Pope Francis is known for having strong opinions and leading one of the most progressive papacies of all time — and, as a Purdue University professor recently found out, he also laughs at his own jokes.

It's the kind of quality one can really only find out about if they're lucky enough to wrangle a private meeting with him. Which, years after initial efforts to make it happen, engineering technology professor John Sheffield just did.

Sheffield, also president of the International Association for Hydrogen Energy, and a group of four colleagues from the association met with Francis at the Vatican on Feb. 7.

The purpose of the meeting was simple: Talk to one of the most powerful men in the world about hydrogen energy technology that Sheffield and his colleagues believe could change the future.

And, as a leading voice in environmental discussions worldwide, the pope's focus on climate-centered messaging made him just the right man for the job.

"His desire is the moral issue of it, like it's a sin if you don’t do something to address the challenge of climate change," Sheffield said. "And our perspective was that hydrogen energy is a sustainable, and we believe, the best solution for energy (issues related to climate change)."

Hydrogen energy involves using hydrogen fuel cell technology to create highly efficient power. Some companies, including Indiana-based engine company Cummins, Inc., are testing zero-emission hydrogen fuel cells as a replacement for diesel- or gas-powered engines.

Sheffield has high hopes for this technology's potential impact on the planet. Being able to discuss this clean-energy option with someone as significant as the pope, he said, was humbling.

"To be honest, I’m still pinching myself," Sheffield said. "The way that the individual handled himself … words can't describe the reaction of seeing somebody like that."

Pope Francis' climate concerns

Francis surprised many when he released an encyclical letter in 2015 calling on people to do what they could to counteract climate change. The 184-page letter is one of the strongest messages a pope has ever sent addressing the issue.

"We are faced not with two separate crises, one environmental and the other social, but rather with one complex crisis which is both social and environmental," he said in the letter.

Since then, the Argentina-born pope has continued to speak publicly on climate change, including hinting in November at making acts of destruction against the Earth an official "ecological sin."

Earlier last year, Francis met with executives of the world's leading energy companies to urge sustainable changes. The recency of this event, Sheffield said, makes it particularly beneficial for him and his colleagues to present the pope with hydrogen energy's potential.

Energy:Controversial coal bill passes off the House floor, heads to Indiana Senate

Opponents:Hoosier National Forest burn could hurt drinking water for 140K people

Also fortuitous is the pope's scientific knowledge. With a background in chemistry, the pope is also familiar with hydrogen energy — he told Sheffield he began studying the topic before publishing his encyclical in 2015.

This combination of Francis' qualities made this meeting all the more valuable, Sheffield said. The pope's influence, he said, is unique in that it can create change faster than it would naturally happen.

'We never imagined that would happen'

Shortly after Francis was elected in 2013, Sheffield and his colleagues got the idea to reach out about setting up a meeting.

Another member of the International Association for Hydrogen Energy named Juan Carlos Bolcich also hailed from Argentina and had connections to the pope, but did not know him personally. Bolcich set about making the meeting happen.

Unsurprisingly, the process to secure an audience with the pope is long. Sheffield got an email with an invitation for a private audience with the pope in December — almost seven years later.

"The lesson there is that it sometimes takes time," Sheffield said, chuckling.

The next few weeks were a whirlwind. Sheffield read as much as he could and watched documentaries to acquaint himself with the pope's history and values.

He also made a point to research protocol. As it's customary to exchange gifts, he decided to bring the pope a copy of 'Ever True: 150 Years of Giant Leaps at Purdue.' In exchange, Francis presented Sheffield and his colleagues with a medal commemorating his seventh year as pope.

Sheffield said he was touched by the pope's humanity and his attention to their conversation and gifts.

"When he opened up the book, he did a double take and relooked and read something in it," Sheffield said. "It was just humbling."

One of those also in the meeting was Sheffield's mentor, Nejat Veziroglu, the 95-year-old former president of the international hydrogen association.

Veziroglu, who retired as president of the international hydrogen association in March, said the meeting was one of the most important he'd had in his life. The pope's position offers a unique voice for promoting the benefits of hydrogen energy worldwide, he said.

"I've met with government leaders and scientists, but I think this was the most memorable," Veziroglu said. "If Pope Francis starts talking about the solution to climate change, it will speed up the conversion from fossil fuels to clean fuels."

Toward the end of the half-hour private meeting, Pope Francis started telling a joke, Sheffield said.

But when he started laughing, everyone was so out of their comfort zone that they didn't get it — and then they realized he hadn't even been able to finish the line.

"He'd already started laughing before he got to the punch line," Sheffield said, laughing himself. "We never imagined that would happen.”

What's next

Closer to home, Sheffield said he has big hopes for hydrogen energy in the Hoosier state. Cummins' actions in acquiring a major hydrogen fuel cell company last year, for example, has put the state in the spotlight for cutting-edge hydrogen technology.

"We should be proud of what Cummins has done," Sheffield said. "To see how that can happen, there’s no better place than to look at than them.”

Sheffield and those who also met with the pope prepared for him a petition, signed by five members of the international hydrogen association, with recommendations to support hydrogen energy as part of the transition to a clean-energy future.

In return, Sheffield said, the pope left them much simpler next steps.

"The pope said he had two things for us to do ... The first one was to pray for him," Sheffield said. "And the second one was to continue our work, and that was it.”

Contact IndyStar reporter London Gibson at 317-419-1912 or lbgibson@gannett.com. Follow her on Twitter @londongibson.

Connect with IndyStar’s environmental reporters: Join The Scrub on Facebook.

IndyStar's environmental reporting project is made possible through the generous support of the nonprofit Nina Mason Pulliam Charitable Trust.