­chem­i­cal cy­cle

Fig 4. A Schematic Di­a­gram of the Bay­o­net-type Re­ac­tor22

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